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Intercellular communication is a critical process that ensures cooperation between distinct cell types and maintains homeostasis. EVs, which were initially described as cellular debris and devoid of biological function, are now recognized as key components in cell–cell communication. EVs are known to carry multiple factors derived from their cell of origin, including cytokines and chemokines, active enzymes, metabolites, nucleic acids, and surface molecules, that can alter the behavior of recipient cells. Since the cargo of EVs reflects their parental cells, EVs from damaged and dysfunctional tissue environments offer an abundance of information toward elucidating the molecular mechanisms of various diseases and pathological conditions. In this review, we discuss the most recent findings regarding the role of EVs in the progression of cancer, metabolic disorders, and inflammatory lung diseases given the high prevalence of these conditions worldwide and the important role that intercellular communication between immune, parenchymal, and stromal cells plays in the development of these pathological states. We also consider the clinical applications of EVs, including the possibilities for their use as novel therapeutics. While intercellular communication through extracellular vesicles (EVs) is key for physiological processes and tissue homeostasis, injury and stress result in altered communication patterns in the tissue microenvironment. When left unchecked, EV-mediated interactions between stromal, immune, and parenchymal cells lead to the development of disease states Video Abstract Supplementary Information The online version contains supplementary material available at 10.1186/s12964-021-00787-y.

Human activities have increasingly impacted rivers globally and influence environmental factors at multiple scales: proximal factors (e.g., light and nutrients) and distal controls (e.g., riparian cover and forest area in the basin) that affect benthic metabolism or algal assemblage structure. Function and structure have been studied separately and thus the linkage between them is not well understood. Furthermore, how much of the variability of benthic metabolism can be explained by changes in the environmental variables is inconclusive. We measured the structural and metabolism of epilithic algae in streams across gradients of catchment forest cover (3.5 to 88.6%) and riparian zone canopy cover (0 to 91.0%) in South East Queensland, Australia. Overall net daily metabolism (NDM) on cobbles was positive (gross primary production, GPP exceeded respiration, R) for all but three of the 34 sites. Chl a biomass of benthic algae on cobbles ranged from 1.37 to 178.05 mg m−2. Shaded streams had the highest percentage of diatoms (95.4%), while the least shaded streams were dominated by cyanobacteria (82.2%). Partial least squares (PLS) regression modeling showed that most of the variation in GPP, R, and NDM was explained by light intensity (photosynthetic photon flux density, PPFD), phosphorus concentration in water, water temperature, and the percentage of mid-dense forest upstream of the site. Path analysis (structural equation modeling, SEM) indicated that, as a surrogate for metabolism, diatom (Bacillariophyta) density was the best predictor of GPP and NDM when we assessed the environmental process in the streams. In comparison, Chl a, which has been traditionally used as a surrogate for measurements of GPP or NDM, performed well only when the percentage of diatoms in the epilithic algal community exceeded 75%. Our results also indicated that both the structure and function of benthic algal communities were sensitive to anthropogenic modifications, including riparian vegetation and forest cover in the catchment in river ecosystems.

Anthropogenic changes to river flows can alter hydrological connectivity and cues necessary for the movement of fish to complete their life cycles. Quantifying flow-related movement ecology of fish and understanding how this varies between species and river systems is important for effective environmental flow management. This study aimed to determine hydroecological factors that influence fish movements in an unregulated, perennial river and to compare these findings to fish from regulated river systems. Broad-scale movements of the endangered Maccullochella ikei and Tandanus tandanus were recorded over 3 years in the unregulated, perennial Nymboida River, Australia. The limited movements both species exhibited were infrequent and over short distances. Although M. ikei movements appeared mostly unrelated to environmental changes, T. tandanus moved on flow pulse peaks and were more likely to move during the breeding season. These findings contrast with previous studies of the same or similar species in differing flow regimes, suggesting that fish in perennial, highly connected rivers may not need to move as frequently as those in more regulated or intermittent systems. Should these disparate behaviours be present in other species occurring among contrasting flow regimes, it will be challenging to define generalisable environmental flow rules to inform river management.

It is often assumed that invertebrate consumers in small tropical streams are dependent on allochthonous sources, although recent studies indicate that algae can form the base of food webs in tropical streams. Fish in tropical streams can feed across several trophic levels and the origin and path of energy and nutrient flow is uncertain for many species. We collected fish, insects, periphyton, and leaf litter from 20 streams across four Atlantic Forest catchments. We analysed stomach contents of fish to define trophic guild and fish dietary trophic position. We also analysed stable isotopes of carbon and nitrogen of fish and their resources to identify the main basal resources of the food web and to estimate trophic positions and identify the path of energy flow. We found that autochthonous sources were the primary resource base for fish communities. Trophic positions estimated from diet and isotopes were similar and correlated for insectivore and algivore–insectivore fish, but not for algivore–detritivore or omnivore fish. Using path analysis, fish classified as algivore–detritivores appear to have derived their biomass through a diet of primary consumer insects and periphytic algae and thus, are more likely to play a trophic role as algivore–insectivores in these streams. However, omnivores probably derived much of their biomass from aquatic insects. Our findings support other studies of tropical systems in which the main basal resource is autochthonous, even in small streams. We also show that the assignment to a specific trophic guild for some fish species, based on gut contents, does not reflect what they assimilate into their bodies. In some species, food sources that are uncommon can make a disproportionately important contribution to their biomass. This study affirms the important role of inconspicuous algal resources in aquatic food webs, even in small forested streams, and demonstrates the effectiveness of taking a combined approach of diet analysis, isotopic tracing, and modelling to resolve food web pathways where the level of omnivory is high.

Translocation of individuals is a widely used tool in the conservation of threatened species. The movement behaviours of translocated individuals in their new environment are a key factor that can influence translocation success (i.e. survival and reproduction). In this study, eel‐tailed catfish (Tandanus tandanus ) and Murray cod (Maccullochella peelii ) movements were monitored using fine‐scale acoustic telemetry over a 5‐month period in two lowland rivers in eastern Australia. Fine‐scale movement and habitat selection were compared among translocated lacustrine and resident riverine T. tandanus and between species. We found no difference in fine‐scale movement behaviours between translocated and resident T. tandanus ; however, there was some variation in movement response to environmental variation between species. In contrast, habitat selection varied not only between the two species, but also between resident and translocated T. tandanus individuals. Notably, activity was significantly lower in the first day after release compared to the remainder of the study period. Although T. tandanus translocated from a reservoir had never experienced the environmental fluctuations of a riverine system, individuals still responded in the same way as resident riverine fish, suggesting an innate behavioural response. However, this was not the case for habitat selection, where translocated individuals preferred habitats more common in their source population's lacustrine environment. The findings of this study are important in guiding future conservation efforts involving the translocation of wild‐caught fish, primarily the importance of the suitable habitat at the release site to ensure the success and persistence of translocated populations.

Closely related species may have different patterns of distribution and spatial population structure in common landscapes, which can have implications for conservation of the species if they are vulnerable. The Australian mountain galaxias species complex has recently been split into 15 described species, 11 of which are threatened. In this study, we assessed the distribution and landscape genetic structure of two mountain galaxias species (Galaxias olidus and Galaxias oliros) in the Goulburn–Broken catchment (Murray–Darling Basin) in relation to hydrographic descriptors of river terrain, network and hydrology. We also assessed small‐scale population structure in juvenile G. oliros in tributary streams of the Goulburn River (the Granite Creeks). Analyses of distribution revealed that G. olidus mostly occurs in smaller streams at high elevations with steep valley slopes and high stream gradients downstream, whereas G. oliros mostly occurs in larger downstream waterways with higher magnitude and variability of stream flow. Population genetic structure in G. olidus was much greater than for G. oliros, with 17 of the 19 sampled locations resolved as genetically distinct populations for G. olidus, but only a single population revealed for G. oliros. Results for both species did not reflect the expectations of the stream hierarchy model or isolation‐by‐distance model. However, small‐scale spatial population structure in juvenile G. oliros in the Granite Creeks reflected the expectations of both stream hierarchy and isolation‐by‐distance models, suggesting that population structure of juveniles of this species reflects some level of spatial aggregation of closely related individuals that probably diminishes as the population ages. Higher landscape genetic structure in G. olidus was correlated with steeper stream and valley slopes, whereas no population genetic correlations were significant for G. oliros. Genetic diversity in G. olidus was higher in areas of greater hydrological variability, suggesting that populations in hydrologically variable areas function as a metapopulation and have a more diverse gene pool than populations in areas with lower hydrological variability. In contrast, genetic diversity in G. olidus was lower in higher elevation areas with steep valley slopes, suggesting that smaller but more stable populations occur in high elevation streams with high perenniality and low interannual variability of flows. Our results are consistent with other studies that highlight the importance of supporting natural hydrological regimes and natural patterns of hydrological connectivity for conservation of vulnerable freshwater fish.

There is growing recognition of the importance of food quality over quantity for aquatic consumers. In streams and rivers, most previous studies considered this primarily in terms of the quality of terrestrial leaf litter and importance of microbial conditioning. However, many recent studies suggest that algae are a more nutritional food source for riverine consumers than leaf litter. To date, few studies have quantified longitudinal shifts in the nutritional quality of basal food resources in river ecosystems and how these may affect consumers. We conducted a field investigation in a subalpine river ecosystem in Austria to investigate longitudinal variations in diet quality of basal food sources (submerged leaves and periphyton) and diet source dependence of stream consumers (invertebrate grazers, shredders, filterers and predators, and fish). Fatty acid (FA) profiles of basal food sources and their consumers were measured. Our results indicate systematic differences between the FA profiles of terrestrial leaves and aquatic biota, that is periphyton, invertebrates and fish. Submerged leaves contained very low proportions of long-chain polyunsaturated fatty acids (LC-PUFAs), which were conversely rich in aquatic biota. While the FA composition of submerged leaves remained similar among sites, the LC-PUFAs of periphyton increased longitudinally, which was associated with increasing nutrients from upstream to downstream. Longitudinal variations in periphyton LC-PUFAs were reflected in the LC-PUFAs of invertebrate grazers and shredders, and further tracked by invertebrate predators and fish. However, brown trout Salmo trutta contained a large proportion of docosahexaenoic acid (DHA, 22:6ω3), a LC-PUFA almost entirely missing in basal sources and invertebrates. The fish accumulated eicosapentaenoic acid (EPA, 20:5ω3) from invertebrate prey and may use this FA to synthesize DHA. Our results provide a nutritional perspective for river food web studies, emphasizing the importance of algal resources to consumer somatic growth and the need to account for the longitudinal shifts in the quality of these basal resources.

Despite providing considerable benefits to society, dams and weirs threaten riverine ecosystems by disrupting movement and migration of aquatic animals and degrading riverine habitats. Whilst the ecological impacts of large dams are well studied, the ecological effects of low-head weirs that are periodically drowned out by high flows are less well-understood. Here we examine the effects of a low-head weir on fine- and broad-scale movements, habitat use, and breeding behaviour of three species of native freshwater fish in the Nymboida River in coastal eastern Australia. Acoustic telemetry revealed that eastern freshwater cod (Maccullochella ikei) and eel-tailed catfish (Tandanus tandanus) made few large-scale movements, but Australian bass (Percalates novemaculeata) upstream of the weir were significantly more mobile than those below the weir. Within the weir pool, all three species displayed distinctive patterns in fine-scale movement behaviour that were likely related the deeper lentic environment created by the weir. No individuals of any species crossed the weir during the study period. Tandanus tandanus nesting behaviour varied greatly above and below the weir, where individuals in the more lentic upstream environment nested in potentially sub-optimal habitats. Our results demonstrate the potential effects of low-head weirs on movement and behaviour of freshwater fishes.

Aquatic macroinvertebrates play an important functional role in energy transfer in food webs, linking basal food sources to upper trophic levels that include fish, birds, and humans. However, the trophic coupling of nutritional quality between macroinvertebrates and their food sources is still poorly understood. We conducted a field study in subalpine streams in Austria to investigate how the nutritional quality (measured by long‐chain polyunsaturated fatty acids, LC‐PUFAs) in macroinvertebrates changes relative to their basal food sources. Samples of macroinvertebrates, periphyton, and leaves were collected from 17 streams in July and October 2016 and their fatty acid (FA) composition was analyzed. Periphyton FA varied strongly with time and space, and their trophic effect on macroinvertebrate FA differed among functional feeding groups. The match between periphyton FA and macroinvertebrate FA decreased with increasing trophic levels, but LC‐PUFA content increased with each trophic step from periphyton to grazers and finally predators. Macroinvertebrates fed selectively on, assimilated, and/or actively controlled their LC‐PUFA, especially eicosapentaenoic acid (EPA, 20 : 5ω3) relative to their basal food sources in the face of spatial and temporal changes. Grazer FA profiles reflected periphyton FA with relatively good fidelity, and especially their EPA feeding strategy was primarily linked to periphyton FA variation across seasons. In contrast, shredders appeared to preferentially assimilate more EPA over other FA, which was determined by the availability of high‐quality food over seasons. Predators may more actively control their LC‐PUFA distribution with respect to different quality foods and showed less fidelity to the basal FA profiles in plants and prey. Overall, grazers and shredders showed relatively good fidelity to food FA profiles and performed as both “collectors” and “integrators” for LC‐PUFA requirements across seasons, while predators at higher trophic levels were more “integrators” with added metabolic complexity leading to somewhat more divergent FA profiles. These results are potentially applicable for other aquatic consumers in freshwater and marine ecosystems.

Sustained water level drawdown followed by rewetting has the potential to shift macrophytes from a nutrient sink to a source, but the fate of macrophyte-derived nutrients after rewetting is not well understood. We investigated the fate of released nitrogen (N) from isotopically labeled macrophyte litter over a 4-week period after rewetting. We used three treatments: (1) different species in the litter (invasive Cabomba caroliniana vs. native Hydrilla verticillata); (2) sediment desiccation history (“dried then rewetted” vs. “constantly wet”); (3) living macrophytes (presence vs. absence). Our results showed that the Cabomba litter treatment had a significantly higher percentage of macrophyte-derived 15N in the dissolved fraction of the water column and assimilated by phytoplankton (higher chlorophyll a concentrations), compared to the Hydrilla litter. The treatments with sediment desiccation also had a significantly higher percentage of 15N in the dissolved fraction and used by phytoplankton after rewetting, but less in the sediment, compared to the constantly wet treatment. The presence of living macrophytes partially reduced the 15N increase in the water column and/or sediment, but the scale of the effect was species- and biomass-dependent. Our study showed that sediment pre-desiccation changed the fate of macrophyte-derived 15N after rewetting, increasing the impact on water quality. This was further exacerbated by the presence of litter from an invasive species. This study highlights the need to manage water levels to maintain healthy macrophyte beds and thereby addressing water quality issues in lakes and reservoirs.

1. Streams and rivers provide crucial sources of long-chain polyunsaturated fatty acids (LC-PUFA, a subset of PUFA with ≥20 C in their acyl chains) for freshwater and terrestrial animals. LC-PUFA are primarily produced by particular algal taxa and are subsequently transferred to fish and higher consumers, including humans. Although research on the trophic transfer of LC-PUFA in streams is very limited, studies on the PUFA composition of organisms at various trophic levels within stream food webs are well documented. Here, we systematically analyze the literature to synthesise LC-PUFA distribution and retention in stream ecosystems. 2. In general, stream food webs are highly enriched in omega-3 LC-PUFA, in particular eicosapentaenoic acid (EPA, 20:5n-3) and docosahexaenoic acid (DHA, 22:6n-3). Algae from temperate rivers had more EPA compared with algae from subtropical rivers. Stream invertebrates from both temperate and subtropical rivers preferentially retain algal EPA and their PUFA tend to vary with algal PUFA. Invertebrates may be capable of regulating their fatty acid (FA) composition, but this ability is limited, to the extent that they must obtain physiologically important LC-PUFA primarily from algae. 3. DHA is selectively and highly retained in all fish taxa irrespective of their feeding sites (i.e. temperate or subtropical rivers). However, DHA content is variable among stream fish taxa and is likely linked to resource utilisation and their endogenous metabolism. Freshwater salmonids have a very high DHA content, which suggests they preferentially retain this FA or biosynthesise it from EPA. The PUFA profiles of other fish taxa more closely reflect their dietary PUFA supply. However, so far very few studies have considered the differences in the PUFA biosynthesis of fish taxa in rivers or compared the dietary PUFA effect on different fish taxa. 4. The supply of LC-PUFA from streams and rivers is known to be affected by altered light availability, greater nutrient inputs and other anthropogenic activities, and any decrease in algal LC-PUFA production will negatively affect the consumers that are nutritionally dependent on these molecules. Further investigations are needed to understand how environmental changes affect LC-PUFA production and retention in stream ecosystems, and how to protect streams from threats to this essential ecosystem service.

The structure of food webs may be strongly influenced by the distribution of top predators in space and time. The Amazon biome is the only region in the world where four alligatorid species are known to occur in sympatry, and they attain high densities in some regions. As top predators with a diverse range of prey species occupying different trophic levels, their impact upon food webs should be substantial, but the degree to which crocodilians differ in their food sources, and potentially avoid competitive exclusion where they occur syntopically is not well understood. Although most crocodilians are considered generalist opportunistic predators that feed on any source of protein available in the environment, Amazonian crocodilians show broad differences in the proportions of prey items they consume. It is believed that these differences may in part reflect habitat use, but it is unknown to what extent they represent interspecific differences in prey preferences or are a direct function of habitat selection. Stable carbon isotope data (δ13C) of crocodilians and their potential prey were used to assess differences in reliance on terrestrial versus aquatic resources. These data were then placed in a spatial context using classified maps that reflect habitat types (headwater streams, mid-order flooded-forest streams and varzea floodplains) to elucidate whether dietary differences are explained by habitat selection or are more likely a reflection of prey preferences. We found evidence for differences in types of basal resources supporting these crocodilians. Mean δ13C values were highest in Paleosuchus trigonatus (Schneider's dwarf caiman, −25.7 ± 1.2‰), intermediate in Caiman crocodilus (spectacled caiman, −27.4 ± 1.2‰) and Paleosuchus palpebrosus (Cuvier's dwarf caiman, −27.7 ± 1.1‰) and lowest in Melanosuchus niger (black caiman, −29.9 ± 1.3‰). A progressive decrease in δ13C values of crocodilian tissues occurred from headwaters to floodplains, which most likely reflects a progressive increase in autochthonous over allochthonous inputs in lower reaches of streams. The shift from terrestrial to aquatic resources sustaining these sympatric predators mirrors their spatial distribution along this ecotone. However, after taking into account the habitat in which pairs of syntopic individuals of distinct species occurred, significant differences in δ13C values suggest that P. trigonatus and P. palpebrosus have different prey bases. Thus, despite being opportunistic predators, our results show that differences in crocodilian diets likely result from prey preferences and not only from habitat selection. These findings suggest that some species of crocodilians may be less generalist than traditionally thought and their influence on terrestrial or aquatic food webs might be species specific.

Tropical floodplains are one of the most productive ecosystems on earth. Studies on floodplain productivity have primarily focused on trees and macrophytes, rather than algae, due to their greater biomass. However, epiphyton—algae and bacteria attached to the submerged portion of aquatic macrophytes—is a major source of energy in many tropical floodplains. Epiphyton productivity rates are unknown for most tropical floodplain wetlands, and spatial variability is not well understood. In this study, we measured primary productivity of epiphyton in Kakadu National Park in northern Australia. We estimated the relative contribution of epiphyton to aquatic production (epiphyton, + phytoplankton + macrophytes). We sampled sites dominated by different macrophyte structural types: vertical emerging grasses, horizontal emerging grasses, submerged macrophytes, and macrophytes with floating leaves. Epiphyton productivity was highly influenced by the structural type of the macrophyte. Highest potential productivity per weight was measured from epiphyton growing on macrophytes with floating leaves and horizontal grasses (1.52 ± 0.53 and 1.82 ± 0.61 mgC/dw g epiphyton/h, respectively) and lowest in submerged macrophytes and vertical grasses (0.57 ± 0.26 and 0.66 ± 0.47 mgC/dw g epiphyton/h, respectively). When considering the areal biomass of the macrophyte and the amount of epiphyton attached, epiphyton on horizontal grasses and submerged macrophytes had productivity values approximately ten times higher (45–219 mgC/m2/d) compared to those on vertical grasses and macrophytes with floating leaves (2–18 mgC/m2/d). Epiphyton contributed between 2 to 13 percent to the aquatic production of these tropical floodplain wetlands.

Article, 1. Many freshwater systems receive substantial inputs of terrestrial organic matter. Terrestrially derived dissolved organic carbon (t-DOC) inputs can modify light availability, the spatial distribution of primary production, heat, and oxygen in aquatic systems, as well as inorganic nutrient bioavailability. It is also well-established that some terrestrial inputs (such as invertebrates and fruits) provide highquality food resources for consumers in some systems. 2. In small to moderate-sized streams, leaf litter inputs average approximately three times greater than the autochthonous production. Conversely, in oligo/mesotrophic lakes algal production is typically five times greater than the available flux of allochthonous basal resources. 3. Terrestrial particulate organic carbon (t-POC) inputs to lakes and rivers are comprised of 80%–90% biochemically recalcitrant lignocellulose, which is highly resistant to enzymatic breakdown by animal consumers. Further, t-POC and heterotrophic bacteria lack essential biochemical compounds that are critical for rapid growth and reproduction in aquatic invertebrates and fishes. Several studies have directly shown that these resources have very low food quality for herbivorous zooplankton and benthic invertebrates. 4. Much of the nitrogen assimilated by stream consumers is probably of algal origin, even in systems where there appears to be a significant terrestrial carbon contribution. Amino acid stable isotope analyses for large river food webs indicate that most upper trophic level essential amino acids are derived from algae. Similarly profiles of essential fatty acids in consumers show a strong dependence on the algal food resources. 5. Primary production to respiration ratios are not a meaningful index to assess consumer allochthony because respiration represents an oxidised carbon flux that cannot be utilised by animal consumers. Rather, the relative importance of allochthonous subsidies for upper trophic level production should be addressed by considering the rates at which terrestrial and autochthonous resources are consumed and the growth efficiency supported by this food. 6. Ultimately, the biochemical composition of a particular basal resource, and not just its quantity or origin, determines how readily this material is incorporated into upper trophic level consumers. Because of its highly favourable biochemical composition and greater availability, we conclude that microalgal production supports most animal production in freshwater ecosystems., final draft, http://purl.org/eprint/status/PeerReviewed

Recent investigators have emphasized that food quality has greater importance than food quantity for herbivore somatic growth. Herbivore growth is thought to be regulated by light and nutrients through changes in food elemental ratios, but our knowledge of the mechanisms driving grazer growth in stream ecosystems is scarce. We manipulated light (through shading) and nutrients (fertilizer addition) in 3 headwater streams in southeastern Queensland, Australia, and measured the responses of natural grazer communities. The growth responses of Austrophlebioides and Helicopsyche to light and nutrients differed with their body size and periphyton food quality. Large larvae were more sensitive to increased light and nutrient availability than were small larvae. Light induced a significant negative effect on Austrophlebioides growth, and this impact increased with increasing nutrient addition. Nutrients had a pronounced positive effect on Helicopsyche growth regardless of light intensity. These responses r...

Algae are higher-quality food resources than allochthonous plant litter for stream invertebrates, in part, because of their higher content of polyunsaturated fatty acids (PUFAs). We tested the hypothesis that algal biofilms on the surfaces of leaf litter improve the nutritional quality of terrestrial inputs for invertebrate shredders. We used a laboratory feeding experiment with 2 light levels (open and shaded) and 2 nutrient regimes (ambient and enriched) to manipulate the algal biofilms that form on leaf surfaces (Lophostemon confertus). We assessed how these treatments affected the fatty acid (FA) composition of these biofilms and the somatic growth of a stream invertebrate shredder (Anisocentropus bicoloratus, Trichoptera) that feeds on the conditioned leaf litter. Shredders reached a significantly larger size when nutrients were added, and leaf mass loss was significantly greater in these treatments than in treatments without nutrients. Shredder growth was affected significantly by leaf PUFA ...

The Australian lungfish, Neoceratodus forsteri Krefft, 1870, is a threatened species whose long-term persistence is at risk due to land-use intensification, water resource development, and other human pressures. Changes to the hydrology of rivers has the potential to alter the availability of certain high-quality food resources for this species, that may impact recruitment success, and contribute to population declines. This study analysed the fatty acid (FA) composition of lungfish eggs and fin tissues from two locations upstream and downstream of a large dam in the Brisbane River. We tested the hypothesis that river impoundment and flow alteration associated with the dam have altered the dietary composition and the FA composition of important dietary items for N. forsteri which translates to the body tissues and eggs. The contribution of each food source was estimated with mixing models using carbon and nitrogen stable isotopes. The total FA content in lungfish fin and eggs was significantly higher in downstream sites compared to upstream. Few significant differences in FA contents of the nine potential lungfish food sources were found between sites upstream and downstream of the dam. Stable isotope analyses of lungfish fin tissues revealed the most likely food sources were gastropods, bivalves, and crustaceans, however their relative importance differed upstream and downstream of the dam. Collectively, these results indicate that the dam did not negatively affect food quality for lungfish downstream. The most likely mechanism for potential FA deficiency and subsequent impacts on recruitment success in N. forsteri would be due to changes to the availability of high-quality food sources. This study highlights the need for future research to determine whether the low FA contents we observed in lungfish, is a function of broader environmental changes, or if FA contents are naturally low for this sub-tropical species.

According to the River Continuum Concept, headwater streams are richer in allochthonous (e.g. terrestrial leaves) than autochthonous (e.g. algae) sources of organic matter for consumers. However, compared to algae, leaf litter is of lower food quality, particularly omega-3 polyunsaturated fatty acids (n-3 PUFA), and would constrain the somatic growth, maintenance, and reproduction of stream invertebrates. It may be thus assumed that shredders, such as Gammarus, receive lower quality diets than grazers, e.g. Ecdyonurus, that typically feed on algae., The objective of this study was to assess the provision of dietary PUFA from leaf litter and algae to the shredder Gammarus and the grazer Ecdyonurus. Three different diets (algae, terrestrial leaves, and an algae-leaf litter mix) were supplied to these macroinvertebrates in a flume experiment for 2 weeks. To differentiate how diet sources were retained in these consumers, algae were isotopically labelled with C-13., Both consumers became enriched with C-13 in all treatments, demonstrating that both assimilated algae. For Gammarus, n-3 PUFA increased, whereas n-6 PUFA stayed constant. By contrast, the n-3 PUFA content of Ecdyonurus decreased as a consequence of declining algal supply., Results from compound-specific stable isotope analysis provided evidence that the long-chain n-3 PUFA eicosapentaenoic acid (EPA) in both consumers was more enriched in C-13 than the short-chain n-3 PUFA alpha-linolenic acid, suggesting that EPA was taken up directly from algae and not from heterotrophic biofilms on leaf litter. Both consumers depended on algae as their carbon and EPA source and retained their EPA from high-quality algae.

Both brown (detrital-based) and green (algal-based) food pathways support freshwater food webs, although the importance of either source may vary within species, regions and different phases of the flow regime. The bony bream (Nematalosa erebi Clupeidae: Dorosomatinae) is one of Australia's most widely distributed freshwater fish species and is a key component of freshwater food webs, especially in northern Australia. We sought to better define the feeding habits of this species, previously classified as a detritivore, algivore or zooplanktivore (or combinations thereof), by undertaking meta-analyses of published accounts based on stomach content analysis and 13C and 15N stable isotope analysis. Stomach content analysis clearly indicated that detritus was the dominant food item, although benthic algae could be an important dietary component in some habitats (inland river flood plains) and during the wet season. Zooplankton were important for small fish (i.e. juveniles 13C enriched, indicating the latter source was not the dominant contributor to the biomass of this species. However, using site-specific data and a regression approach, a significant relationship was revealed between algal carbon and that of large fish, suggesting that carbon derived from benthic algae contributed ~20% of the carbon of adult bony bream. Zooplankton contributed a similar amount. Zooplankton provided the majority of carbon for small fish. We contend that detritus derived from terrestrial vegetation is the likely remaining carbon source for large bony bream, and this interpretation was supported by the outcomes of multiple regression analyses. Although previous studies of aquatic food webs in northern Australia have emphasised the importance of high-quality algal basal resources, this study indicates that terrestrial sources may be important for some species and demonstrates the need to better consider the circumstances that cause biota to switch between different food sources.

Both brown (detrital-based) and green (algal-based) food pathways support freshwater food webs, although the importance of either source may vary within species, regions and different phases of the flow regime. The bony bream (Nematalosa erebi Clupeidae: Dorosomatinae) is one of Australia’s most widely distributed freshwater fish species and is a key component of freshwater food webs, especially in northern Australia. We sought to better define the feeding habits of this species, previously classified as a detritivore, algivore or zooplanktivore (or combinations thereof), by undertaking meta-analyses of published accounts based on stomach content analysis and 13C and 15N stable isotope analysis. Stomach content analysis clearly indicated that detritus was the dominant food item, although benthic algae could be an important dietary component in some habitats (inland river flood plains) and during the wet season. Zooplankton were important for small fish (i.e. juveniles

Food web subsidies from external sources ("allochthony") can support rich biological diversity and high secondary and tertiary production in aquatic systems, even those with low rates of primary production. However, animals vary in their degree of dependence on these subsidies. We examined dietary sources for aquatic animals restricted to refugial habitats (waterholes) during the dry season in Australia's wet-dry tropics, and show that allochthony is strongly size dependent. While small-bodied fishes and invertebrates derived a large proportion of their diet from autochthonous sources within the waterhole (phytoplankton, periphyton, or macrophytes), larger animals, including predatory fishes and crocodiles, demonstrated allochthony from seasonally inundated floodplains, coastal zones or the surrounding savanna. Autochthony declined roughly 10% for each order of magnitude increase in body size. The largest animals in the food web, estuarine crocodiles (Crocodylus porosus), derived ~80% of their diet from allochthonous sources. Allochthony enables crocodiles and large predatory fish to achieve high biomass, countering empirically derived expectations for negative density vs. body size relationships. These results highlight the strong degree of connectivity that exists between rivers and their floodplains in systems largely unaffected by river regulation or dams and levees, and how large iconic predators could be disproportionately affected by these human activities.

Summary While many streams and rivers are dominated by terrestrial inputs of organic carbon, algae are an important trophic base for stream food webs. However, the nutritional importance of algae for stream invertebrates only recently has been highlighted. Algae are acknowledged as higher quality food than terrestrial organic matter for the growth and reproduction of invertebrates. In part, this is because of higher algal polyunsaturated fatty-acid (PUFA) content. Here, we review the important influence of algal food quality, as assessed by PUFA, in stream food webs. Current field investigations have mainly focused on the fatty-acid dynamics of macroinvertebrates, and indicate that algal eicosapentaenoic acid (EPA), α-linolenic acid (ALA) and linoleic acids (LIN) are present in all macroinvertebrates. However, fungal and bacterial tracers have also been observed in a range of macroinvertebrates. The omega-3 (ω3)/omega-6 (ω6) ratio >1 in most macroinvertebrates strongly indicates that dietary energy of algae is highly retained in stream food webs. Interspecific differences in PUFA composition seem to be affected by dietary PUFA and consumer physiology. Some studies have suggested that besides dietary EPA, the shorter chain C18 PUFA LIN and ALA also can improve growth and reproduction of stream invertebrate consumers. Some macroinvertebrates may preferentially retain or synthesise long-chain PUFA from C18 PUFA when experiencing low-quality food. However, this process is controversial since other species have shown very limited ability to synthesise long-chain PUFA. Algal PUFA composition is strongly influenced by abiotic factors, particularly light, nutrients, and temperature. Human disturbance (i.e. riparian vegetation removal and nutrient inputs) on algal PUFA content and their consequent effects on macroinvertebrates and fish clearly warrant further scientific attention. Controlled feeding trials and manipulative studies are required to measure PUFA conversion capacities and reproductive investment of stream macroinvertebrates under different food quality conditions, which will provide insights into how freshwater species can cope with different nutritional food conditions due to human disturbance and climate change.

Trichocorixa verticalis verticalis, a North American water boatman, is the only alien corixid in European fresh waters. It has rapidly spread, becoming the dominant corixid in and around the Doñana protected area in SW Spain. Its high abundance and similar morphology to native corixids suggest that T. verticalis may impact them through competition for food. Here we used stable isotopes of nitrogen (N) and carbon (C) and Bayesian analytical tools to investigate niche partitioning between T. verticalis and the native water boatman Sigara lateralis and Sigara scripta through a combination of experimental and field data. Species sampled from permanent ponds and laboratory aquaria could be separated based on their isotopic values (δ15N and δ13C). S. lateralis consistently showed higher δ15N values than T. verticalis, suggesting that the invasive species may be feeding at a lower trophic position and relying more on herbivory than its native competitors. This was particularly true for the T. verticalis nymph stage, which showed lower δ15N values compared to adults, indicating ontogenetic dietary shifts. In contrast, native corixids and the invasive species showed similar isotopic compositions in temporary habitats at different stages in the flooding-desiccation cycle, suggesting some degree of niche overlap and a slight reduction in trophic level for S. lateralis when inhabiting ephemeral systems. The combination of experimental and field data can help elucidate trophic interactions during a biological invasion. Stable isotopes provided insights into the trophic ecology of this invasive species and into the mechanisms facilitating co-existence with native species

Tropical floodplains are highly productive because of seasonal replenishment of water and nutrients, which substantially boost primary productivity. This study examined how the architecture of aquatic macrophytes affect the light and water quality and consequently the attachment and biomass of epiphytes on a floodplain in northern Australia. Results show that macrophyte structural complexity is not only important for water column light penetration but also for the development of epiphytes on macrophytes. Emergent grasses with simple vertical structure and high plant densities, limit light penetration and consequently the development and biomass of epiphytic algae. In contrast, submerged macrophytes growing just below the water surface, allow greater light penetration. The complex architecture of submerged macrophytes also provides a large surface area for the development of a dense covering of epiphytic algae. Other plant structural forms (e.g., plants with floating leaves) have a simple structure, variable light penetration and low epiphytic algae biomass. The emergent grass Pseudoraphis spinescens (R.Br.) Vickery also had low light penetration but the horizontal alignment of stems across the water surface allow greater exposure to sunlight of the stems and the consequent development of epiphytic algae. We conclude that (1) the complex structure of submerged plants effectively creates a “false bottom” in deeper waters so that they function similarly to the floodplain's littoral zone, and (2) that their extremely large surface area for attachment allows greater production of epiphytic algae than would occur on the sediment surface.

Flooding of the terminal floodplains of northern Australian rivers provides a greatly expanded, productive habitat accessed by both freshwater and estuarine fishes. This study aimed to determine the extent to which sea catfishes (Ariidae) make use of floodplains and the reasons for doing so (i.e. spawning, feeding). Nine species were collected from floodplains and adjacent distributaries of the Mitchell and Flinders rivers; floodplain use was largely restricted to freshwater species. Evidence of prior wet season spawning was recorded for some species, and mesenteric lipid deposits indicated that fish were in good condition. However, little evidence of spawning on floodplains was found. Stomach content analysis and stable isotope analysis indicated dietary partitioning, particularly between freshwater and estuarine species, but also within freshwater species, and indicated that some species were responsive to variations in food availability. Isotope analyses suggest extensive movement between freshwater, estuarine and marine habitats at different life history stages for the catfish assemblage studied. Terminal floodplains of northern Australian rivers provide important temporary habitat for adult sea catfishes to feed upon, but do not appear to be used as spawning grounds.

The trophic position of a top predator, synonymous with food-chain length, is one of the most fundamental attributes of ecosystems. Stable isotope ratios of nitrogen (δ15N) have been used to estimate trophic position of organisms due to the predictable enrichment of 15N in consumer tissues relative to their diet. Previous studies in crocodilians have found upward ontogenetic shifts in their ‘trophic position’. However, such increases are not expected from what is known about crocodilian diets because ontogenetic shifts in diet relate to taxonomic categories of prey rather than shifts to prey from higher trophic levels. When we analysed dietary information from the literature on the four Amazonian crocodilians, ontogenetic shifts in dietary-based trophic position (TPdiet) were minimal, and differed from those estimated using δ15N data (TPSIA). Thus, ontogenetic shifts in TPSIA may result not only from dietary assimilation but also from trophic discrimination factors (TDF or Δ15N) associated with body size. Using a unique TDF value to estimate trophic position of crocodilians of all sizes might obscure conclusions about ontogenetic shifts in trophic position. Our findings may change the way that researchers estimate trophic position of organisms that show orders of magnitude differences in size across their life span.

Water level drawdown followed by rewetting in reservoirs/lakes can shift macrophytes from a nutrient sink to a source, thus changing nutrient dynamics and causing water quality problems. However, there is limited understanding of the effect of macrophyte growth versus decomposition on nitrogen (N) and phosphorus (P) dynamics during water level fluctuations (WLFs). This study examined nutrient release versus uptake by two submerged macrophytes, the invasive Cabomba caroliniana (Cabomba) and the native Hydrilla verticillata (Hydrilla), compared to bare sediments, after drying then rewetting. Overall, we examined the nutrient dynamics in treatments comparing 1) two macrophyte species, 2) macrophyte nutrient uptake versus release, and 3) the presence of macrophytes versus bare sediments. Our study showed that Cabomba litter had a higher nutrient release rate and resulted in higher water column chlorophyll a concentrations compared with Hydrilla litter. The decomposition impact of both species on water column P concentrations was greater than that for N, and proportionally less P was assimilated than released by macrophytes compared with N. This resulted in P accumulation and a decreased N:P ratio in the water column compared to bare sediments without macrophytes. Phosphorus accumulation in the water column during macrophyte decomposition increased phytoplankton growth in our study, highlighting the potential for phytoplankton blooms, especially in P-deficient lakes. Regrown Hydrilla had a higher potential for assimilating sediment N compared to Cabomba. In contrast, regrown Cabomba was more efficient at reducing water column N compared with Hydrilla, suggesting different pathways of assimilation. Neither regrown Cabomba nor Hydrilla significantly reduced water column P concentrations. This study highlights different nutrient impacts of macrophyte beds compared to bare sediments during WLFs, and the role of regrown macrophytes in assimilating the released nutrients from macrophyte decomposition. However, the scale of their role is dependent on macrophyte species, forms of nutrients and drying-wetting regimes.

Mixing models have become requisite tools for analyzing biotracer data, most commonly stable isotope ratios, to infer dietary contributions of multiple sources to a consumer. However, Bayesian mixing models will always return a result that defaults to their priors if the data poorly resolve the source contributions, and thus, their interpretation requires caution. We describe an application of information theory to quantify how much has been learned about a consumer’s diet from new biotracer data. We apply the approach to two example data sets. We find that variation in the isotope ratios of sources limits the precision of estimates for the consumer’s diet, even with a large number of consumer samples. Thus, the approach which we describe is a type of power analysis that uses a priori simulations to find an optimal sample size. Biotracer data are fundamentally limited in their ability to discriminate consumer diets. We suggest that other types of data, such as gut content analysis, must be used as prior information in model fitting, to improve model learning about the consumer’s diet. Information theory may also be used to identify optimal sampling protocols in situations where sampling of consumers is limited due to expense or ethical concerns.

Benthic diatoms have been universally used as indicators to assess water quality in lotic ecosystems. However, most diatom-based indices developed in Europe have not been widely used or tested in other continents such as Asia or Oceania. This study compared the performance of fourteen widely-applied diatom indices in assessing ecological conditions in subtropical streams in South East Queensland (SEQ) in Australia and in the upper Han River in China. Most water quality variables in the upper Han River including dissolved organic carbon (DOC) , total nitrogen (TN) and soluble reactive phosphorus (SRP) had strong relationships with at least one diatom index, with the exception of IDAP (Index Diatom Artois-Picardie) and TDI (Trophic Diatom Index). However, in SEQ, most of the environmental variables including DOC, ammonia nitrogen (NH4-N), TN, SRP and electrical conductivity (EC) showed no significant relationships with diatom indices, and the DI-CH (Swiss Diatom Index) and WAT (Watanabe’s Index) were unrelated to any of the variables examined. Only pH and nitrite or nitrate nitrogen (NOX-N) were significant predictors of several diatom indices in SEQ, especially TID (Rott trophic index). In the upper Han River, much of the spatial variation in most diatom indices was explained by proximate determinants alone, including EC, DOC, dissolved oxygen (DO) or SRP, or a combination of ultimate (canopy, forest) and proximate factors (R2 in most models> 0.75). Most diatom indices performed as predicted in the upper Han River where nutrient and organic matter pollution was relatively high, and variation in pH low. However, the indices performed poorly in SEQ where the water quality gradient was low and instead most responded to spatial variation in pH. This finding serves as a caution to the application of diatom indices in river basins that fall outside of the range of water quality values of the systems in which they originally developed.

Headwater populations of the common shrimp Paratya australiensis were sampled to examine the factors influencing egg and clutch size. Much of the spatial variation in these reproductive traits was associated with differences in altitude, with upper sites having larger eggs and smaller clutches. Mean egg size at high-altitude sites was higher than that previously reported for this species. Temporal variation in egg and clutch size was also observed, with significant increases in egg size during the breeding season. At most sites, this increase was accompanied by a decrease in clutch size. The combined effect of the reciprocal patterns in egg volume and clutch size resulted in relatively little spatial and temporal variation in reproductive effort. In a field experiment, using a unique genetic marker, shrimps were translocated between two sites with significantly different egg sizes. After one generation (18 months), the mean egg size of translocated females was the same as that of females from the `source' population. In contrast, clutch size changed towards that of the resident females. This suggests that egg size is under strong genetic control, while clutch size is influenced by the environment.

Intermittent water flow regimes characterize streams in many world regions, especially those with arid and semiarid climates. During cease to flow conditions, biofilms on streambed sediments may be exposed to desiccation. Environmental conditions and resource availability change with desiccation and may influence biofilm functioning and whole stream ecosystem processes. Rainfall events during the nonflow phase can rehydrate streambed sediments, but the effect of these pulses on biofilm functioning is unclear. This study aimed to analyze the effects of a rehydration event on biofilm functional diversity during the nonflow period in a subtropical Australian stream. Biofilms from three different stream pools on the same reach; one permanently water-covered and the other two differing in their desiccation time were studied. Biofilms initially differed owing to the time they were exposed to dry conditions but rehydration events significantly increased biofilm functional diversity, producing a “reset” effect on the desiccation exposure, as after that bacterial functioning decreased again because of the new dry conditions. The observed rapid biofilm responses to rehydration during flow intermittency might be essential in sustaining biofilm functional diversity in intermittent streams.

Planned indirect potable reuse water treated with advanced wastewater technologies (AWWT) to remove pollutants is increasingly being used to augment drinking water and groundwater supplies. While the treatment process substantially reduces the high nitrogen (N) and phosphorus (P) concentrations typically of wastewater, both nutrients can remain at concentrations and in biologically available forms that may stimulate phytoplankton growth in nutrient-deficient systems. This study examined the short-term effect of N plus P additions, at a range of concentrations, on phytoplankton growth and species composition in mesocosm experiments in a subtropical reservoir. Nitrate (NO(3)) plus orthophosphate (PO(4)) concentrations as low as 50 and 5 μg L(-1), respectively, resulted in significant increases in phytoplankton biomass, with a 3.99 μg L(-1) increase in chlorophyll a concentration with every 10 μg L(-1) increase in N plus 1 μg L(-1) in P. The system was likely to be co-limited because the addition of N or P alone did not result in increased chlorophyll a concentrations compared with the control. However, the toxic cyanobacterium, Cylindrospermopsis raciborskii, had higher growth rates with P addition alone. This study has shown that inputs of AWWT water have the potential to increase the phytoplankton biomass in this subtropical reservoir, at least in the short term. Therefore, the effect of AWWT water on water quality in reservoirs should be further investigated before widespread application occurs.

Natural abundances of stable isotopes in lotic food webs yield valuable information about sources of organic matter for consumers and trophic structure. However, interpretation of isotopic information can be challenging in the face of variability in organisms at the base of food webs. Unionid and dreissenid mussels, commonly used as baseline organisms in lakes, are uncommon in many river settings and can have variable diets, thus making them unsuitable as a universal baseline for many river food web studies and often forcing reliance on more common benthic insects for this purpose. Turnover rates of body carbon and nitrogen in insects are relatively rapid (1 to 50 days half-life). These rapid turnover rates in primary consumers can result in considerable temporal variability in δ13C that rivals that of algae (>10‰ range within a site). This suggests that using primary consumers as a surrogate baseline for algae may not circumvent the problem of temporal variability and the resultant mismatch of sources with longer-lived, slow-growing secondary and tertiary consumers. There are several strategies for reducing the influence of these confounding factors when bivalves with a known diet are not present. These include sampling over large spatial scales and correlating δ13C of consumers with the source of interest (e.g. benthic algae), sampling baseline organisms multiple times in the weeks preceding sampling of larger consumers (particularly in response to large changes in discharge) and using algal-detrital separation methods and multiple tracers as much as possible. Incorporating some of these recommendations and further exploring variability at the base of the food web will potentially provide greater insights into consumer–resource coupling in running waters and more robust conclusions about food web structure and energy flow in these dynamic systems. Copyright © 2012 John Wiley & Sons, Ltd.

Although considerable knowledge has been gathered regarding the role of fish in cycling and translocation of nutrients across ecosystem boundaries, little information is available on how the energy obtained from different ecosystems is temporally allocated in fish bodies. Although in theory, limitations on energy budgets promote the existence of a trade-off between energy allocated to reproduction and somatic growth, this trade-off has rarely been found under natural conditions. Combining information on RNA:DNA ratios and carbon and nitrogen stable-isotope analyses we were able to achieve novel insights into the reproductive allocation of diamond mullet (Liza alata), a catadromous, widely distributed herbivorous-detritivorous fish. Although diamond mullet were in better condition during the wet season, most reproductive allocation occurred during the dry season when resources are limited and fish have poorer body condition. We found a strong trade-off between reproductive and somatic investment. Values of δ13C from reproductive and somatic tissues were correlated, probably because δ13C in food resources between dry and wet seasons do not differ markedly. On the other hand, data for δ15N showed that gonads are more correlated to muscle, a slow turnover tissue, suggesting long term synthesis of reproductive tissues. In combination, these lines of evidence suggest that L. alata is a capital breeder which shows temporal uncoupling of resource ingestion, energy storage and later allocation to reproduction. © 2016 Villamarín et al. This is an open access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.

1. Understanding the processes that structure community assembly across landscapes is fundamental to ecology and for predicting and managing the consequences of anthropogenically induced changes to ecosystems. 2. We assessed the community similarity of fish, macroinvertebrate and vegetation communities against geographic distances ranging from 4 to 480 km (i.e. distance–decay relationships) to determine the balance between local environmental factors and regional dispersal processes, and thus whether species-sorting (niche processes) or dispersal limitation (neutral processes) was more important in structuring these assemblages in Australia’s wet-dry tropics. We investigated whether the balance between niche and dispersal processes depended on the degree of hydrological connectivity, predicting that dispersal processes would be more important at connected sites, and also whether there was spatial concordance among these three assemblage types. 3. There was significant but weak spatial concordance among the study communities, suggesting limited potential for surrogacy among them. Distance–decay in community similarity was not observed for any study assemblage at perennial sites, suggesting dispersal was not limiting and assemblages were structured more strongly by local niche processes at these connected sites. At intermittent sites, weak distance–decay relationships for each assemblage type were confounded by significant relationships with environmental dissimilarity, suggesting that dispersal limitation contributed, albeit weakly, to niche processes in structuring our three study assemblages at disconnected sites. 4. Two environmental factors, flow regime and channel width, explained significant proportions of variation in all three assemblages, potentially contributing to the observed spatial concordance between them and representing local environmental gradients along which these communities re-assemble after the wet season, according to niche rather than dispersal processes.

Understanding the determinants of species’ distributions and abundances is a central theme in ecology. The development of statistical models to achieve this has a long history and the notion that the model should closely reflect underlying scientific understanding has encouraged ecologists to adopt complex statistical methods as they arise. In this paper we describe a Bayesian hierarchical model that reflects a conceptual ecological model of multi-scaled environmental determinants of riverine fish species’ distributions and abundances. We illustrate this with distribution and abundance data of a small-bodied fish species, the Empire gudgeon Hypseleotris galii, in the Mary and Albert Rivers, Queensland, Australia. Specifically, the model sought to address; 1) the extent that landscape-scale abiotic variables can explain the species’ distribution compared to local-scale variables, 2) how local-scale abiotic variables can explain species’ abundances, and 3) how are these local-scale relationships mediated by landscape-scale variables. Overall, the model accounted for around 60% of variation in the distribution and abundance of H. galii. The findings show that the landscape-scale variables explain much of the distribution of the species; however, there was considerable improvement in estimating the species’ distribution with the addition of local-scale variables. There were many strong relationships between abundance and local-scale abiotic variables; however, several of these relationships were mediated by some of the landscape-scale variables. The extent of spatial autocorrelation in the data was relatively low compared to the distances among sampling reaches. Our findings exemplify that Bayesian statistical modelling provides a robust framework for statistical modelling that reflects our ecological understanding. This allows ecologists to address a range of ecological questions with a single unified probability model rather than a series of disconnected analyses.

The estuarine crocodile (Crocodylus porosus) is one of the largest and most widespread crocodilians in the world. Although considered an apex species, the role of the estuarine crocodile in aquatic foodwebs is poorly understood; we know what crocodiles ingest, but not what nourishes them. In this study, we used a combination of stable isotope measurements (δ13C, δ15N, and δ34S) and direct feeding observations to identify the source of nutrition of estuarine crocodiles in Kakadu National Park, Northern Australia. Our results show that most crocodiles sampled (size 850 - 4200mm, with 76% of them being > 2.5 m) consume a large variety of prey, however a large proportion of their nutrition is derived from terrestrial prey. Introduced species such as water buffaloes (Bubalus bubalis) and pigs (Sus scrofa) could contribute between 53 and 84% to the nutrition of the sampled crocodiles. The isotopic composition of large crocodiles (total length > 3 m) suggested possible increase in marine prey consumption with size (R2 = 0.30; p = 0.005). Additionally, we found crocodiles sampled in the dry season had on average higher terrestrial contributions compared to crocodiles sampled during the wet season (84.1 ± 2.4% versus 55.4 ± 7.0%). Overall, we found that terrestrial prey are important source of nutrition for many crocodiles in this region where introduced herbivorous mammals are abundant.

1. Lipids have more negative delta(13)C values relative to other major biochemical compounds in plant and animal tissues. Although variable lipid content in biological tissues alters results and conclusions of delta(13)C analyses in aquatic food web and migration studies, no standard correction protocol exists. 2. We compared chemical extraction and mathematical correction methods for freshwater and marine fishes and aquatic invertebrates to better understand impacts of correction approaches on carbon (delta(13)C) and nitrogen (delta(15)N) stable isotope data. 3. Fish and aquatic invertebrate tissue delta(13)C values increased significantly following extraction for almost all species and tissue types relative to nonextracted samples. In contrast, delta(15)N was affected for muscle and whole body samples from only a few freshwater and marine species and had a limited effect for the entire data set. 4. Lipid normalization models, using C : N as a proxy for lipid content, predicted lipid-corrected delta(13)C for paired data sets more closely with parameters specific to the tissue type and species to which they were applied. 5. We present species- and tissue-specific models based on bulk C : N as a reliable alternative to chemical extraction corrections. By analysing a subset of samples before and after lipid extraction, models can be applied to the species and tissues of interest that will improve estimates of dietary sources using stable isotopes.

Summary 1. We determined whether two sympatric mitochondrial DNA (mtDNA) lineages of freshwater shrimp (Decapoda: Atyidae: Paratya australiensis) represent biological species and if they had concerted or independent population responses to hydrographic factors in small streams (the Granite Creeks) in southeastern Australia. 2. Allozyme data indicated the presence of two gene pools at sites where the P. australiensis lineages were co-occurring and the gene pools were statistically assigned with high probability to each respective lineage. This indicated that these mtDNA lineages in P. australiensis were reproductively isolated and thus biological species. 3. Populations of both lineages were genetically homogeneous among lowland sites within streams, but were isolated by steep stream gradients among upland sites and for lowland–upland site comparisons. However, the magnitude of differentiation was markedly different between the two lineages. Allozyme diversity also differed between the two lineages, suggesting that they have different effective population sizes. Thus, differences in the magnitude of genetic divergence among populations were probably because of different life-history characteristics, including dispersal ability and population size. 4. Genetic population structure was mostly temporally stable, despite the extreme effects of drought during the first year and substantial stream-flow during the second. However, stable isotope analyses revealed greater local movement in both lineages during the second year, as greater hydrological connectivity provided more opportunities for dispersal. Thus, although lowland populations within streams were genetically homogeneous, stable isotope data indicated that connections may be sporadic and result from accumulated small-scale movements among refugial pools. 5. Both lineages were therefore found to have similar small-scale population responses to the unstable habitats of the Granite Creeks. Results highlight the importance of refugia for the capacity of biota to recover from drought and the need for multiple restored patches to reinstate natural population processes (e.g. resilience, recolonization) in degraded systems.

Population connectivity (and isolation) and genetic diversity in declining species are important considerations for species recovery and habitat restoration programs. For lotic species, stream confluences with major rivers may be a determinant of population isolation among streams, although small-scale dynamics in population connectivity within streams may also represent attributes of population functioning (e.g. resilience, recolonisation) that are important for species recovery programs. In this study, we examined the importance of among- and within-stream patterns of population connectivity and genetic diversity in the regionally vulnerable southern pygmy perch (Nannoperca australis) in a series of degraded streams in south east Australia in which population declines have been reported. We used molecular markers (mtDNA and microsatellites) to examine among-stream patterns of population connectivity and genetic diversity, and molecular markers and stable isotopic signatures of nitrogen and carbon to examine within-stream connectivity in the species. Results revealed that populations of N. australis are isolated among streams, and that streams therefore represent functional population units for species recovery and habitat restoration programs. Within-streams, local populations were semi-discrete and connected by various degrees of dispersal, with the degree of connectivity differing between the streams. Genetic bottlenecks were not detected, although the effective population size of N. australis in the more fragmented stream may be lower than in the stream where greater population connectivity was found. Protecting and augmenting refugial habitat of extant populations and restoring corridor functions between refugia within streams will generate the greatest biological response at within-stream scales.

Algal polyunsaturated fatty acids (PUFA), essential for somatic growth and reproduction of aquatic animals, are influenced by ambient environmental conditions, including light and nutrients. Few studies have addressed the extent to which changes in algal PUFA can influence stream herbivore PUFA profiles and the implications for stream food webs. We manipulated subtropical stream periphyton by applying two light levels (open and shaded canopy) and two nutrient regimes (ambient and enriched) to investigate the response of PUFA and somatic growth in stream herbivores. After 6 weeks, the relative content of periphyton PUFA (%) changed distinctly and differed among treatments. Periphyton in the control treatment with open canopy showed a decline in eicosapentaenoic acid (EPA) relative to initial conditions, whereas shading increased EPA and total highly unsaturated FA (HUFA), but decreased α-linolenic acid (ALA), linoleic acid and total C18 PUFA. The interaction of open canopy and added nutrients increased periphyton ALA compared with initial conditions, while the combined effects of shading and added nutrients led to greater total HUFA. FA similarity between stream grazers (the mayfly Austrophlebioides and caddisfly Helicopsyche) and periphyton increased with periphyton HUFA content. In addition, the growth of large instars of both grazers also increased in response to increased periphyton HUFA %. Our findings show that environmental changes, associated with riparian canopy and nutrients, can lead to changes in periphyton PUFA composition that in turn affect growth and PUFA composition in stream grazers.

A combination of stable isotope measurements and gut contents analysis was used to determine the major food sources of the sergestid crustacean Acetes sibogae , in commercial shrimp ponds at two farms in southeast Queensland, Australia. Slight differences were observed between farms but overall the results were consistent. Although gut contents analysis gave a good indication of the range and temporal occurrence of food items consumed by Acetes , it was difficult to ascertain the contribution each item made to the diet. This was mainly due to the large proportion of unidentifiable material in the guts. All specimens examined contained unidentifiable material and about half the Acetes from both farms contained nothing identifiable. This unidentifiable material may be the result of processing by the Acetes gastric mill or the consumption of detritus, sediment or processed material from shrimp pellets. Only resilient items such as crustacean remains, diatoms and tinntinnids were commonly identified from the guts, and although present over the majority of the sampling period, FOCs were never greater than 25%. Stable isotope signals were measured for Acetes and likely food sources including pelleted shrimp feed, zooplankton and macroalgae. The pattern of changes in isotopic signals of Acetes across the season showed that zooplankton was a primary food source. Changes in the signals of zooplankton were reflected by changes in Acetes , but the changes in Acetes signal were less pronounced. At both farms, Acetes were more enriched in 13 C and 15 N (− 15‰ to − 20‰ and 12‰ to 13.8‰) than the zooplankton (− 18.9‰ to − 23.7‰ and 5‰ to 13.1‰), during the whole season. The absolute difference between the δ 13 C values of Acetes and zooplankton were more consistent than for δ 15 N, but both were greater than might be expected based on fractionation over a single trophic level. Furthermore, laboratory feeding trials showed that fractionation could not explain the greater than expected enrichment of the Acetes signal compared to that measured for zooplankton in the ponds. This, together with evidence from gut content analysis, showed that a food source other than zooplankton must also be important to Acetes . Macroalgae are the most likely additional source, although some minor contribution of pellets or microalgae cannot be ruled out entirely. There was no evidence from either gut contents or stable isotope signatures of dramatic dietary changes for Acetes either through a season or as they grew. It would appear unlikely that Acetes would have a great effect on shrimp production in ponds unless they were extremely abundant early in the season when the postlarvae are also feeding on zooplankton.

Summary 1. Episodic floods and extended low or no flow periods characterise dryland river systems in Western Queensland, Australia. During protracted intervals between floods, rivers consist of a series of isolated waterholes, which serve as ‘refugia’ for aquatic species and much of the channel is dry. We categorised these waterholes into ‘main waterholes’, which are located in the main part of the river channel and ‘satellite waterholes’, which are located in distributary river channels. 2. We used mitochondrial sequences and allozymes to investigate levels of genetic diversity and patterns of connectivity among waterholes for two obligate freshwater species: Macrobrachium australiense (Decapoda: Palaemonidae) and Notopala sublineata (Gastropoda: Viviparidae). 3. We sampled 31 waterholes for M. australiense and 12 for N. sublineata. Based on a 505-bp fragment of cytochrome oxidase subunit I, we identified 54 haplotypes in a sample of 232 individuals for M. australiense and based on a 457-bp fragment of the same gene, 36 haplotypes in a sample of 145 individuals for N. sublineata. 4. Both nuclear and mitochondrial genetic data sets indicated that estimates of genetic diversity were not different in populations inhabiting main and satellite waterholes for either species. Also, there was generally very limited genetic differentiation among populations at any site. 5. We suggest that levels of connectivity among populations inhabiting waterholes at most sites are higher than expected. High levels of connectivity may help to maintain overall high levels of genetic diversity as well as low levels of genetic differentiation among waterholes within sites.

Stable isotope studies of food webs in floodplains, large rivers, mangroves, and seagrasses have shown that, although a large proportion of the biomass may come from higher plants, microalgae provide a disproportionate amount of carbon assimilated by metazoan consumers. Evidence is building that this may also be the case for streams, especially those in the tropics. At the level of individual consumer species we also see that the apparent diet may not be reflected in the carbon assimilated. Tropical streams commonly have omnivore- detritivore species that potentially show this phenomenon. We tested these concepts in four moderately shaded sites in a stream in well-preserved Atlantic rainforest at Ilha Grande, Rio de Janeiro. We sampled aquatic insects, shrimps and fish as well as potential terrestrial and aquatic primary food sources. Carbon stocks from terrestrial sources predominated over carbon of algal origin (>99% of total). The primary sources of carbon showed distinctly different isotopic signatures: terrestrial sources had δ 13 C values close to −30‰, microalgae were −20‰ and macroalgae were −25‰. All fauna had δ 13 C values consistent with a carbon source derived from microalgae. Baetid mayflies and atyid shrimps exert a strong grazing pressure on periphyton and organic sediments but appear to assimilate predominantly microalgae. The palaemonid shrimp Macrobrachium olfersi also ingests large amounts of detritus of terrestrial origin, but apparently assimilates animal prey with algal δ 13 C signatures. These results support the growing view that tropical stream food chains are primarily algal based.

The fish assemblages of an arid zone floodplain river, Cooper Creek, Queensland, Australia, were sampled during two dry periods in isolated waterholes and on the inundated floodplain during the early and late phase of a major flood event. Diets were described for nine native species and compared within and between dry and flood periods. In the dry season, when fishes were restricted to waterholes, diets were characteristically simple with narrow diet breadths. Movement onto the floodplain during flooding clearly increased feeding opportunities, with greater diet breadths evident in all species. Despite obvious potential for terrestrial inputs, diets tended to be dominated by aquatic resources in both the waterholes and on the floodplain. Stomach fullness, however, varied little between dry season waterhole and floodplain samples. Fish appear to feed on potentially lower value resources such as detritus and calanoid copepods during the dry season, when waterholes are isolated and food resources were limited. They were then able to capitalize on the "boom" of aquatic production and more diverse food resources associated with episodic flood events.

Stable isotope and ecological stoichiometry investigations of aquatic food webs require separate measurements of microalgae and detritus, but fine particulate matter collected as seston or scraped from biofilms typically is an unknown mixture of these two components plus other material. This paper describes an economical method to partition fine particulate matter into predominantly algal and detrital components by centrifugation in colloidal silica. Centrifugation using this density also worked well to separate algae from inorganic matter. The protocol was tested by sampling a broad range of water bodies in Queensland, Australia. Seston was concentrated using an inexpensive continuous centrifuge and organic matter was collected from surfaces of rocks, mud, or plants. Separation in colloidal silica was achieved using a standard benchtop centrifuge. When the colloidal silica was adjusted to a density of 1.27 g/cm3, the light fraction (supernatant) tended to be dominated by algal organic matter while the heavy fraction tended to be more detrital. Ratios of organic carbon to chlorophyll a indicated the efficacy of the separations. Stable C and N isotope ratios often differed considerably between the algal and detrital fractions, demonstrating the need to perform such separations in many aquatic environments.

We examined genetic structure and levels of connectivity among subpopulations within each of four cryptic species belonging to the freshwater mussel genus Velesunio. We used allozymes and a fragment of the mitochondrial cytochrome c oxidase I gene to examine genetic variation in populations from isolated waterholes, belonging to four major inland drainages in eastern Australia. Based on evidence from other invertebrates in the region we predicted that, in each species, we would find evidence of historical connectivity among populations from different drainages. This was clearly not the case, as for the two species that occurred in more than one drainage there was evidence of both current and past restriction to gene flow. Moreover, given the potential for extensive dispersal of these mussels through the river systems during flood times via their fish hosts, we predicted low levels of genetic variation among populations from waterholes in the same drainage. Contrary to our expectations, all four species showed some evidence of restricted gene flow among waterholes within drainages. This suggests that either (a) mussel larvae are not produced during flood times, when their fish hosts would be free to move between waterholes, or (b) mussel larvae are attached to their hosts at these times, but the fish movement is limited between waterholes.

Stable isotope analysis was used to determine thesources of dietary nitrogen (N) and carbon (C) forshrimp during the rearing phase in extensive rice-shrimppondsinMyXuyenandGiaRaidistricts,Viet-nam. Farm-made feed was added as a food source inshrimp ponds in My Xuyen district, and based onstable isotope analyses, was generallya poor dietarysource.The commercial formulated feed used in GiaRai also appeared to contribute little directly to thenutritional needs of the shrimp. In contrast, the nat-ural biota in all ponds appeared to contribute sub-stantially. In particular, biota from beam trawls andbenthic organic matter were the most likely sourcesofnutritioninMyXuyenponds,whilebenthicorgan-icmatterwasthe mainsource inGiaRaiponds. d 15 Nratios in the natural biota in My Xuyen farms de-creased over the growing season, suggesting in-creased N ¢xation in the case of the benthic organicmatterreachingvaluesaslowas1m.ThissuggestsN-limitationinthepondsandthatnaturalbiotabecomeincreasingly dependent on N ¢xed by algae and/orother microorganisms.There is the potential to pro-motethegrowthoftheplanktonandhence,theothernatural biota, bythejudiciousadditionoffertilizer.Keywords: shrimp, nutrition, natural biota, nat-ural feed, carbon, nitrogenIntroductionInsalineaiectedareasoftheMekongDelta,Vietnam,the traditional wet-season rice crop is often supple-mentedwithadry-seasoncrop ofextensivelyfarmedshrimpwhen salt water intrudes into the waterways(Tran, Dung & Brennan 1999). The adoption ofshrimp as a second crop in the dry season has re-sulted in signi¢cant income gains for some farmers(Tran et al.1999; Brennan, Clayton & Tran 2000). In2000, 10000t of shrimp were produced from40000ha of rice-shrimp farms in Vietnam (Bren-nan,Preston,Clayton T however, the role ofnatural biotais poorlyunderstood.Measurement of stable isotope ratios in foodsources and consumers may be used to determinetheimportanceofdiierentfoodsourcestoprawnnu-trition (Gearing1991; Shearer & Kohl1993). d

1. Recent stable isotope studies have revealed that C4 plants play a minor role in aquatic food webs, despite their often widespread distribution and production. We compared the breakdown of C3 (Eucalyptus) and C4 (Saccharum and Urochloa) plant litter in a small rain forest stream and used laboratory feeding experiments to determine their potential contribution to the aquatic food web. 2. All species of litter broke down at a fast rate in the stream, although Urochloa was significantly faster than Eucalyptus and Saccharum. This was consistent with the observed higher total organic nitrogen of Urochloa compared with the other two species. 3. The breakdown of Urochloa and Saccharum was, however, not associated with shredding invertebrates, which were poorly represented in leaf packs compared with the native Eucalyptus. The composition of the invertebrate fauna in packs of Urochloa quickly diverged from that of the other two species. 4. Feeding experiments using a common shredding aquatic insect Anisocentropus kirramus showed a distinct preference for Eucalyptus over both C4 species. Anisocentropus was observed to ingest C4 plant litter, particularly in the absence of other choices, and faecal material collected was clearly of C4 origin, as determined by stable isotope analysis. However, the stable carbon isotope values of the larvae did not shift away from their C3 signature in any of the feeding trials. 5. These data suggest that shredders avoid the consumption of C4 plants, in favour of native C3 species that appear to be of lower food quality (based on C : N ratios). Lower rates of consumption and lack of assimilation of C4 carbon also suggest that shredders may have a limited ability to process this material, even in the absence of alternative litter sources. Large scale clearing of forest and vegetation for C4 crops such as sugarcane will undoubtedly have important consequences for stream ecosystem function.