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ABSTRACT The ecology of Lake Victoria and rivers draining its 180 000 km2 basin has changed over the past century in response to growing anthropogenic influences that have altered basal resources, trophic status and interactions and river flow regimes. Impacts on the ecology of the lake are well known, but little attention has focused on the ecological status of rivers supplying ecological services to the majority of the basin's over 30 million inhabitants. In this paper, we review existing research on the ecological status of streams and rivers in the Lake Victoria Basin (LVB) and evaluate how they fit into emerging models of riverine ecosystem function in the tropics. Studies to date indicate that allochthonous sources dominate inputs to food webs in forested headwaters and savanna mid-reaches of rivers in the LVB, although transfer pathways vary with position in the river. Although riparian vegetation phenology and hydrologic run-off pathways control the spatial and temporal fluxes of energy inputs in headwater streams; animals play increasingly important roles in savanna mid-reaches. Over the last half century, food web studies have revealed widespread omnivory and a tendency toward increased omnivory among fishes whose diets were more specialized prior to the introduction of Lates niloticus and the ecological changes in the lake. Ecological patterns and processes in LVB rivers generally fit emerging models for the tropics, but studies completed to-date do not find autotrophic energy sources to be important. The findings of this review highlight the importance of specific management actions and needs for future research. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]

In the decisive shift from imperial-states to nation-states after World War Two, two related processes took place. There was a wide scale effort to delegitimize racist ideologies. At the same time, state sovereignty across the world was being nationalized. Nationalist ideologies were rendered not only legitimate but practically mandatory in politics. This talk charts this history in order to understand how racism is organized, practiced, and resisted in an era of postcolonialism (i.e. an era when national sovereignty is the hegemonic state form and when the social and juridical distinction between 'national' and 'migrant' are widely accepted). I examine the growing autochthonization of politics and how nationalisms the world over are increasingly reconfiguring the 'national' as an autochthon, i.e. a 'native' of the national 'soil'. Through a discussion of various autochthonous movements I analyze the double move wherein historic colonizers are re-presented as 'migrants' and today's 'migrants' are made into 'colonizers'. Such a move, I argue, is made possible by postcolonial racisms: the historic articulation between ideas of 'race' and 'nation' wherein ideas of national soil are racialized and racist ideas of blood are territorialized. [ABSTRACT FROM AUTHOR]

Increased incorporation of terrestrial organic matter (t‐OM) into consumer biomass (allochthony) is believed to reduce growth capacity. In this study, we examined the relationship between crustacean zooplankton allochthony and production in a boreal lake that displays strong seasonal variability in t‐OM inputs. Contrary to our hypotheses, we found no effect of allochthony on production at the community and the species levels. The high‐frequency seasonal sampling (time‐for‐space) allowed for estimating the efficiency of zooplankton in converting this external carbon source to growth. From the daily t‐OM inputs in the lake (57–3,027 kg C/d), the zooplankton community transferred 0.2% into biomass (0.01–2.36 kg C/d); this level was of the same magnitude as the carbon transfer efficiency for algal‐derived carbon (0.4%). In the context of the boundless carbon cycle, which integrates inland waters as a biologically active component of the terrestrial landscape, the use of the time‐for‐space approach for the quantifying of t‐OM trophic transfer efficiency by zooplankton is a critical step toward a better understanding of the effects of increasing external carbon fluxes on pelagic food webs. [ABSTRACT FROM AUTHOR]

Frozen tundra soils hold one of the Earth's largest pools of organic carbon. Climate warming and the associated permafrost thaw release a large fraction of this carbon into circumpolar lakes, inducing extreme browning that fuels the heterotrophic microbial food web. How this permafrost carbon affects organisms higher in the food chain remains unknown. Using dissolved organic matter properties, total phosphorus, chlorophylla, fatty acids, and stable isotopes, we investigated the influence of thawing permafrost on primary producers and primary consumers of the plank tonic food web. We sampled four sub-arctic thaw ponds that were affected by permafrost carbon and another four ponds that were not. Our results highlight the stimulating influence of eroding and degrading ice-rich permafrost on nutrients and planktonic algae. Relative to the non-thaw ponds, the permafrost thaw-influenced fresh waters had higher total phosphorus concentrations (14.8 vs. 70.4μg/L, respectively). This in turn led to a higher chlorophylla(2.7 vs. 45.2μg/L) and seston omega-3 fatty acid concentrations (7.3 vs. 53.5μg/L) despite significantly reduced light for primary production. Differences between the thaw and non-thaw ponds were less marked at the primary consumer level. Daphnia pulex, which dominated the crustacean zooplankton community, did not respond to the higher omega-3 availability in the thaw ponds but rather assimilated the high-quality fatty acids equally in all ponds, possibly because their metabolic needs were already saturated. However, some lower quality terrestrial carbon compounds from permafrost ended up in the D. pulex body mass, resulting in a median allochthony of 18% based on fatty acid mixing model; non-thaw ponds had median allochthony mixing model estimates of 8%. The high availability of algal resources seemed to prevent extensive zooplankton allochthony in subarctic thaw ponds. [ABSTRACT FROM AUTHOR]

Energy pathways in stream food webs are often driven by allochthonous basal resources. However, allochthonous dissolved organic carbon (DOC) is generally viewed as a minor if not insignificant basal resource because much of the DOC pool comprises high molecular weight, recalcitrant compounds and is inefficiently incorporated into biomass. Nevertheless, there is increasing evidence that the relatively small, labile fraction of DOC may indeed fuel microbial activity to a level that stimulates productivity across multiple trophic levels, resulting in a trophic upsurge. Here, we tested the trophic upsurge hypothesis by subsidising the labile DOC pool of an Alaskan boreal stream that had relatively high nutrient availability but low levels of naturally occurring DOC.We continuously added ecologically relevant (0.250 mg C/L, c. 10% increase above ambient bulk DOC) concentrations of labile DOC (acetate‐C) for 62 days to a treatment reach that was statistically indistinguishable in its channel form and chemistry from an upstream reference reach. We measured responses of periphyton production and biomass, whole reach metabolism and nutrient uptake, benthic invertebrate abundances, and juvenile salmonid (Dolly Varden, Salvelinus malma) abundance and growth.Measurements of basal ecosystem responses collectively indicated increased energy mobilization at the base of the food web in response to labile DOC addition. Periphyton bacterial production in the treatment reach was generally >1.5× reference reach values, and periphyton ash‐free dry mass, chlorophyll‐a, and chlorophyll‐a:ash‐free dry mass were all greater in the treatment reach by the end of the study. Throughout dosing, ecosystem respiration was 1.3× greater in the treatment reach and dissolved inorganic nitrogen uptake was greater in the treatment reach on eight out of nine measurements.Benthic invertebrate counts, dominated by Baetis spp. and Chironomidae, were c. 4× greater after 28 dosing days and c. 8× greater after 56 days in the upstream portion of the treatment reach. Abundance generally declined with increasing distance from the dosing station. Dolly Varden fry and parr age classes were nearly 2× more abundant in the upstream portion of the treatment reach than in any section of the reference reach and also declined with increasing distance from the dosing station. Further, Dolly Varden tagged with passive integrated transponders prior to the experiment had significantly higher instantaneous growth rates in the treatment reach than those recaptured in the reference reach.The strong consumer responses to small quantities of labile DOC mirrored significant treatment reach increases in basal ecosystem function and therefore demonstrated a response consistent with a trophic upsurge. Terrestrial DOC has historically been viewed as contributing little to metazoan consumers, instead modulating the influence of nutrients and being respired out of a disconnected microbial loop. Because we dosed the treatment reach with a relevant concentration of labile DOC, based on measurements in nearby peatland‐draining streams, we suggest that terrestrial DOC deserves more attention as a basal resource for whole food webs, akin to nutrients fuelling green (autochthonous) pathways. [ABSTRACT FROM AUTHOR]

Understanding the degree to which aquatic and terrestrial primary production fuel tropical aquatic food webs remains poorly understood, and quantifying the relative contributions of autochthonous and allochthonous inputs is methodologically challenging. Carbon and nitrogen stable isotope ratios (δ13C, δ15N) can provide valuable insights about contributions of terrestrial resources and trophic position, respectively, but this approach has caveats when applied in typical complex natural food webs.Here, we used a combination of C, N, and H (δ2H) stable isotope measurements and Bayesian mixing models to estimate the contribution of terrestrial (allochthonous) and aquatic (autochthonous) inputs to fish and invertebrate communities in the Congo River (and some tributaries).Overall, our results show that we gained power to distinguish sources by using a multiple tracer approach and we were able to discriminate aquatic versus terrestrial sources (esp. including hydrogen isotopes). Fish δ2H values were clearly correlated with their food preferences and revealed a high level of variation in the degree of allochthony in these tropical aquatic communities.At the community level, it is clear that terrestrial C3 plants are an important source fueling the Congo River food web. However, in order to better constrain source contribution in these complex environments will require more robust constraints on stable isotope values of algal and methane‐derived C sources. [ABSTRACT FROM AUTHOR]

We investigated ontogenetic and seasonal variations in the diet of the freshwater mullet ( Myxus capensis) across a river-estuary interface using dietary tracer (stable isotopes and fatty acids) and stomach content analyses. Two hypotheses were tested: (A) the freshwater mullet diet shifts as individuals grow and migrate from the estuary to the river, and (B) the dominant food resources utilized by freshwater mullet vary through time, mainly as a function of the seasonal changes in the availability of preferred food items in each habitat. Both hypotheses were supported, as our results indicated broad dietary flexibility by M. capensis, with utilized food items ranging from benthic microalgae to insects depending on habitat and seasonal patterns in availability of resources. Given the unexpected importance of invertebrate-derived prey, including some of terrestrial origin (i.e. aerial or semi-aquatic insects), during the freshwater phase of the M. capensis life cycle, we also emphasize a need for a re-assessment of the trophic designation of this species (previously designated as a strict detritivore). [ABSTRACT FROM AUTHOR]

Paleolimnology is a promising approach to reconstruct past carbon cycle in lakes and its response to global changes. Here, we test the potential of the combined use of sedimentary geochemical proxies and δC analysis of subfossil chironomid (δC) in a sediment core retrieved from a boreal lake. Characteristics of sedimentary organic matter appeared to be strongly variable over time, corresponding to periodic decreases in aquatic organic matter contribution to lake sediments, and this dynamic was attributable to climatic changes occurring during the late Holocene. Results revealed also that δC values were lower than those of organic matter, and these differences were greater when lake sediments were depleted in aquatic organic matter. Thus, chironomid feeding behavior seems to be dependent on the organic matter quality, showing a strong affinity for aquatic organic matter even if this resource is not the most available in sediments. Based on this methodological strategy, our results indicate (i) the relatively poor nutritive quality of allochthonous materials for benthic chironomid larvae, (ii) the strong influence of climate variability on the whole lake functioning, and (iii) the high potential of the combined use of this methodology to reconstruct the past carbon cycle in boreal lakes. [ABSTRACT FROM AUTHOR]

Aquatic food webs are supported by primary production from within the system (autochthony) as well as organic matter produced outside of and transported into the system (allochthony). Zooplankton use allochthonous resources, especially in systems with high terrestrial loading and moderate to low internal primary production. We hypothesized that due to high terrestrial loads and remnant submerged terrestrial material, allochthonous resource use by zooplankton would be significant in all reservoirs and would decline along an increasing reservoir age gradient. Using hydrogen stable isotopes and a Bayesian mixing model, we estimated the contribution of allochthonous sources to organic matter pools and crustaceous zooplankton biomass for ten reservoirs. Dissolved organic matter (DOM) in all systems was dominated by allochthonous sources (posterior distribution median >92% allochthonous), while particulate organic matter (POM) composition varied (2-68% allochthonous) and had a lower allochthonous fraction in older reservoirs. There was no relationship between zooplankton allochthony and reservoir age. Crustaceous zooplankton allochthony varied among systems from 26 to 94%, and Chaoborus allochthony, measured in four reservoirs, was similarly variable (33-94%). Consumer allochthony was higher than POM allochthony in some reservoirs, potentially due to terrestrial DOM pathways being important and/or algal resources being inedible (e.g., cyanobacteria). As with many lakes, in the reservoirs we studied, allochthonous inputs account for a significant fraction of the organic matter of basal consumers. [ABSTRACT FROM AUTHOR]

The zooplankton is a key link in the transfer of energy from primary producers up through aquatic food webs. Previous efforts to quantify the importance of basal resources to aquatic consumers have used stable isotopes ( SI) and simple ternary models, including only 'bulk' phytoplankton, bacteria or terrestrial particulate organic matter (t- POM)., We used a novel Bayesian mixing model based on fatty acids ( FA) to quantify the dietary assimilation of seven basal resources, including five phytoplankton groups, pelagic bacteria and t- POM, to Cladocera in large boreal lakes in Finland. To account for trophic enrichment of FA from the diet to consumers, we parameterised the model with a resource library, from many feeding trials, consisting of Daphnia magna fed 22 diverse basal taxa., The results of the feeding trials show that the distinctive FA profiles of algal groups are transferred to consumers. Moreover, the large number of FA variables ( n = 22) used in the model avoids the limitations of underdetermined mixing problems, common to SI modelling, in cases when the number of resources outnumbers the tracer variables., We show that cladocerans were generally supported by phytoplankton (86-94%), with little use of t- POM (1-9%) and bacteria (1-3%). Cladocerans used primarily high-quality phytoplankton (cryptophytes, diatoms and dinoflagellates) in both summer (51 ± 22%) and autumn (79 ± 12%), and the relative importance of medium-quality resources (cyanobacteria, chlorophytes and chrysophytes) declined from 37 ± 23% in the summer to 8 ± 2% in the autumn., High-quality resources, rich in essential biochemical compounds, are critical in fuelling food webs in large lakes, even those with high concentrations of allochthonous organic matter. [ABSTRACT FROM AUTHOR]

Inputs of terrestrial organic carbon (t-OC) into lakes are often considered a resource subsidy for aquatic consumer production. Although there is evidence that terrestrial carbon can be incorporated into the tissues of aquatic consumers, its ability to enhance consumer production has been debated. Our research aims to evaluate the net effect of t-OC input on Zooplankton. We used a survey of Zooplankton production and resource use in ten lakes along a naturally occurring gradient of t-OC concentration to address these questions. Total and group-specific Zooplankton production was negatively related to t-OC. Residual variation in Zooplankton production that was not explained by t-OC was negatively related to terrestrial resource use (allochthony) by Zooplankton. These results challenge the designation of terrestrial carbon as a resource subsidy; rather, the negative effect of reduced light penetration on the amount of suitable habitat and the low resource quality of t-OC appear to diminish Zooplankton production. Our findings suggest that ongoing continental-scale increases in t-OC concentrations of lakes will likely have negative impacts on the productivity of aquatic food webs. [ABSTRACT FROM AUTHOR]

Terrestrial food subsidies are important energy sources to stream fishes worldwide. However, their importance is not fully understood, except perhaps for some salmonid communities. Using the published literature, we investigated patterns of fish taxonomy and jaw morphology in the consumption of terrestrial food subsidies by non-salmonid stream fishes in the USA. We hypothesised that: (1) in general, non-salmonid fishes would consume terrestrial food items to the same extent as salmonids; and (2) subsidy consumption would be associated with jaw morphology. Cyprinids and fundulids consumed terrestrial subsidies approaching levels observed in salmonids (20-44%) whereas other groups (e.g. catostomids and moronids) consumed far less (<2%) terrestrial food. Fishes with terminal jaw positions tended to consume more terrestrial items than those with subterminal or inferior jaw positions. Within a species, there were highly variable propensities for consuming terrestrial subsidies. Our findings justify focusing on trophic linkages between fishes and riparian systems across a wide range of taxa, especially considering the spatio-temporal variability of environmental conditions across a variety of habitats. It is necessary to assess the relative contribution of terrestrial subsidies in fish growth and population dynamics with a focus on nutritional and energetic benefits across different groups. Terrestrial food subsidies are important to fish across different ecosystems. By assessing the importance of terrestrial subsidies in non-salmonid fishes, we found that cyprinids and fundulids, as well as fishes with terminal jaw positions, tended to consume the most terrestrial subsidies. Our findings justify focussing on trophic linkages between fishes and riparian systems across a wide range of taxa. [ABSTRACT FROM AUTHOR]

Organic carbon inputs from outside of ecosystem boundaries potentially subsidize recipient food webs. Four whole-lake additions of dissolved inorganic 13C were made to reveal the pathways of subsidies to lakes from terrestrial dissolved organic carbon (t-DOC), terrestrial particulate organic carbon (t-POC) and terrestrial prey items. Terrestrial DOC, the largest input, was a major subsidy of pelagic bacterial respiration, but little of this bacterial C was passed up the food web. Zooplankton received <2% of their C from the t-DOC to bacteria pathway. Terrestrial POC significantly subsidized the production of both zooplankton and benthic invertebrates, and was passed up the food web to Chaoborus and fishes. This route supplied 33–73% of carbon flow to zooplankton and 20–50% to fishes in non-fertilized lakes. Terrestrial prey, by far the smallest input, provided some fishes with >20% of their carbon. The results show that impacts of cross-ecosystem subsidies depend on characteristics of the imported material, the route of entry into the food web, the types of consumers present, and the productivity of the recipient system. [ABSTRACT FROM AUTHOR]