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1. We used a 27-year record of Dreissena populations in the freshwater tidal Hudson River to describe interannual variation in population density, body size, and body condition; estimate long-term variation in recruitment, survivorship, and shell growth; and assess possible controls on the populations. 2. Dreissena populations in the Hudson have been highly variable, with interannual ranges of c. 100-fold in abundance and biomass, and 7-fold in mean body mass. This large interannual variation arises from both long-term trends and 2–5-year cycles. 3. Long-term trends include the 2008 appearance of the quagga mussel (Dreissena rostriformis), which still forms a small part (

Summary 1. Nitrogen (N) processing in streams has been investigated using whole-stream 15N addition experiments that, in general, have found that a large proportion of added nitrate removed from the water column appears to be assimilated by the stream benthos. The long-term fate of this retained N is unknown, and of particular interest is the possibility that it becomes denitrified through coupled mineralisation–nitrification–denitrification processes (indirect denitrification). 2. We used in situ chambers to produce highly 15N-enriched benthic biofilms and removed the chambers to allow biofilms to interact with ambient stream conditions. Nitrogen assimilation and direct denitrification were estimated from the first chamber deployment. Chambers were periodically reinstalled over 4 weeks to measure tracer 15N in ammonium (), nitrate () and dinitrogen (N2), from which we estimated subsequent rates of biotic N transformations, including N mineralisation (ammonification), nitrification and indirect denitrification. We also estimated rates of depuration of 15N tracer from benthic biomass compartments. 3. Nitrate uptake was roughly equivalent in the sand and cobble habitats that dominated the stream. Direct denitrification (denitrification of from the water column) was an order of magnitude higher in cobble habitats than in sand habitats, accounting for c. 26 and 2% of total nitrate uptake in cobble and sand, respectively. 4. Mean residence times of actively cycling organic N in stream benthos (algae and microbes) were 16 days in cobble habitats and 9 days in sand habitats. The difference between habitat types was driven by the influence of N residence time in epilithic biofilms (18 days) on cobbles. 5. Release of enriched 15 was the primary flux of remineralised N, while release of enriched 15 was an order of magnitude less. We detected slight 15N enrichment in dissolved nitrogen gas (N2) in post-enrichment sampling, indicating that indirect denitrification was taking place. However, indirect denitrification accounted for

Summary 1. High light availability and stable base flow during the dry season promote primary production in perennial rivers of the wet–dry tropics, in contrast to production during the wet season which is often limited by turbidity and scouring. The Mitchell River of northern Queensland (Australia) was studied to understand controls on aquatic production and respiration in the dry season in relation to spatial and temporal gradients of light and temperature. 2. At three sites along the river, whole-ecosystem gross primary production (GPP) and respiration (ER) were measured from diel changes of dissolved oxygen using the open-channel single station method. Using stable carbon and nitrogen isotope analysis, aquatic consumers and their potential basal food resources were also assessed to determine food web relationships at the beginning and end of the dry season. 3. Nutrient limitation of aquatic net primary production was implied from the oligotrophic conditions and high algal C:N ratios. Rates of GPP were comparable with other tropical and temperate rivers and were regulated by light availability. 4. Respiration rates were high and similar to other tropical and subtropical rivers. Up to 52% of temporal variation of ER was explained by temperature, while P/R was lowest at the downstream site. 5. Benthic algae were the major carbon source for primary and secondary benthic consumers (insects) in the dry season but not for higher consumers (fish and crustaceans). Despite high rates of ER, which were probably supported by decaying terrestrial C3 plant material, this carbon source was not identified as contributing to animal consumer biomass. 6. While benthic algal production in the dry season sustained benthic invertebrates, the importance of external subsidies of carbon along the river, probably from the floodplain, was emphasised for fish and large invertebrates, which evidently were feeding on carbon sources not present in channel waterholes during the dry season.

Summary 1. Mounting interest in ecological restoration of streams and rivers, including that motivated by the Water Framework Directive, has stimulated examination of whether management and restoration measures in streams and their catchments have yielded measurable improvements in ecological status (‘health’). Evidence for the efficacy of diffuse-source pollution reduction (including best management practices on land) has proven elusive. 2. Several hydrological and biogeochemical processes delay the responses of streams and rivers to a decrease in nutrient and sediment inputs, potentially for decades. The implications of such time lags in response to restoration may not be well appreciated by restoration ecologists, regulators, sponsors of restoration work or the broader community. 3. The groundwater time lag results from the long residence time of ground water. This is particularly important with respect to nitrate, but is increasingly important for phosphorus (P) as well. Isotopic tracers and groundwater age dating suggest that stream water often is more than a decade old, and that several decades are required to flush most soluble contaminants from groundwater reservoirs. 4. Sediment movement through river networks can be protracted because of storage and remobilisation processes involving stream beds, impounded reaches and fringing bars and floodplains. In lowland streams and rivers, sediment accretion can be rapid, but its removal is often far slower and can take decades to centuries. 5. Phosphorus availability is subject to time lags because P tends to associate with minerals, resulting in a potentially large yet exchangeable P reserve in upland soils and alluvial and stream-bed sediments. Thus, soils and sediments can remain rich in P for decades after new inputs are reduced, potentially acting as a source of P to surface waters. Phosphorus saturation of soils along groundwater percolation pathways can lead to even longer time lags. Restoration measures that inundate previously dry soils or desiccate previously inundated sediments can induce high rates of P release. 6. These hydrological and biogeochemical time lags can obscure the short-term responses of streams and rivers to restoration measures. In many eutrophic waters, large decreases in nutrient availability would be required to return the ecosystem to a natural nutrient-limited state, and this could take decades.

1.Northern Australia is characterised by a tropical wet-dry climate that regulates the distinctive character of river flow regimes across the region. There is marked hydrological seasonality, with most flow occurring over only a few months of the year during the wet season. Flow is also characterised by high variability between years, and in the degree of flow cessation, or intermittency, over the dry season. 2.At present, the relatively low human population density and demand for water in the region means that most rivers have largely unmodified flow regimes. These rivers therefore provide a good opportunity to understand the role of natural flow variability in river ecosystem structure and processes. 3.This review describes the major flow regime classes characterising northern Australian rivers, from perennial to seasonally intermittent to extremely intermittent, and how these regimes give rise to marked differences in the ecological character of these tropical rivers, particularly their floodplains. 4.We describe the key features of these flow regimes, namely the wet and dry seasons and the transitions between these seasons, and how they regulate the biophysical heterogeneity, primary productivity and movement of biota in Australia's wet-dry tropical rivers. 5.We develop a conceptual model that predicts the likely hydrological and ecological consequences of future increases in water abstraction (e.g. for agriculture), and suggest how such impacts can be managed so that the distinctive ecological character of these rivers is maintained. © 2011 Blackwell Publishing Ltd.

1. Globally, tropical floodplains are highly productive ecosystems. This is largely because of predictable seasonal rains providing replenishing floodwaters that stimulate nutrient turnover which, in turn, substantially boosts both primary and secondary productivity. This is associated with concomitant shifts in the types of primary producers and associated food webs. 2. The Magela Creek floodplain on Kakadu National Park in northern Australia is one of the most studied tropical freshwater ecosystems in Australia and provides an opportunity to collate and examine information on organic carbon sources and pathways through food webs to gain a fundamental understanding of how these systems may function. 3. We reviewed biophysical information published since the early 1980s to construct an assessment of the carbon resources for the channel and floodplain. 4. We conclude that macrophytes, largely in the form of grasses and aquatic plants, produce the greatest above-ground biomass on the Magela Creek floodplain. Although macrophytes provide suitable substrata for the attachment of epiphytes, they do not appear to be an important carbon source for aquatic consumers themselves. Nevertheless, macrophytes do provide critical seasonal food and habitat structure for other producers and consumers on the floodplain, such as the abundant magpie geese. 5. We developed a generalised conceptual food web and carbon budget contrasting the ‘wet’ and ‘dry’ seasons for the Magela Creek system, as a representative of tropical seasonal floodplain systems. 6. Our conceptual model of tropical floodplains indicates that knowledge of the seasonal and spatial links and exchanges between the floodplain and the river is critical in understanding ecosystem function.

Summary 1 Zebra mussels (Dreissena polymorpha) are successful colonisers of lake littoral habitats and they interact strongly with littoral benthos. Previous research suggests that localised areas colonised by zebra mussels may be hotspots of nitrogen (N) cycling. 2 The effects of zebra mussels on nitrification and denitrification rates were examined approximately every other month for 1 year in Gull Lake, Michigan, U.S.A. Littoral sediment was collected from an area free of zebra mussels and distributed into shallow trays; rocks colonised with zebra mussels were placed in half of the trays, while uncolonised rocks were placed in the remaining trays. After an incubation period of 6–8 weeks in the lake, sediment and zebra mussels were collected from the trays, replaced with new sediment and zebra mussels, and placed in the lake for the next interval. In the laboratory, sediment nitrification and denitrification rates were measured for each tray. 3 Sediment nitrification rates did not increase in the presence of zebra mussels; instead nitrification rates were sensitive to changes in water temperature and increased with increasing exchangeable sediment ammonium. In contrast, denitrification rates increased in sediment trays with zebra mussels in the winter when nitrate (NO3−) availability was high and when Chara did not grow in the trays. 4 Sediment denitrification was NO3−-limited in all seasons, regardless of zebra mussel treatment. However, sediment in the presence of zebra mussels responded less to NO3− addition, suggesting that NO3− limitation of denitrification can be reduced by zebra mussel activity. Zebra mussels have a seasonally variable impact on sediment denitrification rates, and this may translate into altered seasonal patterns of N cycling in localised areas of lakes where they are particularly abundant.

SUMMARY 1. The Lotic Intersite Nitrogen eXperiment (LINX) was a coordinated study of the relationships between North American biomes and factors governing ammonium uptake in streams. Our objective was to relate inter-biome variability of ammonium uptake to physical, chemical and biological processes. 2. Data were collected from 11 streams ranging from arctic to tropical and from desert to rainforest. Measurements at each site included physical, hydraulic and chemical characteristics, biological parameters, whole-stream metabolism and ammonium uptake. Ammonium uptake was measured by injection of 15 N-ammonium and downstream measurements of 15 N-ammonium concentration. 3. We found no general, statistically significant relationships that explained the variability in ammonium uptake among sites. However, this approach does not account for the multiple mechanisms of ammonium uptake in streams. When we estimated biological demand for inorganic nitrogen based on our measurements of in-stream metabolism, we found good correspondence between calculated nitrogen demand and measured assimilative nitrogen uptake.

'SUMMARY 1. We studied whole-ecosystem metabolism in eight streams from several biomes in North America to identify controls on the rate of stream metabolism over a large geographic range. The streams studied had climates ranging from tropical to cool-temperate and from humid to arid and were all relatively uninfluenced by human disturbances. 2. Rates of gross primary production (GPP), ecosystem respiration (R) and net ecosystem production (NEP) were determined using the open-system, two-station diurnal oxygen change method. 3. Three general patterns in metabolism were evident among streams: (1) relatively high GPP with positive NEP (i.e. net oxygen production) in early afternoon, (2) moderate primary production with a distinct peak in GPP during daylight but negative NEP at all times and (3) little or no evidence of GPP during daylight and a relatively constant and negative NEP over the entire day. ', 4. Gross primary production was most strongly correlated with photosynthetically active radiation (PAR). A multiple regression model that included log PAR and stream water soluble reactive phosphorus (SRP) concentration explained 90% of the variation in log GPP. 5. Ecosystem respiration was significantly correlated with SRP concentration and size of the transient storage zone and, together, these factors,explained 73% of the variation in R. The rate of R was poorly correlated with the rate of GPP. 6. Net ecosystem production was significantly correlated only with PAR, with 53% of the variation in log NEP explained by log PAR. Only Sycamore Creek, a desert stream in Arizona, had positive NEP (GPP: R > I), supporting the idea that streams are generally net sinks rather than net sources of organic matter.

SUMMARY. 1. During the summer of 1987 we conducted an acidification experiment using large enclosure at Emerald Lake, a dilute, high-elevation lake in the Sierra Nevada, California, U.S.A. The experiment was designed to examine the effects of acidification on the zooplankton and zoobenthos assemblages of Sierran lakes. 2. Treatments consisted of a control (pH 6.3) and pH levels of 5.8, 5.4, 5.3, 5.0 and 4.7; each treatment was run in triplicate. The experiment lasted 35 days. 3. The zooplankton assemblage was sensitive to acidification. Daphnia rosea Sars emend. Richard and Diaptomns signicauda Lilljeborg decreased in abundance below pH 5.5–5.8, and virtually disappeared below pH 5.0. Bosmina longirostris (Muller) and Keratella taurocephala Ahlstrom became more abundant with decreasing pH. although B. longirostris was rare in the pH 4.7 treatment. These species might serve as reliable indicators of early acidification in lakes such as Emerald Lake. 4. The elimination of D. rosea in acidified treatments probably allowed the more acid-tolerant taxa to increase in abundance because interspecific competition was reduced. Even slight acidification can therefore alter the structure of the zooplankton assemblage. 5. In contrast to the zooplankton, there was no evidence that the zoobenthos in the enclosures was affected by acidification.