Anthropogenic increases in nitrogen (N) and phosphorus (P) concentrations can strongly influence the structure and function of ecosystems. Even though lotic ecosystems receive cumulative inputs of nutrients applied to and deposited on land, no comprehensive assessment has quantified nutrient‐enrichment effects within streams and rivers. We conducted a meta‐analysis of published studies that experimentally increased concentrations of N and/or P in streams and rivers to examine how enrichment alters ecosystem structure (state: primary producer and consumer biomass and abundance) and function (rate: primary production, leaf breakdown rates, metabolism) at multiple trophic levels (primary producer, microbial heterotroph, primary and secondary consumers, and integrated ecosystem). Our synthesis included 184 studies, 885 experiments, and 3497 biotic responses to nutrient enrichment. We documented widespread increases in organismal biomass and abundance (mean response = +48%) and rates of ecosystem processes (+54%) to enrichment across multiple trophic levels, with no large differences in responses among trophic levels or between autotrophic or heterotrophic food‐web pathways. Responses to nutrient enrichment varied with the nutrient added (N, P, or both) depending on rate versus state variable and experiment type, and were greater in flume and whole‐stream experiments than in experiments using nutrient‐diffusing substrata. Generally, nutrient‐enrichment effects also increased with water temperature and light, and decreased under elevated ambient concentrations of inorganic N and/or P. Overall, increased concentrations of N and/or P altered multiple food‐web pathways and trophic levels in lotic ecosystems. Our results indicate that preservation or restoration of biodiversity and ecosystem functions of streams and rivers requires management of nutrient inputs and consideration of multiple trophic pathways.

In temporary freshwater systems, the type of vegetation within a system can influence community structure. Vegetation not only provides physical structure, but can also contribute to changes in abundance and quality of food and in water quality through decomposition. An experiment was undertaken using natural and artificial vegetation in small mesocosms to examine the influence of the physical structure of vegetation on invertebrate community structure in terms of water quality, food abundance, and physical structure. It was predicted that invertebrate community structure would be identical in natural and artificial treatments if the effect of vegetative decomposition was negligible.Furthermore, invertebrate community structure in bare ground treatments would be identical to those with vegetation if the physical structure of vegetation has no significant effect. Five treatments were used: a bare ground control, artificial vegetation (92), and natural vegetation treatments (grass, eucalypt leaf litter). Water quality, food abundance, and invertebrate abundance were examined after six weeks of inundation. All treatments had high water temperatures (34-40°C), and natural vegetation treatments had slightly higher conductivity (208-316 mS cm -1 ) and lower turbidity (40-231 NTU) than other treatments (47-156 mS cm -1 and 55-400 NTU, respectively). The physical structure of artificial vegetation did not significantly influence invertebrate community structure compared to the bare ground treatment, whereas treatments with decomposing natural vegetation had relatively low abundances of microcrustaceans (0-96 individuals/ mesocosm) and relatively high abundances of chironomids (192-1576 individuals/mesocosm) compared to other treatments ([100 microcrustaceans/mesocosm if present, and \370 chironomids/mesocosm, respectively). This suggests that food availability had greater importance than physical structure in determining community structure in these small aquatic ecosystems.

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The midge Chironomus tepperi was used in laboratory experiments to assess the relative toxicity of formulated molinate, clomazone, and thiobencarb, three herbicides used in Australian rice crops. Static bioassays were initiated with first-instar larvae at herbicide concentrations between 0.0625 and 2 times the anticipated field concentrations (AFCs) expected from the registered application rates. Adult emergence success, development time, and wing length were used as indices of the effect of each herbicide. Clomazone had no effect on any parameters at concentrations up to 0.288 mg/L (p > 0.05). Molinate significantly increased development time at concentrations equivalent to the AFC (3.6 mg/L) and above (p < 0.05). Thiobencarb reduced emergence success of adult C. tepperi at 0.0625 times the AFC (0.1875 mg/L) as well as decreasing male adult size and increasing development time for males and females at 0.125 times the AFC (p < 0.05). Nontarget effects of the herbicides on aquatic invertebrate communities were assessed in shallow experimental ponds using commercial application rates. One week after treatment, only thiobencarb had a significant effect, suppressing populations of chironomids, calanoids, and cyclopoids (p < 0.05). Four weeks later, all populations had recovered, equaling or exceeding control densities.

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