Proliferations of filamentous chlorophytes and mats of cyanobacteria (hereafter termed metaphyton) are increasingly observed in rivers, lakes, wetlands, and estuaries undergoing eutrophication, but their contribution to invertebrate production and overall ecological significance remains poorly understood. In Lake Saint-Pierre, a shallow widening of the St. Lawrence River (Québec), vascular macrophytes (mainly Vallisneria americana) grow in combination with filamentous chlorophytes (Hydrodictyon, Oedogonium) in the upstream reach, which is fed by nutrient-rich waters from the tributaries, and in association with filamentous cyanobacteria (Lyngbya wollei) in the chronically NO3−-depleted downstream reach. We hypothesized that different vegetation types (macrophytes, filamentous chlorophytes, and cyanobacteria) would support macroinvertebrate communities with different biomasses and taxonomic compositions. We expected a higher invertebrate biomass in the upstream reach and, within the reach, a higher biomass on metaphyton than on macrophytes. Total macroinvertebrate biomass was significantly higher at the enriched stations in the upstream reach (75–100 mg/g vegetation, dry mass) than farther downstream (8–38 mg/g). In addition, macrophytes and metaphyton in the upstream reach sustained taxonomically different invertebrate assemblages. Gastropods dominated the fauna associated with macrophytes throughout the lake (43–73%) and probably benefitted from a structurally simple and solid substratum on which to crawl and feed. Small mobile taxa, such as cladocerans, copepods, chironomids, and ostracods, were more abundant on filamentous metaphyton, both up- and downstream. Amphipods were dominant (59%) in metaphytic mats of L. wollei. At the scale of the river reach, macrophytes supported most of the invertebrate biomass. Chlorophytes in the upstream reach contributed <5% of the total biomass, representing an alternative, albeit temporary, habitat. In contrast, in the downstream reach where macrophytes were scarce, cyanobacterial mats hosted a significant fraction of macroinvertebrates (36%). Shifts in vegetation between the 2 reaches affected the quantity and availability of prey items for fish predators.
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