Macroinvertebrate distribution and production on the floodplains of two lowland headwater streams

SUMMARY. 1 Spatial and temporal distribution, abundance and production of the floodplain macroinvertebrate communities of two low-gradient headwater streams in Virginia, U.S.A., were studied over 1 year. 2 Spatial and temporal distribution patterns of invertebrates were affected by inundation patterns. Numbers, biomass and production were higher in low than high floodplain areas and on a floodplain continuously inundated over 9 months compared with a floodplain completely flooded only occasionally during storms. 3 The predominant species in terms of numbers and production on both floodplains was the harpacticoid copepod Attheyella illinoisensis Forbes. Other species with relatively high production were the sphaeriid clam Pisidium sp., the leptophlebiid mayfly Leptophlebia sp., the isopod Caecidotea racovitzai (Williams), and the chironomids Paratendipes sp. and Polypedilum spp. 4 Annual invertebrate production (dry weight) on the floodplain continuously inundated over 9 months was 6.1 g m−2; production on the periodically inundated floodplain was 1.7 g m−2. Collector-gatherers accounted for 58–61%, and predators 19–25%, of the production. 5 Annual invertebrate production on the floodplains, calculated on the basis of a linear metre of channel length, was 84–490 gm−1, or about one to two orders of magnitude greater than probable production in the channels, suggesting the significance of floodplain invertebrates to stream system trophic dynamics.