Consequences of Fluctuating Discharge for Lotic Communities

Abstract

We used five in situ experiments to test the influence of fluctuations in river discharge on the structure and function of the tailwaters benthos associated with cobble substrata in the Colorado River downstream from Glen Canyon Dam, Arizona, USA. Periods of daily desiccation and freezing during river fluctuation significantly limited community biomass and energy. The permanently submerged channel supported 4-fold higher macroinvertebrate mass than the varial zone. Daily harvests of benthos showed a 50% reduction in mass of Cladophora glomerata after 2 d of repeated 12-h summer exposure. Five days of repeated exposure resulted in >70% reduction in C. glomerata and >50% reduction in epiphyton mass. We observed a ≥85% reduction in benthic macroinvertebrate mass after only one 12-h summer exposure. One night time exposure to subzero winter air temperatures resulted in ≥50% loss of chlorophyll a and mass of C. glomerata and ≥90% loss in macroinvertebrate mass. Gastropod densities on resubmerged cobbles that were subjected to long-term desiccation (∼6 mo) equalled submerged control cobbles within 1 wk, whereas recolonization by C. glomerata, Gammarus lacustris, and chironomid larvae was significantly slower; i.e., ≤30% of controls after 4 mo. Hence, our data showed that two 12-h exposure periods may require >4 mo for recovery to achieve the mass of permanently submerged benthos. Incremental increases in benthic energy (joules/m²) over increased base discharge from 142 to 793 m³/s may result in an increase in trout mass of approximately 42.5 kg/ha in the tailwaters at Lees Ferry.