Grazer Control of Stream Algae: Modeling Temperature and Flood Effects

Abstract

A computer model for epilithic algae and grazer biomass in streams is modified to better predict the effects of temperature and is calibrated for diatoms and mayflies. Mayflies are predicted to maintain low diatom biomass provided that (1) temperatures remain within their preferred range (10-207C); and (2) mayfly populations are not adversely affected by floods. Algal blooms are predicted to occur in mayfly-dominated streams above 207C—temperatures common in pasture streams over summer. We hypothesize that mobile bed streams are susceptible to blooms during summer low flows following floods because (1) they usually lack temperature tolerant snail grazers; and (2) mayfly recovery lags behind algal regrowth, and there is a short period when algae escape from ''top-down'' grazer control. Proliferations of periphytic algae (algal blooms) have been documented in a number of New Zealand rivers with stoney beds draining agricultural catchments (Biggs and Price 1987) and adversely affect ecosystem structure and function, river aesthetics, and some water uses. Most conspicuous are blooms of filamentous green algae, although proliferation of diatoms and blue-green algae are also of management concern. Algal blooms only occur sporadically, when the rates of accrual (au- totrophic fixation plus colonization) exceed the rates of loss (sloughing, flow disturbance, and consumption by grazers). Benthic macroinvertebrate grazers (principally aquatic in- sects) have a significant negative effect on algal biomass; for example, Welch et al. (1992) showed that 3,000 grazers m 22