Mechanisms of algal patch depletion: importance of consumptive and non-consumptive losses in mayfly-diatom systems.

Laboratory experiments were performed to identify the mechanisms by which three mayfly grazers, Baetis tricaudatus, Ephemerella aurivilli and Paraleptophlebia heteronea deplete algae from substrates. Field observations indicated these mayflies foraged predominantly (>70% of all individuals) within small (1-2 cm diameter), low biomass areas where algal biomass was significantly lower than the surrounding algal mat. We postulated four models of algal patch depletion based on the combined effects of a type II functional response consumptive model and four possible forms of nonconsumptive loss. These models were tested in laboratory feeding trials by examining the relative importance of consumptive and non-consumptive removal of the diatom, Navicula sp., by the three common mayfly grazers. The trials were conducted in plexiglass streams that contained substrates with one of five biomass levels (0.11, 0.24, 0.43, 0.65, 0.92 mg/cm dry weight) of the diatom food. After each 1 h feeding trial, consumption was measured, and the remaining algae scraped from the substrates so non-consumption and total patch depletion could be determined. Consumption by all three species followed a type II functional response; mayflies were capable of grazing diatom layers of extremely low biomass (0.11 mg/cm) and reached an asymptotic feeding rate when diatom biomass ranged from 0.24-0.43 mg/cm. Upper asymptotic feeding rates occurred at algal biomasses that were 20 times lower than algal biomass levels within foraging areas in the field and >50 times the overall mean algal biomass on upper stone surfaces in the Bow River. When diatom biomass was low (0.11 mg/cm), the amount of algae ingested accounted for 27%-75% of total depletion of algal patches. Above this level, nonconsumptive, foraging-related losses increased. Thus, depletion of diatom patches was non-linear and positively related with diatom biomass due to the disproportionate increase in non-consumptive losses combined with the type II functional response consumptive model (Case 4). This disproportionate increase in non-consumptive loss may results from (i) a passive process attributable to mechanical limitations of the feeding apparatus, (ii) an active selection process during foraging or (iii) instability of the diatom material resulting in disproportionately high foraging related dislodgement. Regardless of the mechanism, our experiments indicate the importance of considering algal patch depletion by mayfly grazers as a dual product of consumptive and non-consumptive foraging processes. Furthermore, the non-linear increase in nonconsumptive loss with increased algal biomass suggests this process may be a major mechanism of algal patch depletion by mayflies when algal biomass is high.