Productive Capacity of Periphyton as a Determinant of Plant‐Herbivore Interactions in Streams

To investigate the influence of plant productivity on plant-herbivore inter- actions in stream ecosystems, we varied the productive capacity of algal assemblages by exposing periphyton to three levels of irradiance and two levels of grazing. We studied interactions between algal assemblages (grown from algae obtained from four Oregon streams) and herbivorous snails (Juga silicula) in 15 laboratory streams containing either 250 snails/m2 or no snails. Biomass, production, export, and taxonomic structure of the algal community were measured at intervals throughout the 75-d study. Ingestion rate and assimilation efficiency of snails also were measured on six different dates using dual-isotope labeling, and snail growth was measured at the end of the experiment. Rates of primary production, algal biomass accumulation, and dominance by chloro- phytes generally increased with higher irradiance, although these patterns were modified by herbivores. Ungrazed periphyton at low irradiance (photon flux density: 20 ,umol-m-2- s-1) accumulated little biomass, which was further reduced by grazing snails. At inter- mediate (100 ,umol.m-2 s-) and high (400 ,Lmol.m-2-s-1) irradiance, snails delayed the accumulation of algal biomass but did not affect the final biomass attained. After 43 d, net primary production (NPP) at high irradiance was unaffected by grazing, whereas grazing increased NPP at both low and intermediate irradiance. Algal export increased with both irradiance and the presence of grazers and constituted a significant loss of plant biomass from the streams. Grazing by Juga delayed algal succession and altered algal taxonomic structure and assemblage physiognomy by reducing the relative abundance of erect and non-attached algae, while increasing the abundance of adnate diatoms. Snails grew slowly at low irradiance, due to scant food resources, but had high growth rates at intermediate and high irradiance, probably because food was not limiting. Assim- ilation efficiencies for snails generally varied from 40 to 70% and were highest at low irradiance. At low irradiance, 90% ofbenthic production was harvested by grazers, whereas only 10% accumulated as attached biomass or was exported. At higher irradiances, 85% persisted as attached algae or was exported. In these stream ecosystems, the biomass and production of grazers were influenced by abiotic constraints placed on algal productive capacity (i.e., the ability of a plant assemblage to generate biomass). The structure and metabolism of algal assemblages were affected, in turn, by consumptive demand of herbivores. The productive capacity of periphyton mod- ified the nature and outcome of plant-herbivore interactions. This capacity therefore has important implications for the operation of stream ecosystems.

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