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

Abstract

To investigate the influence of plant productivity on plant-herbivore interactions 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/m 2 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 chlorophytes generally increased with higher irradiance, although these patterns were modified by herbivores. Ungrazed periphyton at low irradiance (photon flux density: 20 ~mol·m-2 • s-1 ) accumulated little biomass, which was further reduced by grazing snails. At intermediate (100 ~mol·m-2 ·s-1 ) and high (400 ~mol·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. Assimilation efficiencies for snails generally varied from 40 to 70% and were highest at low irradiance. At low irradiance, 90% of benthic production was harvested by grazers, whereas only 10% accumulated as attached biomass or was exported. At higher irradiances, < 15% of primary production was harvested by grazers, and >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 ofperiphyton modified the nature and outcome of plant-herbivore interactions. This capacity therefore has important implications for the operation of stream ecosystems.

Structure of Macroinvertebrate Communities in Relation to Environmental Variables in a Subtropical Asian River System

“…More than 50% of variability in the macroinvertebrate assemblages still remained to be explained, indicating that unmeasured variables such as resource availability (e.g., riparian features detritus, peryphyton, water chemistry) (CUMMINS et al, 1989;LAMBERTI et al, 1989), biotic interactions (e.g., predation) (ENGLUND et al, 1999) and disturbance could also be important. Thus the importance of riparian variables and disturbance factors will be considered in future studies.…”

Export of fine organic particles from headwater streams: Effects of season, extreme discharges, and invertebrate manipulation

“…This research was supported by National Science Foundation grants BSR 83-16082 and BSR 87-18005. ence periphyton standing crops (Mulholland et al 1983;Lamberti et al 1989) and that detritivores increase the rate of nutrient release (Grimm 1988) and release of dissolved organic carbon (DOC) (Meyer and O'Hop 198 3). Invertebrate shredders increase the rate of conversion of coarse particulate organic matter (CPOM) to smaller, fine particulate organic matter (FPOM) in laboratory streams (Petersen and Cummins 1974) and whole streams (Wallace et al 1982a;Cuffney et al 1990).…”

Crevices as refugia for stream diatoms: Effect of crevice size on abraded substrates

“…Likewise, stream algae are generally small, and the combination of small size and prostrate morphology increases resistance to loss from disturbances such as spateinduced floods (Blenkinsopp and Lock 1994;Peterson 1996) and grazing (Jacoby 1987;Steinman et al 1987;Lamberti et al 1989). Small size has the additional benefit of enabling organisms to fit within crevices.…”