Chemical constraints on grazer-periphyton interactions in streams.

摘要

Periphyton, the microbial community attached to substrates, and its grazing consumers are subject to natural and anthropogenic chemical constraints in streams. A natural chemical constraint falls within the framework of ecological stoichiometry theory when the elemental demand of an individual species constrains the elemental composition of its excretion, which feeds back to affect nutrient availability to primary producers. Many studies have examined elemental constraints in pelagic phytoplankton-herbivore interactions, but few studies have addressed whether these constraints also apply to benthic ecosystems. My objectives were to determine (1) if taxonomic variation exists in macroinvertebrate elemental composition, (2) if grazers differing in elemental composition excrete nitrogen (N) and phosphorus (P) differently, and (3) if grazer identity, via N and P excretion, regulates periphyton elemental composition in streams. In a field survey of 46 taxa collected from 35 streams in two regions (Indiana-Michigan and Wisconsin), benthic macroinvertebrate elemental composition differed taxonomically, but not regionally, for most taxa. Further, field and laboratory experiments revealed that crayfish (Orconectes propinquus, mean body N:P=18) had a higher excretion N:P than did snails (Elimia livescens, mean body N:P=28). Furthermore, in recirculating and in flow-through artificial streams at low nutrient levels, snails and crayfish had differing effects on periphyton elemental composition, which often reflected a difference in grazer N or P excretion. I conclude that elemental constraints on grazer demand and excretion can be important in benthic as well as in pelagic ecosystems. In addition, because grazer identity had no effect on periphyton elemental composition at elevated nutrient levels, grazer excretion may be consequential to periphyton growth at low nutrient levels.; Anthropogenic contaminants may also constrain or alter grazer-periphyton interactions. In two mesocosm experiments, sublethal levels of an ionic liquid [IL, 1-butyl-3-methylimidazolium bromide (bmimBr)], a potential non-volatile replacement for industrial volatile organic solvents, negatively affected snails (Physa acuta) and periphyton and altered their interactions. Further, bmimBr reduced the positive effect of snails on primary productivity, an ecosystem process. Therefore, sublethal levels of an IL can negatively impact aquatic communities and ecosystem processes via complex trophic interactions.