Ecosystem processes of epilithis and epixylic periphyton in a backwater of the Upper Mississippi River

Abstract

In large lotic aquatic systems like the upper Mississippi River (UMR), the majority of primary production is most often attributed to pelagic (free-floating) biological communities, or macrophyte communities. However, the ecological significance of benthic microbial communities is critical when assessing the quality of these aquatic habitats. Benthic systems teem with aquatic microbes, many of which inhabit matrixenclosed biofilms (i.e., periphyton) that contribute to key ecological processes such as primary production, community respiration, nutrient cycling, and secondary production. The objective of this study was to culture periphyton on three different types of artificial substrates (unglazed ceramic tile and two types of wood: poplar and pine) situated inside exclosures developed specifically to inhibit colonization by large macroinvertebrate grazers. The exclosures were deployed in Target Lake, a backwater lake in Pool 8 of the UMR, for three week incubations throughout the summer of 2010. Periphyton growth on the substrates was subsequently analyzed for biomass (i.e., chlorophyll a and ash-free dry mass), primary production, and secondary production. Data from early summer (late May through early July) exclosure deployments showed significant (ANOVA, p < 0.0001) differences in biomass patterns among treatments, and indicated that there may have been more heterotrophic associations with the poplar substrates than with either the tile or pine. Despite the differences in biomass patterns, data from both primary and secondary production were not significantly different among treatments. Complications from flooding at the study site left all substrates from late summer (early August through late September) exclosure deployments unusable. Results from this study suggest that heterotrophic organisms constitute the majority of periphytic biomass, as well as play an important role in the structure and function, of periphyton communities on hard substrata in the UMR. In addition, the novel exclosure design functioned as intended, and could potentially be used for future studies to better understand how periphyton interacts with and influences other attributes of backwater ecosystems.