Biofilm ecoenzymatic activity, organic carbon and nitrogen in Lake Superior tributary streams

Abstract

I compared the ecoenzymatic activity of naturally occurring epilithic biofilm to stream water carbon and nitrogen concentrations in Lake Superior tributary streams along the south and north shores of the western end of Lake Superior. My goal was to determine if the ecoenzymatic activity of biofilm would reflect water chemistry. The streams drain catchments ranging in size from 14-172 km2 of primarily deciduous forest, coniferous forest and woody wetlands. Measurements and samples were collected during base stream flows (June-September). The streams represented a broad range of organic carbon, nitrogen and phosphorous concentrations, which correlated to physical habitat parameters and land usage. I used light absorbance in the ultraviolet and visible light (UV-VIS) spectrum to show differences in dissolved organic matter (DOM) between sites on the north and south shore. The UV-VIS proxies of E2:E3 and SUVA254, which indicate molecular size and degree of aromaticity, were correlated with the percentage of wetlands in the catchment, as well as the specific conductance, pH and the C, N and P concentrations in the water. Eleven different ecoenzymes involved in the breakdown of organic matter were measured. The activities of these enzymes were often positively correlated to each other and correlated to the measured water chemistry. The slopes from Type II linear regression of β-N-acetylglucosaminidase to phosphatase (0.77), β-d-glucosidase to phosphatase (0.68), and -d-glucosidase to β-N-acetylglucosaminidase (0.88) are presented here as metabolic stoichiometric ratios. The molar C:N ratio in epilithic biofilm (11:1) was positively correlated to the ratio of dissolved organic carbon to total dissolved nitrogen (DOC:TDN) in filtered (<0.45 μm) stream water and negatively correlated with biofilm peptidase activity. The expression of the peptidase activity by both L-alanine aminopeptidase and L-leucine aminopeptidase increased in response to dissolved organic nitrogen (DON). Increasing stream-water inorganic nitrogen (DIN) concentrations reduced or inhibited L-alanine aminopeptidase expression and had no apparent influence on L-leucine aminopeptidase. These results along with other studies of biofilm show that the ecoenzymatic activity of biofilm and the C:N ratio in biofilm reflects water chemistry, providing further evidence linking the stoichiometric theory of ecology to metabolic theories of ecology. The ecoenzymatic activity is a measurement of metabolic requirements mediating the stoichiometry of incorporation of nutrients by the biofilm. This study was the first to compare natural epilithic biofilm ecoenzyme activity in the Lake Superior region to water chemistry, land use and habitat characteristics.