Microbial enzyme stoichiometry and nutrient limitation in US streams and rivers

We analyzed water and sediment chemistry, catchment land cover, and microbial dehydrogenase (DHA) and extracellular enzymes activities (EEA) related to microbial C, N, P, and S acquisition in more than 2100 1st–10th order streams. The streams and their catchments represented gradients in water and sediment chemistry (C, N, P, S) and land cover (% forest, % wetland, % row crop agriculture) against which to compare biofilm and sediment DHA and EEA, and to estimate the extent of nutrient limitation in US streams and rivers. Water chemistry was significantly correlated with catchment land cover. Biofilm and sediment DHA and EEA were inversely correlated with water and sediment chemistry, and positively correlated with % forest. Canonical correlation analysis described two environmental gradients to which biofilm and sediment DHA and EEA were significantly correlated. Structural equation modeling (SEM) revealed a significant causal relationship between catchment land cover, chemistry, and biofilm and sediment EEA and DHA. Biofilm and sediment EEA was dominated by phenol oxidase and peroxidase, enzymes related to the degradation of recalcitrant C. However, the hydrolytic enzymes (those which hydrolyze glycoside, peptide, and ester bonds to release C, N, P, S) were better correlated with the chemical variables. This was reflected in the relative apportionment of enzyme activity toward C, N, P, and S acquisition. We compared molar stoichiometry of C, N, and P in water and sediment with biofilm and sediment ratios of GLYC:PEPT:PHOS to gauge how each measure of nutrient limitation compares with the others. Overall, water chemistry suggested many biofilms were P-limited and this was corroborated by allocation of microbial phosphatase activity. Sediments were found to be predominantly C and N-limited, and this was reflected in sediment EEA. We used these measures of nutrient limitation to assess the extent of N or P-limitation in streams and rivers of the conterminous United States and for nine ecoregions of interest to the US EPA's National Rivers and Streams Assessment (NRSA). Enzymatic stoichiometry provides a biological perspective on the influence of catchment scale anthropogenic disturbances resulting in an imbalance of nutrients being transported from those catchments. Enzyme activities represent the interface between microbial demands for, and environmental supplies of, C, N, and P, effectively linking ecological stoichiometric theory with the concept of threshold elemental ratios. The relative activities of the functional classes of extracellular enzymes provide both a measure of nutrient availability and ecosystem metabolism that may be used to assess large-scale phenomena such as regional impacts of climate change or anthropogenic disturbances.