Microbial extracellular enzymes as indicators of riparian and upstream forest cover in headwater streams

Headwater streams flowing through forested landscapes receive much of their energy from terrestrial sources, consisting of coarse, fine, and dissolved organic matter (OM). Much of this OM is broken down via extracellular enzymes used by bacteria and fungi within stream biofilms.. The extracellular enzymatic activities (EEAs) of these different enzymes can potentially indicate the relative importance of the OM fueling these systems. Because spatial cover and composition of surrounding forests influence the OM fueling these systems, I hypothesized that changes in upstream and adjacent forest cover would be reflected in biofilm EEAs. I sampled epilithic biofilms in 46 streams in PA and NY for biomass, nutrients, and the activities of seven extracellular enzymes. For each stream, I measured total nitrogen, total phosphorus, chromophoric dissolved organic matter (CDOM), and both the canopy cover and upstream forest cover. I investigated potential linkages between biofilm EEAs, stream nutrients and OM, and both riparian canopy cover and upstream land cover. Using proportions of nitrogen- and phosphorus-acquiring enzymes and a multiple regressions approach, I explored relative nutrient and carbon limitations. Canopy cover, CDOM, and total phosphorus were found to be predictors of phosphorus vs. nitrogen limitation, while chlorophyll a, biofilm phosphorus, and a proportion phenol oxidase to β-D-1,4-glucosidase activity, which indicates relative recalcitrance, were found to be predictors of carbon vs. nutrient limitation. Canopy cover was found to indicate both relative phosphorus vs. nitrogen and relative nutrient vs. carbon limitation, as well as drive a nutrient spiraling effect correlated with water column total phosphorus concentrations.