Enzyme activity responses to nutrient loading in subtropical wetlands

Abstract

Eutrophication caused by anthropogenic nutrient inputs is one of the greatest threats to the integrity of freshwater wetlands. The resultant changes in organic carbon cycling and nutrient mineralization may be expressed through increased decomposition rates, which are ultimately dependent on the metabolism of the resident microbial community. Specifically, microbial nutrient acquisition is controlled through the activity of enzymes, which are in turn influenced by local biogeochemical conditions. This study examines enzyme activities along distinct North-South P gradients within four distinct hydrologic units of the Florida Everglades. The results indicate that nutrient enriched sites exhibit lower N and P limitations on microbially constrained C mineralization, in addition to enhanced cellulose decomposition rates. Nutrient loading resulted in decreased microbial mobilization of resources for P mineralization, resulting in greater energetic allocation for C mineralization. Additionally, N appears to become less limiting to C mineralization in the enriched sites within Everglades National Park, the least P enriched area within the Everglades. A simple two component model, incorporating total P and the relationship between the enzymes involved in C and P mineralization accounted for between 46 and 92% of the variability in measured cellulose decomposition rates and thus demonstrates the significant influence that P loading plays in these systems. These results also suggest there is an environmental threshold TP concentration below which changes in enzyme-based resource allocation will not occur.