Periphyton nitrogenase activity as an indicator of wetland eutrophication: spatial patterns and response to phosphorus dosing in a northern Everglades ecosystem

The use of periphyton nitrogenase activity (biological N-2 fixation) as an indicator of wetland P impact was assessed using patterns of nutrient content (C, N, P, Ca, Mg, K, Fe, and Mn) and acetylene reduction (AR) in floating cyanobacterial periphyton mat (metaphyton) communities of a P-enriched portion of the Florida Everglades, USA (Water Conservation Area-2A, WCA-2A). Spatial patterns of nutrients indicate the enrichment of floating mat periphyton N, P, Fe, and K, and the reduction of Mn and TN:TP in enriched marsh areas. In highly enriched areas, floating mat periphyton AR was approximately threefold greater than that in less enriched, interior marsh zones. Multiple regression models indicated AR dependence on P in eutrophicWCA-2A areas while the AR of more interior marsh periphyton mats was more closely related to tissue levels of Ca and Fe. Nitrogenase activity of floating mat periphyton from P-loaded mesocosms revealed a significant enhancement of N-2 fixation in samples receiving approximately 2-3 mg P m(-2) of cumulative P dosing or with biomass TP content of 100-300 mg kg(-1). At P contents above the optimum, mat periphyton AR was suppressed possibly as a result of changes in species composition or increased levels of NH4+. After 3 years of dosing, consistently high AR occurred only at low rates of P enrichment (0.4-0.8 g P m(-2) yr(-1)), and the patterns appeared to be seasonal. These findings agree with the hypothesis that P availability is a key determinant of nitrogenase activity in aquatic systems, and thus, may support the use of periphyton nitrogenase to indicate P impacts in P-limited systems. These results also demonstrate the potential existence of a P threshold for biogeochemical alteration of periphyton mat function in the Everglades, and that cumulative loading of limiting nutrients (i.e., P), rather than instantaneous concentrations, should be considered when evaluating nutrient criteria.