Biological Nitrogen Fixation in Periphyton of Native and Restored Everglades Marl Prairies

Abstract

Periphyton mats are an important component of the Everglades ecosystem. These mats are able to fix atmospheric nitrogen; however little attention has been paid to this function throughout much of the Everglades system. The objective of this study was to characterize and quantify periphyton N2 fixation in the Hole-in-the-Donut (HID) region of the southern Everglades, where farmed marl prairie wetlands have been restored through complete soil removal to reduce nutrient levels. Significantly higher N2 fixation rates (measured acetylene reduction) were found in periphyton of the areas cleared in 2000 and 2003 (3–10 nmol g−1 DW h−1) compared to the reference wetland site (less than 1 nmol g−1 DW h−1). Overall rates were stimulated by light (~2 times the measured dark rates). Areal estimates of fixed N were low compared to other Everglades, ranging from 0.1–0.2 g N m−2 yr−1 in the restored sites to 0.05 g N m−2 yr−1 in the reference area. Stable N isotopic ratios (i.e., δ15 N) ranged from −1.0‰ to 0.2‰ and were correlated with nitrogenase activity and TN:TP ratios. These findings suggest that periphyton nitrogenase activity and δ15 N could serve as indicators of nutrient status and restoration success in these systems.