Dynamics of periphyton nitrogen fixation in short-hydroperiod wetlands revealed by high-resolution seasonal sampling

Periphyton N2 fixation plays a key role in N cycling of aquatic systems, but temporal studies of this process are often lacking, especially in systems with only seasonal flooding. We used seven samplings to characterize nitrogenase activity (acetylene reduction method) of periphyton in short-hydroperiod marl prairies and two wetlands restored from agricultural disturbance in Everglades National Park, USA. We hypothesized that the seasonal drying and rewetting would increase the temporal dynamics of the process. All sites showed significant periphyton N2 fixation, but in restored areas highest rates were observed only in the early wet season (July), while in reference sites fixation was spread throughout the summer. Most N2 fixation in the restored areas was confined to a 3-month-period resulting in large underestimates of annual fixation in previous studies with few seasonal measurements. N2 fixation rates correlated with total P, N and TN:TP ratio, and periphyton moisture content in the dry season. N stable isotopic signature was a good indicator of N2 fixation rates between sites, but did not correctly indicate seasonal patterns. These findings improve our understanding of N cycling in wetlands like the Everglades and indicate a need for more detailed measurements of processes in seasonally flooded systems.