Potential N processing by southern Everglades freshwater marshes: Are Everglades marshes passive conduits for nitrogen?

The degree of hydrological connectivity in wetlands plays a vital role in determining the flux of energy, material, and nutrients across these wet landscapes. During the last century, compartmentalization of hydrologic flows in the Florida Everglades by canals and levees has had a profound impact on the natural timing and supply of freshwater and nutrients across the southern Everglades. Nitrogen (N) is an understudied nutrient in the phosphorus-limited Everglades; it plays an important role in many Everglades processes. To gain a better understanding of the overall N-dynamics in southern Everglades' marshes and the role that canals play in the distribution of N across this landscape, we analyzed δ ¹⁵N natural abundance data for the primary ecosystem components (the macrophyte Cladium jamaicense, marl soils, peat soils, and periphyton). Three sample transects were established in the three main basins of the southern Everglades: Shark River Slough, Taylor Slough, and the C-111 basin. Each transect included sample sites near canal inflows, in interior marshes, and at the estuarine ecotone. Natural abundance δ ¹⁵N signatures provided insights into processes that may be enriching the ¹⁵N content of ecosystem components across the marsh landscape. We also conducted a combined analysis of δ ¹⁵N data, tissue N concentrations, and water column N data to provide a broad overview of N cycling in the freshwater marshes of the southern Everglades. The primary trend that emerged from each basin was a significant ¹⁵N enrichment of all ecosystem components at near-canal sites, relative to more downstream sample sites. These data suggest that the phosphorus-limited marshes of the southern Everglades are not inactive conduits for N. Rather, these marshes appear to be actively cycling and processing N as it flows from the canal-marsh interface through downstream freshwater marshes. This finding has important implications to downstream coastal estuaries, including Florida Bay, and to nearshore coastal ocean ecosystems, such as coral reefs, where N is the limiting nutrient.