2. Nitrogen and Phosphorus Exchange Among Tropical Coastal Ecosystems

The concentration and flux of nitrogen (N) and phosphorus (P) through mangrove wetlands, seagrass meadows, and coral reef habitats are mediated by a wide range of hydrodynamic and chemical pathways determined by both natural and anthropogenic drivers. The direct proximity of these coastal habitats to burgeoning urban centers makes them quite susceptible to excessive nutrient loading, subsequent land-use impacts, the related effects of eutrophication and of course the associated loss of ecosystem services. For this reason mangrove, seagrass, and coral reef ecosystems are among the most threatened ecosystems in the tropics. While quantifying the exchange of materials between coastal wetlands and nearshore waters has been the focus of estuarine research for nearly half a century, a concerted effort to understand the net exchange of N and P across these habitats has only begun in the last 20 years. Furthermore, attempts to better understand the interplay of N and P cycles specifically between each of these three habitats has been all but nonexistent. The role mangrove and seagrass ecosystems play in buffering nearshore coral habitats from land-based influences remains a topic of great debate. Critical to understanding the nutrient dynamics between these ecosystems is defining the frequency and magnitude of connectivity events that link these systems together both physically and biogeochemically. In this chapter we attempt to address both N and P water column concentrations and system-level exchanges (i.e., water-mediated fluxes and nutrient loading). We consider how the interactions of N and P between these systems vary with geomorphology, hydrography, seasonal programming, and human influences.