A New Conceptual Model of Tropical Seagrass Eutrophication: Evidence for Single Nutrient Management

Abstract

Nutrient enrichment is one of the greatest threats to seagrass ecosystems globally. Current management is based on a temperate conceptual model of seagrass eutrophication where epiphytes and then phytoplankton cause light limitation and seagrass die-off. However, producers in tropical and subtropical systems respond differently to nutrients, suggesting a need for an updated conceptual model for these ecosystems. Using a long-term multifactorial seagrass enrichment experiment of nitrogen and phosphorus supply rates and ratios, we showed that seagrass growth was enhanced with increased phosphorus but not nitrogen supply and was never inhibited by epiphytes or shifts in seagrass community structure. A parallel experiment showed phytoplankton primarily responded to nitrogen enrichment. Our findings provide evidence for a new nutrient enrichment conceptual model in subtropical and tropical seagrass ecosystems, with a larger role played by phytoplankton. We suggest that management efforts to mitigate nitrogen supply alone may offer an effective starting point for reducing negative impacts of eutrophication, one of the most ubiquitous stressors to these imperiled ecosystems.