Anthropogenic nutrients mitigate importance of fish-mediated nutrient supply for seagrass beds in Haiti

The supply of nutrients from consumer waste products, i.e., consumer-mediated nutrient dynamics, is critical for primary production in marine environments, especially in oligotrophic systems. However, human impacts can influence the effects of consumer-mediated nutrients in two main ways: (1) overfishing can reduce consumer populations and thus the nutrients they supply, (2) anthropogenic nutrient enrichment can shift baselines of nutrient limitation. Previous work has shown that artificial reefs create biogeochemical hot spots from which nutrient responses by surrounding primary producers can be measured. We constructed eight artificial reefs from 88 cinder blocks in shallow seagrass beds near Ile-a-Vache, Haiti, to test for interacting effects of fish-mediated nutrients and anthropogenic nutrients on growth rates of the dominant seagrass, Thalassia testudinum , surrounding the artificial reefs. We found that (1) anthropogenic nutrient enrichment appears to be the primary factor influencing seagrass growth close to the artificial reef, and (2) despite overall anthropogenic nutrient enrichment effects, consumer-mediated nutrients still generate biogeochemical hot spots and the two nutrient sources additively influence seagrass growth rates. We also found that nitrogen and phosphorus similarly predict seagrass growth rates, and better so than their ratio, indicating the supply rates of nitrogen and phosphorus are both limiting seagrass production. Results from this study indicate that the effect of anthropogenic nutrients can strongly influence seagrass growth in these systems, but consumers can supply a source of nutrients that has additive effects on seagrass growth.