Nutrient supply from fishes impacts phase shifts in a Caribbean coral reef ecosystem

Background/Question/Methods

Two of the most pervasive anthropogenic impacts to aquatic ecosystems are the selective harvest of higher trophic level organisms that alter consumer pressure and modified nutrient regimes that change patterns of productivity. Dramatic changes in ecosystem function often follow these alterations of top-down and bottom-up forcing. Importantly, both of these impacts may be mediated through changing the abundance and composition of consumer individuals. That is, in addition to altering primary producer abundance through consumption, consumers may be a significant source of limiting nutrients via their excretion. Despite the important role of both consumers and nutrients in mediating phase shifts on coral reefs, the role of bottom-up forcing by fish excretion has generally been ignored as a mechanism for altering benthic community structure on reefs. Although previous work showed that abundant fishes can facilitate coral growth via nutrient excretion, as coral abundance declines these nutrients may help facilitate increases in macroalgae. Here, we examine the roles of bottom-up (nutrient excretion from fishes) and top-down (herbivore abundance) forces on benthic community structure on coral reefs in the Florida Keys, USA. We used field surveys across forereefs in the Florida Keys to quantify reef fish abundance and benthic community structure. Using bioenergetics models, we calculated the excretion rates of nitrogen and phosphorus by resident fishes as an estimate of bottom-up forcing on benthic communities. Using regression, we evaluated how herbivory and fish excretion as well as other possible explanatory factors such as coral cover and damselfish density correlated with benthic community structure.

Results/Conclusions

We showed that fish excretion supplied 25.5 times more nitrogen to reefs in the Florida Keys, USA than all other biotic and abiotic sources combined. Consequently, there was a positive relationship between fish excretion and macroalgal cover on these forereefs. Herbivore biomass showed an equally important negative relationship with macroalgal cover suggesting strong interactions of top-down and bottom-up forcing. Nutrient supply by fishes also showed a negative correlation with juvenile coral density, likely mediated by competition between macroalgae and corals, suggesting that fish excretion may hinder coral recovery following large-scale coral loss. Thus, the impact of nutrient supply by fishes may be context-dependent and reinforce either coral-dominant or coral-depauperate reef communities depending on initial community states.