The individual and interactive effects of nitrogen and phosphorus enrichment on coral reefs
Human domination of global nutrient cycles is profoundly altering our planet. Yet on coral reefs, the effects of changing nutrient regimes have likely been over-simplified. This dissertation investigates the complexity of animal-nutrient interactions at the organismal level and explores how the outcomes of these interactions cascade through levels of biological organization. To do so, I examined the effects of nitrogen (N) and phosphorus (P) on corals and macroalgae, and how these effects in turn influenced reef communities and entire ecosystems. I show that P consistently increases coral growth rates while N has variable, often negative, effects on coral growth. The majority of this variability was explained by the contrasting responses of corals to ammonium, which had negligible effects on coral growth, versus nitrate, which consistently had negative effects on corals. Experimental manipulations of nutrient regimes revealed that these effects could be attributed, in part, to increased damage to the photosynthetic components of the corals’ endosymbionts. Nitrogen and P-enrichment also impacted macroalgae, increasing the nutrient content of algal tissue and in turn, consumption patterns of herbivorous fishes. Initial phase parrotfishes and juvenile surgeonfishes increased their feeding rates on algae rich in N and P respectively. However, adults from both species were irresponsive to algal nutrient content. At the community level, the effects of N and P on corals, algae and herbivory were linked to the development of distinct benthic communities. Algae cover was lower and coral growth rates higher around reef structures that were consistently enriched with N and P excreted by sheltering fishes. At the ecosystem level, I found that the responses of corals to N and P enrichment were similar to those of other nutrient-sharing mutualists. Across terrestrial and marine environments, I show that N and P enrichment consistently decouples mutualism performance, benefiting one partner at the expense of the other. Thus, collectively this dissertation demonstrates that the impacts of global nutrient loading resonate from single organisms through whole ecosystems.
Shantz, Andrew A, "The individual and interactive effects of nitrogen and phosphorus enrichment on coral reefs" (2016). ProQuest ETD Collection for FIU. AAI10255490.