Improving coral reef conservation requires heightened understanding of the mechanisms by which coral cope with changing environmental conditions to maintain optimal health. We used a long‐term (10 month) in situ experiment with two phylogenetically diverse scleractinians (Acropora palmata and Porites porites) to test how coral–symbiotic algal interactions changed under real‐world conditions that were a priori expected to be beneficial (fish‐mediated nutrients) and to be harmful, but non‐lethal, for coral (fish + anthropogenic nutrients). Analyzing nine response variables of nutrient stoichiometry and stable isotopes per coral fragment, we found that nutrients from fish positively affected coral growth, and moderate doses of anthropogenic nutrients had no additional effects. While growing, coral maintained homeostasis in their nutrient pools, showing tolerance to the different nutrient regimes. Nonetheless, structural equation models revealed more nuanced relationships, showing that anthropogenic nutrients reduced the diversity of coral–symbiotic algal interactions and caused nutrient and carbon flow to be dominated by the symbiont. Our findings show that nutrient and carbon pathways are fundamentally “rewired” under anthropogenic nutrient regimes in ways that could increase corals’ susceptibility to further stressors. We hypothesize that our experiment captured coral in a previously unrecognized transition state between mutualism and antagonism. These findings highlight a notable parallel between how anthropogenic nutrients promote symbiont dominance with the holobiont, and how they promote macroalgal dominance at the coral reef scale. Our findings suggest more realistic experimental conditions, including studies across gradients of anthropogenic nutrient enrichment as well as the incorporation of varied nutrient and energy pathways, may facilitate conservation efforts to mitigate coral loss.

Tropical mangrove forests, seagrass beds, and coral reefs are among the most diverse and productive ecosystems on Earth. Their evolution in dynamic, and ever-changing environments means they have developed a capacity to withstand and recover (i.e., are resilient) from disturbances caused by anthropogenic activities and climatic perturbations. Their resilience can be attributed, in part, to a range of cross-ecosystem interactions whereby one ecosystem creates favorable conditions for the maintenance of its neighbors. However, in recent decades, expanding human populations have augmented anthropogenic activities and driven changes in global climate, resulting in increased frequencies and intensities of disturbances to these ecosystems. Many contemporary environments are failing to regenerate following these disturbances and consequently, large-scale degradation and losses of ecosystems on the tropical seascape are being observed. This chapter reviews the wealth of available literature focused on the tropical marine seascape to investigate the degree of connectivity between its ecosystems and how crossecosystem interactions may be impacted by everincreasing anthropogenic activities and human-induced climate change. Furthermore, it investigates how disruption and/or loss of these cross-ecosystem interactions may impact the success of neighboring ecosystems and consequently, the highly-valued ecosystem services to which these ecosystems give rise. The findings from this review highlight the degree of connectivity between mangroves, seagrasses and coral reefs, and emphasizes the need for a holistic, seascape-wide research approach to successfully protect and preserve these critically important ecosystems and their associated services for future generations.

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Primary production underpins most ecosystem services, including carbon sequestration and fisheries. Artificial reefs (ARs) are widely used for fisheries management. Research has shown that a mechanism by which ARs in seagrass beds can support fisheries and carbon sequestration is through increasing primary production via fertilization from aggregating fish excretion. Seagrass beds are heavily affected by anthropogenic nutrient input and fishing that reduces nutrient input by consumers. The effect of these stressors is difficult to predict because impacts of simultaneous stressors are typically non-additive. We used a long-term experiment to identify the mechanisms by which simultaneous impacts of sewage enrichment and fishing alter seagrass production around ARs across non-orthogonal gradients in human-dominated and relatively unimpacted regions in Haiti and The Bahamas. Merging trait-based measures of seagrass and seagrass ecosystem processes, we found that ARs consistently enhanced per capita seagrass production and maintained ecosystem-scale production despite drastic shifts in controls on production from human stressors. Importantly, we also show that coupled human stressors on seagrass production around ARs were additive, contrasting expectations. These findings are encouraging for conservation because they indicate that seagrass ecosystems are highly resistant to coupled human stressors and that ARs promote ecosystem services even in human-dominated ecosystems.