Coral reefs respond to terrestrial and open ocean processes. Coral reefs and connected ecosystems (including seagrasses and mangroves) are heavily affected by onshore human activities. In the Caribbean, coastal development, urbanization, and daily life cause unnaturally high fluxes of pollutants and potential pollutants into the sea. These inputs include pesticides, pharmaceuticals, oil, metals, plastics, trash, sediments, detritus, nutrients (nitrogen and phosphorus fertilizers), dissolved organic material (DOM, including dissolved sugars), pesticides, and microbes (including viral and bacterial human pathogens) [8-10]. The final concentrations and effects of these inputs depend on a cascading series of physical, chemical, and ecological processes along the land-sea continuum. Terrestrial geology, hydrology, groundwater transport, water run-off, and vegetation affect the initial arrival of these inputs. Next, water column and open-ocean physics, hydrodynamics, ocean chemistry, and biological activity influence the flows, forms, and residence times of terrestrial inputs. Together these processes interact at the interface between land and sea to drive the ecology of coral reef ecosystems and determine their ultimate growth or decline.
Reef science rarely integrates terrestrial or hydrographic knowledge. Despite the links between land, sea, and reef, traditional conservation initiatives generally address the land and sea in isolation from one another. Similarly, and regrettably, scientific research has largely failed to recognize, investigate, and communicate the fundamental links between terrestrial inputs, ocean hydrodynamics, and nearshore ecosystem health (Fig. 2) [8]. This is particularly surprising given that coral reefs in the Caribbean are highly patchy, with some reefs thriving just hundreds of meters from others that are functionally dead [3, 11]. This patchiness and spatial structure is the starkest in known cases of coral reef death that occurred immediately after coastal development [e.g., 12, 13]. Ignoring critical land-sea connections prevents resource managers from identifying clear, targeted, effective scenarios for conservation. For example, a Marine Protected Area may fail to rebuild coral cover if sediment and pollution from coastal construction thwart the arrival and survival of coral larvae. Herbivore protections may fail to reduce algal overgrowth if city wastewater carries heavy fertilizer loads to the sea. Tourists may fail to return for a second trip if their first visit leaves them with an antibiotic-resistant skin infection from swimming at a popular beach near a sewage outfall.
Coral reef growth is patchy and still largely unexplained. In a comprehensive review of Caribbean coral survey data, the IUCN concluded that local factors still outweigh global factors in determining the health of an individual coral reef [1]. Yet most coral reef research to date has ignored the role of terrestrial inputs and hydrodynamics, leaving managers with virtually no ability to predict or control the future trajectory of an individual reef. Why does a thriving coral reef recover in front of a busy, polluted harbor while another coral reef becomes smothered in toxic cyanobacteria along a remote, rural coastline? The urban reef may be highly flushed by a nearby cruise ship terminal, while the remote reef may be poisoned by sewage seeping from rural cesspits. Yet, scientists generally fail to measure these factors. This knowledge gap leaves management recommendations unspecific, untargeted, and largely unsuccessful. It is therefore highly warranted and extremely timely to change research practices in marine ecology by integrating geosciences, ocean sciences, coral reef sciences, computer modeling, and the social science of human activities on land, thereby formally linking coral reef studies to land, sea, and society (Fig. 2).