One of the current threats to coral reef degradation is disease, which in the past few decades has been increasing in frequency (12). Harvell et al. state that the Caribbean is of major concern, as it “has emerged as a disease hot spot”. Although for many coral diseases the causative agent is unknown, the diseases themselves have been determined to be new occurrences due to the severity of their effects and the novelty of their symptoms (12). Recent disease outbreaks are primarily due to an increase in environmental stress, which is often caused by anthropogenic factors. Climate variability, which is an anthropogenic as well as a natural occurrence, causes fluctuating temperatures which can result in modifications to the biological properties of marine populations, and can cause them to become more vulnerable to disease. An increase in severe weather events has been linked to current global temperature increases, which causes increased disturbance and may therefore increase susceptibility to disease (12). Specifically, the El Niño Southern Oscillation (ENSO) has caused unprecendented damage to coral reefs since the increase of its frequency and severity in the 1970’s. The massive coral bleaching event of 1998 is associated with ENSO due to the increased surface water temperature it creates. Furthermore, ENSO has been shown to have caused increased vulnerability to disease in corals due to the environmental stress created by the event (12). Left: Brain coral infected with black band disease. Caroline Rogers, USGS, soundwaves.usgs.gov.

Furthermore, humans have had a more direct role in the increase of disease in marine populations. Specifically, anthropogenic causes such as habitat disturbance and increased pollution have been shown to facilitate disease outbreaks in corals (12, 13). One example of a direct human disturbance is near shore land modification, which often increases erosion and sediment runoff. Rejection of sediment from corals requires energy, resulting in less energy available for the corals to fend off disease. Increased turbidity associated with increased sediment decreases the diffusion of sunlight to symbiotic photosynthetic zooxanthellae on corals which account for much of the corals’ energy. Again, this results in less energy available to the corals; an environmental stress that may increase disease susceptibility (13). Increased nutrient inputs from agricultural and sewage runoff may also have an effect on the vulnerability of corals to disease (14). Like turbidity, increased nutrient levels have been shown to affect interactions between corals and their symbiotic zooxanthellae. Increased nutrient levels are also associated with eutrophication and an increase in algal density, resulting in increased competition between corals and algae (14). Also, pathogens may benefit from high nutrient levels by feeding off extra nutrients, resulting in an increase in disease prevalence (15). Several studies that have examined the effects of increased nutrients on coral reefs attribute coral decline to increased eutrophication and algal competition (14). However, indirect anthropogenic effects include much more than nutrient runoff, including other forms of pollution such as toxins and organic pollutants from both point and non point sources (14). Szmant states that it is often difficult to distinguish between the effects of nutrients and the effects of pollution in terms of cause of coral decline. Right: A reef in Belize dominated by algae during the bleaching event of 1998 (13).

Increased disturbance and prevalence of disease in coral reefs has elicited concern for management of anthropogenic causes. The International Coral Reef Initiative (ICRI) states that although the direct cause of increased coral disease may be unclear, humans are undeniably having an affect on the increased frequency of infection (16). They state that the best way to manage disease outbreaks may be to manage human impacts on marine ecosystems. However, ICRI concludes that limited knowledge on “causal agents and their origins, vectors, and reservoirs” hinders their ability to protect against disease outbreaks. ICRI therefore created a Coral Reef Targeted Research program to link scientific knowledge with management strategies in order to effectively protect against coral reef disease (16).