An understanding of the influence that the environment plays in disease outbreaks can guide the development of useful management strategies. Research indicates a strong relationship between the following environmental drivers and disease outbreaks: temperature, water quality, and sedimentation.
Warming sea surface temperatures is known to influence diseases in two ways:
- Encourage infectious disease by impairing the defense mechanisms of the coral host. Increased temperatures can affect basic biological and physiological properties of corals, particularly the ability to fight infection. This has an influence on the balance between potential pathogen and host.1
- Increasing the virulence or growth rate of disease-causing organisms.2
Seasonal patterns in disease prevalence in the northeastern Caribbean provide further support for a link between warming ocean waters and disease outbreaks. On the Great Barrier Reef, coral disease prevalence increased from winter to summer in all major families of coral. Links between outbreaks or increasing prevalence and warm temperature have thus been detected for black band disease, aspergillosis, yellow band disease, white patch disease and white syndrome. The list of diseases will likely grow as the data set expands. For example, the group of coral diseases known as White Syndromes (WS) has been seen in greatest abundance following periods of anomalously high sea temperatures.7
Research suggests that coral disease is facilitated by a decrease in water quality, particularly due to eutrophication and sedimentation. Recent evidence suggests a synergistic effect between elevated nutrients and disease. High nutrients (Nitrogen and Phosphorus) were associated with accelerated disease signs in both yellow band disease- and aspergillosis-infected corals in field manipulations,3 and in black band disease.4
Sedimentation offers another challenge to host disease resistance. The impacts of land-based sedimentation on nearshore communities are visible and well documented; corals inhabiting silted reefs often possess large patches of dead, exposed skeleton bordered by apparently receding margins of healthy tissue. Opportunistic land-based pathogens (the soil fungus Aspergillus sydowii and the human bacterium Serratia marcescens) have been demonstrated as causal agents for two diseases impacting dominant corals in the Caribbean.5,6 Land-produced silt and run-off may not only cause physical stress for corals, but may also act as a pathogen reservoir. Anthropogenic stressors are linked with disease severity in complex ways. It is important for managers to try to establish and quantify such linkages; understanding these factors may make it possible to lessen stressors through improved reef management and land-use practices.
1 Rosenberg E. et al. 2007
2 Boyett H.V. et al. 2007
3 Bruno J.F. et al. 2003
4 Voss J.D. and Richardson L.L. 2006
5 Patterson K.L. et al. 2002
6 Geiser D.M. et al. 1998
7 Bruno J.F. et al. 2007