Ivana Vu¹, Gillian Smelick¹, Sam Harris¹, Sarah C. Lee¹, Ernesto Weil², Robert F. Whitehead³, John F. Bruno^1*

1 Department of Marine Sciences, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States of America, 2 Department of Marine Sciences, University of Puerto Rico, Lajas, Puerto Rico, United States of America, 3 Center for Marine Science, The University of North Carolina Wilmington, Wilmington, North Carolina, United States of America

Introduction 

Infectious disease outbreaks are a major cause of coral loss and reef degradation. In the Caribbean, outbreaks of white band disease in the early 1980s nearly extirpated the then dominant species Acropora cervicornis and Acropora palmata [1]. The white band pandemic led to the regional collapse of coral cover [2], [3] with wide-raging effects on reef inhabitants, geomorphology and ecosystem processes. Evidence from paleontological studies and ecological monitoring indicate that coral disease prevalence, variety, host range, and impacts have increased substantially over the last 30 years [4]–[6].

There are several potential explanations for the observed increase in the severity and impacts of coral diseases. For example, there is evidence that nutrient pollution [7]–[9] and anomalously high ocean temperature [10]–[12] can increase within- and among-colony spread rates of several coral diseases. These and other environmental stressors could increase pathogen virulence and decrease host resistance [13]–[15]. Another widely discussed yet largely untested explanation for increased coral disease is that decades of overfishing [16] have disrupted the balance of coral reef ecosystems, making corals more susceptible to disease outbreaks and other disturbances [17]–[19]. Specifically, the removal of herbivores has led to substantial increases in benthic macroalgae on some reefs [20], which could facilitate disease outbreaks either by acting as pathogen reservoirs or vectors [21] or by increasing the concentration of Dissolved Organic Carbon (DOC)[22].

A recent study found that algae can cause rapid mortality of small coral fragments in closed containers [22]. Related laboratory studies of the effects of DOC on coral health [23], [24] support a potential mechanism through which algae could indirectly cause coral disease outbreaks. Yet many ecologists remain skeptical of a mechanistic link between fishing, macroalgae and coral disease [25], [3], in part due to the paucity of evidence from field experiments.

The purpose of this study was to test the hypothesis that changes in coral reef trophic dynamics and benthic community structure are a cause of increased coral disease severity. Specifically, we asked whether the presence of macroalgae can influence within- and among-colony spread rates of Caribbean Yellow Band Disease (CYBD) in Montastraea faveolata, a major reef-building species in the region. We also measured the effects of macroalgae on coral growth and survival. Our results suggest that, at least in these short-term field experiments, macroalgae has no effect on the severity and dynamics of CYBD.