Increasing coral cover and decreasing coral species diversity with decreasing light availability at depth: a study of the future effects of sea level rise on fringing reef ecosystems
Global climate change is having widespread effects on the world’s oceans. Particularly vulnerable to changing global conditions are coral reefs, which boast high biodiversity levels though they comprise a small percentage of the oceans. Sea level rise decreases light availability to corals and their symbiotic photosynthetic zooxanthellae, which are responsible for fixing the carbon that corals use to create their carbonate skeleton. Studies have shown how changing light availability with depth can be used as an indicator of coral species diversity. Species richness and diversity of corals is greatest at intermediate depths, with decreasing diversity at greater depths. This study investigated the relationship between coral species diversity and light intensity at depths of 12, 18, and 24 m on the fringing reef ecosystem off the western coast of Bonaire, Dutch Caribbean. Light intensity data were based on theoretical values provided by Beer-Lambert’s law, and video transects were conducted to determine composition of the substrata and to calculate coral species diversity and richness. Strong correlations were found between decreasing coral species diversity with depth (p = 0.041) and percent coral cover and decreasing percent cover of macroalgae and cyanobacteria with depth (p = 0.000, F = 37.60, df = 1). This indicates that while species diversity of corals decreases with lowered light intensity, corals are better able to outcompete macroalgae in environments with decreased light availability. This information is useful in understanding how reefs will respond to environmental changes brought on by sea level rise.