ASSESSING ENVIRONMENTAL CONDITIONS ON REEFS USING STABLE CARBON AND NITROGEN ISOTOPES FROM TWO GORGONIANS

Water quality and overall ecosystem productivity can be negatively impacted on coral reefs from a variety of ocean- and land-based sources of pollution such as hydrocarbons, agriculture, development, and sewage. Stable carbon and nitrogen isotopes may be a useful tool for tracking relative environmental changes in the surrounding seawater. Two gorgonians, the purple sea fan (Gorgonia ventalina) and the sea rod (Pseudoplexaura porosa) from reef locations around Bermuda were analyzed for stable carbon and nitrogen isotopes. G. ventalina was collected from four near shore and five off shore sites for one-time analyses and to determine a range of values around Bermuda. P. porosa grows annual rings and was collected from two locations to assess change in values through time. Nitrogen was more enriched in colonies from near shore compared to those from off shore sites, most likely reflecting higher levels of land-based nitrogen pollution in the seawater of near shore sites. There was a slight west to east trend with carbon becoming progressively more depleted in colonies from more eastern sites. This suggests that currents around Bermuda move water in an easterly direction that becomes depleted in ¹³C and therefore incorporated as an isotopically lighter value in gorgonians. In combination, the isotopic signatures were site specific, even within the relatively small area of Bermuda, and may be useful for monitoring relative changes in water quality at local scales.