d15N AND d13C FROM THE SKELETAL PROTEIN OF GORGONIAN CORALS AS OCEANOGRAPHIC MONITORS: EXAMPLES FROM THE FLORIDA KEYS AND THE GULF OF MAINE

Gorgonian corals secrete a skeletal protein, gorgonin, in concentric rings thought to be annual. Previous measurements of d¹⁵N, d¹³C and ¹⁴C of gorgonin suggest that POM is a significant component of the diet of both shallow and deep-water species. Profiles of d¹⁵N and d¹³C across the annual rings of gorgonian corals could therefore provide information about the temporal variability of surface productivity and trophic structure. We measured d¹⁵N and d¹³C in shallow-water, zooxanthellate Plexaura homomalla and Plexaura flexuosa from the Florida Keys and deep-water, azooxanthellate Primnoa resedaeformis from the NE Channel, north of Georges Bank (450m). Specimens of Plexaura from the Lower Keys had higher d¹⁵N (~4‰) than those from the Middle and Upper Keys (~3‰). The geographic trends parallel d¹⁵N in boring sponges and NO₃ of water. In specimens from the Lower Keys, d¹⁵N increased steadily over the past 25 years. In specimens from the Upper Keys, d¹⁵N did not increase until after the mid-1990s. In Primnoa, isotopic profiles over the past thirty years fluctuate around 9‰ (d¹⁵N) and -21‰ (d¹³C). There was no evidence of protein degradation; therefore we assume these values are stable over at least decadal time scales. Profiles of d¹³C were consistent within- and among-colonies, but d¹⁵N profiles were not. Periodicity in the Primnoa d ¹³C composition, around 4-5 years, could potentially reflect changes in ambient d¹³C originating at the surface.