Limnology and Oceanography In Press Temporal and Spatial Variation in the δ15N and δ13C of coral tissue and zooxanthellae in Montastraea faveolata collected from the Florida reef tract Peter K. Swart, Amel Saied and Kathryn Lamb Marine Geology and Geophysics Rosenstiel School of Marine and Atmospheric Sciences University of Miami Miami Fl 33149 [email protected] Abstract Small pieces of coral skeletons with their associated tissues were collected at monthly intervals, between January 1995 and December 1996, from specimens of Montastraea faveolata at five locations on the Florida Reef tract. The locations (Triangles, Pickles Reef, Crocker Reef, Hen and Chickens, and The Rocks) represent both near shore and off shore environments. During each sampling trip between one to four pieces of coral skeleton and associated tissues were collected from the sides of different colonies living in water depths between 3 to 4 m. At one site (Pickles) samples were also collected from 8 m water depth. The tissue and zooxanthellae were removed from the skeletons, separated, and subsequently analyzed for δ15N and δ13C. The mean δ15N value in the coral tissue of all samples was +6.6 (+/- 0.6 ‰) while the δ13C was -13.3 (+/- 0.5 ‰) (n=197). The δ15N and δ13C of the zooxanthellae were +4.7 (+/-1.1‰) and -12.2 (+/- 1.0 ‰) respectively (n=147). The differences in the δ15N and δ13C between the zooxanthellae and the coral tissue were statistically significant. No statistically significant differences were observed between near shore and offshore stations in either δ15N or δ13C. The absence of a difference between the inshore and offshore stations casts doubt on both whether the δ15N of the coral tissues is related to anthropogenic influences and whether the δ15N value itself can be used as an indicator of sewage contamination in corals. Between 1995 and 1997 there was a long term increase of 1 ‰ in the δ13C of the coral tissue and zooxanthellae and a long term decrease of approximately 0.8 ‰ in the δ15N of the coral tissue and the zooxanthellae. The increase in the δ13C of the organic material (OM) was mimicked in the δ13C of the skeletal material from corals from two reefs in the area suggesting a strong connection between the δ13C of the coral tissues and the skeletal material. There appears to be clear seasonal variations in the δ13C of the coral tissue at certain locations such as Pickles Reef with δ13C of both the coral tissues and the zooxanthellae becoming more positive between July and August. The difference between the δ13C of the zooxanthellae and the coral tissue also varies seasonal with the maximum difference occurring in July of each year. In contrast the maximum δ13C in the skeleton appears to occur later in the year, between September and November. positive influences on the growth of the Introduction coral. Under shallow water conditions the coral-zooxanthellae system is autotrophic It is well established that certain (Muscatine and Cernichiari, 1969). scleractinian corals have symbiotic Evidence of the autotrophic nature of associations with dinoflagellate algae zooxanthellate corals is found in the (zooxanthellae) which are beneficial to the difference in the δ13C of the zooxanthellae host (Goreau, 1959; Muscatine and and coral tissue at various water depths. At Cernichiari, 1969; Wilbur and Simkiss, shallow depths where light intensity is high 1979). The zooxanthellae are able to pass the δ13C of the zooxanthellae and the coral organic compounds to the coral resulting in tissue are relatively similar (Land et al., Page -1- In Press Limnology and Oceanography 1975; Muscatine et al., 1989), and the δ13C 2000a), although Muscatine and Kaplan of the coral tissue is significantly more (1994) also investigated δ15N as an positive (-10 to -14 ‰) than the supposed indicator of autotrophic and heterotrophic food source of the coral, zooplankton (~ -20 responses. The study by Muscatine and ‰). This indicates that sufficient Kaplan (1994) showed a slight decrease in photosynthate is being translocated so that δ15N with increasing depth, although this the δ13C values of the coral tissue and pattern was not always consistent. In contrast to δ13C, the δ15N value was generally enriched in the coral tissue compared to the zooxanthellae. Common to all previous studies on the δ15N and δ13C of coral tissues is the fact that they have ignored any temporal variation in the isotopic composition of the soft tissues of the coral. Usually such 80o 25 samples are taken during the summer 25o 05 #Y 1 months when weather conditions are more favorable. However, in the study of Swart 2 #Y et al. (1996) it was noticed that the δ13C of #Y A o 3 B 25 00 #Y the coral tissues which were collected 7 4 N during the summer months (June -July 1990) #Y 7 #Y 6 W E were isotopically more positive (-15 ‰) C #Y 5 S than those measured in September 1990 (-17 13 10 0 10 Km ‰). The difference between the δ C of the zooxanthellae and coral tissue also changed Figure 1: Location of the Reefs studied in the Florida Keys. Site from about +3 ‰ in June 1990 to +7 ‰ in 2(Triangles), Site 4 (Pickles), Site 5 (Crocker), Site 6 (Hen & September. Based on these data the authors Chickens), and Site 7 (The Rocks). In addition water samples were collected from Marker 2 (Site 1), and Molasses Channel (Site speculated that the changes might be 3). The location of sites from which water samples are collected induced by changes in the partitioning of the for the FKNMS water quality network are shown in the squares (Molasses Channel (A) and Molasses Reef (B)). The coral internal C pool. These data prompted the skeletons analyzed were collected from Crocker Reef (Site 5) and initiation of this study whose goal was to Cheeca Reef (C). The shading denote 20' depth contours. Land and semi-emergent mud-banks are shown in the black shading. investigate temporal changes in tissues and zooxanthellae of corals from the near shore zooxanthellae are similar. With increasing and offshore reefs over an extended period. depth the δ13C of the coral tissues become In particular we were interested in whether more negative and the δ13C approaches that there were any seasonal changes in the δ13C of the zooplankton. Such variations are as previously observed by Swart et al. taken as indicating a change from (1996) and whether there were differences autotrophy to heterotrophy. Studies of the in the δ13C or δ15N relative to the position of δ15N of coral tissue have mainly the reef. concentrated on their potential as indicators of anthropogenic waste (Heikoop et al., Page -2- In Press Limnology and Oceanography Study Site were placed on ice until removal of the tissues (within 24 hours) by air-brushing. The study sites chosen were five patch reefs The zooxanthellae and tissue samples were off Key Largo in the Florida Keys (Figure separated using the methods of Szmant et 1). At each site, small pieces of skeletons al., (1989). Previous work using this with living coral tissue (~2 cm in diameter) method has shown that there is only a small were chipped off the sides of the heads. A amount of cross contamination by total of 197 samples were analyzed for the zooxanthellae in the coral tissue (less than δ15N and δ13C of the coral tissue and 147 for 5%; Fitzgerald and Szmant, 1997). In the δ15N and δ13C of the zooxanthellae. contrast, there can be significant With the exception of Pickles Reef the contamination of the zooxanthellae by the corals were collected at only one depth. At coral tissue during separation (up to 50%; Pickles corals were collected from water Fitzgerald and Szmant, 1997). depths of 8.5, 3.5, and 3 m. For the Isotopic analyses: The isotopic composition purposes of this study the samples from 3.5 (δ15N and δ13C) of organic coral samples and 3 m have been grouped together. The were determined using a CN analyzer corals were never collected from the same interfaced with a continuous-flow individuals which had been sampled during isotope-ratio mass spectrometer (CFIRMS) previous visits. The initial rationale was to (Europa Scientific). External precision protect the corals, but it was later realized determined through the analysis of replicate that continued sampling would not only standard material is 0.1 ‰ for C and 0.2 ‰ eventually destroy the colony, but also for N. severely stress the individual and perhaps Analyses of the dissolved inorganic carbon 13 lead to changes in the δ C induced by (DIC): The CO2 was removed from the stress. Water samples were also collected in sample by acidification in a stream of He order to check for seasonal variation in the gas and analyzed using a Europa 20-20 mass δ13C of the dissolved inorganic carbon (DIC) spectrometer by comparison with a pulse of in the water from the same sites at which the injected reference gas. External precision corals were collected well as two additional for this method as determined by measuring sites (Site 1; Maker 2) and Site 3 (Molasses replicate samples is ~0.08 ‰. Channel) (Figure 1). In order to compare Analysis of the coral skeleton: Samples of changes in the δ13C of OM to changes in the skeletal material from colonies of δ13C of coral skeletons, two corals were Montastraea faveolata at Crocker Reef (8 cored, a specimen of Montastraea faveolata m) and Siderastrea siderea at Cheeca Rocks at Crocker Reef (10m) and a specimen of (3 m) were drilled from the slab using a Siderastrea siderea from Cheeca Rocks hand held drill at a resolution of (Figure 1). approximately 20 samples pre year.