Relationship between light and the S15N of coral tissue: Examples from Jamaica and Zanzibar

Nitrogen isotope values from coral tissue collected over depth-light gradients are reported from Jamaica and Zanzibar. The Jamaica suite consists of multiple specimens of three coral species (Montustrea annularis, Porites astreoides, and Agaricia agaricites) sampled at increasing depths. For each species, combined tissue/zooxanthellae 615N decreases significantly with decreasing availability of photosynthetically active radiation. The Zanzibar sample suite was collected from three coral colonies (all Porites Zobatu). Multiple samples, occupying different depths and light regimes, were collected from each coral corallum. The Zanzibar suite shows a significant decrease in 615N over the relatively small depth range represented by each coral colony. Together, these two sample suites suggest that light is an influence on the nitrogen isotopic composition in corals containing symbiotic zooxanthellae. We propose that in high-irradiance conditions, uptake and assimilation of dissolved inorganic nitrogen (DIN) by symbiotic algae and(or) the host strongly depletes the coral internal DIN pool, leading to reduced fractionation relative to reef DIN. In lower light conditions, less dissolved nitrogen is assimilated and fractionation increases. The autotrophic portion of the diet is thought to be sufficiently abundant in nitrogen and isotopically depleted to result in lower 615N of host tissues under lower light conditions. Measurements of the natural abundance of nitrogen isotopes have been used to trace sewage (Risk et al. 1993, unpubl. data; Dunn 1995; Mendes et al. 1997) and other terrestrial inputs to coral reefs (Risk et al. unpubl. data) and to aid in the identification of nutritional sources in reef settings (Schoeninger and DeNiro 1984; Keegan and DeNiro 1988; Yamamuro et al. 1992, 1995). Nitrogen isotopes are useful in such studies because different sources of dissolved and particulate nitrogen may be isotopically distinct (Sweeney et al. 1980; Owens 1987; Michener and Schell 1994). Nitrogen isotopes generally fractionate in a consistent manner along food chains, with enrichments of -3.5%0 per trophic level Acknowledgments We thank J. Roff for collection of plankton samples, Martin Knyf for his invaluable assistance during stable isotope analysis, Jeremy Woodley for numerous discussions regarding the Discovery Bay reef environment, Ron Allen and Peter Gayle for helping Nigel Waltho with sampling and diving logistics in Jamaica, the Discovery Bay Marine Lab for use of laboratory facilities for preparing samples prior to transport to Canada, Tomas Tomascik, Hillary Stuart-Williams, and Martin Knyf for comments on earlier versions of this manuscript, and Cam Tsujita for drafting Fig. 1. This paper benefited greatly from the comments of three anonymous reviewers. Funding was provided by NSERC operating grants to Mike Risk and Henry Schwartz and an ICOD contract (Zanzibar) held jointly by University of Guelph and McMaster University. (DeNiro and Epstein 198 1; Minagawa and Wada 1984). These fractionations have been attributed to excretion of isotopically light nitrogen (Minagawa and Wada 1984). Nitrogen fractionation during nutrient uptake and assimilation by primary producers (Wada and Hattori 1978; Wada 1980; Montoya and McCarthy 1995) has implications for the nitrogen isotopic composition of zooxanthellate corals. Corals thrive in oligotrophic conditions, largely due to the symbiotic relationship between the coral animal and endosymbiotic zooxanthellae (Symbiodinium microadriaticum) (Muscatine and Porter 1977). This symbiotic relationship allows corals to act, to some degree, as primary producers (photoautotrophs). Zooxanthellae fix inorganic carbon and nitrogen into organic compounds that can be translocated to the coral host (autotrophy) (Muscatine et al. 1984; Falkowski et al. 1984, 1993) to supplement available exogenous food sources (heterotrophy), including dissolved organic matter (DOM) and particulate organic matter (POM, e.g. zooplankton) (e.g. Muscatine et al. 1989; Risk et al. 1994; Dubinsky and Jokiel 1994). Algal symbionts assimilate dissolved inorganic nitrogen (DIN) from the external environment (e.g. Muscatine and D’Elia 1978; D’Elia et al. 1983; Burris 1983; Wafar et al. 1985; Wilkerson and Trench 1986) and from nitrogenous wastes excreted by the coral (Szmant-Froelich and Pilson 1977, 1984; Muscatine and D’Elia 1978; Rahav et al. 1989).