Abstract
Mercury is a widespread and damaging toxic metal that is trophically transferred through food webs. Coral reefs present an interesting dynamic in that they are comprised of complex food webs containing a high number of lateral or horizontal links that do not always end in top predators, thus possibly reducing trophic transfer. Therefore, the objective of the study was to assess the efficiency of trophic transfer in a coral reef food web, using mercury as the tracer. Concentrations of mercury and stable isotopes of nitrogen (δ15N) and carbon (δ13C) were measured in fish from two sites near the coastal waters of Long Key, Florida. The relationship between mercury and δ15N can be used to estimate biomagnification across the food web (i.e., trophic magnification slope, food web magnification factor). Using mercury and stable isotopes of nitrogen and carbon as tracers assisted in quantifying the efficiency at which coral reef ecosystems transfer these and other bioaccumulative toxins (e.g., ciguatoxins, etc.) through the food web, while also increasing our understanding of the associated flow of energy in the system. A total of 242 samples were collected from April 2012 through December 2013 using spear guns, hook and line, and hand collection techniques. Individual Hg concentrations ranged from 17.33 µg/kg in a gray angelfish (Pomacanthus arcuatus) to 3,317 µg/kg in a great barracuda (Sphyraena barracuda) at Long Key Hard Bottom, and 19.01 µg/kg in a rock beauty angelfish (Holacanthus tricolor) to 6,842 µg/kg in a porkfish (Anisotremus virginicus) at Tennessee Reef. Variability in both fish size and δ15N increased the variance in tissue Hg concentration both intra- and inter-specifically. As observed in other systems, the log transformed Hg concentrations in the food web, pooled across species, were significantly related to δ15N. The trophic magnification slope (i.e., slope of Log [Hg] regressed on δ15N), as an estimate of the biomagnification rate of Hg in the subtropical coral reef food web was 0.23 ± 0.03 (±95% confidence interval) at Tennessee Reef and 0.16 ± 0.04 for Long Key Hard Bottom. When δ15N was translated to trophic level, the food web magnification factor (calculated from slope of Log [Hg] regressed on trophic level) were 7.8 and 3.4 for Tennessee Reef and Long Key Hard Bottom, respectively. Although there was some evidence to support significant differences between the two sites (which could have been due to differences in water quality), this difference in slopes could also be a result of unbalanced sampling design. Nonetheless, these results clearly demonstrate that Hg is biomagnified through subtropical coral reef ecosystems and that the transfer efficiency (i.e., slopes) were consistent with previous reports for marine ecosystems.