Trace metals and stable isotopes as tracers of life history and trophic connections in estuarine-dependent fish from Tampa Bay, Florida.

摘要

Florida's estuaries support a wide range of species yet little is known about trace metal cycling among members of this important ecosystem. To examine the flow of trace metals through the Tampa Bay estuary, four fish species representing different trophic levels were analyzed for copper (Cu), zinc (Zn) and stable isotopes of carbon (C) and nitrogen (N). Species selected were the striped mullet (Mugil cephalus), tidewater mojarra (Eucinostomus harengulus), bay anchovy (Anchoa mitchilli), and sand seatrout (Cynoscion arenarius). Juvenile fish were collected from the Alafia, Hillsborough, Palm, and Little Manatee Rivers. Adults were collected from Tampa Bay. Combinations of trace metal and stable isotope analyses were used to evaluate geographic variability in trace metal concentrations among locations in Tampa Bay and to shed light on trophic pathways that lead to trace metal accumulation. In juvenile mullet, significant trends were found between Zn concentrations, stable isotope ratios, and standard length. Animals of the smallest size classes carry greater concentrations of zinc in their tissues and have distinct stable isotope ratios that reflect their recent life history as offshore planktivorous larvae. Interestingly, the ratio of Zn:Cu concentrations was highly conserved. While species-specific differences were observed, relatively small Zn:Cu variations suggest a possible bioregulatory mechanism that maintains an optimal Zn:Cu ratio even in the presence of elevated absolute metal concentrations. Stable isotope ratios proved to be an effective tracer of ontogenetic changes in fish diet and habitat. Carbon and nitrogen stable isotope analyses revealed that trophic relations between species are established very early in an organism's life history. The bay anchovy, a major prey item of the sand seatrout, has delta 15N values very similar to this predator. Although trophic linkages between trace metals and stable isotopes proved difficult to interpret, the relation between zinc concentrations and delta13C values suggested that trace metal concentrations are highest in animals that utilize food webs based on terrestrial carbon.