Relating otolith chemistry to surface water chemistry in a coastal plain estuary

Abstract

Although laboratory studies confirm that otoliths incorporate trace elements and stable isotopes from surrounding waters, few studies explore the relationship of otolith chemistry to water chemistry in the field and none include a larger suite of environmental tracers, such as rare earth elements. Using spotted seatrout (Cynoscion nebulosus) as model species, we tested the hypothesis that otoliths record the water chemistry of seagrass habitats in Chesapeake Bay. In summer 2001, we sampled water and juvenile fish in seagrass beds of the bay. Weighted linear regressions showed that [Ba/Ca]_otolith and [La/Ca]_otolith were best predicted by salinity and were modeled as [Ba/Ca]_otolith (µmol·mol^–1) = –2.25 ± 0.35 × salinity + 59.47 ± 7.01) and [La/Ca]_otolith (pmol·mol^–1) = –8.71 ± 0.65 × salinity + 243.87 ± 12.52. [Ba/Ca]_otolith increased with [Ba/Ca]_water, but the relationship was nonlinear. Salinity did not influence [Mn/Ca]_otolith, but this ratio was positively correlated with [Mn/Ca]_water. Although the partition coefficient of Sr (D_Sr = 0.23 ± 0.019) was similar to that in laboratory experiments, [Sr/Ca] in waters and otoliths was decoupled despite equal temperature exposure, suggesting that [Sr/Ca]_otolith concentration may not be a simple function of water composition. However, there was a predictive relationship between [δ¹⁸O]_otolith and [Sr/Ca]_water ([δ¹⁸O]_otolith = 1.18 ± 0.09 × [Sr/Ca]_water (mmol·mol^–1) – 14.286 ± 0.78) resulting from mixing between fluvial and oceanic waters. Water chemistry showed mixed values as a proxy for otolith chemistry and may not be a surrogate for otolith chemistry in wide estuaries.