Water and otolith chemistry identify exposure of juvenile rockfish to upwelled waters in an open coastal system

Abstract

We present a novel approach to examine the relationship between pelagic juvenile fish and their environment in an open coastal system using a geospatial technique to relate water and otolith chemistries. We compared the chemistries of water and pelagic juvenile rockfish otoliths Sebastes jordani collected from a coastal upwelling system off central California during May and June 2009. To determine the presence and composition of an upwelling chemical signature, the elements Ba, Sr, and Mg expressed as ratios relative to Ca were quantified in the water and outer otolith margin using inductively coupled plasma mass spectrometry. Recently upwelled water, as indicated by a strong inverse relationship with water temperature, had an elevated Ba:Ca concentration. Using all 3 element ratios, cluster analysis and multivariate analysis of variance were used to identify 3 distinct chemical signatures for otolith and water samples. When mapped, these signatures displayed marked geospatial variability that we attributed to mesoscale upwelling dynamics. Canonical discriminant function analysis results indicated that the relative contribution of each element to the 3 signatures was similar between water and otoliths. Interestingly, otoliths containing upwelling signatures (high Ba:Ca) did not match spatially with upwelling signatures in the water. A directional-dependence analysis (spatial cross-covariance) revealed the highest covariance between water and otolith chemistries at a distance between 50 and 100 km apart, suggesting southern movement or transport of fish.

Evaluating chemical signatures in a coastal upwelling region to reconstruct water mass associations of settlement-stage rockfishes

“…Elevated Ba/Ca ratios characterized waters during weak upwelling conditions relative to the other water types. Although strong upwelling water mass types exhibited a lower Ba/Ca ratio relative to weak upwelling conditions, its magnitude was equivalent to measurements by Woodson et al (2013) in nearby locations with persistent upwelling conditions. In addition, surface salinities measured during weak upwelling and relaxation conditions were ~0 to 0.35 psu lower than upwelled surface waters and likely contained a greater contribution of high-Ba river runoff.…”

“…Elemental concentrations in fish otoliths were measured using laser ablation ICP-MS. A right or left otolith was haphazardly selected and mounted to a glass slide covered with Crystalbond Mounting Wax (Buehler Brand), which has negligible elemental concentrations (Woodson et al 2013 Mg isotopes. Samples re ceived 10 cleaning shots prior to sample ablation to remove any surface contaminants.…”

“…This pattern can vary spatially and temporally. Locations with strong upwelling activity have elevated Ba/Ca ratios in surface waters (Woodson et al 2013). Intense transient plankton blooms can significantly deplete surface Ba concentrations below 40 nmol kg −1 ).…”

“…The Ba/Ca ratio in otoliths (Ba/Ca Otolith ) is likely an effective tracer of Ba-rich upwelled waters , Kingsford et al 2009, Walther et al 2013, Woodson et al 2013. For instance, it has been estimated that up to 98% of Ba/Ca Otolith is de rived from ambient water (Walther & Thorrold 2006), and relative to other elements, the short turn-over time of Ba in the bodies of fishes indicates a strong potential for Ba/Ca Otolith to reflect short-term changes in the Ba concentration of the ambient water (Sturrock et al 2012).…”

“…For instance, it has been estimated that up to 98% of Ba/Ca Otolith is de rived from ambient water (Walther & Thorrold 2006), and relative to other elements, the short turn-over time of Ba in the bodies of fishes indicates a strong potential for Ba/Ca Otolith to reflect short-term changes in the Ba concentration of the ambient water (Sturrock et al 2012). Several studies have recognized the potential for otolith chemistry to evaluate fish associations with upwelling regions or water masses; few studies, however have validated these signatures in otoliths or applied them in ecological research (Patterson et al 2004, Kingsford et al 2009, Nishimoto et al 2010, Lin et al 2013, Woodson et al 2013.…”