QUANTIFYING UPWELLING AND FRESHENING IN NEARSHORE TROPICAL ENVIRONMENTS USING STABLE ISOTOPES IN MODERN TROPICAL AMERICAN GASTROPODS

To identify and quantify the influences of upwelling and freshwater to tropical ecosystems we performed stable isotope analyses (δ¹⁸O and δ¹³C) on 13 serially-sampled modern Conus shells collected from coastal waters in the southwestern Caribbean (SWC, non-upwelling) and Gulfs of Chiriquí (non-upwelling) and Panama (upwelling), tropical eastern Pacific (TEP). Most shells reveal seasonal variations in temperature and (or) the δ¹⁸O of the water. Unusually high or low seasonal δ¹⁸O values measure the intensity of seasonal upwelling or freshwater input respectively. To quantify upwelling and freshening signals, baseline δ¹⁸O values that are free of seasonal upwelling and freshening influences were estimated from average temperatures during non-upwelling (wet-season) periods and average salinities during dry-season periods. Shell baseline-normalized δ¹⁸O values reveal little or no upwelling in the SWC and the Gulf of Chiriquí, and strong upwelling in the Gulf of Panama, as well as strong freshwater input in most areas. Dry-season δ¹⁸O values for Gulf of Panama Conus can exceed the baseline by as much as 2‰, which equates to seawater temperatures ~9°C lower than normal. In contrast, rainy-season δ¹⁸O values can be as low as 1.8‰ below the baseline, equivalent to salinities ~7 lower than normal. No shells from the Gulf of Panama had the negative δ¹⁸O-δ¹³C (O-C) correlations that are often associated with upwelling of cool, ¹⁸O-enriched, ¹³C-depleted waters. However, when data from the upwelling seasons (January to April) of Gulf of Panama shells are isolated, four of the five shells tested show significant negative O-C correlations, indicating that the freshwater signal during the rainy, non-upwelling season masks the expected negative correlation. We combine δ¹⁸O range (Δ¹⁸O) and O-C correlation to develop a general model for identifying upwelling and freshening environments in tropical environments. Eutrophic environments like the Gulf of Panama are characterized by high Δ¹⁸O and weak O-C correlation, and oligotrophic environments like the SWC by low Δ¹⁸O and no significant O-C correlation, or moderate Δ¹⁸O and a positive O-C correlation.