STABLE ISOTOPE ANALYSES ON CARBONATE SHELLS OF CULTURED GEODUCK IN NEAH BAY WASHINGTON

The Pacific geoduck, Panopea abrupta, is an economically important species and mainly cultured in the southern Puget Sound of Washington State, USA. There are no examples of geoduck aquaculture along the Washington coast. Since 2007 the Makah Fisheries Management (MFM) has started a pilot project on intertidal geoduck aquaculture in the Port of Neah Bay, the northwest tip of the Olympic Peninsula of Washington. At the same time and location, MFM has established a monitoring program for the geoduck aquaculture beds and collected environmental parameters (e.g., temperature, salinity. pH, dissolved oxygen concentration, and pressure) and seawater samples on a weekly basis. The environmental parameters were measured with a YSI device, and the water samples were analyzed for ¹³C/¹²C ratios (or δ¹³C) from dissolved inorganic carbon (DIC) and ¹⁸O/¹⁶O ratios (or δ¹⁸O) of seawater. These monitoring data and measurements constitute the background information on the Neah Bay beach conditions and a valuable reference for stable isotope analyses on carbonate shells.

In this study I report stable isotope ratio analyses on Neah Bay cultured geoduck shells sampled in the summer of 2015. Two geoduck shell samples were selected, with clear growth increments and annuli on the outside of each shell and a length of about 130 mm from the umbo to the ventral margin. Because the shell is composed of calcium carbonate (CaCO₃) the principles of carbonate geochemistry can be applied to cultured geoduck samples. Preliminary results showed that δ¹³C values of Neah Bay geoduck shells ranged from -0.62 to +1.81‰ VPDB (Vienna Peedee belemnite), whereas δ¹⁸O values of the same samples ranged from -0.23 to +1.84‰ VPDB. Over the geoduck's full life-span (about 8 years) the δ¹³C profile showed seasonal variations, with a distinct difference in δ¹³C patterns after age-1. The δ¹⁸O profile, in contrast, did not show seasonality but a steady decrease in δ¹⁸O values from age-1. The MFM monitoring data indicated that seawater parameters (especially temperature, salinity, and pH) reflect a natural variation and have not changed significantly over the monitoring time. No significant changes were obtained from seawater analyses in δ¹³C from DIC for about one-year collection, and the δ¹⁸O values of seawater largely correlated with seasonal changes of temperature. Therefore, the isotopic composition of cultured geoduck shells in Neah Bay are markedly different from the previously reported isotopic data on subtidal geoduck shells from the Hood Canal of southern Puget Sound. In particular, the seasonal δ¹³C values over the animal's full life-span did not show a steady decrease as the isotopic indicator of climate change and ocean acidification. More sampling and analyses will be needed to verify the isotopic variations and interpretation on these shells.