GEODUCK AQUACULTURE IN NEAH BAY AND THE ISOTOPE ANALYSES OF CARBONATE SHELLS

The Pacific geoduck, Panopea abrupta, is an important clam species especially cultured in the southern Puget Sound of Washington State. Since 2007 the Makah Fisheries Management (MFM) started a pilot project on intertidal geoduck aquaculture in the Neah Bay beaches. At the same time, MFM has established a monitoring program for the geoduck aquaculture beds and weekly collected environmental parameters (e.g., temperature, salinity, pH, dissolved oxygen concentration, and pressure) and seawater samples. All these activities provide an opportunity for chemical analyses on the carbonate shells. In this study we report stable carbon and oxygen isotope analyses (δ¹³C and δ¹⁸O) of geoduck shells from Neah Bay and a comparison with the shells from the southern Puget Sound. Preliminary results showed that the δ¹³C values of Neah Bay geoduck shells ranged from -0.2 to +1.8‰ VPDB (Vienna Peedee belemnite), whereas δ¹⁸O values of the same shell samples ranged from -0.2 to +1.8‰. Over the geoduck clam'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 did not show seasonality but a trend of δ¹⁸O decrease starting 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 (dissolved inorganic carbon), and the δ¹⁸O values of seawater largely correlated with seasonal changes of temperature. Overall, the isotopic comparison showed differences between the Neah Bay geoduck shells and samples from the southern Puget Sound, where distinct effects of ocean acidification were observed. In particular, the seasonal variations over the geoduck clam's life-span in Neah Bay did not show a steady decrease of δ¹³C as isotopic signatures of ocean acidification. Thus, we concluded that the carbonate shells of Pacific geoduck may be a good proxy for climate change and marine environmental studies, but the effects of ocean acidification along the Washington coast need more investigations.