The role of phytoplankton in the modulation of dissolved and oyster cadmium concentrations in Deep Bay, British Columbia, Canada

We previously identified dissolved cadmium (Cddiss) as the main source of this metal in cultured Pacific oysters, Crassostrea gigas, in Deep Bay, British Columbia, Canada (Lekhi et al., 2008). Total suspended particulate Cd (Cdpart) was not found to be a significant source of oyster Cd (Cdoys), with Cdpart > 20 μm negatively correlated with Cdoys concentration. High phytoplankton abundance in spring and summer was hypothesized to reduce Cdoys indirectly by drawing down Cddiss and increasing oyster growth. In the present study we expanded on these results by examining specifically how the phytoplankton community composition modulates both Cddiss and Cdoys concentrations in Deep Bay. Based on calculations of nutrients and Cddiss drawdown, phytoplankton accounted for approximately 90% of the overall summer reduction in Cddiss in the bay. Diatoms were the dominant phytoplankton group, being correlated negatively with Cdoys and positively with Cdpart. This suggests that diatom growth mediates the transfer of Cd from the dissolved to the particulate phase, resulting in lower Cdoys. Spring blooms and sporadic harmful algal blooms may mediate a large flux of Cdpart to the sediments. Thus, phytoplankton act as a sink, rather than a source, of Cd to oysters in Deep Bay and have a crucial role in the seasonality of Cdoys by reducing the concentration of Cddiss during the summer. Based on environmental variables, two descriptive models for annual Cdoys concentrations were developed using multiple linear regression. The first model (R2 = 0.870) was created to explain the maximum variability in Cdoys concentrations throughout the year, while the second (R2 = 0.806) was based on parameters that could be measured easily under farm conditions. Oyster age heavily affected both models, with the first model being secondarily affected by temperature and the second one being more sensitive to changes in salinity.

► Phytoplankton and oceanographic processes modulate dissolved and oyster Cd throughout the year. ► Phytoplankton account for the loss of dissolved Cd during summer while upwelling replenishes it in winter. ► Summer accumulation of dissolved Cd and the subsequent sedimentation of phytoplankton reduced its availability to oysters. ► Harmful algae created short-lived pulses of dissolved Cd reduction which limited acquisition of particulate Cd by oysters. ► Two descriptive models explain most of the variability observed in oyster Cd throughout the year.