Filter feeders are an important link between benthic and pelagic environments. By sequestering pelagic resources (phytoplankton) in the benthos pacific oysters (Crassostrea gigas) can have ecosystem wide impacts in an estuary. In Willapa Bay, Washington, mathematical models suggest that, filter feeders may be at or near their effective carrying capacity; thus on a baywide scale, oysters may be food limited. At a local scale then, is food limitation a simple result of local resource depletion through the mechanism of competition? Initial sampling confirmed that oyster filtration significantly reduces chlorophyll a concentrations over distances of 100 to 1000m. On this scale, we intensively examined water circulation and oyster growth at one intertidal oyster bed in Willapa Bay, WA. At this site, we measured the growth of oyster spat out-planted to 8 different locations across the bed for three field seasons from 2006-2008. We spent 3 summers tracking surface water flows using field drifters with GPS units. During the same time period we also placed chlorophyll and salinity loggers on the oyster bed at known locations to trace the changes in water column properties over time and space. During the field season of 2008 we deployed 2 Aquadopp current profilers to gauge the flux of water over the tide flat.
Results/Conclusions
We found that oysters lower in the intertidal (closer to where water exchange occurs) had a better condition index (ratio of shell length to dry meat weight) than those higher in the intertidal (where less water exchange occurs). Using salinity as a passive tracer, we have shown that chlorophyll depletion occurs at our study site. From previous work we have calculated an average field filtration rate of 0.48 L/g*h (shucked wet weight) for Pacific oysters (Crassostrea gigas), applying this filtration rate to observed chlorophyll concentrations we consistently underestimated the magnitude of depletion (by 10 to 30 percent). However, by combining our field filtration rate with data on water retention from surface and subsurface water flows and salinity measurements, we have built a model that more closely predicts chlorophyll depletion over multiple tidal cycles. Our results suggest that oyster condition is impacted by local food depletion compounded by tidal circulation patterns which create long water residence times. This project helps further our understanding of how large-scale (bay-wide) circulation patterns and oceanographic influences combine to influence oyster growth the small scale of an intertidal oyster bed.
