Oysters as ecosystem engineers: Top down effects

Filter feeders represent an ecologically dynamic link between benthic and pelagic environments. The capacity of filter feeders to reduce seston concentrations in water column has important implications for restoration and management. However, there is significant contention, especially theoretically, about the capacity of current or historical populations of bivalves to improve water clarity. I directly measured changes in water properties across intertidal oyster beds in Willapa Bay, Washington, a macrotidal estuary, where 10% of U.S. oysters are grown. The project takes 2 perspectives: Lagrangian (recording water properties in a particular parcel of water as it moves across a tide flat using small drifters) and Eulerian (recording water properties at a single location over time as water flows past using YSI 6600 EDS sondes). Chlorophyll a levels were measurably reduced as water passed over oyster beds (~200m), but only when adult oyster density exceeded 100 oysters/m² and water depth was less than 1m. Additionally, the Eulerian data suggest that overall, significant depletion occurs because water on the tide flat must flow over the oyster beds on both the flood and ebb tide. Of particular concern in this system is the impact of oyster aquaculture on critical eelgrass habitat. This study explores the existence of a potential indirect positive effect of oysters on eelgrass. Our results suggest that oysters significantly reduce seston and Chl a concentrations and that this filtration activity results in increased local water clarity in and around commercial oyster beds.