Ability of Eelgrass to Alter Oyster Growth and Physiology Is Spatially Limited and Offset by Increasing Predation Risk

Factors limiting oyster growth in Willapa Bay (Washington, USA) evaluated with in situ feeding experiments

“…(2003) showed that oyster spat ( C. gigas ) grew ~8 mm month −1 in the upper estuary and ~12 mm month −1 in the lower estuary in summer. Lowe et al. (2018) found a similar spatial pattern for juvenile oysters: 7.6 vs. 11.2 mm month −1 at upper and lower estuary sites, respectively.…”

“…For this experiment, a site was chosen within Willapa Bay characterized by low pH ( Ruesink et al., 2015 ) and poor oyster growth relative to other locations in the bay ( Lowe et al., 2018 ; Ruesink et al., 2003 ). The experiment was conducted in mid-summer over 27 days in July–August 2016.…”

“…Willapa Bay, Washington, contributes a substantial portion (10–20%) of total United States oyster production. The primary aquaculture oyster, Crassostrea gigas, exhibits differential growth along the estuarine gradient ( Lowe et al., 2018 ; Ruesink et al., 2003 ). This differential was first recognized in the scientific literature almost eight decades ago by the identification of a “fattening line” dividing the bay into an upper (southern) region where oysters recruit naturally from the lower (northern) region of rapid oyster growth and successful conditioning ( Chapman and Esveldt, 1943 ).…”

“…Field experiments have attempted to ameliorate harmful effects of carbonate chemistry on oysters by co-locating them with seagrass, which scrubs CO 2 during daytime photosynthesis (although any net effect on pH may be small; Koweek et al., 2018 ). In a recent study of oysters in Willapa Bay, positive effects of associating with seagrass were weak and generally restricted to lower estuary habitats ( Lowe et al., 2018 ), whereas low-pH conditions typically occur up-estuary associated with river inputs ( Ruesink et al., 2015 ). Patterns of fatty acid concentrations provided greater support for food limitation than carbonate chemistry stress ( Lowe et al., 2018 ), a mechanism consistent with other studies of shellfish growing in seagrass ( Allen and Williams, 2003 ; Bologna and Heck, 1999 ; Reusch, 1998 ).…”

“…In a recent study of oysters in Willapa Bay, positive effects of associating with seagrass were weak and generally restricted to lower estuary habitats ( Lowe et al., 2018 ), whereas low-pH conditions typically occur up-estuary associated with river inputs ( Ruesink et al., 2015 ). Patterns of fatty acid concentrations provided greater support for food limitation than carbonate chemistry stress ( Lowe et al., 2018 ), a mechanism consistent with other studies of shellfish growing in seagrass ( Allen and Williams, 2003 ; Bologna and Heck, 1999 ; Reusch, 1998 ). Direct manipulation of food availability in the field is challenging, especially considering the multitude of factors that can reduce ingestion rate by suspension feeders: phytoplankton concentration, species composition, interference from inorganic seston, or sedimentation.…”