AEI prepress abstract  -  DOI: https://doi.org/10.3354/aei00245

Pelagic oxycline in sheltered shallow coastal waters: the nature and damage potential of hypoxia to the Pacific oyster Crassostrea gigas suspended in longline aquaculture systems

Sang Jun Lee, Qtae Jo*, Jong-Cheol Han, Yeong-Cheol Park, Tae Gyu Park

*Email: qtjo@korea.kr

ABSTRACT: A depletion in dissolved oxygen (DO) occurred in 2 semi-enclosed shallow bays in Korea in which Crassostrea gigas were suspended for aquaculture. DO depletion started at the bottom and expanded rapidly upwards for days until it was blocked by a temperature-driven density differential. This resulted in the formation of pelagic oxyclines at a depth of 2–4 m in hypoxic site-1 (HS-1) and 2–5 m in hypoxic site-2 (HS-2). Water above the oxyclines was normoxic (>4 mg l–1), while water below the oxycline was hypoxic (<1 mg l–1). The oxyclines accompanied significant (p < 0.05) changes in environmental variables and phytoplankton composition, but the changes were not fatal to the oysters. However, the oxyclines caused oyster mortality: complete mortality below the oxyclines and depth-dependent mortality within the oxycline. Interestingly, mortality was observed in a significant number of oysters above the oxyclines compared with the reference site (p < 0.05), and greater mortality was observed in HS-1 than HS-2 (p < 0.05). These findings suggest the existence of toxic compounds that diffused up from below the oxycline in shallow waters and exerted effects that overshadowed those of DO. The higher mortalities in the HS-1 normoxic layer supported this influence from the bottom layer. In a subsequent experiment, we witnessed additional mortalities among the surviving oysters from the upper normoxic waters after the bays had completely reoxygenated due to fall turnover. These data provide useful observations of hypoxia in highly sheltered shallow waters and can be used to guide site selection for oyster longline aquaculture.