Pelagic oxycline and damage potential of hypoxia to the Pacific oyster Crassostrea gigas suspended in longline aquaculture systems

Abstract

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 2ñ4 m in one bay (Hypoxic Site 1, HS-1) and 2ñ5 m in the other (Hypoxic Site 2, HS-2). Water above the oxycline was normoxic (>4 mg l-1), while water below the oxycline was hypoxic (<1 mg l-1). The oxyclines were accompanied by significant changes in environmental variables and phytoplankton composition, but these changes were not fatal to the oysters. However, the oxyclines themselves 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, and greater mortality was observed in HS-1 than HS‑2. These findings suggest the existence of toxic compounds that diffuse up from below the oxycline in shallow waters and exert effects that overshadow those of DO. The higher mortalities in the HS-1 normoxic layer supported this influence from the bottom layer. In a subsequent experiment, we observed additional mortalities among the surviving oysters in the upper normoxic waters after the bays had completely reoxygenated during 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.