Hypoxia and acidification and their impact on zooplankton in the southern coastal waters of Korea

Abstract

Perpetual summer bottom water hypoxia has become a norm of the southern coastal bays of Korea over the past decades. Although we do not know if the hypoxia has gotten worse over the years, efforts controlling the input of human induced nutrient have not resulted in any significant improvements in water quality of the bays. Copepods, a key trophic link between plankton and fish in the bays, are sensitive to hypoxia and subsequent acidification of water, with lethal and sublethal consequences. We examined how hypoxia and acidification influence the distribution of the copepods and mesozooplankton in the water column and hatching success of the eggs laid on the sediments, the refuge of copepod population. Calanoid copepods were negatively affected by hypoxia with reduced abundance, whereas the smaller cyclopoid copepods seem to be more resistant and thrive in the low oxygen waters with increased abundance. Copepod egg hatching was severely impaired under hypoxia with much lower hatching success than in normoxic sediment. Lower pH also had adverse effects on egg hatching success of the copepods. Placing the eggs exposed to lower pH back to seawater of normal pH yielded a rather complex pattern of recovery, but it was clear that exposure to lower pH could cause irreversible damage to some copepod eggs.ient have not resulted in any significant improvements in water quality of the bays. Copepods, a key trophic link between plankton and fish in the bays, are sensitive to hypoxia and subsequent acidification of water, with lethal and sublethal consequences. We examined how hypoxia and acidification influence the distribution of the copepods and mesozooplankton in the water column and hatching success of the eggs laid on the sediments, the refuge of copepod population. Calanoid copepods were negatively affected by hypoxia with reduced abundance, whereas the smaller cyclopoid copepods seem to be more resistant and thrive in the low oxygen waters with increased abundance. Copepod egg hatching was severely impaired under hypoxia with much lower hatching success than in normoxic sediment. Lower pH also had adverse effects on egg hatching success of the copepods. Placing the eggs exposed to lower pH back to seawater of normal pH yielded a rather complex pattern of recovery, but it was clear that exposure to lower pH could cause irreversible damage to some copepod eggs.