In Situ Hatching Success of Calanoid Copepod Eggs in Hypoxic Sediments of a Coastal Bay

Abstract

Hypoxia can have adverse effects on recruitment of marine copepods via its effects on egg hatching and survival of nauplii. This study investigated the effects of hypoxia on in situ hatching success of calanoid copepods in Gamak Bay, Korea, a shallow (9-m) microtidal coastal bay with long water residence times exceeding a month. Plankton emergence trap/chambers were placed into seafloor-surface sediments in July at a hypoxic site (mean = 0.6 mL L-1) in the inner part of the bay and at a normoxic site in the mid bay. Nauplii that hatched and left the sediment were collected daily for 12 days, and the top 1 cm of sediment was preserved at the end of incubation. The number of eggs in the sediment was >3 X 10(5) eggs m(-2) at the hypoxic site, about six times that at the normoxic site. Most of the eggs appeared to be subitaneous. Egg-hatching rate in the hypoxic site, however, was only 4% of that at the normoxic site, where 57% of the eggs present hatched. Cumulative numbers of eggs hatching into the hypoxic chambers were about half the numbers hatching into the normoxic chambers. The pH level in the sediment was 7.1 to 7.3 at the hypoxic site, slightly lower than in the normoxic sediment (pH > 7.5). That difference seemed to have little effect on egg hatching. High sulfide concentration at the hypoxic site (> 300 mu M), combined with low dissolved oxygen in summer were likely responsible for the depressed egg hatching. However, the cumulative number of eggs that hatched in the hypoxic chambers was still about half of that in the normoxic chambers. Thus, it appears that the hypoxic site might still be a significant source of juveniles for the next generation of the copepods.