Award: OCE-0961924

To develop an understanding of how copepods are affected by hypoxia (i.e., low dissolved oxygen), we compared top-down controls of Chesapeake Bay copepods in waters experiencing seasonal hypoxia and in those that were not hypoxic. During 6 week-long research cruises, we collected both biological and hydrographic data from two stations in the main channel of Chesapeake Bay: the North (more hypoxic) station (38° 31.32Æ N, 076° 24.48Æ W) and the South (typically less hypoxic) station (37° 43.68Æ N, 076° 12.0Æ W). Samples were collected from late spring to early fall of 2010 (May 26-31, August 19-25, September 21-27) and of 2011 (May 25-31, July 19-25, September 22-28). Specifically, the goal of this project was evaluate hypoxia-induced changes in mortality of copepods due to gelatinous zooplankton (i.e., comb jellies and sea nettles). Simultaneously, other investigators working on this collaborative project determined the influence of hypoxia on fish predators of copepods and prey, as well as the direct effects of hypoxia on copepod mortality and reproduction. We found that gelatinous zooplankton populations were highest in the warm months of 2011, which was a year categorized by lower oxygen levels relative to 2010. However, we also found that gelatinous zooplankton avoided the most severely hypoxic bottom waters of Chesapeake Bay, which resulted in higher jellyfish concentrations in the mid-depths to surface waters, where oxygen concentrations ranged from moderately hypoxic to normoxic. We estimated the predatory impact of the predominate gelatinous species, the ctenophore Mnemiopsis leidyi, by estimating the rate at which they consume copepods at various dissolved oxygen levels. In addition, we found an inverse relationship between gelatinous zooplankton and their copepod prey throughout the study, which suggests that predation by jellyfish controlled copepod populations during our study. During the warm summer months, we estimated that ctenophores populations were sufficiently large that they could consume as much as 58% of the adult copepods present. Although the ctenophore bloom is relatively short-lived in Chesapeake Bay each summer, our results indicate that when present, predation by M. leidyi can cause considerable copepod mortality. Last Modified: 06/04/2014 Submitted by: Mary Beth Decker