The occurrence of low-oxygen waters, often called "dead zones" in coastal ecosystems throughout the world is increasing. Despite these increases, the pelagic food-web consequences of low-oxygen waters remain poorly understood. Laboratory research has demonstrated that hypoxic water (< 2 mg l-1) can result in mortality, reduced fitness and lower egg production of planktonic copepods, a major link in food webs supporting pelagic fish. Observations in the sea indicate that hypoxic bottom waters usually have depressed abundances of copepods compared to normoxic waters (> 2 mg l-1). The gradient of declining oxygen concentration with respect to depth (oxycline) can be a critical interface in coastal pelagic ecosystems by altering the migratory behavior and depth distribution of copepods and their spatial coherence with potential predators and prey. This project will result in a mechanistic understanding of how behavior and fitness of copepods are affected by hypoxia. The PIs will compare bottom-up and top-down controls on the ecology of copepods in Chesapeake Bay waters experiencing seasonal hypoxia and those that are normoxic.
Specific objectives of this project are to: 1) analyze changes in migratory behavior and fine-scale (meter) distribution of copepods across the oxycline over hourly and diel time scales while simultaneously examining the distribution and abundance of their food (phytoplankton and microzooplankton) and predators (fish, gelatinous zooplankton); 2) estimate effects of hypoxia on the "fitness" of copepods using a suite of measurements (length/weight ratios, feeding, egg production, and egg hatching success) to develop condition indices of copepods captured at different times and depths in hypoxic and normoxic waters; and 3) evaluate effects of hypoxia on copepod mortality by hypoxia-induced, stage-specific copepod mortality in hypoxic bottom waters and by changes in top-down control of copepods from predation by fish and gelatinous zooplankton.
Oxyclines may be a barrier to vertical migration of copepods and thus disruptive to predator avoidance behavior. Faced with increased predation risk from fish and jellyfish, copepods may seek refuge in hypoxic waters for part of the day and/or make short-term vertical excursions between hypoxic and normoxic waters. By regulating vertical migrations, copepods may increase utilization of microzooplankton prey concentrated in the oxycline. Hypoxic waters may elevate consumption of copepods by jellyfish and depress consumption by pelagic fish. This project will evaluate copepod distribution and migration behavior, individual fitness and stage-specific mortality in hypoxic and normoxic waters. It will examine food-web consequences of increased or decreased spatial coherence of copepods and their predators and prey in regions with hypoxic bottom waters and will contribute to fundamental understanding of food-web processes in eutrophic coastal ecosystems.
Broader Impacts: As hypoxia becomes more prevalent in estuarine and shelf waters, increased understanding of its effects on planktonic food-webs becomes essential for effective, ecosystem-based management. The effects of eutrophication and hypoxia are areas of research emphasized in the JSOST Ocean Research Priorities Plan. Information gained from this project will be critical for food-web modeling in development of fisheries ecosystem plans for Chesapeake Bay. In a broader sense, the research is needed to achieve goals in the Chesapeake Bay Program's "Chesapeake 2000" Agreement. The proposed research will support two graduate students and a postdoc. In addition, the Horn Point Laboratory is part of the mid-Atlantic NSF-COSEE program and this project will support the participation of two summer teacher interns. The Horn Point Laboratory also participates in the NSF Research Experience for Undergraduates (REU) program. REU undergraduate students will be involved in the proposed research. Dissemination of results to the public and environmental managers will be facilitated by the infrastructure of University of Maryland Center for Environmental Science's Integration and Application Network (www.ian.umces.com).
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° 31.32’ N, 076° 24.48’ W) and the South (typically less hypoxic) station (37° 43.68’ N, 076° 12.0’ 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.
