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Recent evidence suggests that mesoscale eddies are an important mechanism for supplying nutrients to the surface waters of oligotrophic gyres. However, little is known about the biological response to these physical perturbations. Because mesozooplankton play a key role in food-web interactions and the flux of carbon and other elements from surface waters, changes in mesozooplankton community structure can affect biogeochemical cycling. During the summers of 2004 and 2005, respectively, we followed the development of a cyclonic eddy and an anti-cyclonic mode-water eddy in the Sargasso Sea. Zooplankton tows were conducted across both eddies using a Multiple Opening and Closing Net Environmental Sensing System (MOCNESS), which sampled 9 discrete depth intervals between 0-700 m. Comparison of the abundance of major taxa of mesozooplankton in the upper 150 m at eddy center and outside the eddies (day and night), indicated that the cyclone and mode-water eddy supported similar mesozooplankton communities, with little difference inside vs. outside the eddies. However, a comparison with the Bermuda Atlantic Time-series Study (BATS) site, used as an alternative outside station, indicated significantly higher abundance of several zooplankton taxa inside both eddies. In both eddies copepod peak abundance occurred in the 50-100 m depth interval, coincident with the chlorophyll a maximum, suggesting elevated food concentration in the eddies may be influencing zooplankton vertical distribution. The two eddies differed in the strength of diel vertical migration of zooplankton, as indicated by the ratio of night:day abundance in the epipelagic zone which was higher at the center of the mode-water eddy for most taxa. Over the sampling interval of 1-2 months, abundance of the three most common taxa (copepods, chaetognaths, and ostracods) decreased in the cyclone, and increased in the mode-water eddy. This further supports previous findings that the cyclone was in a decay phase over the sampling period, while the mode-water eddy was sustaining high nutrient and phytoplankton concentrations for the duration of sampling. A more detailed analysis of community structure in the mode-water eddy (0-700 m) indicated no significant difference between eddy center vs. outside the eddy in the abundance of any taxa at any specific depth interval. However, the 0-700 m integrated abundance of doliolids was significantly higher inside the eddy. The presence of a mesopelagic (200-700 m) layer of lepadid barnacle cyprids highlights the potential of these eddies for transport and dispersal of biota. We conclude that eddies can influence zooplankton behavior and alter zooplankton community structure in ways which affect biogeochemical cycling in the open ocean.

Although zooplankton carcasses can be quite prevalent within aquatic systems, they have largely been overlooked in most zooplankton population studies. Anthropogenic stressors can potentially increase the overall abundance of carcasses on a local scale. Once a carcass is present within a system, the fate of its biomass is of considerable interest as it may be remineralized within the water column or transported to depth. Through the collection of field samples I assessed the possibility of an anthropogenic stressor (boat-generated turbulence) as a potential source of nonconsumptive mortality. I also conducted a series of laboratory experiments to monitor the decomposition of representative crustacean and non-crustacean zooplankton carcasses and determined the fate of carcass-derived organic matter. Higher carcass abundances were found within boat wakes than outside boat wakes, indicating that boat-generated turbulence could have induced non-consumptive mortality of zooplankton. Copepod carcasses decomposed at a much faster rate than rotifer carcasses, suggesting that crustacean zooplankton carcasses would likely be decomposed within the water column and support bacterial production while non-crustacean zooplankton could serve as a transport mechanism of high quality POM to depth.