Forays and Foraging by Marine Zooplankton

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Intellectual Merit: A major challenge for biological oceanography is to understand and predict the vertical distribution and foraging behavior of planktonic animals. These organisms play central roles in pelagic ecosystems as consumers and producers of particles and as crucial links in pelagic food webs as prey for higher trophic types such as fish. A predictive understanding of the roles of zooplankton in these processes hinges on knowledge of their depth selection and foraging behavior. For example, where a planktonic animal forages in the water column can profoundly affect not only its feeding rate and growth rate, but also where it produces particles associated with its feeding. Moreover, in vertically structured advective fields, the dispersal of individuals depends on their position in the water column that may be dictated by other properties. Although diel vertical migration is the most conspicuous expression of depth selection and foraging behavior in zooplankton, there is much evidence indicating that zooplankton make excursions of substantial vertical extent on time scales shorter than the diel period. The biological significance of such behavior is currently unknown, but if frequent and widespread, its inclusion in models of zooplankton population dynamics and dispersal will be essential. The goal of this research is to test, in the ocean, the hypothesis that when occupying near surface waters planktonic copepods make repeated vertical excursions (forays) into and out of the food-rich layer. The Investigators will test the hypothesis by directly intercepting (trapping) vertically migrating individual adults and the last pre-adult stage (copepodid stage 5) of three species of planktonic copepods. Copepods will be sampled on sufficiently short time scales to detect repeated forays and to assess whether the forays are for the purpose of feeding. The research team will measure a small number of key indicators of physiological state and condition to assess the physiological consequences for individual copepods of such foraging behavior. The observations will be obtained in different seasons with different feeding conditions, water column physical structure (intensity of thermal stratification) and nocturnal period. Finally, they will integrate the observations in an individual-based model of copepod forays and foraging. The results of the research will be important for identifying and assessing processes and their potential significance in determining the vertical distribution and foraging behavior of zooplankton. These considerations are fundamental not only to understanding and modeling the population dynamics of marine zooplankton, but also to predicting the possible response of zooplankton to environmental changes, such as those associated with global climate change, and to understanding the vertical flux of materials, nutrients, and energy from surface waters to depth in the ocean.Broader impacts: The project will involve direct participation of two graduate student research assistants, who will pursue related research projects. The research will also develop a partnership with NOAA's Pacific Fisheries Environmental Laboratory in Pacific Grove, CA. Relevant to that partnership, the insights gained in the proposed research should also be valuable to fisheries management in terms of advancing understanding of how environmental change may affect the distribution and availability of a major class of prey items for many managed living marine resources, such as juvenile salmon and other small pelagic fish.