Schnetzer¹, Astrid, and Deborah K. Steinberg²

¹Department of Biological Sciences, 3616 Trousdale Parkway AHF 301, University of Southern California, Los Angeles, CA 90089-0371, USA, Tel: 213 821 2123, Fax: 213 740 8123, E-mail: astrids@usc.edu and ²Virginia Institute of Marine Science, P.O. Box 1346, Gloucester Pt, VA 23062, USA

The impact of diel vertical migration on surface export of particulate organic matter and cycling of energy in the mesopelagic zone

Diel vertical migration is a common strategy for mesopelagic zooplankton to access food-rich surface layers and avoid being visible to predators. Migrators ascend at night to feed, and descend to residence depths of several hundred meters before dawn. Our understanding of how diel vertical migration impacts biogeochemical cycling and planktonic community structure has grown tremendously during JGOFS. Migrant communities actively export substantial amounts of dissolved inorganic and organic matter by respiring and excreting surface-ingested material at depth. We recently demonstrated that a portion of the material transported inside migrators’ guts is also released as feces at depth. Based on data from the Bermuda Atlantic Time-series Study (BATS) we discuss how active flux of particulate organic matter (POM) changes seasonally and interannually, and compare the relative importance of active POM flux and passive vertical particle flux amongst BATS and other JGOFS study sites. Gut content analyses of common migrant species from the Sargasso Sea indicate that actively exported POM is a nutritious food source for mesopelagic organisms. Although all species examined have a mixed diet, differences in feeding preferences and in the degree of food selectively were apparent. This implies an individual species approach can help to understand how migrating zooplankton affect resource partitioning and carbon cycling in pelagic food webs and helps elucidate how zooplankton ‘make a living’ in the mesopelagic realm. JGOFS is unique in enabling us to study ocean processes on a global scale concurrently with the role of individual species in cycling of energy.