MEPS 299:19-31 (2005)  -  doi:10.3354/meps299019

Columbia River plume fronts. I. Hydrography, zooplankton distribution, and community composition

Cheryl A. Morgan1,*, Alex De Robertis2,4, Richard W. Zabel3

1Oregon State University, Hatfield Marine Science Center, Cooperative Institute for Marine Resources Studies, 2030 S. Marine Science Drive, Newport, Oregon 97365, USA
2NOAA Fisheries, Northwest Fisheries Science Center, Hatfield Marine Science Center, Newport, Oregon 97365, USA
3NOAA Fisheries, Northwest Fisheries Science Center, 2725 Montlake Boulevard East, Seattle, Washington 98112-2097, USA
4Present address: Alaska Fisheries Science Center, 7600 Sand Point Way NE, Seattle, Washington 98115, USA

ABSTRACT: Well-defined fronts develop at the leading edge of the Columbia River (USA) plume. Convergent flow at these frontal boundaries may concentrate zooplanktonic organisms, which may in turn increase local prey availability to planktivorous fishes. In May 2001 and 2002, we compared the density, biomass and community structure of planktonic and neustonic zooplankton among plume fronts, low-salinity plume waters, and within the more saline, coastal marine waters. Fronts were characterized by distinct color discontinuities and high wave energy and were usually accompanied by foam and flotsam. The surface manifestation of the fronts was narrow and formed a thin lens of warm, low-salinity water overlying colder and more saline shelf waters. Overall, neither zooplankton nor neuston densities were higher in frontal regions. However, some zooplankton taxa were more abundant at fronts, and plankton biomass was 4 to 47 times higher in the frontal regions than in the neighboring plume and more oceanic shelf waters. The zooplankton community in the front habitat was distinct, particularly in the near-surface neuston, and was comprised of more surface-oriented organisms compared to the adjacent ocean and plume waters. We conclude that convergence zones at frontal regions at the leading edge of the Columbia River plume concentrate organisms that, either through active swimming or positive buoyancy, are maintained near the surface. Time scales of these fronts are much shorter than generation times in these organisms and therefore we believe that the observed changes in biomass and community composition in the front habitat are due to physical concentrating mechanisms and not to in situ growth. Increased zooplankton biomass at plume fronts may provide a unique and valuable food resource for planktivorous fishes, including juvenile salmonids as they transition from freshwater to the ocean environment.

KEY WORDS: Frontal regions · Zooplankton distribution · Community structure · Columbia River plume

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