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

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 plumeResale or republication not permitted without written consent of the publisher

Columbia River plume fronts. II. Distribution, abundance, and feeding ecology of juvenile salmon

“…We sampled fishes with a surface trawl, made conductivity, temperature and depth (CTD) casts, collected water samples, and collected zooplankton with plankton and neuston nets at a series of blocked triplicate stations (hereafter 'station block'). Methods were comparable in both years, but due to interannual differences in river flows the distance between the habitats sampled in each station block was larger in 2002 (~14 km compared to ~8.5 km in 2001; see Morgan et al 2005 for details). Each station block consisted of 1 station in each of the 3 distinct habitats: the low-salinity plume, along the surface front at the leading edge of the plume, and in the adjacent more saline coastal marine water (hereafter referred to as plume, front, and ocean habitats, respec-tivly).…”

“…Each station block consisted of 1 station in each of the 3 distinct habitats: the low-salinity plume, along the surface front at the leading edge of the plume, and in the adjacent more saline coastal marine water (hereafter referred to as plume, front, and ocean habitats, respec-tivly). Sampling design, physical measurements, and prey sampling are reported in Morgan et al (2005), and are only briefly described here.…”

“…The strength of these fronts fluctuates with freshwater outflow and tidal period. These fronts are transient but recurrent features, forming near the river mouth with the ebb tide as low-salinity water exits out of the estuary, and persisting for several hours as they propagate out to sea with the tide (Morgan et al 2005). As with other river plumes (Grimes & Finucane 1991, Kingsford & Suthers 1994, the front can be clearly identified by sharp horizontal gradients in salinity and water clarity, as well as by accumulations of foam and flotsam.…”

“…Well-defined fronts develop at the seaward edge of riverine plumes, where suspended materials and planktonic organisms are concentrated by convergent flow (Le Févre 1986, Grimes & Finucane 1991, Franks 1992, Govoni & Grimes 1992. Riverine plume fronts are often hypothesized to be favorable habitats for planktivorous fishes because of their elevated concentrations of planktonic prey, particularly near-surface neustonic species (Le Févre 1986, St. John et al 1992, Grimes & Kingsford 1996, Morgan et al 2005.…”

“…In the frontal areas of the Columbia River plume, planktonic organisms are concentrated by convergent flow, particularly in nearsurface waters, where the biomass of plankton is 4 to 47 times higher than in adjacent waters (Morgan et al 2005). If juvenile salmon, which are surface-oriented feeders (Brodeur 1989), are able to locate and maintain themselves within these food-rich fronts, they will encounter higher prey densities in frontal regions, which could increase ingestion rates.…”