Abstract
MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 182:77-93 (1999) - doi:10.3354/meps182077 Foraging at a front: hydrography, zooplankton, and avian planktivory in the northern Bering Sea Robert W. Russell*, Nancy M. Harrison**, George L. Hunt Jr Department of Ecology and Evolutionary Biology, University of California, Irvine, California 92697-2525, USA Present addresses: *Museum of Natural Science, 119 Foster Hall, Louisiana State University, Baton Rouge, Louisiana 70803-3216, USA. E-mail: migrants@hotmail.com **Anglia Polytechnic University, East Road, Cambridge CB1 1PT, United Kingdom ABSTRACT: We studied hydrographic structure, zooplankton distributions, and foraging by planktivorous seabirds in the Anadyr Strait, northern Bering Sea, during 4 summer cruises (1984-1986, 1993). The western portion of the strait was occupied by cold, dense Anadyr water that was mixed from top to bottom. This mixed water was separated from the stratified Bering Shelf water on the eastern side of the strait by a sharp surface front (the 'Anadyr Front'). Net sampling indicated that calanoid copepods were the numerically dominant component of the zooplankton, and that densities of several species were elevated in the frontal zone, apparently due to mechanical accumulation resulting from surface convergence. Hydroacoustic surveys showed that overall zooplankton biomass was concentrated along the thermocline and at the front. Although the location of the Anadyr Front was highly variable over time scales as short as 1 d, large numbers of least auklets Aethia pusilla often flew 25 to 50 km from their breeding colonies to feed at the front. Diet samples indicated that the copepod Neocalanus plumchrus was the principal prey taken by least auklets both at the front and away from it, indicating that heavy use of the distant frontal habitat was due to the higher densities of their preferred prey (i.e. rather than absence of suitable prey species closer to shore). Whenever aggregations of least auklets were found away from the front, there was evidence that they were exploiting near-surface high-density patches of zooplankton, though the exact mechanisms responsible for the formation of such patches are unclear. In contrast to least auklets, crested auklets Aethia cristatella were usually found away from the front. In several cases, compact aggregations of crested auklets were located over acoustically observed epibenthic layers of zooplankton. Hydrographic data suggested that intense subsurface jets and/or upwelling along the eastern side of the strait might have increased the availability of the crested auklets' preferred euphausiid prey. Thus, spatial segregation of the 2 principal planktivores in Anadyr Strait likely arises because different physical mechanisms cause concentrations of preferred prey originating at different depths. KEY WORDS: Front · Foraging · Bering Sea · Copepods · Neocalanus plumchrus · Least auklet · Aethia pusilla · Crested auklet · Aethia cristatella Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 182. Publication date: June 11, 1999 Print ISSN:0171-8630; Online ISSN:1616-1599 Copyright © 1999 Inter-Research.
Highlights
Oceanic fronts are complex fluid structures often characterized by sharp sea-surface gradients in density, temperature, and/or salinity
We studied the relationships between auklet foraging, zooplankton distributions, and the Anadyr Front during cruises to the northern Bering Sea in 1984, 1985,1986, and 1993.This paper focuses on the following questions: (1) How predictably are auklets found near the front and where are they distributed in relation to the hydrographic structure? (2) Do predictable zooplankton distributions at the front draw the birds? and (3) What are the implications of frontal structure and location for the feeding ecology and life history of the birds?
Net tows indicated that high densities of Neocalanus plumchrus were present in the vicinity of the concentration of auklets at the front (Fig. 2A)
Summary
Oceanic fronts are complex fluid structures often characterized by sharp sea-surface gradients in density, temperature, and/or salinity. But are not limited to, boundaries between different water masses. A growing body of evidence indicates that oceanic fronts support high levels of biotic activity across a wide range of trophic levels (reviewed by Le Fevre 1986, Olson et al 1994). Frontal enrichment or concentration has been documented in a wide variety of marine plankton, including microbes (e.g.Floodgate et al 1981, Fernandez et al 1994), phytoplankton. The northward flow, known as the feed on the plankton concentrations
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