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Marine Ecology Progress Series

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MEPS 393:189-198 (2009)  -  DOI:

Shortened duration of the annual Neocalanus plumchrus biomass peak in the Northeast Pacific

Sonia D. Batten1,*, David L. Mackas2

1Sir Alister Hardy Foundation for Ocean Science, c/o 4737 Vista View Cr., Nanaimo, British Columbia V9V 1N8, Canada
2Fisheries and Oceans Canada, Institute of Ocean Sciences, PO Box 6000, Sidney, British Columbia V8L 4B2, Canada

ABSTRACT: The calanoid copepod Neocalanus plumchrus (Marukawa) is a dominant member of the spring mesozooplankton in the subarctic North Pacific and Bering Sea. Previous studies have shown interdecadal and latitudinal variation in seasonal developmental timing, with peak biomass occurring earlier in years and places with warmer upper ocean temperatures. Because N. plumchrus normally has a single dominant annual cohort, its seasonal timing can be indexed from measurements of total population biomass or by following progressive changes in stage composition. Early studies empirically found that peak upper ocean biomass occurred when about half of the pre-dormant population had reached copepodite stage 5 (C5). However, more recent comparisons derived from recent Continuous Plankton Recorder (CPR) data now show peak biomass when a larger fraction (>80%) of the population is at C5. CPR samples the surface 10 to 15 m, but comparisons to depth-resolved BIONESS data show that this discrepancy is not an artefact of sampling depth. Other causes are either a prolongation of duration of pre-dormant C5 or a narrowing of the age range making up the annual cohort. We assessed changes in cohort width using a modification of Greve’s cumulative percentile method, and found that average cohort widths in the Alaska Gyre were significantly narrower in 2000–2007 than in 1957–1965 (1968–1980 were intermediate). Net tow sampling of Strait of Georgia populations showed a similar significant narrowing of cohorts in the 2003–2005 sampling period. This study provides evidence that in addition to the shift to an earlier occurrence of peak biomass reported previously, the duration of the peak has also decreased in the last decade.

KEY WORDS: Neocalanus plumchrus · Copepod · Biomass · Continuous Plankton Recorder

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Cite this article as: Batten SD, Mackas DL (2009) Shortened duration of the annual Neocalanus plumchrus biomass peak in the Northeast Pacific. Mar Ecol Prog Ser 393:189-198.

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