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Marine Ecology Progress Series

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MEPS 440:27-40 (2011)  -  DOI:

Size-dependent photosynthetic variability in the North Pacific Subtropical Gyre

Binglin Li1, David M. Karl1, Ricardo M. Letelier2, Matthew J. Church1,*

1Department of Oceanography, University of Hawaii, 1000 Pope Road, Honolulu, Hawaii 96822, USA
2College of Oceanic and Atmospheric Sciences, Oregon State University, 104 COAS Administration Building, Corvallis, Oregon 97331, USA

ABSTRACT: Photosynthesis-irradiance (P-E) relationships and in situ measurements of primary production for 2 phytoplankton size fractions (>2 µm and 0.2 to 2 µm) were used to evaluate variability in photophysiology over a 5 yr period (2004 to 2009) in the North Pacific Subtropical Gyre (NPSG). Picophytoplankton (0.2 to 2 µm) were dominant contributors to euphotic zone chlorophyll a (chl a) concentrations (averaging 91 ± 2% [mean ± SD] of the 0 to 125 m depth-integrated inventories) and accounted for a major fraction (averaging 74 ± 7%) of the in situ, depth-integrated, 14C-based primary production. Short-term in vitro P-E experiments were conducted to examine the photophysiology of both phytoplankton size classes. Results from these experiments demonstrated that in the well-lit ocean (0 to 45 m) chl a normalized maximum rates of photosynthesis, Pchlmax, were significantly greater among the larger phytoplankton size class than in the smaller size fraction (1-way ANOVA, p < 0.01), while in the dimly lit region (125 m) there were no significant size-dependent differences in Pchlmax (1-way ANOVA, p > 0.05). Neither the initial slope of the P-E ­relationships, α, nor the light intensities required to saturate photosynthesis, Ek, varied significantly between the 2 size fractions. Although larger phytoplankton appear to constitute a relatively small fraction of phytoplankton biomass and production in this ecosystem, the photophysiological responses of plankton in this size class demonstrated considerable variability, suggesting these larger size phytoplankton experience time-variable changes in growth despite persistently oligotrophic habitat conditions.

KEY WORDS: Photosynthesis · Photophysiology · Phytoplankton · Primary production· North Pacific · Oligotrophic

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Cite this article as: Li B, Karl DM, Letelier RM, Church MJ (2011) Size-dependent photosynthetic variability in the North Pacific Subtropical Gyre. Mar Ecol Prog Ser 440:27-40.

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