MEPS 562:1-18 (2016)  -  DOI: https://doi.org/10.3354/meps11954

FEATURE ARTICLE
Temporal and vertical variability in picophytoplankton primary productivity in the North Pacific Subtropical Gyre

Yoshimi M. Rii1,2,*, David M. Karl1,2, Matthew J. Church1,2,3 

1Department of Oceanography, University of Hawai‘i at Mãnoa, Honolulu, HI 96822, USA
2Daniel K. Inouye Center for Microbial Oceanography: Research and Education, University of Hawai‘i at Mãnoa, Honolulu, HI 96822, USA
3Present address: Flathead Lake Biological Station, University of Montana, Polson, MT 59860, USA
*Corresponding author:

ABSTRACT: Picophytoplankton (≤3 µm) are major contributors to plankton biomass and primary productivity in the subtropical oceans. We examined vertical and temporal variability of picophytoplankton primary productivity at near-monthly time scales (May 2012–May 2013) in the North Pacific Subtropical Gyre (NPSG) based on filter size-fractionated and flow cytometric sorting of radiolabeled (14C) picoplankton cells. Primary productivity by picophytoplankton comprised ~68 to 83% of total (>0.2 µm) particulate 14C-based productivity, and was lowest between September and December and highest between March and August. Group-specific rates of production by Prochlorococcus, Synechococcus, and photosynthetic picoeukaryotes (PPE) averaged ~39, ~2, and ~11% of the total 14C-productivity, respectively. Average cell-specific rates of production by PPE (15.2 fmol C cell-1 d-1) were 25- to 90-fold greater than Prochlorococcus (0.36 fmol C cell-1 d-1) and Synechococcus (1.56 fmol C cell-1 d-1). Prochlorococcus dominated (61-78%) the summed picophytoplankton biomass, while PPE and Synechococcus contributed 21-36% and 2-8%, respectively. Rates of production normalized to biomass were nearly equivalent amongst Prochlorococcus, Synechococcus, and PPE, averaging 0.6, 0.5, and 0.4 d-1, respectively. Over our study period, Prochlorococcus and PPE production varied 4- to 5-fold, while biomass varied ~3-fold. In contrast, Synechococcus production varied ~30-fold, with peak rates in March 2013 accompanied by ~13-fold increase in biomass. Combined, our results provide evidence for rapid growth by picophytoplankton in this persistently low-nutrient ecosystem, highlighting the importance of cell loss processes responsible for mediating organic matter cycling in the euphotic zone of the NPSG.


KEY WORDS: Picoplankton · 14C primary production · Picophytoplankton · Picoeukaryotes · Prochlorococcus · Synechococcus · Flow cytometry · Time series · North Pacific


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Cite this article as: Rii YM, Karl DM, Church MJ (2016) Temporal and vertical variability in picophytoplankton primary productivity in the North Pacific Subtropical Gyre. Mar Ecol Prog Ser 562:1-18. https://doi.org/10.3354/meps11954

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