AME 49:267-279 (2007)  -  DOI: https://doi.org/10.3354/ame01151

ABSTRACT: Autotrophic picoplankton (cells <2 µm) have been comparatively little studied in coastal waters. I present here the seasonal variation of picophytoplankton photosynthetic parameters, primary production and growth rates determined using the ¹⁴C method at a shelf station in the southern Bay of Biscay sampled monthly during 2003. Picophytoplankton chlorophyll a (chl a) (11.5 ± 1.7 mg m^–2, mean ± SE) amounted to a mean of 42% (6 to 63%) of euphotic-layer integrated total values. Although prokaryotic cyanobacteria (Synechococcus and Prochlorococcus) generally outnumbered eukaryotic cells, picoeukaryotes were the greatest contributors to total picophytoplankton biomass (54 to 914 mg C m^–2) for most of the year. Mean C:chl a ratios ranged from 7 to 89. The initial slope α^B (0.01 to 0.11 mg C mg chl a^–1 [µmol photons m^–2 s^–1]^–1) was always higher at depth than at the surface, while the maximum chl a-normalized photosynthetic rate P^B_m (0.6 to 17.4 mg C mg chl a^–1 h^–1) was only significantly higher at the surface during summer, with vertical differences positively correlated to stratification. In contrast, maximum growth rates (μ_max, 0.1 to 3.1 d^–1) were remarkably similar in surface and deep samples, with maxima in spring and minima in summer and autumn. μ_max values were significantly correlated with the relative abundance of picoeukaryotes, indicating their prevalence over prokaryotes in coastal ecosystems. Integrated picophytoplankton primary production ranged from 35 to 345 mg C m^–2 d^–1 and accounted for a mean annual value of 51% (4 to 76%) of total rates, thus confirming the essential role of small cells in the carbon flux of this temperate coastal ecosystem.

KEY WORDS: Picophytoplankton · Photosynthetic parameters · Primary production · Growth rates · Seasonal variability · Coastal waters