Annual cycle of picophytoplankton photosynthesis and growth rates in a temperate coastal ecosystem: A major contribution to carbon fluxes

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 [pmol 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.