Winter pelagic photosynthesis in the NW Mediterranean

There is a large gap in our knowledge of winter pelagic primary production in offshore Mediterranean waters, despite the widespread observation of phytoplankton blooming during that season. On two cruises performed in the NW region in March 1999 and January-February 2000, primary production was estimated by means of concurrent in situ incubations and photosynthesis-irradiance (P-E) experiments with the 14C technique. Although the water column was generally well-mixed below the euphotic zone, photosynthetic parameters displayed vertical differences within the mixed layer, indicating that photoacclimation rates were faster than mixing. Their values were similarly variable during both cruises, except for the maximum chlorophyll a-normalized photosynthetic rate (PmB), which was significantly higher in March. A strong relationship was found between the two methods of estimating integrated primary production rates (log P-E PPint=0.36+0.87 log in situ PPint; r2=0.88, p<0.001, n=18), which ranged from 0.1 to 2 g C m−2 d−1. March 1999 values (1.0±0.5 g C m−2 d−1) were notably higher than in other more intensively sampled periods. Water column chl a-normalized rates were similar on both cruises (mean 8.6 mg C mg chl a−1 d−1) despite great differences in phytoplankton biomass and composition, but were also positively related to incident irradiance. This finding, together with the marked seasonality of surface photosynthetic parameters obtained in previous surveys in the western Mediterranean, stresses the fundamental role played by irradiance in determining the photosynthetic response during the annual cycle.