Abstract
Photoacclimation of picophytoplankton was studied in the mixed layer of 3 stations in the central Cantabrian Sea (southern Bay of Biscay). Picophytoplankton chl a: carbon ratios (theta) presented minimum values during summer, when irradiance, temperature, and biomass of prokaryotes reached maximum values and inorganic nutrient concentrations were low. Conversely, the maximum theta were reached during winter, coincident with lowest annual irradiance but maximum concentration of inorganic nutrients and higher relative biomass of eukaryotes. Changes in theta were modeled using irradiance as an independent variable. Exponentially decreasing functions of theta with irradiance were significant only when the mean temperatures in the mixed layer were above 14 degrees C. These functions presented light- saturated minimum ratios (theta min) that decreased linearly with temperature and low- light maximum ratios (theta max) that increased exponentially with temperature. Such relationships were used to establish an empirical model that reproduced the seasonality of picophytoplankton theta in the mixed layer, with minima in summer and maxima in winter. A maximum potential theta(r)theta(Nr) (T-max), was determined to estimate picophytoplankton growth rates in the central Cantabrian Sea. Combinations of picophytoplankton growth rates and biomass in the mixed layer were used to estimate areal picophytoplankton primary production rates in the euphotic zone that presented a bimodal seasonal cycle, with maxima in late winter (ca. 100 mg C m(-2) d(-1)) and in late autumn (> 200 mg C m(-2) d(-1)) and mean annual values around 120 mg C m(-2) d(-1).