Seasonal and latitudinal dependencies of phytoplankton carbon-to-chlorophyll a ratios: results of a modelling study

A realistic description of acclimation of the growth rate and carbon-to-chlorophyll a ratio (C:chl a ) of phytoplankton to irradiance, nitrate concentration and temperature was incorporated into a l-dimensional model of phytoplankton production dynamics. The properties of the model's steadystates are described as a guide to its behaviour and to parameter sensitivity. Seasonal cycles of chlorophyll a and C :chl a within the upper 200 m of the ocean were predicted at latitudes ranging from 0" to 60" N. Although limited to a consideration of physiological acclimation of a single taxon, our results compared well with observations. Predicted values of C : chl a ranged from 20 to >l60 g C g-l chl a. Lowest values of C :chl a were predicted for the top of the nutricline within the seasonal thermocline in mid-summer. Highest values of C :chl a were predicted for the nutrient-depleted surface mixed layer in mid-summer. The seasonal range of C :chl a was greatest in tropical and subtropical waters and least at 60°N. Predictions of the vertical distribution of C:chl a at 20°N were consistent with published observations for the subtropical North Pacific Ocean. Predictions of the relationship between C : chl a and chlorophyll a In surface waters showed qualitative agreement with published observations for the North Atlantic Ocean. The value of C:chl a within the surface mixed layer depends on the mean irradiance within the surface mixed layer and the extent of draw-down of the limiting nutrient (nitrate) from winter/spring maxima. A balance between phytoplankton growth and loss to grazing led to quasisteady-state conditions in the mixed layer In summer. Parameterisation of grazing was a critical determinant of summer C:chl a. Another cntical parameter was the physiologically determined minimum value of C : chl a. Implications of physiological acclmation of C : chl a to our .understanding of the role of phytoplankton in the ocean carbon cycle are discussed.

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