SEASONAL AND LATITUDINAL DEPENDENCIES OF PHYTOPLANKTON CARBON-TO-CHLOROPHYLL A RATIOS - RESULTS OF A MODELING STUDY

A realistic description of acclimation of the growth rate and carbon-t o-chlorophyll a ratio (C:chl a) of phytoplankton to irradiance, nitrat e concentration and temperature was incorporated into a 1-dimensional model of phytoplankton production dynamics. The properties of the mode l's steady-states are described as a guide to its behaviour and to par ameter sensitivity. Seasonal cycles of chlorophyll a and C:chl a withi n the upper 200 m of the ocean were predicted at latitudes ranging fro m 0 degrees to 60 degrees N. Although limited to a consideration of ph ysiological acclimation of a single taxon, our results compared well w ith observations. Predicted values of C:chl a ranged from 20 to >160 g C g(-1) chi 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 mi xed layer in mid-summer. The seasonal range of C:chl a was greatest in tropical and subtropical waters and least at 60 degrees N. Prediction s of the vertical distribution of C:chl a at 20 degrees N were consist ent with published observations for the subtropical North Pacific Ocea n. Predictions of the relationship between C:chl a and chlorophyll a i n surface waters showed qualitative agreement with published observati ons for the North Atlantic Ocean. The value of C:chl a within the surf ace mixed layer depends on the mean irradiance within the surface mixe d 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 laye r in summer. Parameterisation of grazing was a critical determinant of summer C:chl a. Another critical parameter was the physiologically de termined minimum value of C:chl a. Implications of physiological accli mation of C:chl a to our understanding of the role of phytoplankton in the ocean carbon cycle are discussed.