LIGHT AND PRODUCTIVITY OF PHYTOPLANKTON IN POLAR MARINE ECOSYSTEMS - A PHYSIOLOGICAL VIEW

This study deals with the modelling of photosynthesis and growth of polar phytoplankton and variations in relevant parameters. Polar regions arc characterised by low sun elevations (< 40-50-degrees), extreme seasonal variations in irradiance and day length, and low sea temperatures (-1.8 to 6-degrees-C). Due to the latter, maximum phytoplankton growth rates are low (< 0.6 d-1). Light absorption by phytoplankton is strongly dependent on spectral composition (blue oceanic versus green coastal waters), but absorption characteristics (and thus chlorophyll a-normalised photosynthetic efficiency alpha(B)) do not differ appreciably between polar and other phytoplankton. The maximum chlorophyll-normalised photosynthetic rate P(m)B is, however. lower and dependent on the irradiance to which the cells are adapted. Chla:C ratios vary widely. but within ranges known for other phytoplankton. The carbon-normalised coefficient P(m)C varies little with irradiance, but is clearly dependent on day length and nutrient supply. The corresponding coefficient alpha(C) is somewhat higher in shade-adapted than in light-adapted cell, Polar species exhibit a high tolerance for strong light and long days in combination with low temperature relative to other species. The interpretation of P-I functions is discussed, and an empirical formulation is suggested that does not need the Chla:C ratio for predicting thc light-limited gross growth rate of polar phytoplankton. Mathematical simulations of the spring bloom indicate that the depth of thc mixed layer and the attenuation of light are the most important variables for determining thc photosynthetic rate. The spectral composition of light is of particular importance in low light, e.g. in deeply mixed layers. Generally, the deeper the mixing, the more sensitive thc development of a spring bloom becomes to any algal or environmental variable.