Light and productivity of phytoplankton in polar marine ecosystems:
a physiological view. Pp. 6S-85 in Sakshaug. E.. Hopkins. C. C. E. & Britsland. N. A. (eds.): Proceedings
of the Pro Mare Symposium on Polar Marine Ecology. Trondheim. 12-16 May 1990. folur Research 10(1).
This study deals with the modelling of photosynthesis and growth of polar phytoplankton and variations
in relevant parameters. Polar rcgions are chardctcriscd by low sun elevations (< 4k50"). extreme seasonal
variations in irradiance and day length. and low sea temperatures (- I .R to 6°C). Due to the latter. maximum
phytoplankton growth rates are low (< 0.6d-').
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 $) do not differ appreciably between polar and othcr phytoplankton. The maximum chlorophyllnormalised
photosynthetic rate Pt 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 carbonnormalised
coefficient P', varies little with irradiance. but is clearly dependent on day length and nutrient
supply. The corresponding coefficient a' is somcwhat higher in shade-adaptcd than in light-adapted cells.
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-l functions is discussed, and an empirical formulation is suggested that does not
need the Chla:C ratio for predicting the light-limited gross growth rate of polar phytoplankton. Mathematical
simulations of the spring bloom indicate that the depth of the mixed layer and the attenuation of
light are the most important variables for determining the 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, t