Deep-Sea Re~earch. Vol. 38. No. 4, pp. 415--4311. 19ql. 0198-0140/91 $3.011 -*- 0.fin Prmted ill Great Bntam. ~ lt~O[ Pergamon Prexs plc Modeling of light-dependent algal photosynthesis and growth: experiments with the Barents Sea diatoms Thalassiosira nordenskioeldii and Chaetoceros furcellatus EGIL SAKSHAUG,* GEIR JOHNSEN,* KJERSTI ANDRESEN* and MARIA VERNETS" (Received 12 March 1990; in revised form 6 September 1990; accepted 1 October 1990) Abstract--The models by SAKSHAUG et al. (1989, Limnology and Oceanography. 34. 198-205) and WEBB et al.( 1974, Oecologia, 17, 281-291), for prediction of the gross growth rate of phytoplankton and short-term photosynthesis, respectively, have been modified on the basis of experiments with cultures of the centric diatoms Thalassiosira nordenskioeldii and Chaetocerosfurcellatus grown at 0.5°C at combinations of two irradiances (25 and 400~mol m -z s -t ) and two day-lengths (12 and 24 h). The models have one spectrum. *o. which represents chlorophyll a (Chin) specific absorption of photosynthetically usable light, and introduces a factor q which represents Chin per PSU. functionally defined. The models describe phytoplankton growth in terms of physiologically relevant coefficients. A properly scaled fluorescence excitation spectrum (°F) represents a more appropriate estimate for °tl than the Chin-specific absorption spectrum °a~ judging from calculations of c!%,,~ (=t~/°o). On the basis of °F. ~lJ,,,,~ is I).114 g-at C(mol photons)-i for gross growth and about 0.115-0.08 for short-term carbon uptake (unfiltered samples). Calculations based on *a~ yield values for 'l~m,,~ which on average are 44% lower. P vs I (photosynthesis vs irradiance) parameters are relatively independent of day-length and highly dependent on growth irr:tdiance. The product of q [mg Chin (mol PSU) -I] and r (the minimum turnover time of the photosynthetic unit, h) increases 2-3-fold from high to low irradiance, thus p u (=Cl~m~x/qr) and Ik (=l/qr°o) decreased. °F decreases from high to low irradiance. Carbon-specific dark respiration rates are <0.09 day- t. Pigment ratios vary inversely with irradiance and day-length. The Chin: C ratio is particularly low under high, strong continuous light; Chic:Chin ratios are higher for shade- than for light- adapted cells, while the converse is true for the ratio of the sum of the photoprotective pigments diadinoxanthin and diatoxanthin to Chin. The fucoxanthin : Chin ratio is virtually independent of the light regime. The two species are similar with respect to variations in growth rate (0.09--11.33 day- t ) and/~ (31-36 vs 49-1(X)l~mol m -z s-i at low and high irradiance, respectively). P~mand a a for growth as well as °F are systematically higher for C. furcellatus than for T. nordenskioeldii, while the product qr is lower. C. furcellatus is considerably more plastic than T. nordenskioeldii with respect to pigment composition. INTRODUCTION MATHEMATICAL models of algal photosynthesis and growth are important in the prediction of global- and regional-scale variations in marine primary productivity and are used in the "Trondhjem Biological Station. The Museum, University of Trondheim, Bynesveien 46, N-7018 Trondheim, Norway. * Polar Research Program, A-002, Scripps Institution of Oceanography, University of California at San Diego, La Jolla. CA 921193, U.S.A. 415