Values of net compensation irradiation and their dependence on photosynthetic efficiency and respiration in marine unicellular algae

Daily compensation irradiation for net photosynthetic rates (ΣI _comp) of Skeletonema costatum (Greville) Cleve, Chaetoceros ceratosporum Ostenfeld, Nitzschia sp., Thalassiosira nordenskiöldii Cleve, and Chroomonas salina (Wislouch) Butcher were measured during 1979 to obtain values for use in ecological models describing compensation and critical depths of marine phytoplankton. Batch cultures of these unicellular algae were exposed to temperatures and photoperiods varying from 6° to 15°C and 8.4 to 16.0 h, conditions typical of surface water in Saanich Inlet, a fjord in Vancouver Island, British Columbia, Canada. Results obtained with S. costatum and T. nordenskiöldii provided estimates of ΣI _comp varying between 1.8 and 13 J cm^-2 d^-1, from which a mean value of 7.0 J cm^-2 d^-1 was calculated for use in ecological models with neritic phytoplankton. No seasonal variation in compensation irradiation was noted because photosynthetic efficiencies, which increased as division rates increased, were balanced by respiration rates, which increased as temperature increased. Results obtained with Chaetoceros ceratosporum, Nitzschia sp., and Chroomonas salina were difficult to interpret, because respiration rates were stimulated and depressed by light, respectively, for the first two species and the last one. This light effect was greatest when cells grew under conditions of low temperature and short photoperiod. Also, under winter and spring conditions, cells of Nitzschia sp. appeared to fix CO₂ in the dark and with low irradiances by mechanisms other than photosynthesis.