Autoradiographic detection of species-specific thermal stress effects on natural phytoplankton assemblages

Species-specific photosynthetic responses to stress were detected in mixed phytoplankton assemblages using autoradiography. In laboratory experiments subjecting a mixture of the diatom Phaeodactylum and the flagellate Pseudoisochrysis to control and transient thermal stress treatments of +5, +10, +15 and +20°C, both species responded negatively with Phaeodactylum appearing more sensitive. Whole community measures of stress ( 14C uptake and DCMU-fluorescence) showed alterations in the photosynthetic response of the two-species mixtures with increasing ΔT, but did not sort out species-specific differences. In two field trials at an estuarine power plant, after entrainment through the plant's cooling system, the photosynthetic rate of a diatom, Cyclotella sp., was reduced whereas the photosynthetic rate of a thecate dinoflagellate, Prorocentrum minimum, was enhanced. Field DCMU-fluorescence measurements of stress failed to detect the observed species-specific entrainment effects.