Phytoplankton from the hypolimnetic biomass peak in Lake 266SW (Experimental Lakes Area, northwestern Ontario) was subjected to conditions of increased light or phosphorus in continuous culture. Phosphorus enrichment increased total carbon fixation 16% relative to the control; however, the particulate fraction was initially depressed and the phytoplankton biomass was less than 2% greater after 10 d. The high light culture displayed 21% higher final biomass despite a 13% decline in total carbon fixation which was primarily due to reduced extracellular release. The 21 most common species demonstrated a wide range of positive and negative biomass responses to the experimental perturbations; however, the three dominant chrysophyte species all responded positively to higher light. Detection of population biomass changes provided a direct and accurate means of quantifying perturbation effects provided the species response was stable over several days. Measurement of carbon uptake rates of species through track autoradiography provided the additional sensitivity and temporal resolution needed to monitor low level or variable responses. Synedra radians exhibited a lengthy lag period before demonstrating a strong positive response to phosphorus enrichment, while Dinobryon sertularia shifted from neutral to strongly negative and then strongly positive within a 3-d period. These results illustrate the inadequacy of short-term bioassays for the prediction of long-term perturbation effects.Key words: growth rate, primary production, Ankistrodesmus falcatus, Dinobryon sertularia, Synedra radians
Actinide bioimaging in tissues: Comparison of emulsion and solid track autoradiography techniques with the iQID camera
