Comparison of radiocarbon uptake and DCMU-fluorescence techniques in evaluating dispersed oil effects on phytoplankton photosynthetic activity

Abstract

In a mesocosm study of dispersed-oil effects on cold seawater natural phytoplankton, two methods were compared for assessing algal photosynthesis. The radiocarbon ( 14C) uptake method seemed to be affected by elevated heterotrophic activity caused by a bacterial increase a few days after oil addition and by the presence of small dispersed oil droplets retained on the filters. These artefacts are thought to be the cause of an apparent stimulation of the specific photosynthetic rate (P/B, or photosynthetic rate per unit algal biomass) following oil contamination, although actual photosynthetic rates declined in all cases after contamination. Such artefacts did not affect the DCMU-fluorescence response, showing a decrease in photosynthetic ability that followed the decline in chlorophyll a concentration and was very sensitive, whatever the initial algal biomass. The latter technique thus seems more appropriate for assessment of deleterious effects on phytoplankton photosynthesis, especially in oligotrophic environments. Addition of dispersed crude oil at concentrations of the order of 10 mg 1 −1 in April and 4 mg l −1 in December resulted in a decline in phytoplankton biomass and photosynthetic ability. Progressive dilution of the contaminated environment helped recovery from toxic effects after 1 week.