Résumé
The present study demonstrates the effects of the water-soluble fraction (WSF) of a crude oil, enhanced ultraviolet-B radiation (UVBR: 280 to 320 nm), and the combination of WSF and enhanced UVBR on a natural plankton assemblage (<150 μm) isolated from the lower St. Lawrence Estuary. To study the separate and dual effects of WSF and UVBR, 12 microcosms (9 l) were immersed in the water column of larger mesocosms (polyethylene bags; 1800 l), providing 4 treatments, each in triplicate: (1) NUVBR + WSF (natural UVBR with WSF), (2) HUVBR + WSF (enhanced UVBR with WSF), (3) NUVBR (natural UVBR without WSF), and (4) HUVBR (enhanced UVBR without WSF). During 5 d we monitored the incident radiation, WSF and nutrient concentrations, abundance and production of heterotrophic bacteria and phytoplankton. Strong deleterious effects of WSF and lower effects of UVBR were observed on the phytoplankton assemblage, with a decrease in growth rates accompanied by an increase in mean cell size which reflected a perturbation of the cell division cycle. Using the NUVBR treatement as reference conditions, the above effects resulted in a reduction of 84, 79 and 60% of total abundance of the phytoplankton fraction <20 μm in the HUVBR + WSF, NUVBR + WSF and HUVBR treatments, respectively. Significant higher values of bacterial abundances were observed in the WSF-added treatments compared to NUVBR without WSF. However, bacterial thymidine incorporation exhibited diel variations, suggesting cumulative UVBR-induced DNA and/or PAH-induced DNA damages, and possible repair mechanisms with the co-occurrence of more available growth substrates from stressed phytoplankton. The absence of significant differences between both WSF-added treatments under the 2 different UVBR conditions suggests that there is no additive interaction between WSF and UVBR. This study provides therefore the first evidence of a non-synergistic interaction between both stresses, and suggests that UVBR-induced effects on marine microorganisms can be completely masked by the strong deleterious effects of soluble petroleum hydrocarbons.