No abstract

No abstract

Ultraviolet-B (UVB, 280-320 nm) radiation is a natural component of sunlight that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (Ͻ240 m) was studied in a mesocosm experiment for 7 d under varying conditions of UVB radiation: UVB excluded, natural radiation, and UVB enhanced at two different levels. The dynamics of several populations at different trophic levels comprising heterotrophic bacteria (Ͻ1 m), heterotrophic flagellates (2-10 m), small phytoplankton (Ͻ5 m), large phytoplankton (5-20 m), and ciliates (15-35 m) were monitored during the experiment. Enhanced UVB provoked a significant decrease in the number of ciliates (66%) and large phytoplankton (63%) relative to natural UVB conditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly limit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a positive feedback between enhanced UVB radiation and prey abundances, shown by increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon partitioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, respectively) under enhanced UVB exposure was balanced by an increase in the carbon biomass of heterotrophic bacteria (48%), heterotrophic flagellates (126%), and autotrophic flagellates (162%). As a manifestation of enhanced UVB at the community level, the ecosystem develops toward a microbial food web in preference to an herbivorous food web. Thus, enhanced UVB radiation can change the structure and dynamics of the pelagic food web.The pelagic planktonic community functions through a web of energy and nutrient exchanges mediated by a diverse array of producers and consumers, which ultimately depend on the energy supplied by sunlight. Following the discovery of stratospheric ozone depletion (Farman et al. 1985) and the resulting increase in intensity of biologically harmful UVB radiation (280-320 nm) reaching Antarctic waters, the majority of UVB studies have focused on phytoplankton be- AcknowledgmentsWe thank F. Rassoulzadegan, F. Azam, and T. Sime-Ngando for comments; C. Lovejoy and L. Bérard for help with the identification of some planktonic species; and D. Bourget and N. Lafontaine for nutrient analyses.This work was supported by NSERC of Canada, Fonds FCAR of Québec, and FODAR (University of Québec). International collaboration was made possible by NATO collaborative research grant (CRG 95139) to S.D. and P.M. This investigation is a contribution to the research programs of the Groupe de Recherche en Environnement Côtier.

A natural planktonic assemblage from the St. Lawrence Estuary was isolated in eight 1,500-liter outdoor mesocosms and subjected to combinations of fast or slow mixing regimes with natural solar radiation or natural solar radiation artificially enhanced with ultraviolet-B (UVB, 280-320 nm) radiation. The interdependent evolution of dissolved organic carbon (DOC), absorption by chromophoric dissolved organic matter (CDOM), chlorophyll a (Chl a), particulate organic carbon (POC), and bacterial abundance in the mesocosms was followed over a 10-d period. There was a net increase of Chl a, POC, and DOC in all systems over time; however, the slower mixing treatments had less accumulation than the systems with faster mixing. All systems displayed weak correlations of DOC with POC and Chl a. A significant effect of enhanced UVB radiation on concentrations of these bulk properties was not observed in any of the mesocosms. A strong correlation of CDOM absorbance loss (photobleaching) with absorbed radiation dose was observed in all treatments, with the fast mixing systems having larger absorbance losses and faster loss rates. Photobleaching was wavelength dependent, resulting in an increase in the spectral slope of CDOM absorption over time. Thus, although CDOM photobleaching may result in deeper penetration of light at all wavelengths, the ratios of UVB to ultraviolet-A (UVA) and photosynthetically active radiation (PAR) are reduced. The effect of enhanced UVB radiation was unexpected, with no proportional increases in CDOM photobleaching in the ϩUVB treatments. Comparisons of the different treatments indicate that interactions of biological activity, mixing, and the in situ light field can influence CDOM absorbance properties and/or photoreactivity and that there is a possible role for UVB in the production of CDOM.Numerous recent reports indicate that dissolved organic matter (DOM) plays a more active role in the biogeochemical carbon cycle than had previously been thought and that its fate in the ocean is intimately related to photochemical processes in the upper water column (Miller 1994). Photo-