MICROBIAL FOOD-WEB RESPONSES TO PHOSPHORUS SUPPLY AND SOLAR UV-RADIATION IN A SUB-ARCTIC LAKE

We examined the effects of solar ultraviolet radiation (UVR) and phosp horus enrichment on microbial food web structure in a subarctic oligot rophic lake (Lac l'Eau Claire, Canada; latitude 56 degrees 10' N, long itude 74 degrees 30' W) using replicated, 201 polyethylene containers incubated in situ. The experiments were run for between 6 and 9 d at t he surface of the lake at 3 times during the summer open water period. Chlorophyll a (chi a) production of the >2 pm fraction of the phytopl ankton community was stimulated by P; this response was reduced on ave rage by 31% in the presence of UV-A, and by an additional 29% in the p resence of UV-B. Conversely the <2 mu m chl a fraction declined with P enrichment, but to a lesser extent in the presence of UV-B. Cell coun ts of picocyanobacteria showed a decrease under P enrichment, with sig nificantly greater declines induced by UV-A and additionally by UV-B. The nanoflagellate community increased in cell concentration in respon se to P addition, but only in the absence of UV-A. Bacterial cell conc entrations in the P enrichments remained close to or slightly above (u p to 20 %) the initial values, but sharply declined in the treatments without P addition; the positive effect of enrichment was not influenc ed by UV-A or by UV-B. This diverse array of responses implies that sh ifts in underwater UVR caused by chancres in spectral penetration thro ugh the atmosphere (ozone depletion) or water column (changes in disso lved organic carbon) will influence microbial trophic structure as wel l as productivity within subarctic lakes. Such effects, however, will be limited to aquatic environments that are naturally exposed to high UVR such as shallow water columns or surface waters during diurnal str atification.