AME 12:239-249 (1997)  -  doi:10.3354/ame012239

We examined the effects of solar ultraviolet radiation (UVR) and phosphorus enrichment on microbial food web structure in a subarctic oligotrophic lake (Lac à l'Eau Claire, Canada; latitude 56°10'N, longitude 74°30'W) using replicated, 20 l polyethylene containers incubated in situ. The experiments were run for between 6 and 9 d at the surface of the lake at 3 times during the summer open water period. Chlorophyll a (chl a) production of the >2 μm fraction of the phytoplankton community was stimulated by P; this response was reduced on average by 31% in the presence of UV-A, and by an additional 29% in the presence of UV-B. Conversely the <2 μm chl a fraction declined with P enrichment, but to a lesser extent in the presence of UV-B. Cell counts of picocyanobacteria showed a decrease under P enrichment, with significantly greater declines induced by UV-A and additionally by UV-B. The nanoflagellate community increased in cell concentration in response to P addition, but only in the absence of UV-A. Bacterial cell concentrations in the P enrichments remained close to or slightly above (up to 20%) the initial values, but sharply declined in the treatments without P addition; the positive effect of enrichment was not influenced by UV-A or by UV-B. This diverse array of responses implies that shifts in underwater UVR caused by changes in spectral penetration through the atmosphere (ozone depletion) or water column (changes in dissolved organic carbon) will influence microbial trophic structure as well 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 stratification.

Microbial food webs · Phosphorus · Polar · Subarctic lakes · UV