The effect of ultraviolet B on phytoplankton populations in clear and brown temperate Canadian lakes

Beaverskin and Pebbleloggitch lakes are located 2 km apart in Kejimkujik National Park (KNP) and share many limnetic characteristics: pH 5.0, shallow depth, and low nutrient concentrations. The lakes differ mainly in DOC concentrations, as Pebbleloggitch has 13 mg L 21 and Beaverskin 3.5 mg L 21 . Consequently, 95% ultraviolet B (UVB) extinction occurs in relatively clear water at 50 cm, whereas in brown water it occurs at 4 cm below surface. Two treatments (UV-B excluded with Mylar and UV-B exposed) were used with experimental enclosures in the two lakes. In each lake, nine replicates of both treatments were sampled every 2 weeks from 2 July to 14 August 1996. Beaverskin lake phytoplankton differed fundamentally from that of the brown-water Pebbleloggitch. The community in Beaverskin was relatively simple, consisting of few taxa, mainly cyanobacteria, dominated by Merismopedia tenuissima. Pebbleloggitch, in contrast, hosted many taxa, from all major algal divisions, e.g., Chlamydomonas angulosa, Mougeotia spp., Cryptomonas czosnowskii, Spherocystissp., and Tabellaria quadriseptata. Phytoplankton species composition and cell densities in Beaverskin Lake did not differ significantly between the exposed and Mylar covered enclosures, except in the final week of collection. In contrast, phytoplankton populations in covered enclosures in Pebbleloggitch Lake were sharply different from those that were not covered. We attribute differences in response to the influence of the brown waters of Pebbleloggitch where the higher rates of absorbance of light by the brown waters results in slower mixing and greater damage by UV-B radiation. This conclusion is in contrast to the generally accepted view that brown waters provide protection for phytoplankton.