Effect of UVB radiation on ecosystems of selected lakes in the Canadian High Arctic

Abstract

Two studies on the effects of enhanced ultraviolet-B radiation (UVB; 280--320 nm) on planktonic organisms of Canadian High Arctic lakes are presented. In the first study, the long-term effects of a moderate increase in UVB levels on the planktonic community of a lake were evaluated using in situ mesocoms. Four mesocosms (3 m square and 3 m deep) were placed in Two Basin Lake, a small lake (14.2 ha) located on Ellesmere Island (79°55.5'N, 84°40'W; Nunavut, Canada). For 27 days, two mesocosms were exposed to full sunlight (including ambient UVB) while two others were exposed to sunlight plus artificially enhanced UVB. Chlorophyll a, zooplankton mean length and carbon allocation into macromolecular constituents were not affected by enhanced UVB. Phytoplankton productivity displayed diverse and inconsistent responses to enhanced UVB. Picocyanobacteria abundance decreased in the enhanced UVB mesocosms, but only at the surface. Enhanced UVB generally increased heterotrophic bacterial abundance and activity. Heterotrophic nanoflagellates and zooplankton abundances increased in the enhanced UVB treatment after 14 days. The cladocerans and rotifers were positively affected by UVB, while the copepods were negatively affected. The high levels (≥ 5 mg L -1) of dissolved organic carbon (DOC) in this lake combined with vertical mixing generally protected the planktonic community from direct damage by enhanced UVB. It is hypothesized that UVB may have indirectly stimulated the microbial food web and the rest of the food chain through increased photodegradation of high molecular weight refractory dissolved organic matter into more bioavailable nutrients. In the second study, the short-term (2 to 24 h) effects of enhanced UVB on carbon uptake rates, photosynthetic fractionation into three size classes (picoplankton [0.2-2 mum], nanoplankton [2-20 [mum] and netplankton [> 20 mum]) and carbon assimilation into the four main end-products (low molecular weight metabolites [LMW], lipid, polysaccharide and protein) were assessed for nine lakes located near Resolute (74°15'N, 94°50'W) on Cornwallis Island (Nunavut, Canada). These lakes have low DOC levels (≤ 2 mg L-1). For each lake, 14C-inoculated water samples were exposed to 6, 25, 50 and/or 100% surface irradiance levels (E o) under natural solar radiation (including ambient UVB) or solar radiation plus artificially enhanced UVB. (Abstract shortened by UMI.)