Ecological effects of blooms of filamentous green algae in the littoral zone of an acid lake

Abstract

Ecological effects of blooms of filamentous green algae (FGA) were studied in an experimentally acidified lake (pH 4.5) at the Experimental Lakes Area in northwestern Ontario, Canada. Blooms of FGA influenced the energy balance, chemical cycling, physical features, and biological conditions in the littoral zone. Photosynthetic capacity of the FGA, dominated by Zygogonium, was greater than that of acidified epilithon, the normally dominant littoral algal association, partially offsetting acidification-induced oligotrophication in the littoral zone. Intra- and inter-annual variability of FGA growth was large, however, so that FGA were an unreliable energy source for the littoral food web. Nutrient uptake varied with the degree of FGA growth; e.g., FGA were occasionally the largest phosphorus pool in the epilimnion. Nitrogen dynamics of FGA varied seasonally causing acidification in spring and summer, and alkalinization in fall. The blooms also affected epilimnetic cycling of dissolved inorganic carbon. Peak blooms prevented as much as 90% of the light from reaching the lake bottom. Local depletions of oxygen resulting from FGA decomposition also posed potential risks for animals using the FGA as habitat.