Disruption of littoral algal associations by experimental lake acidification

The effects of lake acidification on algal associations in the littoral zone were examined at the Experimental Lakes Area, northwestern Ontario. Net photosynthesis of periphyton declined, partly because of increased carbon limitation, as Lake 302S was experimentally acidified with sulfuric acid from pH 6.7 to 4.5. The ratio of respiration to photosynthesis increased in epilithon (periphyton on rock surfaces) as pH declined, largely because respiration increased rapidly. Changes in Lake 302N, which was acidified with nitric acid, were similar to those in Lake 302S. Major changes also occurred in epilithic algal composition, especially as pH decreased below 5. Abundance of filamentous green algae increased as pH decreased below 6. The filamentous green algae became metaphyton (no longer attached to the substratum) and bloomed at pH 5.5 and below; Zygogonium dominated at pH 4.5. Metaphytic filamentous green algae sometimes covered nearly the entire littoral zone bottom during blooms. In contrast to epilithon, metaphytic filamentous green algae did not display an increasing respiration to photosynthesis ratio with declining pH. The functional changes seen in littoral algal associations resulting from acidification, among the most sensitive ones known in aquatic ecosystems, became more conspicuous as acidification proceeded.