Characteristics of three acidic lakes in Kejimkujik National Park, Nova Scotia, Canada.

This report summarizes a study of the chemical and biological characteristics of three oligotrophic lakes located in a region that receives a moderately acidic precipitation (mean annual pH 4.5-4.6), and a sulfate deposition of about 20 kg/ha/yr. The two brownwater lakes are relatively acidic (pH 4.5 and 4.8), and much of their acidity is attributable to organic anions. The brownwater lakes also have a large concentration of aluminum and iron, but these are bound to dissolved organic matter and are relatively non-toxic to biota. Average phytoplankton production was largest in the clearwater lake. This was due to its relatively deep euphotic zone, since the average unit-volume productivity did not differ much among the lakes. In fact, productivity at light optimum was largest in the most acidic brownwater lake, probably because of its larger phosphorus concentration. The clearwater lake had extensive macrophyte vegetation, which covered its bottom to a depth of 6.5 m. In the brownwater lakes, macrophytes were confined to shallow nearshore water because of the limited water transparency. Zooplankton density and biomass were largest in the most acidic brownwater lake, probably because of allochthonous organic particulates and little fish predation. Benthic invertebrates were abundant in all three lakes, and were dominated by insects, especially Chironomids. Lakes in the study area appear to be sustaining fish populations at more acidic pHs than elsewhere. This may be due to the large concentration of dissolved organic matter in many lakes, which complexes and partially detoxifies metals such as aluminum.