Relative importance of clarity determinants in Lakes Okaro and Rotorua

Abstract

Water clarity, as indicated by the beam attenuation coefficient, c, was measured monthly on 16 successive occasions during 1983–84 in the eutrophic Lakes Okaro and Rotorua. Measurements were also made of several water constituents including measures of both algal and non‐algal effects on c. Average and extreme clarities, as well as the temporal cycles of clarity, were similar in the two lakes. Relationships describing the dependence of c on chlorophyll a (B) and non‐standing crop‐related inorganic suspensoids (S) were derived for each lake. These accounted for most (80%) of the variation in c in each lake. When the relationships were used to calculate the relative contributions to predicted c (c) of both B and S, however, the results for the two lakes differed markedly. Algal biomass was the dominant influence on clarity in Okaro, accounting for 68% of coverall. In contrast, although c was occasionally dominated by B in Rotorua, both average and maximum (i.e., lowest clarity) values were dominated (40–62%) by inorganic suspensoids. Unusually high phaeopigment concentrations were found to covary with S in Rotorua. This indicated that wind‐related resus‐pension of bottom sediments was probably the source of the material which generally dominated the lake's clarity. In markedly smaller Lake Okaro, wind‐related mixing forces are unlikely to be significant. It is not clear how the supply of the clarity‐dominating inorganic suspensoids will be affected by the proposed reduction of sewage nutrient load at Lake Rotorua.