Phytoplankton in six hydroelectric lakes on the Waikato River, New Zealand, 1970–72

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Abstract
Phytoplankton were studied in six lakes (Aratiatia, Ohakuri, Atiamuri, Whakamaru, Waipapa, and Karapiro) on the Waikato River, New Zealand, 1970–72. The standing crop of phytoplankton showed a progressive increase as the Waikato River flowed northwards from Lake Taupo through eight dams to Lake Karapiro. Ths dominant algal species was Melosira distans (Ehrenb.) Kütz, with M. granulata var. angustissima O. Müll, and Asterionella formosa Hass. as the subdqminants. The increase in standing crop of the phytoplankton was associated with increase in the nitrogen and phosphorus loading on the river. The role of M. distans, M. granulata var. angustissima, and Asterionella formosa as indicators of trophic status of lakes is discussed. Application of the Shannon‐Weaver index of diversity showed that the index was very sensitive to changes in the relative abundance of M. distans and M. granulata var. angustissima, but was insensitive to changes in the relative abundance of the rest of the phytoplankton species. The analysis of trend showed a tendency for the algal standing crop to increase during the study period. This trend was most noticeable in those lakes with high nutrient inputs. A comparison of the chlorophyll standing crop value found in Lake Ohakuri to previously reported values confirmed this trend and showed that there had been a recent change in the algal species composition of the lake.

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