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Abstract
Lake Okaro is a eutrophic lake on the central volcanic plateau of North Island, New Zealand. Its hypolimnion has become progressively depleted of dissolved oxygen between 1955 and 1964. Since then hypolimnetic anoxia has occurred annually. Secchi disc transparency changed little since 1955. Higher values in 1977/78 and 1983/84 appeared to be caused by greater proportions of the cladoceran Ceriodaphnia dubia in the zooplankton. Despite large interannual changes in biological and chemical properties of Lake Okaro, trophic state has changed since 1955. The phyto‐plankton species composition changed from no cyanobacteria in 1955/56 to persistent cyanobac‐terial blooms in 1963 and abundant and extensive blooms of Anabaena spiroides and A. flos‐aquae in 1979/80. Elodea canadensis dominated the macro‐phyte assemblage and its abundance has changed little since 1963. Zooplankton numbers decreased between 1979/80 and 1985/86 because of a decline in the calanoid copepod population. The zooplankton assemblage, dominated by rotifers in 1979/80, was crustacean‐dominated in 1982/83. Eutrophi‐cation coincided with a higher proportion of chi‐ronomids, particularly the littoral Polypedilum pavidus, which became abundant in the presence of cyanobacterial blooms.
