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Some effects of agricultural development and fluctuations in water level on the phytoplankton productivity and zooplankton of a New Zealand reservoir
Author(s) -
MITCHELL S. F.
Publication year - 1975
Publication title -
freshwater biology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.297
H-Index - 156
eISSN - 1365-2427
pISSN - 0046-5070
DOI - 10.1111/j.1365-2427.1975.tb00155.x
Subject(s) - phytoplankton , zooplankton , productivity , plankton , environmental science , daphnia , ecology , ecosystem , standing crop , nutrient , biology , biomass (ecology) , economics , macroeconomics
Summary In Lake Mahinerangi, a New Zealand hydro‐electric supply reservoir, phytoplankton productivity increased from an annual average of 76 mg earbon m −2 day^ −1 in 1964–66 to 210 mg carbon m −2 day −1 in 1968–70. It is likely that the increase was due partly to water levels being higher in 1968–70. Results of multiple regression analyses on the annual productivity curves for 1964–66 had suggested that water level, with temperature, was a major factor influencing the productivity at near‐optimal light intensity. In the interval between the two studies the first agricultural land‐development, amounting to 3% of the drainage basin, was carried out, and there is evidence that this development also contributed to the increase in phytoplankton productivity. Streams which drain the developed land were richer in phosphate and nitrate than the unmodified ones. Phosphate concentrations in the lake increased, but other chemical changes were slight. There were no major changes in the species composition of the phyto‐plankton. None of the three dominant species of zooplankton increased, but there were substantial increases in two less abundant taxa, Daphnia carinata King and Cyclopoida. The relative increase in Daphnia is consistent with the hypothesis that large zooplankton enjoy a competitive advantage over smaller species. Similar changes are noted from elsewhere.