Some effects of river discharges and currents on phytoplankton in the sea off Otago, New Zealand

Abstract

Major rivers on the south‐eastern coast, South Island, New Zealand create plumes that are deflected north between the Southland Current and the coast. Surveys of the continental shelf water mass off the Otago coast over a 2‐year period, May 1996‐April 1998, confirmed that when the Southland Current surfaced it contained less chlorophyll a than mixed water subject to terrestrial influence, implying that river‐borne nutrients stimulated primary production in the mixed water. The greatest boost occurred in winter when diatom abundance increased. The band of chlorophyll‐rich water extended through the photic zone and was several kilometres wide suggesting that riverine influence on marine biomass is substantial. In summer, the Southland Current often lay beneath oceanic water, with no increase in chlorophyll a concentration in the overlying water, due possibly to heavy grazing by zooplankton. In summer 1997–98, seawater temperatures were higher and phytoplankton biomass lower than in summer 1996–97, consistent with a strong El Niño event in the Pacific Ocean.

Keywords:

• Clutha
• river plume
• El Niño