Weekly observations at a sampling station in Storm Bay, south-east Tasmania (43°10'S, 147°32'E) over a period of 4 years from March 1985 to March 1989 have shown a 2.5°C warming of sea surface temperature (SST) since the cool, windy summer of 1986–87. The sampling station lies close to the northern edge of the subtropical convergence (STC) so that warm years were characterized by an increased north-easterly, subtropical influence and cool years by an increased south-westerly, subantarctic influence. Intrusions of subtropical water could be detected by increased salinities and very low dissolved inorganic phosphorus The increase in subtropical influence was coincident with a reduction in the westerly wind stress in the period 1987–89. As SST increased, the depth of winter mixing decreased in the offshore oceanic waters Interannual variability in the westerly wind stress was only observed as a change in the power of a significant 40 day oscillation in the wind Mean wind speeds did not change from year to year Calm years were marked by a reduction in the power of the 40 day oscillation in the westerlies. Phosphate concentrations in Storm Bay in the summer of 1988–89 were one-half of the concentrations measured in 1985 Winter nitrate concentrations in 1988 were also one-half of average Total nitrogen, phosphorus and dissolved organic carbon concentrations in surface waters fell steadily throughout the sampling period. Pulses of algal growth (measured as chlorophyll) followed peaks in the 40 day wind oscillation and resulted from the resuspension of nutrients regenerated by decomposition in bottom waters Windy periods (such as the summer of 1986–87) were marked by high nitrate and dissolved organic nitrogen concentrations in surface waters. Episodes of ‘new’ production therefore followed periods of increased westerly wind stress. There was thus a strong interaction between the time scales of wind events, nutrient uptake, phytoplankton growth, grazing, sedimentation, decomposition and resuspension Particle size data and data on the size fractionation of the zooplankton biomass indicated a strong trend towards more obgotrophic conditions in surface waters during the warm, calm period after 1987 The phytoplankton community was dominated by small flagellates, indicative of regenerated production rather than the large diatoms characteristic of new production in the cool, windy years. The very oligotrophic conditions of the summer of 1988–89 led to small copepods dominating, and the elimination of all large zooplankters (especially the euphausiid Nyctiphanes auslralis) from the zooplankton community. Nyctiphanes occurs in dense swarms on the shelf in Tasmanian coastal waters in years of high ‘new’ production (windy years) and supports the largest single-species fishery in Australia. The disappearance of Nyctiphanes in the summer of 1988-89 led to the collapse of this jack mackerel fishery because the fish could no longer be caught in purse seines in surface waters The entire chain of events appears to have been associated with the major La Nina ‘Cold Event’ of 1988. Long-term SST records (1945–88) from these waters indicate that these conditions have occurred before, most noticeably in the mid 1950s and the late 1970s.
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Harris et al. (1991) studied this question.