Seasonal and interannual variability in physical processes, nutrient cycling and the structure of the food chain in Tasmanian shelf waters

• Published in: Journal of plankton research 1991-01, Vol.13 (suppl.), p.109-131
• Main Authors: Harris, G P, Griffiths, F B, Clementson, LA, Lyne, V, van der Doe, H
• Format: Article
• Language: English
• Subjects: Marine; Nyctiphanes australis; Trachurus declivis
• ISSN: 1464-3774; 0142-7873; 1464-3774
• DOI: 10.1093/oxfordjournals.plankt.a042363

Weekly observations at a sampling station in Storm Bay, Tasmania, over a period of 4 years from Mar 1985 to Mar 1989 have shown a 2.5 degree C warming of sea surface temperature (SST) since the cool, windy summer of 1986-87. The samplings station lies close to the northern edge of the subtropical convergence. 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. 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 1/2 of the concentrations measured in 1985. Winter nitrate concentrations in 1988 were also 1/2 of average. Total nitrogen, phosphorus and DOC 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. The very oligotrophic conditions of the summer of 1988-89 led to small copepods dominating, and the elimination of all large zooplankters.