Nutrient and plankton distribution near a shelf break front in the region of the Bass Strait cascade

Abstract

Nutrient enrichment of surface water to the east of a shelf break front at the eastern end of Bass Strait occurs in winter. This area of enrichment is more than 100 nautical miles (E-W) by 150 nm (N-S). From east of Banks Strait (40º 45'S,148ºE), some of the nutrient-rich water is carried northwards with the northward flow of Bass Strait water which later forms the well-known 'cascade' below the warmer waters of the Tasman Sea. In September 1984, the chlorophyll a concentration increased along the line of this northward flow, producing a maximum off the Victorian coast near where the cascade occurs. In contrast to nutrient and chlorophyll a distributions, zooplankton biomass (dry weight) was higher in the shallow water of Bass Strait than over the continental slope. This suggests that the plankton growth observed in shallow Bass Strait waters in late winter had ceased by September, but was continuing to the north-east and over the slope in waters with a shallow mixed depth. We propose that the northward flow of water along the shelf break maintains plankton in a nutrient-rich environment, so that they continue to grow until they are carried below the photic zone by the cascade.