The water masses of the east coast of Tasmania: Seasonal and interannual variability and the influence on phytoplankton biomass and productivity

Abstract

The seasonal and interannual variability of the water masses, nutrients, phytoplankton biomass and primary productivity of the waters off the eastern coast of Tasmania are described. The seasonal and interannual variability in the water masses on the east coast could be explained by the varying influence of tropical and subantarctic waters and the presence of the northern edge of the subtropical convergence north-east from Maria Island. The physical oceanography was dominated by mesoscale events and the influence of the two parent water masses was highly episodic. Subtropical water rarely extended as far south as Tasman Island in summer and subantarctic water never extended as far north as Flinders Island. Data from satellite Advanced Very High Resolution Radiometer (AVHRR) images confirmed the hydrographic data and were used to interpret the seasonal and interannual variability. Interannual variability in maximum summer temperatures at Maria Island was correlated with the southerly extension of subtropical waters and with El Nino/Southern Oscillation events at least until the mid-1970s. There was evidence of a long-term warming at Maria Island and a cyclic fluctuation of 10-15 years in smoothed maximum summer temperatures. The decline in nitrate, and hence the phytoplankton spring bloom, occurred a month earlier inshore than offshore. Long-term data records from the inshore station at Maria Island showed strong interannual changes in the timing of the nitrate decline. The timing of the spring bloom varied from year to year by as much as 4 months. The seasonal cycle of phytoplankton biomass in 1984 and 1985 showed spring (October) and autumnal (April) blooms, with an early summer bloom in December. Low chlorophyll levels in February and March coincided with the influence of subtropical water. The seasonal cycle of phytoplankton biomass was therefore a complex function of seasonal and episodic events. Primary productivity data from the spring of 1984 showed low productivity on the west coast of Tasmania but high productivity on the east coast around Maria Island and the islands in Bass Strait.

New evidence links changing shelf phytoplankton communities to boundary currents in southeast Tasmania

“…1). This observation was supported by an earlier study conducted by Harris et al (1987), whereby physical evidence of the EAC was absent from temperature to salinity (TS) plots in shelf waters south of Tasman Island.…”

“…The physical signature of the EAC has been studied thoroughly and is known to be warm ([18°C), highly saline ([35.6) and poor in nutrients, especially silicate (Harris et al 1987;Ridgway 2007). Ridgway (2007) described the penetration of the EAC into east Tasmanian waters after examining data collected at the MINRS, and proposed that the current spills onto the shelf in January and peaks in influence during February and March.…”

The effect of the Leeuwin Current on phytoplankton biomass and production off Southwestern Australia

“…[43] A similar mechanism has been described for several western boundary currents, where eddies and meanders over the continental slope ''pump'' nutrients onto the continental shelf and thereby promote a significant proportion of the shelf productivity off the southeast USA [Yoder et al, 1981;Lee and Atkinson, 1983;Lee et al, 1989Lee et al, , 1991, Brazil [Campos et al, 2000], Japan [Kimura et al, 1997], and eastern Australia [Tranter et al, 1986;Harris et al, 1987]. The western boundary currents are often notably low in nutrients, yet the eddy and meander structures promote shelf productivity through eddy-induced upwelling along the shelf break, because the nitricline is lifted upward toward the continental shelf.…”

“…Meanders and eddies spin up during the autumnwinter period and often continue to evolve through winter, to be either reabsorbed or shed into the interior of the eastern Indian Ocean and drift westward Fang and Morrow, 2003]. Western boundary current eddies and meanders have been shown to contribute significantly to the productivity of the continental shelves off the southeastern USA [Yoder et al, 1981;Lee and Atkinson, 1983;Lee et al, 1989Lee et al, , 1991, Brazil [Campos et al, 2000], Japan [Kimura et al, 1997], and eastern Australia [Tranter et al, 1986;Harris et al, 1987], but the impact of eddies off the coast of Western Australia on shelf productivity has not been previously examined.…”

Oceanographic context of the First Aerosol Characterization Experiment (ACE 1): A physical, chemical, and biological overview

“…Harris et al [1987] have shown that the onset of the spring bloom can occur at any time between September and January off Tasmania's east coast, and this is controlled primarily by the strength of the westerly winds. They also show that the onset of the spring bloom is delayed in E1 Nino years, and 1995 was an E1 Nino year.…”