Subtidal macroalgal studies in East and South Eastern Tasmanian coastal waters

This thesis is concerned with (1) the introduced Japanese macroalga Undaria pimzatifida in Tasmania and (2) the growth patterns of major components of macroalgal communities at two sites on Tasmania's coast, Bicheno in the east and George III Reef in the south The algae studied were Macrocystis pyrifera, Ecklonia radiata, Phyllospora comosa at both sites plus the red algal community at George III Reef. Surveys ofUndaria in 1988 found that it inhabited approximately 15 kilometres of the east coast from Triabunna southwards to Lachlan Is. The alga is very fecund and smothers the bottom in 'barren' areas particularly in sheltered waterways at certain times of the year. Effects on the local biota are unknown but the environmental tolerances of the alga reveal a potential distribution from Cape Leeuwin in south west Western Australia to Wollongong on the New South Wales Coast. Environmental parameters and growth rates were monitored at George III Reef from 1985-88 and at Bicheno from 1987-88. Over the joint period, there was little difference in temperature, saliitity and nutrients of the seawater between the two sites. In 1988, the water masses of the south and east coasts of Tasmania were distinguished by unusually warm waters. In March, the warm nutrient - depleted waters of the East Australian Current were detected further south than usual and in winter, coastal waters were relatively warm and nutrient poor compared to previous years (Harris et. al1991). For Ecklonia radiata, light appears to be the most significant factor affecting the rate of increase in lamina length and thus production at Bicheno and George III Reef. At George III Reef, high temperatures and to a lesser extent large swells negatively affect growth rates. It is postulated that swell may be acting indirectly through its affect on the amount of suspended particles in the water column and thus reducing light. Productivity estimates suggest 1.8 kg wet wt/m\\(^2\\)/yr at 14m depth at George III Reef and 4.7 kg wet wt/m\\(^2\\)/yr at 10 m. depth at Bicheno in areas of continuous cover. For Macrocystis, production appears to be best determined from rate of blade production because change in length may be affected extrinsically by factors of swell and light. Productivity of M. pyrifera at the two sites suggests similar frond productivities of 6 kg wet wt/frond/yr. For a typical density of 4 fronds per m\\(^2\\) of sea floor this results in estimates of 24kg wet wt/m\\(^2\\)/yr weight. Macrocystis disappeared almost entirely from both sites during the warm year of 1988. For Phyllospora, regression of growth increment against environmental parameters indicates light availability as the principal factor affecting growth for most of the year though nutrients are likely to limit growth in late summer. High rates of plant mortalities and lower growth rates late in the study coincide with the incursion of the EAC and the warm winter of 1988. The calculated production at George III Reef of approximately 7.4 kg wet wt\\m\\(^2\\) /yr at 7.7m depth compares with the combined Phyllospora comosa and Ecklonia radiata annual production figure of 4.8 kg/m\\(^2\\)/yr at Bicheno at 10m depth. For the red algal community at George III Reef, filamentous and foliose species increasing in number in the summer and outside of the Macrocystis canopy. The presence of the Macrocystis canopy results in a halving of the red algal biomass/m\\(^2\\).