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.

The effects on UVB radiation on a subtidal, cohesive-sediment biofilm dominated by the diatom Gyrosigma balticum (Ehrenberg) Rabenhorst were investigated. Chlorophyll fluorescence parameters (F v /F m , PSII ), pigment concentrations, cell densities, and carbohydrate fractions were measured in four treatments (no UVBR, ambient UVBR, ؉7%, and ؉15% enhancement with UVBR). Enhanced UVBR was provided by a computer-controlled system directly linked to natural diel UVBR levels. Increases in PSII values in the UVBR-enhanced treatments and a decrease in the steady-state fluorescence yield (F s ) from the surface of the biofilms during the middle and latter part of daily exposure periods suggested that G. balticum responded to enhanced UVBR by migrating down into the sediment. Diatoms in the ؉15% UVBR treatment also had significantly higher concentrations of ␤-carotene after 5 days of treatment. Although G. balticum responded to enhanced UVBR by migration and increased ␤carotene concentrations, significant reduction in maximum quantum yield of PSII (F v /F m ) and in minimal fluorescence (F o ) and decreases in cell densities occurred after 5 days. Concentrations of different carbohydrate fractions (colloidal carbohydrate, glucan, exopolymers [EPS]) associated with diatom biomass and motility also decreased in the UVBR-enhanced treatments. Short-term responses (migration) to avoid UVBR appear insufficient to prevent longer-term decreases in photosynthetic potential and biofilm carbohydrate concentration and biomass.

The effect of ultraviolet-B radiation (UVBR) on a microphytobenthic community, dominated by diatoms, was studied under different nutrient conditions in a 9 day outdoor experiment on the Swedish west coast. The microalgal assemblage was isolated from the sediment and resettled onto acid-cleaned sand placed in petridishes. The experimental treatments were: ''ambient'' or ''enhanced'' UVBR with no nutrient addition, ''ambient'' or ''enhanced'' UVBR with nutrient addition (N, P, Si). Enhanced UVBR (ϩ15%) was provided by a computer-controlled system. Primary productivity, carbon allocation, chlorophyll a (Chl a) concentrations, composition of algae and pigments and intra-and extracellular carbohydrate fractions were measured. Most UVBR effects were seen in treatments without nutrient addition. Exposure to enhanced UVBR resulted in statistically significant decreases in primary productivity, Chl a and in the biomass of Haslea ostrearia and Nitzschia spp. The relative amount of carbon allocated to proteins increased when exposed to enhanced UVBR. The effect of enhanced UVBR on microalgae subjected to nutrient addition was less pronounced, and observed for Chl a, algal intracellular storage of carbohydrates (glucans) and concentration of extracellular ''colloidal'' carbohydrates. Enhanced UVBR ϩ nutrients resulted in a significantly greater ratio between glucan and total carbohydrates and a decreased concentration of colloidal carbohydrates compared to the ambient UVBR ϩ nutrients treatment. These results indicate that availability of inorganic nutrients acts to mitigate the negative effects of UVBR on microphytobenthic communities and that UVBR acts as a selective force during early growth and succession.

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