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1. The effect of mechanical and natural mixing of water on phytoplankton community assemblages was compared in one reservoir with a destratification unit operating during the wet summer months (North Pine Reservoir) with two adjacent reservoirs without artificial mixing (Wivenhoe and Somerset Reservoirs) over 6 years in subtropical Australia. 2. All three reservoirs were dominated by cyanobacteria, with the same three genera representing 75-80% of the abundance, i.e. Aphanocapsa/Merismopedia/Cyanodictyon group, Cylindrospermopsis raciborskii (Wołoszyń ska) Seenayya et Subba Raju and Planktolyngbya. Associations between the dominant genera were consistent across all three reservoirs although there were reduced seasonal differences in abundance in North Pine Reservoir compared with the other reservoirs. 3. Peaks in abundance of the solitary filamentous species C. raciborskii and Planktolyngbya occurred earlier and the densities of the colonial species Aphanocapsa/Merismopedia/ Cyanodictyon were lower in the reservoir where the destratification unit was switched on in spring compared with the naturally mixed reservoirs, possibly reflecting the differential effect of artificial mixing on colonial versus solitary filamentous species. 4. Phosphate concentrations were positively correlated with algal densities in the two naturally mixed reservoirs but not the artificially mixed reservoir where phosphate concentrations at the surface were near the limit of detection (0.06 lM M P). Artificial mixing may, therefore, promote the growth of species able to utilise and store low concentrations of phosphate, such as C. raciborskii.

Rates of nitrogen fixation in seagrass beds (Zostera capricorni) were determined with(15)N and reduction of acetylene in intact cores of sediment and seagrass. There was good agreement in the results from the two techniques, with a molar ratio of 3∶1.9 ethylene: ammonia produced. Fixed nitrogen was rapidly utilized by the plants, with significant amounts of(15)N found in the roots and rhizomes and 50% of fixed(15)N apparently translocated to the leaves. Rates of fixation were high in summer (25 to 40 mg N m(-2) day(-1)) and lower in winter (10 mg N m(-2) day(-1)) and were estimated to supply between one-third and one-half of the nitrogen requirements of the seagrass. Rates of nitrogen fixation were greater in the light than in the dark, and in cores of intact seagrass than in defoliated cores, indicating that the bacteria were dependent on organic compounds secreted by the plants.

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