African Journal of Microbiology Research Vol. 6(20), pp. 4277-4291, 30 May, 2012 Available online at http://www.academicjournals.org/AJMR DOI: 10.5897/AJMR11.735 ISSN 1996-0808 ©2012 Academic Journals Full Length Research Paper Seasonal examination of phytoplankton abundance and assemblages in Myall Lake and Bombah Broadwater, New South Wales, Australia Nita RUKMINASARI 1 * and Anna REDDEN 2 1 Faculty of Marine Science and Fisheries Hasanuddin University, Jl. Perintis Kemerdekaan Km. 10, Makassar - 90245, South Sulawesi - Indonesia. 2 Acadia Centre for Estuarine Research, Acadia University Box 115, Wolfville, NS B4P 2R6 Canada. Accepted 29 November, 2011 The Myall Lakes are a series of interconnected, shallow and poorly flushed coastal lakes located within the Myall Lakes National Park in NSW, Australia. The following field study aims to uncover patterns in temporal variability in water quality and phytoplankton abundance, diversity and assemblage structure, in the upper and lower water bodies of the Myall Lakes system. The result showed that the two lakes differ greatly in their catchment to lake area ratios, freshwater inputs, degree of saline intrusion, light attenuation and submerged macrophyte biomass. The broadwater, which receives large inputs of freshwater from the catchment, and is influenced by saline inputs from the Port Stephen’s estuary, exhibited highly variable water quality and dissolved nutrient concentrations, and phytoplankton assemblages with much temporal variation in taxa present, biovolume, cell size and abundance, and taxonomic diversity. In contrast, the upper lake, Myall Lake, shows much greater stability in water chemistry and nutrient availability; the phytoplankton community is diverse, with relatively low and constant biovolume, and is dominated year-round by small-celled cyanobacteria. Differences in phytoplankton between the two lakes were greatest during the autumn and winter, and least during the summer, when the phytoplankton assemblages in both lakes were dominated by similar cyanophyte taxa. Key words: Seasonal variation, phytoplankton abundance and assemblages, Myall Lake and Bomba Broadwater. INTRODUCTION Concentrations of available nitrogen and phosphorus in aquatic ecosystems represent contributions from both autochthonous and allochthonous sources. In river estuaries, it is allochthonous sources of nutrients that are generally most responsible for the supply of nitrogen and phosphorus to aquatic primary producers. Catchment activities have a major impact on the export of nutrients to rivers, lakes and estuaries. Deforestation, agricultural *Corresponding author. E-mail: nita.r@unhas.ac.id or nitasari_02@hotmail.com. Tel: +62-0411-588828. Fax: +62- 0411-586025. practices (fertilisers and land clearing), urban developments, industry and stormwater run-off lead to the accumulation of nutrients and sediments in poorly flushed water bodies (Entry and Emmingham, 1996; Mander et al., 1997; Jorgensen et al., 2000; Edward et al., 2003). Activities in the catchment also impact on the N:P ratios found in lakes and streams, with forested watersheds contributing to very high TN:TP ratios in lakes (Barbanti et al., 1995). In general, Australia’s freshwater, marine and estuarine systems are lower in nutrients than European and North American systems, partly due to lower rates of atmospheric deposition of N, and partly because Australian catchments, by comparison, are less disturbed