Degree Name

Doctor of Philosophy


School of Earth & Environmental Sciences - Faculty of Science


Phytoplankton, as one of the most important primary producers in aquatic ecosystems, have been widely used to indicate the health of such ecosystems, but they are a poorly understood component of coastal plant communities in Australia. The primary objective of this study was to address a number of gaps in the current knowledge of phytoplankton in the estuaries of southeastern Australia.

An extensive literature review on phytoplankton diversity and ecology in the estuaries of Australia was conducted to identify knowledge gaps. There are more than 130 estuaries in New South Wales, Australia (NSW), but almost no single set of comprehensive information on phytoplankton taxonomy has been established, and very few studies on the seasonal and annual patterns of phytoplankton distribution have been carried out. The thresholds or standards to assess the changes of aquatic environments using phytoplankton are therefore difficult to establish, even though an increasing number of bloom events indicates an urgent need to monitor the dynamics of phytoplankton assemblages. Thus, seven representative estuaries, which cover about 400 km of the southeastern NSW coast, were chosen for study of the general characteristics of phytoplankton diversity and ecology. A more detailed comparative study of the phytoplankton assemblages in two lakes with different nutrient status and a historical study of environmental change using diatom frustules in sediments of one lake as indicators were also completed. In addition, the growth of key species was studied to explore the governing environmental factors.

In the seven estuaries, 145 phytoplankton species were identified, covering five groups - Cyanophyta, Chlorophyta, Chromophyta, Diatom and Pyrrophyta. Diatom and Pyrrophyta species were the main contributors to the phytoplankton diversity. Phytoplankton assemblages were characterized by a large number of temperate water species combined with a smaller number of warm water species, reflecting the warm temperate climate in the studied areas. The proportions of brackish and freshwater to brackish species corresponded to the gradient of salinity in the estuaries. A combination of euplanktonic, tychoplanktonic, epiphytic and periphytic species was observed in the phytoplankton assemblages, which was related to specific estuarine habitats. The seasonal patterns of phytoplankton biomass displayed winter or spring maxima, and a positive relationship with the nutrient levels in the studied estuaries. The horizontal distribution of phytoplankton assemblages within each lake can be significantly impacted by seagrass beds or large freshwater discharges caused.

A more detailed comparison of the phytoplankton assemblages between a heavily impacted lake (Lake Illawarra) and a less impacted lake (Burrill Lake), was completed, based on the bimonthly observations from April 2005 to April 2007. Lake Illawarra displayed higher DIN and DIP concentrations compared to Burrill Lake, particularly the DIP concentration. The former was characterized by a marked N-limitation condition, while the later displayed variable DIN:DIP ratios. Diatoms were the main dominant species in Lake Illawarra, combined with a limited dinoflagellate dominance in some seasons. In contrast, dinoflagellates dominated in Burrill Lake in most seasons. The proportion of toxic and red tide species in Burrill Lake was notably higher compared to Lake Illawarra. The different nutrient ratios between the two lakes were regarded as an important controlling factor. The seasonal succession of phytoplankton in the two lakes did not completely follow a spring maximum that is generally observed in temperate waters. In Burrill Lake, phytoplankton biomass increased from autumn and reached the maximum in winter, while the maxima in Lake Illawarra could occur in autumn, winter and spring. Factors influencing the seasonal maximum include nutrient concentrations and ratios, temperate and grazing pressure. In addition, seagrass beds significantly impacted the horizontal distribution of phytoplankton assemblages within lakes.

Long-term environmental changes that have occurred in Lake Illawarra were investigated using dated (210Pb and 137Cs) sediment core studies. The diatom frustules deposited in the sediment post-World War II were analyzed along with the chronology. The results indicated that significant changes had occurred in the phytoplankton communities in the lake over the past 60 years. There was evidence of increasing eutrophication through changes in diatom concentrations and the ratio of planktonic:benthic diatoms, and also of increasing salinity supported by the development of marine planktonic diatoms and a decrease of freshwater and brackish water species.

Growth characteristics of two key phytoplankton species, Prorocentrum lima and Thalassionema nitzschioides isolated from Lake Illawarra, representing key dinoflagellate and diatom species, respectively, were studied. Their life strategies under the different nutrient and temperature conditions were compared. P. lima displayed a higher temperature preference and greater nutrient demand in contrast to Th. nitzschioides with a lower temperature preference and nutrient demand. N-limitation can significantly impact the growth of P. lima because of its K-strategy preferring to utilize all available nutrients in a short time, and then ceasing growth. The growth of Th. nitzschioides displayed less impact under N-limitation because of the preferred r-strategy which uses the nutrient in a more sustainable manner. In competitive experiments, P. lima displayed a great dominance in the f/2 enriched medium compared to the Th. nitzschioides at both 18 and 25ºC. In the N-limitation condition, Th. nitzschioides displayed dominance at 18ºC. Two explanations are suggested to explain these results. One is that the different strategies of two species to the nutrient uptake and temperature preference played important roles in determining their competitive ability. The other is that an allelopathic substance reported to be produced by P. lima could inhibit the growth of Th. nitzschioides.

This research has provided important new information to assist understanding the characteristics of phytoplankton assemblages in estuaries of southeastern NSW. The indicative role of phytoplankton in estuaries was assessed and linked to the relevant physical and chemical factors, human activities and climate change. The need for further research and the application of phytoplankton studies in the environmental management has also been highlighted.

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Unless otherwise indicated, the views expressed in this thesis are those of the author and do not necessarily represent the views of the University of Wollongong.