The use of diatoms as biological indicators of water quality, and for environmental reconstruction, in south-east Tasmania, Australia

Water quality around the globe has been in serious decline for many decades. To reverse the degradation of waterways there must be a significant improvement in the way the coastal zone is managed. The effective management of the coastal zone requires the ability to effectively monitor and assess changes in water quality, and the ability to identify past, current and potential impacts on water quality. In recent years, water quality monitoring and assessment programs have been significantly improved by the inclusion of biological indicators. Diatoms have been used extensively as biological indicators in water quality monitoring and assessment studies, and in palaeo-environmental reconstruction of water quality, in many areas of the world. This study documents the use of diatoms as biological indicators of water quality, and for environmental reconstruction, in south-east Tasmania, Australia. The biomass (chlorophyll a) of marine benthic algal mats was determined along a depth gradient at two sites within the near-shore marine environment approximately fortnightly for 3 months, to determine whether depth significantly influenced biomass. Average chl a levels ranged from approximately 9 to 60 mg/m2 , and varied inconsistently with depth. Physical disturbance of the substrata may account for the greatest variations in biomass observed. Diatoms were found to contribute significantly to the productivity of the near-shore, subtidal marine environment of south-east Tasmania, comprising approximately 95% of the benthic algal community. Canonical correspondence analysis (CCA) was used to identify causative relationships between the species composition of diatom communities and the corresponding physical and chemical variables from 51 sites within the near-shore, sub-tidal marine zone of south-east Tasmania. The composition of micro-algal communities within these habitats was found to be most strongly influenced by nutrient concentrations. Transfer functions were generated to infer nitrite/nitrate, silicate and sediment size at other sites within the geographic region of the study area. The determination of environmental optima and tolerance ranges for south-east Tasmanian diatom species, and the generation of transfer functions, provides a valuable water quality monitoring and assessment resource for this region. The environmental history of Pittwater Lagoon, an impacted Ramsar wetland site, was reconstructed from the late 18th century using sediment-core fossil diatoms, 2I0Pb dating, transfer functions and historical environmental data. A significant change in the diatom flora of the lagoon was found to have occurred during the past 100 years. The future health of south-east Tasmanian coastal ecosystems will depend on the ability of responsible stakeholders and caretakers to incorporate effective biological monitoring and assessment into their management strategies.