The Suitability of Diatoms for the Biomonitoring of Surface Water Salinity and Trophic Status

peter.gell@adelaide.edu.au

Diatoms are unicellular, microscopic algae of the class Bacillariophyceae. They have an intricate siliceous frustule (valve or shell), the morphology of which is the basis for their taxonomy. They satisfy all the conditions to qualify as suitable indicators (Dixit et al., 1992) in that they are simple, capable of quantifying changes in water quality, applicable over large geographic areas and can furnish data on background conditions and natural variability. They provide advantages over direct chemical monitoring in that they respond over periods of days avoiding unrepresentative sampling. They are easily prepared and biomonitoring results can be returned in several days. Also, as an important component of the biota, they represent the chemical/biological interface.

Unlike many other algal groups, diatoms are readily identifiable to species level and beyond (Round, 1991). While their identification is difficult - numerous comprehensive taxonomic texts and floras dealing with taxonomy are readily available which give the diagnostic characteristics of many species and their varieties. Taxonomic workshops are being regularly staged in Australia to facilitate the exchange of information and to provide taxonomic quality control for those developing data sets (a simple beginners key to the more common genera is included here). Diatoms are ubiquitous in both lakes and rivers as well as in other moist conditions where there is sufficient light for photosynthesis - even moist soil. Diatoms therefore can provide bioindication of water conditions which are beyond the tolerance of many other biota used for monitoring.

Significantly also, diatom species are cosmopolitan. Many diatom taxa have been identified from a range of sites throughout the world, and furthermore, are sensitive to, and appear to have a consistent tolerance of, a wide range of environmental parameters such as light, moisture, current velocity, pH, salinity, oxygen and inorganic and organic nutrients (Van Dam et al., 1994). Diatoms often occur in large numbers and often show considerable species richness: even a simply collected, small surface mud sample or a scraping from a rock can yield > 106 valves, often of between 20 and 50, and even up to 100, taxa (Battarbee, 1988). These characteristics provide diatomists considerable advantage over ecologists reliant on macroinvertebrates or other bioindicators.

A further asset of diatoms in water quality biomonitoring is the preservation of their valves in sites of sediment accumulation. While they may dissolve in waters undersaturated in silica, they regularly preserve in good condition enabling past assemblages to be quantified. This provides perhaps the best opportunity to place modern lake conditions in a longer term context and can provide evidence for 'natural', pre-disturbance, lake water conditions.

Where once the simplest approach, one focusing only on the abundant, ecologically well known taxa was used, it was soon recognised that this abundance and diversity provided data which could refine the indicator-based environmental predictions. This shift encouraged the development of large data sets of one to several hundred samples and hence a semi-quantitative, statistical approach to water quality analysis and monitoring, but one still dominated by index groups within parameters. One of the most comprehensive of these methodologies (Watanabe et al., 1988) was based on 548 diatom taxa identified from 1343 samples taken from Japanese rivers. Here a Diatom Assemblage Index of Pollution (DAIpo) was produced for each taxon and site based on the relative abundance of taxa in each of three components relating to saprobity. The index score represents the species' optimum tolerance to BOD (Biological Oxygen Demand: a proxy indicator for organic content) on a scale of 0 - 100. Taxa giving scores of less than 30 are considered to be saprophilous, or pollution intolerant, while those with scores greater than 70 are saprophobic, that is, antagonistic to pollution. A simple Weighted Averaging approach can provide DAIpo values for sites and samples (Reid et al., 1995).

Given the transmissivity of the indices of most diatom taxa from one part of the world to another, the DAIpo index and the numerous indices such as those of Van Dam et al. (1994) have provided a ready made vehicle by which to evaluate the trophic condition of natural waters. The formal integration of ecologically comparable, but geographically disparate, data sets is promoted by the development of applicable multivariate statistical techniques in the early 1980s (Smol, 1990). The application of techniques such as direct gradient analysis has greatly enhanced the potential for environmental monitoring and reconstruction by directly relating species assemblages to water parameters through the statistical determination of the optimum and tolerance of species to particular water parameters.

Large data sets of many taxa and samples manipulated by statistically intensive computer programs have now become the norm in diatom-based environmental and palaeoenvironmental analyses and have elevated the status of diatoms to one of the most informative indicators of past and present limnological conditions.

This approach is being applied to salinity monitoring and palaeosalinity reconstructions using salt lake diatoms (Fritz et al., 1993; Juggins et al., 1994) derived from some of the world's major salt lake fields. An Australian data set (Gell, 1995, 1997) is presently being used to monitor salinity-threatened wetlands in the Sunraysia region of Victoria and to reconstruct salinity changes over millenia from Lake Alexandrina and from lakes on Kangaroo Island and at Cooke's Plain in South Australia. It is also intended that this set be directly applied to the biomonitoring of salinity in the Barwon system. The DAIpo, or Van Dam et al.'s, indices, can provide a simple scale for the biomonitoring of trophic status.

Diatom biomonitoring has been used by the author and colleagues to examine periodic changes over intervals from two weeks to two months and to compare the impact on biota of outfalls by monitoring immediately up and downstream of the release point (eg. Erewash; Test, Babingly & Wissey Rivers in the UK; Gellibrand & Ovens Rivers in Victoria; Torrens Rivers in SA). After a brief training visit, a local field officer can be guided on the sampling of diatoms which can then be sent for preparation and analysis at The University of Adelaide (eg. EPA (SA) have supplied samples as part of the Monitoring River Health Initiative). A brief video illustrating the river diatom sampling techniques of Bruce Chessman and Peter Gell will soon be available. 

Contact Peter Gell (peter.gell@adelaide.edu.au) for further details on introductory courses and taxonomic workshops.
 

References

Dixit, S.S., Smol, J.P., Kingston, J.C. & Charles, D.F. 1992. Diatoms: powerful indicators of environmental change. Environ. Sci. Technol., 26 (1): 23-33.

Fritz, S.C., Juggins, S, & Battarbee, R.W. 1993. Diatom assemblages and ionic characterization of lakes of the northern Great Plains, N.A.: a tool for reconstructing past salinity and climate fluctuations. Canadian Journal of Fisheries and Aquatic Sciences, 50: 1844-1856.

Gell, P.A. 1995. The Development and Application of a Diatom Calibration Set for Lake Salinity, Western Victoria, Australia. Unpublished PhD thesis, Monash University.

Gell, P.A. 1997. The development of a diatom data base for inferring lake salinity: towards a quantitative approach for reconstructing past climates. Australian Journal of Botany, 45.

Gell, P.A., Fluin, J. & Sluiter, I.R.K. in prep. Testing a diatom-based transfer function for lake salinity using an external data set. for Lakes & Reservoirs: Research and Management.

Juggins, S., Battarbee, R.W., Fritz, S.C. & Gasse, F. 1994. The CASPIA project: diatoms, salt lakes, and environmental change. Journal of Paleolimnology, 12: 191-196.

Reid, M.A., Tibby, J., Penny, D. & Gell, P. 1995. The use of diatoms to assess past and present water quality. Australian Journal of Ecology, 20: 57-64.

Round, F.E. 1991. Use of diatoms for monitoring rivers. In: Whitton, B.A., Rott, E. & Friedrich, G. (Eds). Use of Algae for Monitoring Rivers. Institut fur Botanik, Universitat, Innsbruck: 25-32.

Smol, J.P. 1990. Are we building enough bridges between paleolimnology and aquatic ecology? Hydrobiologia, 214: 201-206.

Van Dam, H., Mertens, A. & Sinkeldam, J. 1994. A coded checklist and ecological indicator values of freshwater diatoms from the Netherlands. Netherlands Journal of Aquatic Ecology, 28 (1): 117-133.

Watanabe, T., Asai, K. & Houki, A. 1988. Numerical water quality monitoring of organic pollution using diatom assemblages. In: Round, F. E. (Ed.) Proceedings of the Ninth International Diatom Symposium, Bristol, August 24-30, 1986. Biopress, Bristol & Koeltz, Koenigstein: 123-141.