Report No SR3439/1

ALGAL BIOASSAYS: A REVIEW OF THEIR POTENTIAL FOR MONITORING AND MANAGING THE TROPHIC STATUS OF NATURAL WATERS

SR3439/1

Nov 1993

SUMMARY

The River Purification Boards and Department of the Environment (Northern Ireland) are charged with identifying sensitive and less sensitive waters (under the Urban Waste Water Treatment Directive) and vulnerable zones (under the Nitrates Directive). Guidelines to identify such areas have been laid down (SOEnD 1993) which rely on chemical analysis of nutrient, chlorophyll and dissolved oxygen levels, taking into account retention time and changes in aquatic flora and fauna.

These criteria may protect enriched waters from further nutrient pollution, but pristine waters in particular may not be adequately protected. It has been suggested that a suite of algal growth potential bioassays could be used to better define and manage those waters which are at risk of eutrophication. This report reviews the published information on algal bioassays in terms of broad methodological approaches and discusses how the results of such bioassays can be used to manage water quality.

Only in situ bioassays take any account of natural environmental conditions and so give a result which could be used to predict the extent of likely bloom formation. Unfortunately these are impractical for many waters. In vitro bioassays offer other the oretical advantages in terms of the basis of the test; and of these methods. microplate-based techniques appear to offer the largest advantages in terms of cost, simplicity and the large amounts of replicate assays which can easily be undertaken.

As with chemical monitoring and the predicted algal response to measured nutrient levels, algal bioassay results cannot be used alone to classify waters; chemical and physical data must also be considered. Standard algal bioassays already exist, but these methods are not suited to large numbers of replicates and require modifications if they are to be used for monitoring all types of waters (lakes, rivers, estuaries and coastal waters).

Algal bioassays cannot be used to replace chemical analysis of nutrient levels, so must be used as an additional determinand in current monitoring programmes. Because of the extra expense involved with this, there is only limited justification for the further development of algal bioassays for use as routine monitoring tools. However, the selective use of algal bioassays as water quality management tools has clear advantages over more traditional methods in a number of situations, and it is for this purpose that method development is recommended. It is suggested that a suite of methods are developed which can be fine-tuned by using different algal species and growth conditions, thereby increasing the relevance of bioassays for the management of individual waters.

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