Factors affecting the algal growth potential in small ponds

Abstract

The Algal Assay Procedure-Bottle Test (AAP-BT), developed by the Environmental Protection Agency, has been used to determine the nutritional status (algal growth potential - AGP), limiting nutrients, and the presence of inhibitory substances in many freshwater environments. However, the AAP-BT has been used only to a limited extent in small ponds. The AGP of these bodies of water can vary significantly not only between seasons, but also from day to day depending upon rainfall events, sediment disturbances, and other factors. Short-term variability in the AGP depends upon the particular stage or compartment in which the nutrients exist at the time of sampling. Nutrients incorporated in detritus and plankton, and also those in dissolved form, after pretreatment have varying degrees of availability to the assay organism. Surface water samples from different locations within a small pond during periods of calm weather exhibit minimal differences with respect to AGP. In contrast, subsurface samples may vary greatly between locations, and may be extremely different from the surface AGPs. The AGP of small ponds can vary significantly from one season to the next, or within a single season, depending upon rainfall events, wind conditions, condition of the watershed, etc. Care must be taken when storing water samples in order to prevent significant changes in the AGP. Samples kept cold (and in the dark) with no pretreatment, or those samples which have been autoclaved on the day of collection, can be stored for periods of up to 2 weeks with little or no change in AGP. The use of a distilled water blank as specified by the EPA could result in erroneously high chlorophyll or fluorescence values since the change in background fluorescence is not accounted for in water samples (autoclaved or filtered).