THE DUCKWEED LEMNA-MINOR COMPARED WITH THE ALGA SELENASTRUM-CAPRICORNUTUM FOR BIOASSAY OF POND-WATER RICHNESS

In an initial comparison of Lemna and Selenastrum bioassay, on water f rom two ponds, both organisms made greater growth in the more nutrient -rich water. However, when the comparison was extended to 57 diverse p onds, the two bioassay organisms responded to different aspects of wat er quality. Lemna and Selenastrum growth were not correlated, and were not necessarily related to the same water-quality variables: e.g. Lem na dry weight was correlated with Ca, Mg and K while Selenastrum cell concentration was correlated with PO4, NH4, NO2 and (negatively) with pH. Neither Lemna nor Selenastrum growth were related to the number of hydrophyte species or their average trophic ranking score. Thus, bioa ssay to assess pond richness, either using Lemna or Selenastrum, is no t straightforward and is probably best used alongside water analysis a nd/or vegetation survey. (C) 1998 Elsevier Science B.V.