Implications of cyanophyte toxicity on biological monitoring of calcareous streams in north-east Spain

There is increasing evidence that benthic cyanophytes in Mediterranean calcareous streams can influence the macroinvertebrate community, probably directly due to toxicity. A study was therefore planned to establish whether this also influenced the results of indices used for the biological monitoring of stream water quality. Two indices (BMWPc based on macroinvertebrates; IBD based on diatoms) widely used to assess water quality based on the diversity of macroinvertebrate and diatom assemblages were calculated for two unpolluted Mediterranean calcareous streams in north-east Spain. The toxicity levels of the cyanophyte communities were quantified by the Microtox test. There was a clear inverse relationship between the dominance of cyanophytes and the values obtained for the biological indices. It is recommended that cyanophyte biomass should be incorporated in some way in the calibration of index values; the use of phycobiliprotein concentration and/or the chlorophyll a:phycobiliprotein ratio is proposed.