D665/D665a INDEX VS. FREQUENCIES AS INDICATORS OF BRYOPHYTERESPONSE TO PHYSICOCHEMICAL GRADIENTS

The chlorophyll-to-phaeophytin (D665/D665a) ratio is an index of physiological stress in aquatic bryophytes. In the present study, the usefulness of this index for evaluating water contamination was investigated. Samples of bryophytes and water were collected from 188 stretches of river in northwest Spain. Water quality was characterized by standard procedures. Ecological profile methods were used to investigate (for each species and each water quality variable) whether stress index or frequency of occurrence varied significantly along environmental gradients. Most stress profiles showed significant departures from uniformity, whereas many frequency profiles did not, indicating that the stress index approach is more sensitive to differences in water quality than are approaches based simply on presence/absence data. To further investigate relationships between water quality variables and D665/D665a ratio, canonical correspondence analysis (CCA) was used. The first axis extracted by CCA was clearly closely related to degree of organic pollution. The second axis was most strongly correlated with pH. Taken together, these results suggest that D665/D665a ratio in aquatic bryophytes may be of value as an indicator of river water pollution.