Induced metal tolerance in microbenthic communities from three lowland rivers with different metal loads

The response of microbenthic communities to sustained metal stress was stud ied in three lowland rivers with different levels of pollution. Tolerance a gainst zinc and cadmium was determined in short-term toxicity tests with mi crobenthic assemblages colonizing glass discs. Photosynthetic activity serv ed as an endpoint in tests for algae, whereas for bacteria thymidine incorp oration was determined. For bacterial assemblages from unpolluted locations , EC50 values in short-term tests ranged between 6.7 and 56.2 mu M zinc, an d 8.7 and 25.5 mu M cadmium, respectively. Bacterial assemblages from the t wo most polluted sites were significantly more tolerant for zinc (EC50: 994 mu M and >1,000 mu M) and cadmium (EC50: 218 mu M and 154 mu M). Results i ndicated a shift in community composition toward pollution-adapted organism s when a threshold concentration of 1 mu M zinc is exceeded. Although an in creasing community tolerance was also indicated for algae, EC50 values for microbenthic algae from all sites exceeded in most cases the highest metal concentrations tested (Zn: 1,000 mu M; Cd: 320 mu M). Since species composi tion of algal assemblages was found to change at much lower metal levels, i t is concluded that short-term toxicity tests measuring photosynthesis inhi bition do not reflect well the long-term effects of these metals. Toxic eff ects of metals on both algal and bacterial assemblages are attenuated by pr ecipitation and complexing capacities of the biofilm.