COPPER DYNAMICS AND THE MECHANISM OF ECOSYSTEM-LEVEL RECOVERY IN A STANDARDIZED AQUATIC MICROCOSM

The Standardized Aquatic Microcosm (SAM) was used to assess the effect s and behavior of copper at the ecosystem level, The concentration of algal cells and Daphnia magna, pH, dissolved organic carbon (DOC), and dissolved and ionic copper concentration were measured for 489 d and used to explain the recovery sequence of a community of organisms. The results indicate that a resistant algal species was crucial for initi ating the recovery sequence in these microcosms and that the timing of D. magna blooms was variable but highly correlated with decreasing io nic copper. In order to explain copper toxicity and the success of the recovery phase, a stepwise analysis of the functional role of the res istant algal species Oocystis pusilla, and the tolerance of D. magna w as undertaken. These process studies determined that O. pusilla was a suitable food for D. magna, and that this algal species could also act as a major ligand for copper, although sorption was probably importan t only for a limited time during the bloom. These studies also conclud ed that copper bioavailability controlled toxicity because it was show n that D. magna from the microcosms exhibited no resistance to copper toxicity, even though the dissolved copper concentration was 5 times t he LC50 value (concentration lethal to 50% of the population). DOC and pH, which were controlled by algal metabolism, were probably importan t for decreasing ionic copper, which allowed the recovery sequence to begin. Additional studies showed that the microcosm-derived DOC was ab le to complex copper at low pH and that DOC was highly correlated with reduced concentrations of ionic copper.