INFLUENCE OF TROPHIC STATUS ON THE TOXIC EFFECTS OF A HERBICIDE - A MICROCOSM STUDY

Naturally derived microbial communities were developed in the laborato ry under three nutrient regimes by manipulating phosphate and nitrate concentrations. Resulting communities differed in both functional and structural attributes. Low nutrient microcosms (0.05 mg N-NO3-/L + 0.0 1 mg P-PO4-3/L) showed the sharpest differences. Medium (0.5 mg N-NO3- / L + 0.1 mg P-PO4-3/L) and high (5.0 mg N-NO3-/L + 1.0 mg P-PO4-3/L) nutrient treatments differed in total algal biomass and algal communit y composition. After a 25-day acclimation period, a single dose of the herbicide diquat (3.5 mg/L) was added to test the response of the mic robial communities to herbicide stress. Regardless of nutrient regime, diquat-dosed microcosms had decreased electron transport system activ ity (ETSA), an almost complete absence of cyanobacteria, and reduced g ross photosynthesis (GP), respiration, and pH relative to undosed micr ocosms, inorganic nutrients (PO4-3, NO3-) were released from the stres sed algal communities, probably as a result of their altered metabolis m. Alkaline phosphatase activity (APA), total microbial biomass (estim ated as protein), algal biomass (estimated as chlorophyll), and relati ve abundance of green algal taxa proved highly insensitive to herbicid e action. Nutrient treatments had a small influence on toxicant effect s; the magnitude of the herbicide effects was comparable across nutrie nt levels. Only the capacity of recovery from the toxic stress was aff ected by trophic status. At the end of the study period, ETSA had reco vered to control values in high nutrient microcosms but not in medium and low ones. Microcosm pH, and to a lesser extent GP, showed recovery under both high and medium nutrient treatments. Trophic status affect ed the diquat disappearance rate; the herbicide persisted longer in lo w nutrient microcosms than in high and medium nutrient ones. Differenc es in recovery capacity may stem from higher nutrient level microcosms reaching less toxic herbicide levels in a shorter period of time.