Influence of trophic status on the toxic effects of a herbicide: A microcosm study

Naturally derived microbial communities were developed in the laboratory 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 + 0.01 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 community 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 microbial communities to herbicide stress. Regardless of nutrient regime, diquat-dosed microcosms had decreased electron transport system activity (ETSA), an almost complete absence of cyanobacteria, and reduced gross photosynthesis (GP), respiration, and pH relative to undosed microcosms. Inorganic nutrients (PO4-3, NO3-) were released from the stressed algal communities, probably as a result of their altered metabolism. Alkaline phosphatase activity (APA), total microbial biomass (estimated as protein), algal biomass (estimated as chlorophyll), and relative abundance of green algal taxa proved highly insensitive to herbicide action. Nutrient treatments had a small influence on toxicant effects; the magnitude of the herbicide effects was comparable across nutrient levels. Only the capacity of recovery from the toxic stress was affected by trophic status. At the end of the study period, ETSA had recovered 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 affected the diquat disappearance rate; the herbicide persisted longer in low nutrient microcosms than in high and medium nutrient ones. Differences in recovery capacity may stem from higher nutrient level microcosms reaching less toxic herbicide levels in a shorter period of time.