Responses of Planktothrix agardhii and Pseudokirchneriella subcapitata to Copper Sulfate (CuSO4 · 5H2O) and a Chelated Copper Compound (Cutrine®-Ultra)

Reliable viability measures are needed to predict responses of algae to phytotoxic exposures. Non-axenic laboratory cultures of Planktothrix agardhii and Pseudokirchneriella subcapitata were exposed to a series of concentrations of CuSO4 · 5H2O (0.1–0.5 and 1.0–8.0 mg Cu/L) and a chelated copper compound with an adjuvant, Cutrine®-Ultra (0.05–0.25 and 1.0–8.0 mg Cu/L) in 7-day static laboratory experiments. Algal viability measures in terms of cell density, uptake of mortal stain (erythrosin b), chlorophyll a concentration, pheophytin a concentration, and respiration (measured as 2-(p-iodophenyl)-3-(p-nitrophenyl)-5-phenyl tetrazolium formazan absorbance (INT)) were discerned daily. The time and concentration required to achieve algal control, 96-h EC50s, potency slopes, and doubling times were calculated from algal responses if algal viability measures were accurate and precise. Cell densities and erythrosin b-stained cells had sufficient accuracy and precision to differentiate responses of P. agardhii and P. subcapitata from resident algae while chlorophyll a concentrations and INT formazan absorbances did not discriminate. In this study, pheophytin a concentrations lack precision and accuracy. P. agardhii was an order of magnitude more sensitive than P. subcapitata, and Cutrine®-Ultra was more than twice as potent as CuSO4 · 5H2O. This laboratory study corroborates previous laboratory and field studies suggesting that chelated copper-based algaecides when applied in the field can selectively control specific target algae at a lower copper concentration than CuSO4 · 5H2O. Individual algal viability measures such as cell density and erythrosin b staining can measure the relevant change in algal assemblage as a result of exposure.