Higher alkalinity increases copper toxicity in phytoplankton: The importance of pH flux when treating algal blooms

• Copper treatments were more effective at reducing algae as alkalinity increased. • Low copper treatments reduced cyanobacteria and promoted beneficial chlorophytes. • High alkalinity-based copper treatments reduced phytoplankton but caused high total ammonia nitrogen. For more than a century, copper sulfate pentahydrate (CuSO 4 ·5H 2 O) has been used to combat harmful algal blooms in freshwater systems. Dosing recommendations for copper sulfate are often at 1% of the ambient alkalinity, thus increasing the applied dose with higher alkalinity. This methodology is based on the premise that higher ambient alkalinity makes copper sulfate less toxic to aquatic organisms due to the expected precipitation of copper-carbonate complexes. Using a 21-day, gradient-design, replicated field experiment in a eutrophic aquaculture pond initially dominated by cyanobacteria, this study showed the opposite is true: copper sulfate becomes more toxic to phytoplankton as alkalinity increases. To evaluate the effect of sodium bicarbonate-based alkalinity on copper sulfate toxicity to phytoplankton, four target alkalinity concentrations (50, 125, 200, and 275 mg/L CaCO 3 ) across three copper sulfate treatments (an untreated control (0 mg/L CuSO 4 ·5H 2 O); a low, constant dose (0.324 mg/L) across all alkalinities; and increasingly higher alkalinity-based doses (0.25-1.375 mg/L)) were used. Both the low and high copper sulfate treatments effectively eliminated cyanobacteria (regardless of copper dose or alkalinity), while the low copper treatment allowed beneficial chlorophytes to quickly return to initial concentrations. Notably, the results show that as the ambient alkalinity in the enclosures increased, the effect of the constant low copper sulfate dose on phytoplankton was larger (i.e., less phytoplankton abundance; more negative algal growth rate) contradicting long-standing copper sulfate dosing methodologies. Higher pH at lower alkalinities appears to mediate these effects given that toxic Cu 2+ availability decreases with higher pH. These results suggest that water resource managers should consider pH and alkalinity when making decisions about copper applications to avoid overdosing of copper sulfate that can cause other water quality problems, such as heavy metal accumulation, anoxia, non-target effects on zooplankton, and recurring treatment-resistant algal blooms.