RELATIVE TOLERANCE OF MAT‐FORMING ALGAE TO COPPER AND ITS IMPLICATIONS

Abstract

The major method for algae control in the U.S. is the application of copper. Certain groups of phytoplankton, such as the green algae, are known to be more tolerant to copper than groups such as the cyanobacteria. Through experience, commercial applicators suggest that mat‐forming algae also differ in their susceptibility to copper, but to this point, data to support these observations have not been available. Laboratory studies in which unialgal cultures of mat‐forming algae were exposed to a range of copper concentrations show a clear progression in copper tolerance. The EC50 values (concentration of copper in mg Cu L−1 that results in 50% reduction of biomass) were less than 0.003 (Spirogyra, Oedogonium), approximately 0.050 (Hydrodictyon, Pithophora, Rhizoclonium), and greater than 0.200 (Oscillatoria). These data are important to us in the midwest because Oscillatoria is becoming a more frequent component of mat‐forming algae infestations. The Oscillatoria infestations are caused by at least five different species, each of which is very tolerant to copper. Circumstantial evidence suggests that the increasing frequency of this mat‐forming cyanobacterium is due to elimination of more sensitive genera (such as Spirogyra or Rhizoclonium) with heavy copper dosing. In addition to concerns often expressed about copper use, such as buildup in the sediments, toxicity to invertebrates, and possible (although not well verified) resistance of cyanobacteria, another compelling reason to seek alternative approaches to copper is the buildup of copper‐tolerant mat‐forming algae.