The control of algal biodeterioration of a marble petroglyph site

Several blue-green algae, including Calothrix sp., Chroococcus sp., Oscillatoria sp., Gloeocapsa sp., Lyngbya sp., and the green alga Chlorococcum sp., were identified in a green and black accretion on a rock art site at Petroglyphs Provincial Park, Ontario, Canada. The accretion covered several hundred petroglyphs of Algonkian origin which were pecked into a gently sloping marble outcrop. The algae promoted frost weathering of the marble by retaining water, and caused pitting and colouring of the rock surface. An enclosed, protective structure, with a fully glazed southeast wall, was constructed over the site to exclude atmospheric precipitation and groundwater run-off, while allowing maximum natural ventilation and passive solar heating of the rock surface. In laboratory tests, the algae showed a high tolerance to dryness, and algae viability within the protective structure was expected to drop only slowly. The algicidal and algistatic effectiveness of ethanol, silver nitrate and five commercial algicides was tested on laboratory cultures of the indigenous algae in case it proved necessary to stop continued growth and expansion of the accretion within the structure. Silver nitrate prevented growth of Chlorococcum sp. at the lowest concentration and shortest contact time; two of the commercial algicides, one containing copper chelates and the other, dichlorophen, also gave good results.