MINIMUM LIGHT REQUIREMENTS FOR SUBSTRATE COLONIZATION BY CLADOPHORA-GLOMERATA

Cladophora glomerata, an attached filamentous green alga that inhabits streams and lakes in North America, can produce large quantities of standing crop in areas of the Laurentian Great Lakes. An integrated laboratory and field survey approach was used to define the minimum light intensity required to support substrate colonization by Cladophora. In a laboratory light gradient experiment, negligible growth was observed at light intensities less than 29-mu-Em-2S-1. Field surveys of the vertical distribution of Cladophora were undertaken in western Lake Erie to determine if the minimum laboratory light intensity was also observed in nature. Surveys were conducted utilizing SCUBA techniques during the peak biomass periods in late June over 3 years. The depth of Cladophora colonization varied from site to site depending on light attenuation, with the maximum depth corresponding to areas having the highest Secchi transparencies and lowest light extinction coefficients. Paired observations of photosynthetically active radiation (PAR) profiles and Cladophora deepest depth of colonization indicated that average daily intensities below 28-mu-Em-2S-1 were limiting to Cladophora colonization. A model is presented that predicts the maximum depth of Cladophora colonization based on the laboratory minimum light level and site-specific field light attenuation coefficients.