Factors Influencing Photosynthetic Productivity ofChara vulgarisL. in a Moderately Productive Hardwater Lake

In situ seasonal patterns and ecophysiology of photosynthesis were investigated in the macroalga Chara vulgaris L. growing in Shoe Lake, Michigan, in an effort to understand controlling factors and to determineé the role of photorespiration. C. vulgaris was shown to be a C3 plant with inducible photorespiration. Its photosynthetic productivity in Shoe Lake in early summer was maximal for submersed macrophytes. Subsequent seasonal declines and dialy fluctations in productivity were correlated with presence of photorespiration, but the photorespiration represented a minor percentage of photosynthesis. In the shallow light-saturated water both nitrogen and carbon availability were limiting factors at times during mid- and late summer.