Factors Controlling Photosynthetic Productivity in a Population of Cladophora Fracta (Chlorophyta)

Cladophora fracta is the dominant filamentous alga in the littoral zone of Shoe Lake, Oakland County, Michigan. Seasonal and diurnal in situ productivity studies were combined with perturbations of several environmental parameters conducted in situ and in the laboratory to determine the factors that control primary productivity in this population. In situ net photosynthetic carbon fixation ranged from 0.26 to 5.39 mg°g— ¹°h— ¹, with seasonal maxima in spring and fall and diurnal patterns generally, but not always, correlating with daily insolation patterns. Phosphorus and nitrogen were above limiting concentrations throughout the year, and C. fracta was unresponsive to experimental P and N enrichments. The seasonal productivity correlated most strongly with total alkalinity (inorganic carbon content) and pH, and experimental enrichment of inorganic carbon stimulated photosynthesis during the summer. C. fracta was characterized in the laboratory as basically a C₃ plant on the basis of very low levels of C₄ acids in early ¹ ⁴C photosynthetic products and light saturation of photosynthesis below 30 klx. In situ experiments showed oxygen inhibition of photosynthesis, but photorespiration was not directly demonstrable by evaluation of ¹ ⁴CO₂ release, and the CO² compensation point was low. Release of organic photosynthate was stimulated by high oxygen, but the effect was maximal during spring and fall growth peaks rather than in the summer. Optimal photoperiod was experimentally determined to be relatively short (8 h), which correlated with the fall and spring maxima in productivity and carbon availability. Temperature in situ was below the experimentally determined optimum for photosynthesis in both spring and summer, and was associated with daily patterns of photosynthesis more closely than with the seasonal pattern. Epiphytic flora on C. fracta reduced organic carbon release to a small extent and contributed to a slight enhancement of community productivity. The productivity of C. fracta during the ice—free growing season in Shoe Lake appears to be limited principally by inorganic carbon and by oxygen inhibition of photosynthesis. C. fracta competes significantly on the basis of rapid growth capability early in the season coincident with maximal carbon availability and later possibly by minimization of photorespiration.