Photosynthesis-Irradiance Relationships for Three Species of Submersed Macrophytes in the Tidal Freshwater Hudson River

Net photosynthesis under a range of natural light intensities was determined for three common macrophytes of the tidal freshwater Hudson River: Vallisneria americana Michx., Potamogeton perfoliatus L., and Myriophyllum spicatum L. Light-saturated net photosynthetic rates did not differ among species, nor were there differences among species in the light intensity at which respiration balanced photosynthesis. The initial slope (α) of the photosynthesis-irradiance (P-I) curve was greater for V. americana than the other two species. As a result, maximum photosynthetic rates were reached in V. americana at significantly lower irradiances than those required to saturate photosynthesis in P. perfoliatus and M. spicatum. This characteristic would give this species a competitive advantage under conditions in which growth is limited by light availability. Macrophyte biomass distribution showed no clear correlation with depth. V. americana was the most abundant of the three species, both in frequency of occurrence and absolute biomass. The low irradiance required for the saturation of photosynthesis in V. americana may contribute to its predominance in the turbid Hudson River.