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Pigment concentrations and photosynthetic rates of phytoplankton in Chesapeake Bay were examined to determine shortand long-term responses to light. Samples were collected horizontally along a turbidity gradient and vertically relative to the depths of the pycnocline and photic zone. This approach provided phytoplankton assemblages that were exposed to light differing in both ~ntensity and time-scale of change. Photosynthesis-irradiance (P-I) curves and the parameters aB and PmB were assayed at light intensities and wavelength bands that occur in situ. Short-term (minutes to hours) changes in light elicited responses in the distribution of aB and PmB coordinate pairs along a single line. Long-term (days to weeks) differences in light elicited a change in the relation of a* to PmB, manifested as a shifted slope of the linear correlation of these parameters. Photosynthetic efficiencies were similar in light sources of narrow ('blue-green' or 'orange') and broad spectra ('white'), indicating the successful harvesting of light of these spectral qualities by photosynthetic pigments in resident phytoplankton. This similarity in photosynthetic efficiency represents a long-term response that may depend on floral composition and differences In pigmentation. Concentrations and ratios of photosynthetic pigments revealed seasonal and spatial differences in floral composition of phytoplankton in Chesapeake Bay. The presence of chrysophytes, diatoms, and dinoflagellates was indicated by low molar ratios of chl a and c during spring and summer, while the abundance of cyanobacteria (bluegreen algae) was documented by high a:b and a:c ratios in autumn. These findings have implications for the use of P-I curve parameters in diagnosing photoadaptive responses by phytoplankton and for the seasonal abundance of cyanobacteria possibly associated with eutrophication of Chesapeake Bay.

Photosynthesis patterns in Chesapeake Bay phytoplankton: short- and long-term responses of P-l curve parameters to light