Water column transparency and the distribution of spectrally distinct forms of phycoerythrin-containing organisms

Predominance of Type I (phycourobilin-containing) and Type II (phycourobilin-lacking) phycoerythrins (PE) was examined using scanning fluorescence spectroscopy at 176 stations in the northwestern Atlantic off the northeast coast of the United States. Simultaneous optical measurements were made at 75 stations, permitting an analysis of the distribution of spectral types of PE-containing organisms based on geographic position of the stations and on the relative penetration of blue and green wavelengths of light. Stations dominated by Type I PE occurred almost exclusively in very transparent water with high transmissivity for blue light [domnwelling attenuation coefficient; K-d(440) < 0.12] and relatively low attenuation of blue light relative to green light. This pattern was reversed for Type II PE, which dominated in less transparent waters with relatively high attenuation of blue light relative to green light. Type II PE tended to dominate on the continental shelf and slope, and Type I PE tended to dominate in the Sargasso Sea. Regardless of geographic location, there was a transition from dominance by Type I PE to Type II PE as the ratio K-d(440)/K-d(550) exceeded 1.25. Our data suggest that optical parameters are important niche dimensions for marine Synechococcus and that nearshore waters may be classified optically by phycoerythrin characterization.