Observations of a Synechococcus-dominated cyclonic eddy in open-oceanic waters of the Arabian sea

The optical, chemical and biological characteristics of a cyclonic eddy were investigated in the Arabian Sea during November 1995. This eddy was 3,000 km(2) in area and located 350 km offshore of the coast of Oman. The mixed layer of this feature extended to a depth of 17 m, below which oxygen concentrations were depleted to 10% of surface values. Chlorophyll a concentrations within the mixed layer averaged 1.7 mg m(-3). Microscopic observations and flow cytometric measurements revealed that the algal community was dominated by the coccoid cyanobacterium Synechococcus. Detailed pigment analyses documented additional phytoplankton biomass contributions by Prochlorococcus, diatoms, dinoflagellates, prymnesiophytes, cryptophytes, chlorophytes and pelagophytes. This algal community caused a marked attenuation of the blue to bluegreen wavelengths of light, resulting in a preferential transmittance of green light with increasing depth. Measurements of photosynthetic performance and the spectral absorption coefficient document that the phytoplankton community was photo- and chromatically-adapted to the light environment within the eddy. The results of this field work support a previous laboratory study which found that the nonphotosynthetic carotenoid zeaxanthin produces significant decreases in the maximum quantum yield of photosynthesis of the marine cyanobacterium Synechococcus.