Abundance and cellular characteristics of marine Synechococcus spp. in the dilution zone of the Changjiang (Yangtze River, China)

Abstract

Concentrations of the marine cyanobacterium Synechococcus were estimated in the dilution zone of the Changjiang (Yangtze River, China) using epifluorescence microscopy while cell size and photosynthetic pigment content were measured with flow cytometry. Cyanobacteria concentrations (10 2–10 3 cell ml −1 in winter and 10 2–10 5 cell ml −1 in summer) increased in the offshore direction and reached their maximum at the most marine stations in winter and at stations of intermediate salinities (25–30 g l −1) in summer. In the latter season, concentrations in excess of 10 4 cell ml −1 developed in the surface layer when the euphotic depth increased beyond 10 m as a result of suspended matter sedimentation. Cell concentrations fell by an order of magnitude below the thermocline in comparison to the surface layer and the highest values were associated with the saline waters of the Taiwan Current. In summer Synechococcus cell size (as measured by light scattered at 90° angle) was the smallest in the region of maximum cell concentration, probably in relation to the high population growth rate there. Cell phycoerythrin content (measured by fluorescence) was related to light availability and to a lesser extent to inorganic nitrogen concentration.