Synechococcus growth and mortality rates in the East China Sea: range of variations and correlation with environmental factors

During the period between December 1997 and November 1998, four cruises were conducted in the East China Sea with the purpose of investigating the growth and grazing mortality rates of an autotrophic picoplankton, Synechococcus spp. In the euphotic zone, the mean growth rate of individual cruises varied from 0.20 day−1 in March to 0.68 day−1 in June. By comparison, the mean grazing rate varied from 0.07 day−1 in March to 0.40 day−1 in June. Both the mean growth and the mean grazing rates apparently varied with seasonal changes in water temperature. With respect to spatial variations, the pattern during the cold period of the year (cruises in December and March) was characterized by low growth rate in the coastal zone (<0.2 day−1) and a moderate-to-high growth rate in the shelf-break zone. In contrast, a reversed pattern was observed on cruises in June and October. The growth rate in the coastal zone increased to greater than 0.6 day−1, and a decreasing trend was observed along the inshore-to-offshore direction. In addition to temperature, nutrients also might be important in controlling the spatial distribution of Synechococcus growth, especially in regions with water temperatures higher than 16°C. On the other hand, the grazing mortality rate did not show a clear pattern of spatial distribution. On average, Synechococcus spp. in the East China Sea contributed 5–63% of total primary production. However, a comparison between the growth rate and grazing mortality revealed that more than 37% of Synechococcus production was not consumed by grazers in the euphotic zone.