Differences in cell viabilities of phytoplankton between spring and late summer in the northwest Pacific Ocean

Abstract

Cell viabilities of phytoplankton in the Oyashio and Kuroshio-Oyashio transition regions of the northwest Pacific Ocean were examined in September 2003 (late summer) and May 2005 (spring) using a membrane permeability test. Specific lysis rates of the phytoplankton during late summer were also assessed by an esterase activity assay. In late summer, cyanobacteria Synechococcus spp. were > 2 × 10 4 cells ml − 1 and numerically dominated the phytoplankton communities. The cell viabilities of Synechococcus spp. and eukaryotic ultraphytoplankton (< 10 μm in size) were 60–79% and 26–41% in surface waters, respectively. The specific lysis rates of the phytoplankton were 0.12–0.67 d − 1 in late summer. By contrast, in spring, eukaryotic cells were predominant in the phytoplankton communities. The cell viabilities of surface eukaryotic ultraphytoplankton in spring were > 70% and significantly higher than those in late summer. During spring, Synechococcus spp. only occurred with < 1 × 10 4 cells ml − 1 in the Kuroshio-Oyashio transition region, and their viabilities were 80%. In the Oyashio region where a spring diatom bloom developed, the viability of fucoxanthin-containing algae (mainly diatoms and prymnesiophytes) was ca. 90%. These results suggested that the cell viability of phytoplankton could vary seasonally with their community structure in the study area. The phytoplankton cell death in late summer was particularly significant for their loss process and could support the microbial food webs by supplying dissolved organic carbon (DOC) derived from the dead cells.