Seasonal variations in trophic dynamics of nanoflagellates and picoplankton in coastal waters of the western subtropical Pacific Ocean

Abstract

This investigation was undertaken between August 2002 and July 2003 at a coastal station on the southern edge of the East China Sea. We found a 2-phase (warm season, > 25°C [June to October] and cold season, < 25°C [November to May]) seasonal cycle with a 10-fold variation in the growth of bacteria (heterotrophic bacteria only) and picophytoplankton, primarily coccoid cyanobacteria (Synechococcus spp.), and nanoflagellate grazing rates upon them. Growth rate in bacteria and Synechococcus spp. appeared to be affected by changes in temperature, and the nanoflagellate grazing rate was controlled by concentrations of bacteria and Synechococcus spp. The seasonal cycles of abundance in bacteria and Synechococcus spp. were a reflection of their changing net growth rates (i.e. picoplankton growth rates -nanoflagellate grazing rates), which were highest at the beginning of the warm season. During the warm season, nanoflagellates consumed an equal amount of bacteria and Synechococcus spp.; therefore, growth in both groups was affected equally by grazing in the warm season. However, during the cold season, bacteria contributed more to nanoflagellate carbon consumed than did Synechococcus spp. because the growth rate of Synechococcus spp. was low. We conclude that during the warm season a significant part of bacteria and Synechococcus spp. carbon is channeled through the microbial loop, possibly making it an important link between primary production and higher trophic levels. KEY WORDS: Synechococcus spp. · Picoplankton · Nanoflagellate · Microbial loop Resale or republication not permitted without written consent of the publisherAquat Microb Ecol 51: [263][264][265][266][267][268][269][270][271][272][273][274] 2008 nutrient concentration and seawater temperature (Tsai et al. 2005), these observations suggest that bacterial production rates are weakly related to temperature over the summer, but strongly affected by temperature during cold months. Similarly, Ochs et al. (1995) found that bacterial growth rates were unrelated to temperatures above 14°C, which was further confirmed by Shiah & Ducklow (1994). Their findings suggest that temperature might control bacterial growth activity during the colder months. Growth in the warmer seasons seems to be controlled by factors such as substrate supply or availability of nutrients (Keil & Kirchman 1991). Tsai et al. (2005) found a significant diel variation in bacterial growth, but there is a lack of data on the seasonal variation in bacterial growth and of nanoflagellate grazing rates, and on factors controlling the effect of nanoflagellate grazing on bacteria throughout the year. One study (Choi 1994), however, suggested that water temperature and prey density are among the most important factors regulating the seasonal grazing rate on bacteria by protists.Picophytoplankton, predominantly coccoid cyanobacteria (Synechococcus spp.), on the other hand, can make up a major proportion of the phytoplankton biomass and production in oceanic waters (Olson et al. 1990), and...

Population dynamics and diversity of Synechococcus on the New England Shelf

“…This may indicate a grazer saturation concentration for which grazers (if only ingesting Synechococcus) are no longer food-limited and can achieve their maximum division rates. For nanoflagellates observed in other studies, these end-bloom concentrations appear to be just slightly below ingestion saturation concentrations [20,109].…”

“…Strong relationships between division rate and temperature and abundance and temperature have been demonstrated for Synechococcus in temperate regions [114,1,58,60,109]. Studies investigating grazing on Synechococcus over seasonal cycles have demonstrated that heterotrophic grazers are a signification source of mortality and, depending on the season, can control Synechococcus abundances [1,109,8]. Our investigation of one annual cycle of the Synechococcus population at the Martha's Vineyard Coastal Observatory (MVCO) also found that temperature and loss rates are important.…”

Importance of the Viral Shunt in Nitrogen Cycling in Synechococcus Spp. Growth in Subtropical Western Pacific Coastal Waters

“…Specific resources (inorganic nutrients) and temperature are reported to be the main limiters of growth in Synechococcus spp. (Chang et al 1996;Carlsson and Caron 2001;Tsai et al 2005), although in warmer seasons the availability of nutrients appears to be the strongest factor (Tsai et al 2008). Data from nutrient dilution experiments suggest that Synechococcus spp.…”

“…Therefore, Synechococcus spp. abundance is less than 1 × 10 4 cells mL -1 during low temperatures when there are no clear diel variations in abundance (Tsai et al 2008). Christaki et al (2002) reported that heterotrophic nanoflagellate (HNF) grazing on Synechococcus spp.…”

“…Furthermore, during warm seasons, Synechococcus spp. shows distinct diel changes in abundance, with higher division rates at dusk (Dolan and Šimek 1999;Christaki et al 2002;Tsai et al 2009) and maximum abundances at night (Christaki et al 2002;Tsai et al 2008Tsai et al , 2009). Therefore, Synechococcus spp.…”