We analyzed seasonal and diel fluctuation patterns of heterotrophic bacteria, Synechococcus spp., and nanoflagellates at a coastal station at the southern edge of the East China Sea. Synechococcus spp. and nanoflagellates exhibited diel fluctuation at water temperatures above 25ЊC. Cell concentrations of Synechococcus spp. were significantly higher during the evening, whereas those of nanoflagellates were higher during the day. The day and night amounts of heterotrophic bacteria did not differ significantly, and we did not observe diel rhythms in these organisms below 25ЊC. The fractionation experiments we performed between August and October showed that growth rates of bacteria were high (0.73-1.00 g C L Ϫ1 h Ϫ1) during the day. However, because there was an increase in nanoflagellate grazing, there was no change in the abundance of bacteria over the day. Synechococcus spp. was not actively consumed by nanoflagellates during the day, but its rate of production was exceeded by the rate of grazing by nanoflagellates during the night. This out-of-phase Synechococcus spp. growth and mortality caused by grazing created diel variations in its abundance. We also found that picoplankton contributed 24-36 g C L Ϫ1 d Ϫ1to the microbial loop, and Synechococcus spp. and bacteria contributed equally to this carbon flux.

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...

We investigated temporal variations in the effects of bacterivory by different sizes of heterotrophic nanoflagellates (HNF) and pigmented nanoflagellates (PNF) during a warm period (May to September) in oligotrophic coastal waters of the subtropical western Pacific. Short-term experiments with fluorescently labeled bacteria (FLB) demonstrated ingestion rates of 0.3 to 5.8 bacteria HNF -1 h -1 by HNF in the size range 3-6 µm -rates that were higher than observed for other sizes of HNF. Rates of ingestion by PNF ranged between 0.9 and 15.5 cells PNF -1 h -1, and, as for HNF, were greatest for PNF in the 3-6 µm size group. Nanoflagellates of size < 6 µm removed about 98% of the total amount of bacteria consumed. The 3-6 µm PNF, 2-3 µm HNF, and 3-6 µm HNF were major consumers in the nanoflagellate community and were responsible for an average of 52, 28 and 16% of the total consumption of bacteria, respectively. The smallest PNF (2-3 µm) consumed only about 2% of the total and were considered to be primarily autotrophic. Despite ingestion rates in the range of those reported elsewhere, the low abundance of nanoflagellates observed resulted in relatively low grazing impacts (<10% of bacterial standing stock). We found a significant negative correlation between PO 4 concentrations and ingestion rates of the 3-6 µm PNF, suggesting that the PNF ingestion rate increased under nutrient-deficient conditions.

Since viral lysis and nanoflagellate grazing differ in their impact on the aquatic food web, it is important to assess the relative importance of both bacterial mortality factors. In this study, an adapted version of the modified dilution method was applied to simultaneously estimate the impact of both virus and nanoflagellate grazing on the mortality of heterotrophic bacteria. A series of experiments was conducted monthly from April to December 2011 and April to October 2012. The growth rates of bacteria we measured ranged from 0.078 h⁻¹ (April 2011) to 0.42 h⁻¹ (September 2011), indicating that temperature can be important in controlling the seasonal variations of bacterial growth. Furthermore, it appeared that seasonal changes in nanoflagellate grazing and viral lysis could account for 34% to 68% and 13% to 138% of the daily removal of bacterial production, respectively. We suggest that nanoflagellate grazing might play a key role in controlling bacterial biomass and might exceed the impact of viral lysis during the summer period (July to August) because of the higher abundance of nanoflagellates at that time. Viral lysis, on the other hand, was identified as the main cause of bacterial mortality between September and December. Based on these findings in this study, the seasonal variations in bacterial abundance we observed can be explained by a scenario in which both growth rates and loss rates (grazing + viral lysis) influence the dynamics of the bacteria community

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