DNA synthesis and cell division of Prochlorococcus are tightly synchronized to the daily light cycle, therefore, cell division rates can be estimated from the fraction of cells in each cell cycle stage during a 24 h sampling period. The total mortality rate of Prochlorococcus can also be estimated from the difference between the observed cell abundance and the expected cell number projected from growth rate in that sampling period, providing an estlmate of grazing impact. Growth and mortality rates of Prochlorococcus were investigated at 2 statlons In the equatorial Pacific, as well as Station ALOHA in the subtropical North Pacific Ocean. Growth rate of Prochlorococcus remained high at all sites independent of the nitrate concentration. The maximum growth rate (up to 1 doubling d-') occurred at 70 m depth at the western equatorial Pacific site (166' E) and at 40 to 45 nl at the eastern equatorial Pacific site (150' W) and at Station ALOHA (22" 45' N, 158' W). Total mortality rates were roughly balanced by Prochlorococcus growth at all stations. Because of the phased cell cycle and continuous (if not evenly distributed) mortality, the abundance of Prochlorococcus at each depth could vary up to 2-fold between afternoon and midnight. Prochlorococcus production was estimated to contribute 9 and 39% to the total gross primary production in the eastern and western equatorial Pacific, respectively, and up to 82% in the subtropical North Pacific Ocean at Station ALOHA. Our results suggest Prochlorococcus are not severely nutrient-limited in the oligotrophic environment. Rapid nutrient recycling by grazing activity permits Prochlorococcus to contribute a significant fraction of the total primary production.

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[1] A coupled three-dimensional physical model and a nitrogen-based dissolved inorganic nitrogen, phytoplankton, zooplankton, and detritus (NPZD) ecosystem model was used to study the ecosystem responses to the wind-driven summer upwelling and to the Pearl River plume over a distinctly widened shelf in the northeastern South China Sea (NSCS). Forced with an idealized, but representative, upwelling-favorable wind and the river discharge for the purpose of process-oriented study, we identified two high chlorophyll centers that are typically observed over the NSCS shelf and stimulated by nutrient enrichment from intensified upwelling over the widened shelf and from the river plume. The nutrient enrichment has strong along-shore variability involving the variable cross-isobath nutrient transport between the middle and the inner widened shelf during the upwelling and an eastward expansion of the nutrient-rich plume. About 20% of the upwelled nutrientrich deep water from the outer shelf reaches the inner shelf where algal blooms occur. Nutrient enrichment in the plume stretches over a broad extent of the shelf and produces significant biomass on the NSCS shelf. The plume is physically governed by intensified surface Ekman dynamics that leads to a strong offshore nutrient transport and eventually offsets the shoreward transport caused by the upwelling in the NSCS. Biological forcing and circulation dynamics of the surface Ekman layer jointly form the spatial dislocation and temporal variation of NO 3 , phytoplankton, and zooplankton biomasses in the upwelled and plume waters. The simulated results qualitatively resemble field and satellite measurements and demonstrate the physically modulated biological responses to the intensified upwelling and plume-influenced NSCS shelf.

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We conducted 28 dilution experiments during August-September 2007 to investigate the coupling of growth and microzooplankton grazing rates among ultraphytoplankton populations and the phytoplankton community and their responses to habitat variability (open-ocean oligotrophy, eddy-induced upwelling, and the Mekong River plume) in the western South China Sea. At the community level, standing stocks, growth, and grazing rates were strongly and positively correlated, and were related to the higher abundance of larger phytoplankton cells (diatoms) at stations with elevated chlorophyll concentration. Phytoplankton growth rates were highest (.2 d 21 ) within an eastward offshore jet at 13uN and at a station influenced by the river plume. Among ultraphytoplankton populations, Prochlorococcus dominated the more oceanic and oligotrophic stations characterized by generally lower biomass and phytoplankton community growth, whereas Synechococcus became more important in mesotrophic areas (eddies, offshore jet, and river plume). The shift to Synechococcus dominance reflected, in part, its higher growth rates (0.87 6 0.45 d 21 ) compared to Prochlorococcus (0.65 6 0.29 d 21 ) or picophytoeukaryotes (0.54 6 0.50 d 21 ). However, close coupling of microbial mortality rates via common predators is seen to play a major role in driving the dominance transition as a replacement of Prochlorococcus, rather than an overprinting of its steady-state standing stock.

bSeasonal variation in the phylogenetic composition of Synechococcus assemblages in estuarine and coastal waters of Hong Kong was examined through pyrosequencing of the rpoC1 gene. Sixteen samples were collected in 2009 from two stations representing estuarine and ocean-influenced coastal waters, respectively. Synechococcus abundance in coastal waters gradually increased from 3.6 ؋ 10 3 cells ml ؊1 in March, reaching a peak value of 5.7 ؋ 10 5 cells ml ؊1 in July, and then gradually decreased to 9.3 ؋ 10 3 cells ml ؊1 in December. The changes in Synechococcus abundance in estuarine waters followed a pattern similar to that in coastal waters, whereas its composition shifted from being dominated by phycoerythrin-rich (PE-type) strains in winter to phycocyanin-only (PC-type) strains in summer owing to the increase in freshwater discharge from the Pearl River and higher water temperature. The high abundance of PC-type Synechococcus was composed of subcluster 5.2 marine Synechococcus, freshwater Synechococcus (F-PC), and Cyanobium. The Synechococcus assemblage in the coastal waters, on the other hand, was dominated by marine PE-type Synechococcus, with subcluster 5.1 clades II and VI as the major lineages from April to September, when the summer monsoon prevailed. Besides these two clades, clade III cooccurred with clade V at relatively high abundance in summer. During winter, the Synechococcus assemblage compositions at the two sites were similar and were dominated by subcluster 5.1 clades II and IX and an undescribed clade (represented by Synechococcus sp. strain miyav). Clade IX Synechococcus was a relatively ubiquitous PE-type Synechococcus found at both sites, and our study demonstrates that some strains of the clade have the ability to deal with large variation of salinity in subtropical estuarine environments. Our study suggests that changes in seawater temperature and salinity caused by the seasonal variation of monsoonal forcing are two major determinants of the community composition and abundance of Synechococcus assemblages in Hong Kong waters. Members of the Synechococcus group of widely distributed and abundant picocyanobacteria are important primary producers in the surface waters of global oceans (1). Strains of Synechococcus are both phenotypically and phylogenetically diverse and dynamic (2, 3). Based on gene markers, like the 16S rRNA gene, marine Synechococcus strains form a well-defined clade termed cluster 5 (4, 5), which is divided into 3 subclusters: 5.1, 5.2, and 5.3. Of these, subcluster 5.1 is the most abundant and diverse subcluster in marine environments and is further divided into at least 9 clades (6). The high genetic diversity of Synechococcus is reflected in the ecogeographic and temporal distribution of different ecotypes. Subcluster 5.1 clade I mainly dominates in temperate mesotrophic ocean waters, while clade II is mainly present in offshore, continental shelf, and oligotrophic warm waters (4, 7). Subcluster 5.2 Synechococcus strains are phycocyanin-enriched (PC-type) euryhaline s...