We compile two data sets from ¹⁴C uptake and dilution experiments conducted in surface waters of the global ocean to investigate the relationship between phytoplankton mass‐specific growth rate and cell size. After temperature correction, both data sets suggest that this relationship might be described by a unimodal quadratic curve with the modal size (the size corresponding to the maximal growth rate) being 2.8 and 5.4 µm in the ¹⁴C and dilution data sets, respectively. Nutrient enrichment does not change the qualitative nature of the relationships, and we conclude that inherently low maximal growth rates of picophytoplankton, not ambient nutrient effects, play the major role in determining the positive relationships over the size range where phytoplankton size is below the modal size. Temperature‐corrected phytoplankton grazing mortality rate is positively correlated with phytoplankton average size, but the proportion of daily primary production consumed by microzooplankton is negatively correlated with cell size, suggesting a reduced grazing effect as size increases. The unimodal relationship between phytoplankton growth rate and cell size is consistent with theoretical considerations and might reflect an adaptive response of phytoplankton to varying extents of nutrient limitation and grazing effect in marine systems.

We examined the dynamics of dissolved inorganic carbon (DIC) and pH in the Pearl River Estuary (PRE) and the adjacent northern South China Sea (NSCS) shelf in summer, aiming for a better understanding of the interaction between eutrophication, hypoxia, and ocean acidification. Using a semi‐analytical diagnostic approach based on validated multiple end‐member water mass mixing models, we showed a −191 ± 54 μmol kg⁻¹ deficit in DIC concentrations in an extensive surface plume bulge, corresponding to a significant pH increase of ∼ 0.57 ± 0.19 units relative to conservative mixing. In contrast, DIC additions in the bottom hypoxic zone reached ∼ 139 ± 21 μmol kg⁻¹, accompanied by a decrease in pH of −0.30 ± 0.04 units. In combination with stable carbon isotopic compositions, we found biological production and CO₂ outgassing to be responsible for DIC deficits in surface waters, while degradation of organic matter (OM) accounted for DIC additions in bottom waters. The PRE‐NSCS plume system as a whole served as a net source of atmospheric CO₂ from the perspective of Lagrangian observations, because strong CO₂ outgassing in the inner estuary overwhelmed the CO₂ uptake in the plume despite strong phytoplankton blooms. Using a two‐layer box model, we further estimated that at least ∼ 45 ± 13% of eutrophication‐driven OM production in the surface plume accounted for 67 ± 18% of the DIC addition and oxygen consumption in bottom waters. Eutrophication also buffered ocean acidification in surface waters while hypoxia enhanced it in bottom waters, but their effects on acid‐base buffering capacity were secondary to the amplification of coastal ocean acidification caused by freshwater inputs.

Diatoms are often a major food source for zooplankton and contribute significantly to vertical POC flux through sinking of dead cells, aggregates and zooplankton fecal pellets. The silica content of diatoms varies among different species and within a species growing under different environmental conditions and physiological status. However, to-date there has been no investigation of the effect of diatom silica content on zooplankton grazing, growth and reproduction. We conducted a series of experiments using the diatom Thalassiosira weissflogii with different silica content achieved by growth under high and low light and these cells were fed to a copepod, Parvocalanus crassirostris. Our results show that this copepod strongly preferred cells with low silica content over high silica-containing cells, with the ingestion rate on low silica diatoms being 2-3 times higher than that on high silica diatoms. Fecal pellet production rate was significantly higher for copepods feeding on highly silicified cells. Furthermore, copepod growth rate (measured as an increase in wet weight), egg production rate and hatching success were all severely compromised under a high silica diatom diet. Females of P. crassirostris feeding on a low silica diatom diet produced an average 90 eggs during a one day incubation, while those fed with high silica diatoms produced only 11 eggs per day. Similarly, the hatching success during a 3-day period was 82 ± 17% and 23 ± 36% for the low and high silica diatom treatments, respectively, with zero success observed in ~65% of the females feeding on high Si diatoms. Our findings have important ecological implications for the biological pump and may alter our previous view of the role of diatoms in planktonic food webs and the role of the degree of silicification in controlling amount of POC flux to deeper waters.

Marine diazotrophs convert dinitrogen (N₂) gas into bioavailable nitrogen (N), supporting life in the global ocean. In 2012, the first version of the global oceanic diazotroph database (version 1) was published. Here, we present an updated version of the database (version 2), significantly increasing the number of in situ diazotrophic measurements from 13 565 to 55 286. Data points for N₂ fixation rates, diazotrophic cell abundance, and nifH gene copy abundance have increased by 184 %, 86 %, and 809 %, respectively. Version 2 includes two new data sheets for the nifH gene copy abundance of non-cyanobacterial diazotrophs and cell-specific N₂ fixation rates. The measurements of N₂ fixation rates approximately follow a log-normal distribution in both version 1 and version 2. However, version 2 considerably extends both the left and right tails of the distribution. Consequently, when estimating global oceanic N₂ fixation rates using the geometric means of different ocean basins, version 1 and version 2 yield similar rates (43–57 versus 45–63 Tg N yr⁻¹; ranges based on one geometric standard error). In contrast, when using arithmetic means, version 2 suggests a significantly higher rate of 223±30 Tg N yr⁻¹ (mean ± standard error; same hereafter) compared to version 1 (74±7 Tg N yr⁻¹). Specifically, substantial rate increases are estimated for the South Pacific Ocean (88±23 versus 20±2 Tg N yr⁻¹), primarily driven by measurements in the southwestern subtropics, and for the North Atlantic Ocean (40±9 versus 10±2 Tg N yr⁻¹). Moreover, version 2 estimates the N₂ fixation rate in the Indian Ocean to be 35±14 Tg N yr⁻¹, which could not be estimated using version 1 due to limited data availability. Furthermore, a comparison of N₂ fixation rates obtained through different measurement methods at the same months, locations, and depths reveals that the conventional ¹⁵N₂ bubble method yields lower rates in 69 % cases compared to the new ¹⁵N₂ dissolution method. This updated version of the database can facilitate future studies in marine ecology and biogeochemistry. The database is stored at the Figshare repository (https://doi.org/10.6084/m9.figshare.21677687; Shao et al., 2022).

Seasonal variation in the phylogenetic composition ofSynechococcusassemblages in estuarine and coastal waters of Hong Kong was examined through pyrosequencing of therpoC1gene. Sixteen samples were collected in 2009 from two stations representing estuarine and ocean-influenced coastal waters, respectively.Synechococcusabundance in coastal waters gradually increased from 3.6 × 10³cells ml⁻¹in March, reaching a peak value of 5.7 × 10⁵cells ml⁻¹in July, and then gradually decreased to 9.3 × 10³cells ml⁻¹in December. The changes inSynechococcusabundance 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-typeSynechococcuswas composed of subcluster 5.2 marineSynechococcus, freshwaterSynechococcus(F-PC), andCyanobium. TheSynechococcusassemblage in the coastal waters, on the other hand, was dominated by marine PE-typeSynechococcus, 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, theSynechococcusassemblage compositions at the two sites were similar and were dominated by subcluster 5.1 clades II and IX and an undescribed clade (represented bySynechococcussp. strain miyav). Clade IXSynechococcuswas a relatively ubiquitous PE-typeSynechococcusfound 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 ofSynechococcusassemblages in Hong Kong waters.

Th , Qingdao 266071, P. R. China To investigate the effects of enhanced nutrient loading in estuarine waters on phyto- plankton growth and microzooplankton grazing, we conducted monthly dilution experiments at 2 stations in Hong Kong coastal waters with contrasting trophic conditions. The western estuarine sta- tion (WE) near the Pearl River estuary is strongly influenced by freshwater discharge, while the east- ern oceanic station (EO) is mostly affected by the South China Sea. Growth rates of phytoplankton were often limited by nutrients at EO, while nutrient limitation of phytoplankton growth seldom occurred at WE due to the high level of nutrients delivered by the Pearl River, especially in the sum- mer rainy season. Higher chlorophyll a, microzooplankton biomass, phytoplankton growth and microzooplankton grazing rates were found at WE than at EO. However, the increase in chlorophyll greatly exceeded the increase in phytoplankton growth rate, reflecting different response relation- ships to nutrient availability. Strong seasonality was observed at both stations, with temperature being an important factor affecting both phytoplankton growth and microzooplankton grazing rates. Picophytoplankton, especially Synechococcus, also exhibited great seasonality at EO, with summer abundances being 2 or 3 orders of magnitude higher than those during winter. Our results confirm that in eutrophic coastal environments, microzooplankton grazing is a dominant loss pathway for phytoplankton, accounting for the utilization of >50% of primary production on average.