Quantitative evaluation of the copepod feeding process is critical for understanding the functioning of marine food webs, as this provides a major link between primary producers and higher trophic levels. In this study, a molecular protocol based on quantitative polymerase chain reaction (qPCR) targeting 18S rDNA was developed and used to investigate the feeding and digestion rates of the copepod Acartia erythraea in a laboratory experiment using microalgae Thalassiosira weissflogii, Prorocentrum shikokuense, and Alexandrium catenella as prey. Although offered an equal encounter rate based on biovolume, prey uptake varied substantially among the three algal species, with the ingestion rate (IR) and digestion rate (DR) of A. erythraea differing significantly (P < 0.001) based on both cell counting and qPCR detection. Acartia erythraea showed the highest IR (2.79 × 10⁴ cells/(ind.·h)) and DR (2.43 × 10⁴ cells/(ind.·h)) on T. weissflogii, and the lowest amounts of ingested P. shikokuense were detected. The highest assimilation rate (~90.64%, IR/DR) was observed in copepods fed with P. shikokuense. The qPCR method used here can help determine the digestion rate and assimilation rate of copepods by detecting cells remaining in the gut hence providing the possibility to examine trophic links involving key species in the marine ecosystem. Our results indicate that A. erythraea has diet-specific feeding performance in different processes, and a quantitative assessment of copepod feeding is needed to accurately determine its functional role in the energy and matter uptake from marine food webs.

Prokaryotic diversity and community composition in the water column of eight stations (63 samples) around the Antarctic Peninsula of the Southern Ocean were investigated. Through pyrosequencing of the V3–V4 hypervariable regions of the 16S ribosomal RNA gene, we characterized 4 720 089 valid reads representing 48 188 operational taxonomic units (OTUs, 97% similarity). The community was dominated by the phyla Pseudomonadota (original name: Proteobacteria, 47%), Oxyphotobacteria (26%), and Bacteroidota (original name: Bacteroidetes, 18%), which comprised an average of 91% of the total OTUs in all samples. The prokaryotic community composition varied vertically within the water column. Water column prokaryotic communities exhibited a clear depth profile, with higher microbial richness and higher diversity observed with increasing water depth. Cluster analysis of the community composition of water column samples exhibited a similar trend with depth. Correlation with environmental factors suggested distinct variation in prokaryotic community composition with changes in depth, salinity, temperature and dissolved oxygen levels. Functional prediction showed presence of active nitrogen, sulphur and methane metabolic cycles along the vertical transect of the studied region. These results will improve our knowledge of prokaryotic diversity and community composition at different depth of water column for better understanding of the microbial ecology and nutrient cycles in Antarctic Peninsula region of the Southern Ocean.

It is widely acknowledged that the distribution of macrobenthos is affected by salinity, but the degree of influence varies in different areas. To explore the distribution pattern of macrobenthic assemblages in the Hangzhou Bay, 12 stations were sampled to collect macrobenthos and the corresponding bottom water. Changes in the general characteristics of macrobenthos along the salinity gradient in the Hangzhou Bay and its adjacent waters were considered. Three dominant species were identified, including the polychaetes Sternaspis chinensis, the crustacea Oratosquilla oratoria and the echinoderm Ophiuroglypha kinbergi . And the macrobenthic assemblages showed a zonal distribution along with the salinity change. The correlation analyses showed that salinity, depth, temperature, suspended solids and dissolved oxygen had concurrent significant correlations with carnivorous group, Margalef species richness (d), Brillouin index (H) and Shannon-Wiener diversity index (H’). In light of the strong correlation between salinity and Changjiang River diluted water, which produces considerable disturbances by freshwater inflows, the deposition of suspended solids and the resuspension of seabed sediments, the combined environmental disturbances, instead of salinity alone, should be adopted to explain the zonation distribution pattern of macrobenthic assemblages.

Channel dredging in estuaries increases water depth and subsequently impacts sediment dynamics and morphology. The Jiaojiang River Estuary is dredged frequently owing to heavy shipping demands. In this study, the effects of different dredging schemes on siltation were assessed through numerical modeling. The sediment model of the Jiaojiang River Estuary utilized an optimized bottom boundary layer model that considered the bed sediment grain size and fluid mud, and this model was calibrated using field data. Result reveal that channel dredging modifies the flow velocity inside and around the channel by changing the bathymetry; subsequently, this affects the residual current, bed stress, suspended sediment concentration, and sediment fluxes. Increasing the dredging depth and width increases the net sediment fluxes into the channel and dredging depth has a greater influence on the channel siltation thickness. When the dredging depth is 8.4 m or11.4 m, the average siltation thickness of the channel is 0.07 m or 0.15 m per mouth respectively. The parallel movement of the channel has small effects on the siltation volume during the simulation period. The sediment deposits in the channel primarily originates from the tidal flats, through bottom sediment fluxes. Vertical net circulation has a dominant impact on siltation because the difference of horizontal current of each layer on the longitudinal section of the channel increases, which intensifies the lateral sediment transport between the shoal and channel. The influence of vertical frictional dissipation on the lateral circulation at the feature points accounts for more than 50% before dredging, while the non-linear advective term is dominant after dredging. Tidal pumping mainly affects the longitudinal sediment fluxes in the channel. These results can be used for channel management and planning for similar estuaries worldwide.

Sand waves in the Taiwan Shoal are characterized by two distinct spatial scales. Giant sand waves have a length of 2 kilometers with height between 5 m and 25 m, whilst small sand waves is less than 100-m long with height less than 5 m between giant sand wave peaks (crests). A series of five high-resolution multi-beam echo-sounding surveys between 2012 and 2020 in the middle of Taiwan Shoal indicated that artificial dredging on the giant sand waves had caused sand wave reform and evolution. Overall, the removal of giant sand waves significantly affected the migration of small sand waves adjacent to the dredging site, with the latter on both sides of the former appear to migrate towards the dredging pit. Moreover, in the dredging area, new sand waves emerged with wavelength much smaller than the original giant sand waves, while the convergent pattern of the small sand waves tends to store and form the giant sand waves, which might spread far beyond the survey period.

In the open ocean, radium isotopes are useful tracers of residence time and water-mass mixing. However, limited by the measurement resolution of commonly used gamma counters, the low activity of radium in the open ocean makes it necessary to enrich radium from large volumes of seawater and pretreat radium-enriched carriers prior to measurements. The commonly applied method of radium enrichment and pretreatment, however, has limitations of uneven coating of MnO₂ on cartridges, relatively expensive cartridges, time-consuming issues during cartridge-ashing, ash loss during transfer, and changes of gamma counters efficiency caused by different ash weights. To address these issues, in this study we optimized the enrichment and pretreatment of low-activity radium prior to measurements. Firstly, we replaced commonly used acrylic cartridges with cheaper polypropylene cartridges, which took 6 h to be ashed, 42 h shorter than for acrylic cartridges. Secondly, MnO₂-coated cartridges were prepared with a circulating hot acidic KMnO₄ solution to ensure homogeneous coating. The radium extraction efficiency of this MnO₂-coated cartridge was 20%–61% higher than that prepared by directly immersing cartridges in the solution. The radium delayed coincidence counter efficiency for MnO₂-coated cartridge was stable with a moisture content of 0.05–1. Lastly, after ashing cartridges, instead of directly transferring the ash to a measurement vial, a mixture of hydroxylamine hydrochloride and hydrochloric acid was used to completely leach the ash for long-lived radium, followed by coprecipitation by BaSO₄, to avoid potential loss of ash during transfer and variations in measurement geometry due to different ash weights. And the recovery of long-lived radium pretreatment was 94%–102%, which improved by 11% compared with the common method. In addition, the radium extraction efficiency of the MnO₂-coated cartridge varied from 3% to 4% within the in situ pump working flow rate of 4–7 L/min, which fell within the measurement errors.

Submarine groundwater discharge (SGD) plays a major role as a conveyor of metals to coastal waters. However, the seasonal change of metal fluxes derived through SGD is unclear. Here, we evaluated the behaviours and fluxes of trace metals (Mn, Fe, Ba, Pb, U, Cr, Zn, Cu) in an estuary under different seasonal conditions. The behaviours of trace metals revealed that SGD was the source of Mn (3.51 mmol/(m²·d)), Fe (0.174 mmol/(m²·d)) and Ba (0.024 mmol/(m²·d)), but the Cu sink (−0.55 μmol/(m²·d)) and other metals exhibited a seasonal source‒sink conversion. The seasonal variation of dissolved organic matter and the fresh groundwater proportion in subterranean estuaries may have an important effect on metals fluxes especially for the Fe, Mn and Ba. Our result shows that the single seasonal metal fluxes estimation applied to the annual scale will cause a large deviation, up to 3.6 times for Fe, 5.5 times for Mn, and 15 times for Ba. Therefore, the influence of seasonal fluctuations on SGD-derived metal fluxes cannot be ignored, and our findings will be important for comprehending the metal budget and cycle in nearshore environment.

Radon (Rn) is a naturally occurring radioactive inert gas in nature, and ²²²Rn has been routinely used as a powerful tracer in various aquatic environmental research on timescales of hours to days, such as submarine groundwater discharge. Here we developed a new approach to measure ²²²Rn in discrete water samples with a wide range of ²²²Rn concentrations using a Pulsed Ionization Chamber (PIC) Radon Detector. The sensitivity of the new PIC system is evaluated at 6.06 counts per minute for 1 Bq/L when a 500 mL water sample volume is used. A robust logarithmic correlation between sample volumes, ranging from 250 mL to 5000 mL, and system sensitivity obtained in this study strongly suggests that this approach is suitable for measuring radon concentration levels in various natural waters. Compared to the currently available methods for measuring radon in grab samples, the PIC system is cheaper, easier to operate and does not require extra accessories (e.g., drying tubes etc.) to maintain stable measurements throughout the counting procedure.

Phosphorus (P) is an essential nutrient for many organisms in the ocean, which plays a central role in the stability of ecosystems and the evolution of the environment. The distribution, occurrence and source-sink process of P in offshore waters are highly influenced by mariculture activities. P transformation in water-sediment system is the key process in P cycling, however, the mechanism is poorly documented in the coastal sea which is influenced by human activities. Based on the comprehensive surveys in the adjacent waters outside Rushan Bay in May, July and August 2014 and February 2015, the form and transformation of P in the suspended particulate matter (SPM) and surface sediment were analyzed. The results showed that contents of total P, authigenic P and organic P of SPM increased with the increase in distance from the shoreline off Rushan Bay, and the detrital-P decreased. The partition coefficient of P between water and SPM was related to the chemical activity of different forms of P, and a higher reactivity of inorganic P associated with SPM was observed. Hypoxia induced by mariculture changes the distribution and morphological composition of P in SPM and sediment in this typical aquaculture area, which can result in a conversion of sink to source of P in the sediment, thereby having a significant impact on the regional nutrient budget and associated with eutrophication.

Submarine groundwater discharge (SGD) is an important part in the land-sea interactions, which mainly contains three components: submarine fresh groundwater discharge (SFGD), tidal flat recirculated saline groundwater discharge (tidal flat RSGD) and subtidal recirculated saline groundwater discharge (subtidal RSGD). In order to make a more accurate assessment of the impact of SGD on coastal ecological environment, it is necessary to distinguish the main components of SGD. In this study, the Maowei Sea, located in the northern part of the Beibu Gulf, was selected as the study area. Based on the radium (Ra) tracing method, we present a new analytical method for distinguishing the three main components of SGD in this area combined with field data. The average daily flow along the coastline of the Maowei Sea of tidal flat RSGD was slightly higher than that of SFGD, and both two were on the magnitude of 1×10⁵ m³/d. The average daily flow for the subtidal RSGD of the entire subtidal zone of the Maowei Sea reached to the magnitude of 1×10⁶–1×10⁷ m³/d. The long-term variation trend of terrestrial SGD is a valuable information for the study of the influence of terrigenous material on the coastal ecological environment. Based on the results of four sampling periods, it is found that the fluxes of SFGD and tidal flat RSGD in the Maowei Sea had good linear correlation with the net precipitation. As an example, January 2015 to August 2022 were selected as the study periods, and the variation trends of SFGD and tidal flat RSGD were calculated by linear function with net precipitation as the independent variable. The results showed that the flux of tidal flat RSGD was slightly higher than that of SFGD, and the difference between the two is larger in flood season while smaller in dry season. In general, in the coastal range of China, the total SGD flux in the Maowei Sea area is at a high level, and the SFGD flux is at a medium level.