During the past decades of years, the widespread restrictions and ban on the use of legacy persistent organic pollutants (POPs) such as organochlorine pesticides (OCPs) and polychlorinated biphenyls (PCBs) around the world have resulted in gradual decrease in their concentration in various environmental media. As an open sea area, the Northwest Pacific Ocean is far away from the continental area without obvious pollution point source, the ocean currents play an important role in the transportation and diffusion of POPs. In this study, the surface waters (2−5 m) and subsurface waters (150 m) in the Kuroshio area of the Northwest Pacific Ocean were collected and analyzed for dissolved OCPs and PCBs. The results showed that the concentrations of HCHs, DDTs, CHLs and PCBs in the surface waters ranged from 30.7 pg/L to 68.8 pg/L, 6.16 pg/L to 23.8 pg/L, 1.07 pg/L to 5.75 pg/L and 49.8 pg/L to 124 pg/L, respectively; the concentrations in the subsurface waters ranged from 27.3 pg/L to 68.4 pg/L, 7.06 pg/L to 14.1 pg/L, 0.518 pg/L to 10.1 pg/L and 34.1 pg/L to 68.4 pg/L, respectively. The concentration levels of OCPs in different water bodies were comparable, which may be related to the huge water body and internal homogeneity of the Kuroshio. The ratios of the various isomers of HCHs indicated that the sea area was dominated by lindane input, while the ratio of DDTs and CHLs showed that the DDTs and CHLs in the waters of the sea area were mainly derived from the residues of legacy. PCBs were dominated by tri-PCBs and tetra-PCBs, which consistented with the use of PCBs in East Asia. The large volume and uniform internal distribution of Kuroshio water mass have an important impact on the vertical distribution of pollutants, that is, the concentration and composition of OCPs in water at different depths are similar. At the same time, Kuroshio brings high temperature and high nutrient salt water masses, forming potential risks to the enrichment of pollutants in the marine organisms.

In order to understand the distribution characteristics, relationships and diffusion fluxes of nutrients and metal elements across the sediment-water interface, we investigated the vertical profiles of nutrients and metal elements through the diffusive gradient in thin-films (DGT) technology in Shenzhen Bay (SZB) and Maozhou River (MZR). The two-dimensional high-resolution available sulfur (DGT-labile S) distributions were also synchronously measured. The results showed that the mean concentrations of ${\rm{NH}}_4^+ $, P and S²⁻ and heavy metals of Cd, Co, Cu, Fe, Ni, Pb and Zn in porewater of MZR were significantly higher than those in the overlying water and the SZB, except for ${\rm{NO}}_3^- $ and Mo. The pollution degree of nutrients and metal elements in porewater of MZR was generally higher than that in SZB, but the pollutants of nutrients and metal elements in SZB were more than those in MZR. The spatial distribution of DGT-labile S in MZR indicated the distinguish bioturbation-induced tubes, which enhanced the spatial heterogeneity and led the increasing of solute concentrations with sediment depths. The diffusion fluxes of nutrients and metal elements ranged from −0.27 μg/(cm²·d) to 0.0065 μg/(cm²·d) in SZB, and from −0.061μg/(cm²·d) to 0.069 μg/(cm²·d) in MZR.

The outbreak of crown-of-thorns starfish has seriously threatened the ecological health of coral reefs, and the supplement of nutrients in seawater may be a key factor leading to the outbreak of starfish. Sandy sediments play a key role in regulating the nutrient concentration and structure in coral reefs. Therefore, crown-of-thorns starfish and sandy sediments were incubated using flow-through reactor, to analyze the effects of crown-of-thorns starfish excretion and dead body degradation on nutrient concentration and structure, and to explore the role of sandy sediments under activities of crown-of-thorns starfish. The results were as followed: (1) crown-of-thorns starfish excreted dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphorus (DIP) at rates of (83.55±4.74) μmol/(ind.·h) and (2.53±0.03) μmol/(ind.·h), respectively, which maight provide nutritional conditions for the continuous outbreak of starfish; (2) sandy sediments could regulate the nutrient concentration changed by the excretion of starfish, about 70.7% of DIN and 91.4% of DIP were trapped in the sediments, but the increase of DIN/DIP caused by nutrient exchange at the sediment-seawater interface may be detrimental to coral growth; (3) the degradation of starfish organisms could promote the release of nutrients at the sediment-seawater interface. Based on the density of starfish, the released nutrients could cause the concentrations of DIN and DIP in the overlying water to increase by 0.32 μmol/L and 0.01 μmol/L, respectively, which maight promote the rapid growth of macroalgae and hinder the self-restoration of corals.

The temporal and spatial variation characteristics and influencing factors of euphotic depth and primary productivity in the Zhanjiang Bay were studied by applying voyage data of four seasons from 2016 to 2017．The results showed that the average value of euphotic depth was (6.95±3.17) m, and the spatial variations were more obvious than the seasonal variations. There was a significant positive correlation between K_d (PAR) and turbidity, as the R² of established linear regression model was 0.73 (p<0.01), indicating that suspended particles on euphotic depth was the dominant attenuation factor in the Zhanjiang Bay. The average value of primary productivity obtained by VGPM model was (639.53±427.95) mg/(m²·d), and its temporal and spatial characteristics were basically consistent with euphotic depth, which mean euphotic depth explained the temporal and spatial distribution pattern of primary productivity better than chlorophyll a concentration.

In order to improve the study of the interaction between internal solitary waves and submarine slope sediments, the stage of continued motion on the slope after internal solitary waves fragmentation is focused on in this paper, and conducts physical simulation experiments to analyse the changes in earth pressure and super-pore water pressure in response to the slope to reveal the process of internal wave action. The results show that the sediment particles on the slope are resuspended under the combined action of vortex and seepage caused by the internal solitary waves fragmentation, and the change in slope gradient does not change the dominant dynamic role of the sediment in generating the dynamic response; the amplitude of the internal solitary waves affect the ratio between vortex and seepage, i.e. the vortex is dominant under small amplitude conditions and the seepage is dominant under large amplitude conditions; the fragmented fluid forms a new dynamic role when it rushes out along the slope. The dynamic response of the sediment to the new vorticity is influenced by the slope of the slope. The results of this paper are useful for the study of internal solitary waves resuspension transporting seafloor sediments and modifying seafloor topography.

The applicability of environmental DNA (eDNA) metabarcoding fragments to relative fish species had not been compared. There was a risk of “species loss” in diversity surveys. In order to screen out the best fragments, we compared the resolution rate differences of 15 eDNA metabarcoding fragments in 106 genera (a total of 935 species). The results were as follows: (1) the protein-coding gene (COI, fragment 15) had the highest resolution rate, but the universality of its corresponding primer pairs was the worst; the overall mean distance based on primer pair sequence of fragment 09, fragment 11, fragment 07, fragment 03 and fragment 12 were obviously large, suggesting their eDNA amplification efficiency were possibly low. (2) The resolution rates were significantly affected by the length of fragments, and the fragment 05, fragment 06, fragment 01, fragment 02 and fragment 13 of ribosomal genes had a higher resolution rate except fragment 15. (3) Non-metric multidimensional scaling analysis (NMDS) showed that there were great differences among different genes and different fragments belonging to the same gene. Therefore, the combination application of multi-fragment and multi-gene should be considered; besides, fragment 01 and fragment 02, and fragment 05 and fragment 06 were close to each other in the NMDS plot. They function of fish resolution were overlapped. (4) Species groups affected the resolution results, and eDNA studies stilled need to develop fragments with higher resolution rates. Based on the resolution rate of relative species, the universality of primer pairs and NMDS analysis, this study recommended fragment 01 (Mifish-U) for 2×150 bp sequencing platform and fragment 05 (Ac12S) for 2×250 bp sequencing platform, supplemented by fragment 13 (Vert-16S-eDNA) to investigate the diversity of relative fish. This study provided some support for improving the accuracy of fish eDNA survey results.