2023 Vol. 42, No. 1

2023, 42(1)
2023, 42(1): 0-1.
The South China Sea Annual Meeting 2021
Nitrate isotope dynamics in the lower euphotic-upper mesopelagic zones of the western South China Sea
Zixuan Li, Chao Xu, Minfang Zheng, Mengya Chen, Yusheng Qiu, Hantao Zhou, Min Chen, Run Zhang
2023, 42(1): 1-11. doi: 10.1007/s13131-022-2091-4
The dual isotopes (N and O) of nitrate were measured using a denitrifier bacterial method in the western South China Sea (WSCS) during September 2015 to elucidate key information during N transformation in the lower euphotic zone (LEZ)-upper mesopelagic zone (UMZ, down to 500 m in this study) continuum, which is a vital sub-environment for marine N cycle and sequestration of atmospheric CO2 as well. The N isotopic composition (δ15N) of nitrate generally decreased from 500 m toward the base of the euphotic zone (~100 m), reaching a value of ~4.6‰ (vs. air N2) at the base of the LEZ, suggesting the imprint of remineralization (nitrification) of isotopically light N from atmospheric source. The δ15N and δ18O of nitrate only generally conform to a 1:1 line at 50 m and 75 m, suggesting that nitrate assimilation is a dominant process to shape nitrate isotope signature in this light-limited and relatively N-replete lower part of the euphotic zone. The fractionation factors of N and O isotopes during nitrate fractionation (15εASSIM, 18εASSIM) using a steady-state model were estimated to be 4.0‰±0.3‰ and 5.4‰±0.3‰, respectively. The occurrence of nitrification at the base of the LEZ and most of the UMZ is corroborated by the decoupling of δ15N and the oxygen isotopic composition (δ18O) of nitrate. Our results will provide insights for better understanding N cycle in the South China Sea from a perspective of present and past.
Declined trends of chlorophyll a in the South China Sea over 2005−2019 from remote sensing reconstruction
Tianhao Wang, Yu Sun, Hua Su, Wenfang Lu
2023, 42(1): 12-24. doi: 10.1007/s13131-022-2097-y
Chlorophyll a concentration (CHL) is an important proxy of the marine ecological environment and phytoplankton production. Long-term trends in CHL of the South China Sea (SCS) reflect the changes in the ecosystem’s productivity and functionality in the regional carbon cycle. In this study, we applied a previously reconstructed 15-a (2005−2019) CHL product, which has a complete coverage at 4 km and daily resolutions, to analyze the long-term trends of CHL in the SCS. Quantile regression was used to elaborate on the long-term trends of high, median, and low CHL values, as an extended method of conventional linear regression. The results showed downward trends of the SCS CHL for the 75th, 50th, and 25th quantile in the past 15 a, which were −0.004 0 mg/(m3·a) (−1.62% per year), −0.002 3 mg/(m3·a) (−1.10% per year), and −0.001 9 mg/(m3·a) (−1.01% per year). The negative trends in winter (November to March) were more prominent than those in summer (May to September). In terms of spatial distribution, the downward trend was more significant in regions with higher CHL. These led to a reduced standard deviation of CHL over time and space. We further explored the influence of various dynamic factors on CHL trends for the entire SCS and two typical systems (winter Luzon Strait (LZ) and summer Vietnam Upwelling System (SV)) with single-variate linear regression and multivariate Random Forest analysis. The multivariate analysis suggested the CHL trend pattern can be best explained by the trends of wind speed and mixed-layer depth. The divergent importance of controlling factors for LZ and SV can explain the different CHL trends for the two systems. This study expanded our understanding of the long-term changes of CHL in the SCS and provided a reference for investigating changes in the marine ecosystem.
Distribution and risk assessment of heavy metals in surface sediments of coastal mudflats on Leizhou Peninsula, China
Tingting Li, Lili Jia, Xin Zhu, Min Xu, Xinchang Zhang
2023, 42(1): 25-34. doi: 10.1007/s13131-022-2101-6
Mudflats play a vital role in maintaining the dynamic balance between sea and land. To understand the characteristics, sources, and pollution risks of six heavy metals (As, Cd, Cr, Cu, Hg, and Pb) in the coastal mudflats on the Leizhou Peninsula, 257 surface sediment samples were studied using mathematical statistics, correlation analysis, and factor analysis. The results show that the overall concentrations of these heavy metals are low although there are several high abnormal points in the local areas. The strong correlation between these heavy metals indicates that the sources of some of the metals are similar, yet their elemental combinations in different cities (counties) varied. According to the calculated enrichment factor (EF), anthropogenic activity-induced heavy metals were determined in order of decreasing influence: As, Cd, Pb, Cr, Cu, and Hg. The low EF values of Hg indicate that it does not present as a contaminant in the study area, while low values of Cr and Cu from the Lianjiang City suggest that these two metals were also attributed to natural sources. The presence of As, Cd, Cr, Cu, and Pb from the remaining cities (counties) should be influenced by anthropogenic activities. The overall potential ecological risk index indicates that the ecological risks posed by the six analyzed heavy metals to the Leizhou Peninsula mudflats, in order of decreasing risk, are Cd, As, Hg, Pb, Cu, and Cr. It is noteworthy that only Cd in Lianjiang City demonstrated substantial ecological risk. Other examined heavy metals in other cities of the study area showed slight ecological risk.
The influence of lipid-extraction on the δ13C of mesopelagic and demersal fish in the South China Sea: modification and application of lipid normalization models
Linyu Wang, Fuqiang Wang, Zuozhi Chen, Ying Wu
2023, 42(1): 35-43. doi: 10.1007/s13131-022-2045-x
Mesopelagic fish, the most important daily vertically migrating community in the oceans, are characterized by high lipid content which may obscure the interpretation of stable isotopes analysis. Demersal fish, which are important consumers in the food web dominated by mesopelagic fish, also have a high lipid content. Here we collected 127 fish samples from the South China Sea and evaluated the effect of lipid contents on δ13C of mesopelagic and demersal fish. In lipid-extracted mesopelagic fish, the C/N content ratio (<5.5) shows a clear correlation with Δδ13C (the offset of bulk and lipid-extracted δ13C values), especially in non-migratory and semi-migratory species; these values were less correlation in demersal fish. Based on our results, we suggest that mesopelagic and demersal fish in different regions of the South China Sea should be studied separately using appropriate correction models and less fit for the traditional model. Moreover, the C/N content ratio should be used cautiously for establishing the lipid normalization model, especially for the fish in migratory mesopelagic fish and demersal fish. Our results also reveal that mesopelagic fish across nearby regions could be analyzed together. The new models described here can be applied in future studies of mesopelagic and demersal fish in the South China Sea.
Articles$Physical Oceanography, Marine Meteorology and Marine Physics
An assessment of the subduction rate in the CMIP6 historical experiment
Shan Liu, Xueyi Jing, Xingrong Chen, Huijun Wang
2023, 42(1): 44-60. doi: 10.1007/s13131-022-2108-z
Subduction process is a dynamical bridge for the exchanges of heat between the atmosphere and subsurface ocean water, which is regarded as a central proxy for the ocean climate studies. Given its key indicator in climate signals, it is of importance to examine the ability of a model to simulate the global subduction rate before investigating the climate dynamics. In this paper, we evaluated the ability of 21 climate models from Coupled Model Intercomparison Project Phase 6 (CMIP6) in simulating the subduction rate. In general, the simulation ability of the models to the subduction climatology is better than that to the long-term variation trend. Based on the comprehensive analysis of climatology distribution and long-term trend of the subduction rate, GISS-E2-1-G performs better in reproducing the subduction rate climatology and IPSL-CM6A-LR can simulate positive long-term trend for both the global mean subduction rate and the lateral induction term in the Antarctic Circumpolar Current (ACC) region. However, it is still challenging to capture both the distribution characteristics of the subduction climatology and the long-term temporal trend for the 21 CMIP6 models. In addition, the model results demonstrate that, the ACC area is the major region contributing to the long-term trend of the global mean subduction rate. The analysis in this paper indicates that the poor simulation ability of reproducing the long-term trend of global mean subduction rate might be attributed to the ocean dynamics, for example, the zonal velocity at the bottom mixed layer and zonal gradient of mixed layer depth.
Articles$Marine Chemistry
Lateral downslope transport and tentative sedimentary organic carbon box model in the southern Yap Trench, western Pacific Ocean
Dong Li, Jun Zhao, Chenggang Liu, Jianming Pan, Ji Hu
2023, 42(1): 61-74. doi: 10.1007/s13131-022-2043-z
Sediment collapse and subsequent lateral downslope migration play important roles in shaping the habitats and regulating sedimentary organic carbon (SOC) cycling in hadal trenches. In this study, three sediment cores were collected using a human-occupied vehicle across the axis of the southern Yap Trench (SYT). The total organic carbon (TOC) and total nitrogen (TN) contents, δ13C, radiocarbon ages, specific surface areas, and grain size compositions of sediments from three cores were measured. We explored the influence of the lateral downslope transport on the dispersal of the sediments and established a tentative box model for the SOC balance. In the SYT, the surface TOC content decreased with water depth and was decoupled by the funneling effect of the V-shaped hadal trench. However, the sedimentation (0.002 5 cm/a) and SOC accumulation rates (~0.038 g/(m2·a) (in terms of OC)) were approximately 50% higher in the deeper hadal region than in the abyssal region (0.001 6 cm/a and ~0.026 g/(m2·a) (in terms of OC), respectively), indicating the occurrence of lateral downslope transport. The fluctuating variations in the prokaryotic abundances and the SOC accumulation rate suggest the periodic input of surficial sediments from the shallow region. The similar average TOC (0.31%–0.38%), TN (0.06%–0.07%) contents, and SOC compositions (terrestrial OC (11%–18%), marine phytoplanktonic OC (45%–53%), and microbial OC (32%–44%)) of the three sites indicate that the lateral downslope transport has a significant mixing effect on the SOC composition. The output fluxes of the laterally transported SOC (0.44–0.56 g/(m2·a) (in terms of OC)) contributed approximately (47%–73%) of the total SOC input, and this proportion increased with water depth. The results of this study demonstrate the importance of lateral downslope transport in the spatial distribution and development of biomes.
Comparative assessment of nitrogen fixation rate by 15N2 tracer assays in the South China Sea
Danyang Li, Minfang Zheng, Yusheng Qiu, Limin Lai, Nengwang Chen, Hongmei Jing, Run Zhang, Min Chen
2023, 42(1): 75-82. doi: 10.1007/s13131-022-2092-3
Nitrogen fixation is one of the most important sources of new nitrogen in the ocean and thus profoundly affects the nitrogen and carbon biogeochemical processes. The distribution, controlling factors, and flux of N2 fixation in the global ocean remain uncertain, partly because of the lack of methodological uniformity. The 15N2 tracer assay (the original bubble method → the 15N2-enriched seawater method → the modified bubble method) is the mainstream method for field measurements of N2 fixation rates (NFRs), among which the original bubble method is the most frequently used. However, accumulating evidence has suggested an underestimation of NFRs when using this method. To improve the availability of previous data, we compared NFRs measured by three 15N2 tracer assays in the South China Sea. Our results indicate that the relationship between NFRs measured by the original bubble method and the 15N2-enriched seawater method varies obviously with area and season, which may be influenced by incubation time, diazotrophic composition, and environmental factors. In comparison, the relationship between NFRs measured by the original bubble method and the modified bubble method is more stable, indicating that the N2 fixation rates based on the original bubble methods may be underestimated by approximately 50%. Based on this result, we revised the flux of N2 fixation in the South China Sea to 40 mmol/(m2·a). Our results improve the availability and comparability of literature NFR data in the South China Sea. The comparison of the 15N2 tracer assay for NFRs measurements on a larger scale is urgently necessary over the global ocean for a more robust understanding of the role of N2 fixation in the marine nitrogen cycle.
Spatiotemporal changes of biogenic elements in the Changjiang River Estuary and adjacent waters in summer over the last decade
Lu Yang, Yujia Zhang, Xiaoli Wang, Qiulu Wang, Long He, Xiao Li
2023, 42(1): 83-90. doi: 10.1007/s13131-022-2104-3
The long-term spatiotemporal changes of surface biogenic elements in the Changjiang River Estuary and adjacent waters during the summer of 2008−2016 were analyzed in this study. The concentrations of dissolved inorganic nitrogen (DIN), soluble reactive phosphate (\begin{document}${{\rm {PO}}_4^{3-}} $\end{document}) and silicate (\begin{document}${{\rm {SiO}}_3^{2-}} $\end{document}) were generally stable, with a slight decrease of DIN and \begin{document}${{\rm {PO}}_4^{3-}} $\end{document}, and a slight increase of \begin{document}${{\rm {SiO}}_3^{2-}} $\end{document}, which mainly occurred in the estuarine waters. The grey correlation analysis was carried out between biogenic elements and chlorophyll a (Chl-a). Results showed that compared with the absolute values of biogenic elements, the correlations between the concentration ratio of nitrogen to phosphorus (N/P), ratio of silicon to nitrogen (Si/N) and Chl-a were closer, indicating the important influence on phytoplankton by the structure of biogenic elements. The study area was generally in a state of potential P limitation, and could have potential impact on the phytoplankton community, triggering the shift of red tide dominant species from diatoms to dinoflagellates.
Sedimentary record of climate change in a high latitude fjord—Kongsfjord
Hang Wu, Binbin Deng, Jinlong Wang, Sheng Zeng, Juan Du, Peng Yu, Qianqian Bi, Jinzhou Du
2023, 42(1): 91-102. doi: 10.1007/s13131-022-2098-x
The sedimentary record of climate change in the Arctic region is useful for understanding global warming. Kongsfjord is located in the subpolar region of the Arctic and is a suitable site for studying climate change. Glacier retreat is occurring in this region due to climate change, leading to an increase in meltwater outflow with a high debris content. In August 2017, we collected a sediment Core Z3 from the central fjord near the Yellow River Station. Then, we used the widely used chronology method of 210Pb, 137Cs, and other parameters to reflect the climate change record in the sedimentary environment of Kongsfjord. The results showed that after the mid-late 1990s, the mass accumulation rate of this core increased from 0.10 g/(cm2·a) to 0.34 g/(cm2·a), while the flux of 210Pbex increased from 125 Bq/(m2·a) to 316 Bq/(m2·a). The higher sedimentary inventory of 210Pbex in Kongsfjord compared to global fallout might have been caused by sediment focusing, boundary scavenging, and riverine input. Similarities between the inventory of 137Cs and global fallout indicated that terrestrial particulate matter was the main source of 137Cs in fjord sediments. The sedimentation rate increased after 1997, possibly due to the increased influx of glacial meltwater containing debris. In addition, the 137Cs activity, percentage of organic carbon (OC), and OC/total nitrogen concentration ratio showed increasing trends toward the top of the core since 1997, corresponding to a decrease in the mass balance of glaciers in the region. The results of δ13C, δ15N and OC/TN concentration ratio showed both terrestrial and marine sources contributed to the organic matter in Core Z3. The relative contribution of terrestrial organic matter which was calculated by a two-endmember model showed an increased trend since mid-1990s. All these data indicate that global climate change has a significant impact on Arctic glaciers.
Application of an optical nitrate profiler to high- and low-turbidity coastal shelf waters
Yanpei Zhuang, Yangjie Li, Xizhen Liu, Shichao Tian, Bin Wang, Zhongqiang Ji, Haiyan Jin, Jianfang Chen
2023, 42(1): 103-108. doi: 10.1007/s13131-022-2038-9
Here, we report the results of high-resolution nitrate measurements using an optical nitrate profiler (in situ ultraviolet spectrophotometer, ISUS) along transect across a high-turbidity shelf (East China Sea) and a low-turbidity shelf (Chukchi Sea). The ISUS-measured nitrate concentrations closely reproduced the results measured by conventional bottle methods in low-turbidity waters. However, for high-turbidity waters of the East China Sea (salinity<30), a correction factor of 1.19 was required to match the standard bottle measurements. The high-resolution ISUS data revealed subtle spatial variability (e.g., a subsurface nitrate minimum) that may have been missed if based solely on bottle results. Four main structures of the nitracline on the East China Sea are apparent from the ISUS nitrate profile. High-resolution nitrate data are important for studying nitrate budgets and nutrient dynamics on continental shelves.
A simple guideline to apply excitation-emission matrix spectroscopy (EEMs) for the characterization of dissolved organic matter (DOM) in anoxic marine sediments
Shuchai Gan, Verena B. Heuer, Frauke Schmidt, Lars Wörmer, Kai-Uwe Hinrichs
2023, 42(1): 109-119. doi: 10.1007/s13131-022-2050-0
Marine sediments represent a major carbon reservoir on Earth. Dissolved organic matter (DOM) in pore waters accumulates products and intermediates of carbon cycling in sediments. The application of excitation-emission matrix spectroscopy (EEMs) in the analysis of subseafloor DOM samples is largely unexplored due to the redox-sensitive matrix of anoxic pore water. Therefore, this study aims to investigate the interference caused by the matrix on EEMs and propose a guideline to prepare pore water samples from anoxic marine sediments. The parameters determined by fluorescence spectra include 3D-index derived from EEMs after parallel factor analysis (PARAFAC), fluorescence index (FI) (contribution of terrigenous DOM), biological index (BIX) and humification index (HIX) derived from 2D emission spectra. First, we investigated the impacts of extensively-presented ions as typical electron acceptors, which are utilized by anaerobic microbes and stratified in marine sediments: Fe(II), Fe(III), Mn(II) and sulfide in anoxic pore water resulted in biases of fluorescent signals. We proposed threshold concentrations of these ions when the interference on EEMs occurred. Effective removal of sulfide from sulfide-rich samples could be achieved by flushing with N2 for 2 min. Second, the tests based on DOM standard were further verified using pristine samples from marine sediments. There was a significant change in the fluorescence spectra of DOM in anoxic sediments from the Rhône Delta. This study demonstrated that the change was caused by oxidation of the matrix rather than the intrinsic alteration of DOM. It was confirmed by extracted DOM via both EEMs analysis and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). Slight oxidation of sulfur-containing compounds (e.g., sulfhydryl) and polyphenol-like compounds occurred. Finally, a sample preparation sequence is proposed for pore water from anoxic sediments. This method enables measurement with small volumes of the sample (e.g., 50 µL in this study) and ensures reliable data without the interference of the redox-sensitive matrix. This study provides access to the rapid analysis of DOM composition in marine sediments and can potentially open a window into examining the carbon cycling of the marine deep biosphere.
Articles$Marine Biology
Otolith microchemical evidence revealing multiple spawning site origination of the anadromous tapertail anchovy (Coilia nasus) in the Changjiang (Yangtze) River Estuary
Zhongya Xuan, Tao Jiang, Hongbo Liu, Xiubao Chen, Jian Yang
2023, 42(1): 120-130. doi: 10.1007/s13131-022-2135-9
The estuarine tapertail anchovy (Coilia nasus) is a high-value commercial fish. Estimating the spawning site or hatchery origin and habitat is essential for its conservation. This study aimed to determine the habitat use and life history characteristics of C. nasus from the Changjiang River Estuary. We investigated the environmental signatures of strontium (Sr) and calcium (Ca) in the otoliths of the collected specimens using electron probe microanalysis; additionally, we examined their gonadal maturity stage. Our results indicate that the 31 adult C. nasus specimens used in this study could be classified into two types based on their otolith Sr:Ca concentration ratios and their gonadal maturity stage. The long freshwater early life history type (Type LF) had wider central region in the otolith with low Sr:Ca concentration ratios ranging from 1.24±0.62 to 1.92±0.78 and a bluish pattern of low Sr content level. These fish are of riverine origin and had a relatively long early life history in freshwater and low gonadal maturation when captured. The short freshwater early life history type (Type SF) had a shorter central region in the otolith with low Sr:Ca concentration ratios ranging from 1.35±0.5 to 2.82±0.97 and a correspondingly bluish pattern. These fish also had a relatively short-term early life history in freshwater and high gonadal maturation when captured. The results of the otolith microchemical analysis indicated that Type LF and Type SF originated in spawning/hatching sites far from and close to the estuary, respectively. The mature gonads of Type SF fish indicated that they may breed in areas close to the estuary, whereas the immature gonads of Type LF fish indicated that they may breed in areas far from the estuary. This study is the first to reveal that the Changjiang River Estuary contains stocks of anadromous C. nasus originating in different spawning sites during the same season. The estuarine habitat plays a critical role in the connectivity between freshwater recruitment and the marine resources available to adult spawners of this commercially valued species. From a conservation perspective, this study provides important information for identifying anadromous C. nasus stocks originating in different spawning sites in the Changjiang River Basin.
Articles$Marine Technology
A speckle noise suppression method based on surface waves investigation and monitoring data
Jingwei Gu, Xiuzhong Li, Yijun He
2023, 42(1): 131-141. doi: 10.1007/s13131-022-2103-4
The internal energy distribution of waves can be described using ocean-wave spectra. In many ways, obtaining wave spectra on a global scale is critical. Surface waves investigation and monitoring onboard the Chinese-French oceanography satellite is the first space-borne instrument for detecting wave spectra specially, which was launched on October 29, 2018. It can avoid the shortage of synthetic aperture radar detection results while still having some problems, especially with the effects of speckle noise. In this study, a method to suppress the speckle noise is proposed. First, the empirical formula for background speckle noise is established. Second, many spatio-temporal representative fluctuation spectra are classified and averaged. Third, rational transfer function filtering is used to obtain speckle noise close to the along-track direction. Finally, a signal-to-noise ratio threshold is used to suppress the abnormal speckle noise. This method solves the problems existing in previous denoising methods, such as excessive denoising in the along-track direction and the inability of some abnormal noises to be denoised in the two-dimensional directional wave spectra.