Anthropogenic nutrient inputs drove shifts in phytoplankton productivity and community structure in the northern South China Sea over the past century.

Changes in phytoplankton productivity and community structure in the northern South China Sea during the past 260 ka

East Asian winter monsoon controlling phytoplankton productivity and community structure changes in the southeastern South China Sea over the last 185 kyr

Based on a fully-coupled, high-resolution regional climate model, this study analyzed three-dimensional temperature and momentum changes in the South China Sea (SCS) from 1970 to 2000, during which period the climate shifts from a decadal La Nina-like condition (before 1976/77) to a decadal El Nino-like condition afterward. With a set of partially-coupled experiments, sea surface temperature (SST) and kinetic energy (KE) changes during this period are first decomposed into two components: those induced by lateral boundary forcing and those induced by atmospheric surface fluxes. The results showed that the total SST and KE changes show an increasing trend from 1970 to 2000. The two decomposed components together determined 96% and 89% of the SST and KE changes respectively, implying their dominant roles on the SCS's surface variability. Spatially, a sandwich pattern of air-sea forcing relationship is revealed in the SCS basin. The increased KE, represented by a cyclonic flow anomaly in the northern SCS, was induced by enhanced cold water intrusion from Pacific into the SCS via the Luzon Strait (boundary forcing). This cold-water inflow, however, resulted in SST cooling along the northern shelf of the SCS. The maximal SST warming occurred in the central SCS and was attributed to the wind-evaporation-SST (WES) positive feedback (surface forcing), in which a southwestward wind anomaly is initialized by SST gradients between the northern and southern SCS. This wind anomaly decelerates the southwestly summer monsoons and in turn increases the SST gradients. Over the shallow Sunda shelf, which is far from the Luzon Strait, the SST/KE variability appeared to be determined primarily by local air–sea interactions. Furthermore, analyses on subsurface components indicated that the subsurface temperature changes are primarily induced by internal ocean mixing, which becomes significantly important below the thermocline. The enhanced subsurface flow is driven by the Luzon Strait inflow as well, and exits the SCS via the Mindoro-Sibutu passage. This article is protected by copyright. All rights reserved.

Observations from two Bio‐Argo floats deployed in the northern and central South China Sea (SCS) show distinct seasonal patterns of vertical chlorophyll distribution. There is a permanent subsurface chlorophyll maximum (SCM) located between 60 and 80 m throughout the year and a weak seasonality of surface chlorophyll in the central SCS. In the northern SCS, the SCM shoals to the upper mixed layer in winter and surface phytoplankton shows a clear winter bloom pattern. The mechanism driving the spatial and seasonal differences in phytoplankton dynamics in the euphotic zone remains unclear. Here a coupled physical‐biological model is developed and applied to the northern and central SCS. With model and satellite data, we show that the contrasting patterns in chlorophyll are induced by the spatial variability in winter mixing dynamics. In the northern SCS, the buoyancy flux‐induced mixing plays a dominant role in controlling the seasonal variability of vertical nutrient transport and phytoplankton production, which leads to the peak of surface chlorophyll and the significant shoaling of the SCM in winter. In the central SCS, the intensity of the buoyancy flux is reduced and both buoyancy flux‐ and wind‐induced mixing control the winter mixing dynamics. However, the combination of these two mixing processes is weaker than in the northern SCS and the vertical nutrient transport is limited to the layer above the SCM, resulting in the reduced seasonality of surface chlorophyll and the relatively stable SCM all year round in the central SCS.

The present study documents the atmosphere–ocean interaction in interannual variations over the South China Sea (SCS). The atmosphere–ocean relationship displays remarkable seasonality and regionality, with an atmospheric forcing dominant in the northern and central SCS during the local warm season, and an oceanic forcing in the northern SCS during the local cold season. During April–June, the atmospheric impact on the sea surface temperature (SST) change is characterized by a prominent cloud-radiation effect in the central SCS, a wind-evaporation effect in the central and southern SCS, and a wind-driven oceanic effect along the west coast. During November–January, regional convection responds to the SST forcing in the northern SCS through modulation of the low-level convergence and atmospheric stability. Evaluation of the precipitation–SST and precipitation–SST tendency correlation in 24 selected models from CMIP5 indicates that the simulated atmosphere–ocean relationship varies widely among the models. Most models have the worst performance in spring. On average, the models simulate better the atmospheric forcing than the oceanic forcing. Improvements are needed for many models before they can be used to understand the regional atmosphere–ocean interactions in the SCS region.

A cruise was conducted in the summer of 2023 from the Pearl River Estuary (PRE) to the adjacent waters of the Xisha Islands in the northern South China Sea (NSCS) to investigate the distribution, community structure, and assembly patterns of eukaryotic and prokaryotic phytoplankton using high-throughput sequencing (HTS) and microscopic observation. Dinophyta were the most abundant phylum in the eukaryotic phytoplankton community based on HTS, accounting for 92.17% of the total amplicon sequence variants (ASVs). Syndiniales was the most abundant order among eukaryotic phytoplankton, whereas Prochlorococcus was the most abundant genus within cyanobacteria. The alpha diversity showed the lowest values in the PRE area and decreased gradually with depth, while cyanobacteria exhibited higher alpha diversity indices in the PRE and at depths ranging from 75 m to 750 m. The morphological results were different from the data based on HTS. Diatoms (37 species) dominated the phytoplankton community, with an average abundance of 3.01 × 104 cells L-1, but only six species of dinoflagellate were observed. Spearman correlation analysis and redundancy analysis (RDA) showed that the distribution and community structure of phytoplankton were largely influenced by geographical location and environmental parameters in the NSCS. The neutral community model (NCM) and null model indicated that deterministic processes played a significant role in the assembly of eukaryotic phytoplankton, with heterogeneous selection and homogeneous selection accounting for 47.27 and 29.95%, respectively. However, stochastic processes (over 60%) dominated the assembly of cyanobacteria and undominated processes accounted for 63.44%. In summary, the formation of eukaryotic phytoplankton was mainly influenced by environmental factors and geographic location, but the assembly of cyanobacteria was shaped by both stochastic processes, which accounted for over 60%, and environmental selection in the NSCS.

The South China Sea (SCS) plays an important role in global marine ecology. Studies of phytoplankton diversity promote the sustainable utilization of resources in the SCS. From July to August 2020, the phytoplankton community structure at 47 stations in the northern SCS was investigated. Species composition and distribution of phytoplankton, water quality, diversity index, main influencing factors, and succession characteristics of the community structure were analyzed in combination with the survey results from previous years. A total of 332 separate taxa from 83 genera and three phyla were identified, including 142 species and 45 genera of Bacillariophyta, 188 species and 36 genera of Dinophyta, and two species and two genera of Chrysophyta. Average phytoplankton cell abundance was 649.97 cells/L. Nitzschia spp., Thalassionema nitzschioides, and Scrippsiella spp. were the dominant species. Scrippsiella spp. was found for the first time as a dominant species in the northern SCS. Meanwhile, Nitzschia spp. was associated with organic-polluted water. The high-value areas of Nitzschia spp. also indicated eutrophication, and water was slightly polluted. The Shannon–Weiner diversity index of the surface layer was 0.99–4.56 (with a mean of 3.57), and the evenness index was 0.23–0.96 (with a mean of 0.83). The phytoplankton community structure in the northern SCS was deemed to be stable. Pearson correlation analysis showed that the sum of nitrate and nitrite was significantly negatively correlated with the abundance of dinoflagellate, which indicated restrictions as a result of the sum of nitrate and nitrite, with no significant correlation between ammonium salt and various groups. Small- and medium-sized phytoplankton are usually dominant in the SCS, where nitrogen is limited.

We have generated a record of alkenone sea surface temperatures (SSTs) during the last 28000 years from Core MD97-2146 for the northern South China Sea (SCS). The SST record showed a typical pattern for change in the northern SCS SST. The SST during the LGM was ~25℃, this decreased to ~24℃ to 17 ka, increased to ~25.5℃ to 14.5 ka, decreased again to ~24.5℃ to 11.8 ka, increased gradually to ~27℃ to 6 ka, and then increased more gradually to reach ~27.5℃ at present. The SST difference (△SST(subscript NSCS)=SST(subscript MD97-2146-SST(subscript MD97-2141)) between Cores MD97-2146 (the northern SCS; this study) and MD97-2141 (the Sulu Sea; Rosenthal et al. 2003) was used to characterize the SST changes in the northern SCS relative to changes in the adjacent WTP region. The △SST(subscript NSCS) decreased from 21 to 11.8 ka and increased after 11.8 ka, indicating slower warming of the northern SCS during the last deglaciation than that of the adjacent western tropical Pacific region. We infer that the slow warming of the northern SCS was principally a result of stronger winter monsoon during the last deglaciation and early Holocene. In addition, the cool water inflow through the Taiwan Strait after 13 ka and the warm water inflow through the Sunda Shelf after 11 ka could influence the SST in the northern SCS.

Li, J.; Sun, L.; Yang, Y.; Yan, H., and Liu, S., 2020. Upper ocean responses to binary typhoons in the nearshore and offshore areas of northern South China Sea: A comparison study. In: Zheng, C.W.; Wang, Q.; Zhan, C., and Yang, S.B. (eds.), Air-Sea Interaction and Coastal Environments of the Maritime and Polar Silk Roads. Journal of Coastal Research, Special Issue No. 99, pp. 115–125. Coconut Creek (Florida), ISSN 0749-0208.Changes in sea surface temperature (SST) and sea surface salinity (SSS) in response to the binary typhoons Sarika and Haima (2016) in the offshore and nearshore areas along the Guangdong Province Coast (GPC) and Hainan Island Coast (HIC) in the northern South China Sea are comprehensively investigated using multi-satellite observations and ocean reanalysis data. The results show that the maximum SST cooling was 2.5°C (average 1.5°C) and 6°C (average 2.5°C) after the passage of typhoon Sarika in the HIC nearshore and offshore, respectively. In contrast, the average SST cooling was 1°C in the GPC offshore and very marginal in the GPC nearshore after the passage of typhoon Haima. For SSS, typhoon Sarika induced changes of 0.1 psu and 0.35 psu in the HIC nearshore and offshore, respectively, while typhoon Haima caused changes of -0.1 psu and 0.3 psu in the GPC nearshore and offshore, respectively. The responses of both SST and SSS are similar in the offshores of both the GPC and the HIC. However, they are quite different in the nearshores of the GPC and the HIC. It is found that the nearshore responses to typhoon are quite different depending on the coast conditions: river discharge and advection of coastal current. In the GPC nearshore, the river discharge due to typhoon can lead to larger SSS decrease than that in the HIC nearshore. Besides, the advection of coastal high SSS water from the GPC to the HIC along coast made SSS decrease in the GPC but increase in the HIC. As a result, the SST and SSS responses are easily to be influenced, while they could exist much longer in the offshores.

Air-sea interactive forcing on phytoplankton productivity and community structure changes in the East China Sea during the Holocene

Palaeoceanographic and palaeoclimatic reconstructions, particularly on the spatial and temporal evolution across different regions, can offer valuable insights into global changes. At present, abundant data recorded in sediments indicate a phase asynchrony from land to sea during the Holocene. This has raised great debate about the forcing mechanisms of paleoclimatic evolution. In this study, we reconstructed sea surface temperature and salinity during the Holocene from the northern South China Sea (SCS) by the Mg/Ca ratios and δ18O values of Globigerinoides ruber sensu stricto (s.s.) in the core SH-CL38. By comparing the results with records from other cores in the SCS, it indicates that during the Holocene, the climatic changes in the SCS are mainly influenced by the East Asian summer monsoon driven by Northern Hemisphere summer insolation. The lower salinity in the early Holocene compared to the mid-late Holocene is mainly controlled by palaeogeographic changes in the SCS Basin related to sea level. The fitted sea surface temperature anomaly results from the northern and southern SCS show that the climate evolution in the entire SCS during the early Holocene was asynchronous. The multi-year mean air mass backward trajectory results indicate that the northern SCS is significantly influenced by moisture originating from the tropical western Pacific, while the southern SCS exhibits notable local or regional contributions. Therefore, the differences in the composition of moisture contributions caused by changes in the strength and path of the summer monsoon may be a factor driving the different spatial climate patterns in the SCS.

High-resolution sea surface temperature records derived from foraminiferal Mg/Ca ratios during the last 260 ka in the northern South China Sea

Phytoplankton productivity and community structure in marginal seas have been altered significantly during the past three decades, but it is still a challenge to distinguish the forcing mechanisms between climate change and anthropogenic activities. High time-resolution biomarker records of two 210Pb-dated sediment cores (#34: 28.5°N, 122.272°E; CJ12-1269: 28.861 9°N, 122.515 3°E) from the Min-Zhe coastal mud area were compared to reveal changes of phytoplankton productivity and community structure over the past 100 years. Phytoplankton productivity started to increase gradually from the 1970s and increased rapidly after the late 1990s at Site #34; and it started to increase gradually from the middle 1960s and increased rapidly after the late 1980s at Site CJ12-1269. Productivity of Core CJ12-1269 was higher than that of Core #34. Phytoplankton community structure variations displayed opposite patterns in the two cores. The decreasing D/B (dinosterol/brassicasterol) ratio of Core #34 since the 1960s revealed increased diatom contribution to total productivity. In contrast, the increasing D/B ratio of Core CJ12-1269 since the 1950s indicated increased dinoflagellate contribution to total productivity. Both the productivity increase and the increased dinoflagellate contribution in Core CJ12-1269 since the 1950–1960s were mainly caused by anthropogenic activities, as the location was closer to the Changjiang River Estuary with higher nutrient concentration and decreasing Si/N ratios. However, increased diatom contribution in Core #34 is proposed to be caused by increased coastal upwelling, with higher nutrient concentration and higher Si/N ratios.

The development of late Holocene coastal cooling in the northern South China Sea

Internal wave (IW) events occur rapidly and have a short duration, but they have a great impact on nearshore ecosystems. To address the problems of short observation time, limited range based on measured data, and low accuracy based on mesoscale satellite data for the study of IW-induced sea surface temperature (SST) change, this paper introduce high-frequency geostationary orbit satellite data combined with SST data of different times and analyze and discuss the changes and mechanisms of immediate and long-term spatio-temporal SST distributions in the northern South China Sea (SCS) caused by IWs. The results show that high-precision satellite data can reflect SST changes caused by IWs in the northeastern SCS, these being particularly significant at the Dongsha Atoll (DA) and along the northwestern continental slope, where SST can be reduced by 1°C–1.5°C, which is caused by the vertical transport of internal waves and the turbulent mixing effect of the broken internal waves, respectively. The discontinuity between the two cold centres is due to the short duration of the vertical transport of internal waves. Whereas turbulent mixing due to IW fragmentation on the continental shelf at shallower depths of 200 m, the duration of the constantly fragmented wave packets is sufficient to maintain low temperatures on the continental shelf, although the turbulent mixing effect is weaker than the vertical transport. Long-term IW activity has deepened the SST depression caused by shallow topography (shallower than 300 m) in the northeastern SCS, especially at a water depth of 200 m. Fragmentation and dissipation of IWs caused SST valleys on the continental slope as shallow as 160 m. This study validates the conclusions from methods such as moorings and modeling and has important implications for the study of IW biology.