Upper water structure and mixed layer depth in tropical waters: The SEATS station in the northern South China Sea

Abstract. We conducted a comprehensive investigation on the microzooplankton herbivory effect on phytoplankton in the northern South China Sea (SCS) using the seawater dilution technique at surface and deep chlorophyll maximum (DCM) layers in two cruises (July–August of 2009 and January of 2010). We compared vertical (surface vs. DCM), spatial (onshore vs. offshore), and seasonal (summer vs. winter) differences of phytoplankton growth (μ0) and microzooplankton grazing rates (m). During summer, both μ0 and m were significantly higher at the surface than at the layer of DCM, which was below the mixed layer. During winter, surface μ0 was significantly higher than at DCM, while m was not significantly different between the two layers, both of which were contained within the mixed layer. Surface μ0 was, on average, significantly higher in summer than in winter; while average surface m was not different between the two seasons. There were no significant cross-shelf trends of μ0 in summer or winter surface waters. In surface waters, μ0 was not correlated with ambient nitrate concentrations and the effect of nutrient enrichment on phytoplankton growth was not pronounced. There was a decreasing trend of m from shelf to basin surface waters in summer, but not in winter. Microzooplankton grazing effect on phytoplankton (m/μ0) did not increase with distance offshore, suggesting that the importance of microzooplankton as grazers of phytoplankton may not decrease in onshore waters. On average, microzooplankton grazed 73% and 65% of the daily primary production in summer and winter, respectively.

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Characteristics of the surface mixed layer depths in the northern South China Sea in spring

Journal of Oceanography ◽

10.1007/s10872-016-0351-7 ◽

2016 ◽

Vol 72 (4) ◽

pp. 567-576 ◽

Cited By ~ 3

Author(s):

Chunhua Qiu ◽

Yongsheng Cui ◽

Jie Ren ◽

Qiang Wang ◽

Dan Huo ◽

...

Keyword(s):

South China Sea ◽

Mixed Layer ◽

South China ◽

Northern South China Sea ◽

Surface Mixed Layer ◽

China Sea

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Ocean Response to Successive Typhoons Sarika and Haima (2016) in the Northern South China Sea

10.5194/egusphere-egu2020-6882 ◽

2020 ◽

Author(s):

Han Zhang

Keyword(s):

South China Sea ◽

Mixed Layer ◽

South China ◽

Model Simulation ◽

Heat Budget ◽

Northern South China Sea ◽

Heat Content ◽

Sea Surface ◽

Surface Cooling ◽

China Sea

<p>Tropical cyclones (TCs) are natural disasters for coastal regions. TCs with maximum wind speeds higher than 32.7 m/s in the north-western Pacific are referred to as typhoons. Typhoons Sarika and Haima successively passed our moored observation array in the northern South China Sea in 2016. Based on the satellite data, the winds (clouds and rainfall) biased to the right (left) sides of the typhoon tracks. Sarika and Haima cooled the sea surface ~4 and ~2 &#176;C and increased the salinity ~1.2 and ~0.6 psu, respectively. The maximum sea surface cooling occurred nearly one day after the two typhoons. Station 2 (S2) was on left side of Sarika&#8217;s track and right side of Haima&#8217;s track, which is studied because its data was complete. Strong near-inertial currents from the ocean surface toward the bottom were generated at S2, with a maximum mixed-layer speed of ~80 cm/s. The current spectrum also shows weak signal at twice the inertial frequency (2f). Sarika deepened the mixed layer, cooled the sea surface, but warmed the subsurface by ~1 &#176;C. Haima subsequently pushed the subsurface warming anomaly into deeper ocean, causing a temperature increase of ~1.8 &#176;C therein. Sarika and Haima successively increased the heat content anomaly upper than 160 m at S2 to ~50 and ~100 m&#176;C, respectively. Model simulation of the two typhoons shows that mixing and horizontal advection caused surface ocean cooling, mixing and downwelling caused subsurface warming, while downwelling warmed the deeper ocean. It indicates that Sarika and Haima sequentially modulated warm water into deeper ocean and influenced internal ocean heat budget. Upper ocean salinity response was similar to temperature, except that rainfall refreshed sea surface and caused a successive salinity decrease of ~0.03 and ~0.1 psu during the two typhoons, changing &#160;the positive subsurface salinity anomaly to negative.</p>

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High-resolution distributions of O<sub>2</sub>/Ar on the northern slope of the South China Sea and estimates of net community production

10.5194/os-2020-69 ◽

2020 ◽

Author(s):

Chuan Qin ◽

Guiling Zhang ◽

Wenjing Zheng ◽

Yu Han ◽

Sumei Liu

Keyword(s):

High Resolution ◽

South China Sea ◽

South China ◽

Inorganic Nitrogen ◽

Mixed Layer Depth ◽

Northern South China Sea ◽

Net Community Production ◽

Study Region ◽

China Sea ◽

Community Production

Abstract. Net community production (NCP) is a proxy of carbon export from the surface ocean and can be estimated based on O2/Ar. In order to obtain the high-resolution distribution of NCP and improve our understanding of its regulating factors in the slope region of the Northern South China Sea (SCS), we conducted continuous measurements of dissolved O2, Ar, and CO2 by membrane inlet mass spectrometry during cruises in October 2014 and June 2015. An overall autotrophic condition was observed in the study region in both cruises with an average ∆(O2/Ar) of 1.1 % ± 0.9 % in October 2014 and 2.7 % ± 2.8 % in June 2015. NCP was on average 11.5 ± 8.7 mmol C m−2 d−1 in October 2014 and 11.6 ± 12.7 mmol C m−2 d−1 in June 2015. Correlations between dissolved inorganic nitrogen (DIN), ∆(O2/Ar), and NCP were observed in both cruises, indicating that NCP is subject to the nitrogen limitation in the study region. In June 2015, we observed a rapid response of the ecosystem to the episodic nutrient supply induced by eddies. Eddy-entrained shelf water injection, which supplied large amounts of terrigenous nitrogen to the study region, resulted in high productivity along a transect. In addition, upwelling brought large uncertainties to the estimation of NCP at the core region of the cold eddy (cyclone) in June 2015. The correlation between the volumetric NCP (NCPvol) and the mixed layer depth (MLD) indicated that light availability may have also been a factor in influencing NCP in the SCS.

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Diel to Seasonal Variation of Picoplankton in the Tropical South China Sea

Frontiers in Marine Science ◽

10.3389/fmars.2021.732017 ◽

2021 ◽

Vol 8 ◽

Author(s):

Tzong-Yueh Chen ◽

Chao-Chen Lai ◽

Jen-Hua Tai ◽

Chia-Ying Ko ◽

Fuh-Kwo Shiah

Keyword(s):

Seasonal Variation ◽

South China Sea ◽

South China ◽

Heterotrophic Bacteria ◽

Mixed Layer Depth ◽

Northern South China Sea ◽

Euphotic Zone ◽

Cold Season ◽

Coefficient Of Determination ◽

China Sea

Eight diel surveys on picoplankton (Prochlorococcus, Synechococcus, picoeukaryotes, and heterotrophic bacteria) abundance at the South East Asian Time-Series Station (SEATS; 18°N; 116°E) were conducted during the period of 2010 to 2014. The results indicated that Prochlorococcus and picoeukaryotes showed a subsurface maximum in warm seasons (spring, summer, and fall) and were abundant at the surface in the cold season (winter). Synechococcus and heterotrophic bacteria exhibited higher cell numbers at the surface and decreased with depth throughout the year. Although not all, some clear diel patterns for picoplankton were observed. Picophytoplankton usually peaked in the nighttime; picoeukaryotes peaked at ~7 to 8 p.m., followed by Synechococcus (peaking at 1 a.m.) and Prochlorococcus (peaking at 2 a.m.). Unlike these picoautotrophs, heterotrophic bacteria could peak either at dusk (i.e., 7 p.m.) or at noon. Seasonally, Prochlorococcus was more abundant in the warm than the cold seasons, while Synechococcus and picoeukaryotes showed blooms in the winter of 2013 and 2011, respectively. Heterotrophic bacteria showed no significant seasonality. Regression analysis indicated that ~73% of the diel-to-seasonal variation of the euphotic zone depth-integrated picophytoplankton biomass (i.e., PicoBeu) could be explained by the changes of the mixed-layer depth (MLD), and this suggested that inorganic nutrient supply could be the major controlling factor in their growth. The strong linear relationship (coefficient of determination, R2 of 0.83, p < 0.01) between sea surface temperature (SST) and PicoBeu implied, for the first time, a potential of using satellite-based SST to trace the biomass of picophytoplankton in the pelagic areas of the northern South China Sea.

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Seasonal and spatial comparisons of phytoplankton growth and mortality rates due to microzooplankton grazing in the northern South China Sea

Biogeosciences ◽

10.5194/bg-10-2775-2013 ◽

2013 ◽

Vol 10 (4) ◽

pp. 2775-2785 ◽

Cited By ~ 27

Author(s):

B. Chen ◽

L. Zheng ◽

B. Huang ◽

S. Song ◽

H. Liu

Keyword(s):

South China Sea ◽

Mixed Layer ◽

South China ◽

Surface Waters ◽

Northern South China Sea ◽

Phytoplankton Growth ◽

China Sea ◽

Grazing Effect ◽

Basin Surface ◽

Microzooplankton Grazing

Abstract. We conducted a comprehensive investigation on the microzooplankton herbivory effect on phytoplankton in the northern South China Sea (SCS) using the seawater dilution technique at surface and deep chlorophyll maximum (DCM) layers on two cruises (July–August of 2009 and January of 2010). We compared vertical (surface vs. DCM), spatial (onshore vs. offshore), and seasonal (summer vs. winter) differences of phytoplankton growth (μ0) and microzooplankton grazing rates (m). During summer, both μ0 and m were significantly higher at the surface than at the DCM layer, which was below the mixed layer. During winter, surface μ0 was significantly higher than at the DCM, while m was not significantly different between the two layers, both of which were within the mixed layer. Surface μ0 was, on average, significantly higher in summer than in winter, while average surface m was not different between the two seasons. There were no cross-shelf gradients of μ0 in summer or winter surface waters. In surface waters, μ0 was not correlated with ambient nitrate concentrations, and the effect of nutrient enrichment on phytoplankton growth was not pronounced. There was a decreasing trend of m from shelf to basin surface waters in summer, but not in winter. Microzooplankton grazing effect on phytoplankton (m/μ0) was relatively small in the summer basin waters, indicating a decoupling of microzooplankton grazing and phytoplankton growth at this time. On average, microzooplankton grazed 73% and 65% of the daily primary production in summer and winter, respectively.

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