Transparent Exopolymer Particles in a Coastal Frontal Zone of the Northern South China Sea and the Associated Biogeochemical Implications

• Published in: Journal of geophysical research. Biogeosciences Vol. 126; no. 2
• Main Authors: Li, Qian P., Ge, Zaiming, Liu, Zijia, Zhou, Weiwen, Shuai, Yiping, Wu, Zhengchao
• Format: Journal Article
• Language: English
• Published: Washington Blackwell Publishing Ltd 01-02-2021
• Subjects: Biogeochemical cycle; Biogeochemical cycles; Biogeochemistry; Carbon; Carbon cycle; Chlorophyll; Chlorophyll a; Coastal waters; Coastal zone; Distribution; Dynamics; Environmental factors; Estuaries; Estuarine dynamics; Eutrophication; Floating barriers; Mineral nutrients; Nutrients; Organic carbon; Particulate organic carbon; physical‐biological interaction; Physics; Phytoplankton; phytoplankton size‐classes; Plankton; Resuspension; Sediment; sediment resuspension; Sediments; Surface layers; Surface water; Temperature; temperature front; transparent exopolymer particles; Xanthan; South China Sea; Pearl River Estuary
• ISSN: 2169-8953; 2169-8961
• DOI: 10.1029/2020JG005893

Transparent exopolymer particles (TEP) are essential for the ocean carbon cycle. However, their relationships with physical and biogeochemical processes remain inadequately understood in the coastal ocean. Here, we focus on TEP dynamics across a temperature front in the coastal waters of the Northern South China Sea (NSCS). Our results suggested that TEP concentration was relatively high across the frontal system with 35.0–160.5 μg Xeq L−1 (μg Gum Xanthan equivalent per liter) in the surface waters, 70.0–197.2 μg Xeq L−1 in the bottom waters, and 12.1–16.3 mg Xeq g−1 in the surficial sediment. In the surface layer of the front, TEP distributions appeared to be driven by cross‐front physical and biogeochemical variations, as it showed significant correlations with both temperature and nutrients, as well as chlorophyll‐a along with its size‐fractionated components. TEP dominated on the shore side of the front with high phytoplankton biomass and intense TEP production in the surface layer. In contrast, there were much lower TEP concentrations and higher TEP production efficiencies on the seaside driven by nutrient limitation. In the bottom layer of the frontal zone, sediment resuspension associated with the front's secondary circulation was found to play a crucial role in enhancing the levels of TEP‐rich aggregates with prominent distribution patterns characterized by the localized accumulation in the bottom waters at the frontal zone. Our results provide valuable insights about the controlling factors that modulate TEP dynamics in a coastal frontal system and highlighted their potential role in the carbon cycling of the NSCS shelf‐sea.
Plain Language Summary
Transparent exopolymer particles (TEP) have gained increasing attention due to their important roles in the biogeochemical cycles of the coastal ocean. Intense phytoplankton booms driven by increasing eutrophication has led to a very high level of TEP in the Pearl River Estuary (PRE). Meanwhile, TEP dynamics in the coastal waters of the Northern South China Sea (NSCS) outside the PRE, particularly in the nearshore frontal zone over the inner shelf, remained mostly unknown. Physical dynamics of the frontal system may be an important driver for TEP distribution and the related biogeochemical processes such as particulate organic carbon export in the coastal ocean, as particles including TEP can be accumulated there due to frontal physics. In this paper, we focused on TEP concentrations in water‐columns and sediments as well as related environmental factors across a nearshore temperature front to explore the physical and biogeochemical mechanisms regulating TEP dynamics in the coastal regions of the NSCS shelf‐sea.
Key Points
Phytoplankton is an important source of surface transparent exopolymer particles (TEP) across the frontal system
Higher surface TEP production efficiencies on the seaside of the front driven by nutrient limitation
Sediment resuspension associated with the front's secondary circulation causes an enhanced TEP concentration in the bottom layers of the frontal zone