Origin of the Particulate Organic Matter in a Monsoon-Controlled Bay in Southern China

Abstract

In this study, the isotopic composition (δ13C and δ15N), total organic carbon content, total nitrogen content, and C/N ratios of suspended particulate organic matter (POM) in Zhanjiang Bay, which is a semi-enclosed bay with concentrated artificial activities in Southern China, were analyzed in order to investigate the seasonal variations in the principal POM sources in the monsoon region. In summer, the δ13C and δ15N values showed a weak correlation with the chlorophyll a (Chl a), suggesting that terrigenous sources were dominant. However, in winter, the particulate organic carbon and particulate nitrogen values were correlated with the Chl a in the middle bay and bay mouth. Moreover, the δ13C values showed a significant correlation with Chl a during the winter, indicating that the contribution of the in situ phytoplankton was relatively important and was affected by the monsoon in winter. Compared with the corresponding δ13C values, the δ15N values exhibited a complex spatial distribution. By using a Bayesian mixing model, in the upper bay, the source of POM was mainly from marine organic matter (49%) in summer, and almost an equilibrated contribution of all sources in winter. In the middle bay and bay mouth, the POM contribution mainly originated from marine organic matter (53%) during the winter. In contrast, the POM source was mainly soil organic matter (63%) in summer, suggesting that the POM was sourced from the runoff from the upstream basin. Our results suggest that the seasonal shifts of the source of POM should be taken into account when estimating C or N mass balance in the monsoon-controlled bay.

Unveiling the eutrophication crisis: 20 years of nutrient development in Zhanjiang Bay, China

“…The bay experiences a subtropical oceanic monsoon climate (Zhou et al, 2020) with a water retention time of approximately 10 days (Shi et al, 2020). ZJB is fed by several rivers; the northern Suixi River is the primary source of freshwater with a runoff of 7.9×10 9 m 3 /yr, accounting for over 90% of the total river inflow in the bay (Li et al, 2021). Additionally, the discharge of land sewage is approximately 1.5×10 8 m 3 /yr (Li et al, 2021), accounting for less than 2% of river runoff.…”

“…ZJB is fed by several rivers; the northern Suixi River is the primary source of freshwater with a runoff of 7.9×10 9 m 3 /yr, accounting for over 90% of the total river inflow in the bay (Li et al, 2021). Additionally, the discharge of land sewage is approximately 1.5×10 8 m 3 /yr (Li et al, 2021), accounting for less than 2% of river runoff. The injection of sewage, mariculture, and river water introduces considerable amounts of nutrients and pollutants, leading to a substantial terrestrial pollution burden (Fu et al, 2020).…”

Remote Sensing Estimates of Particulate Organic Carbon Sources in the Zhanjiang Bay Using Sentinel-2 Data and Carbon Isotopes

“…Concentrations of TSM were calculated using the weight method [45]. After being removed and carbonated for more than 48 h in steam-containing condensed HCl, the filter membranes used for the POC concentration and δ 13 C analyses were washed three times with deionized water [46]. The filter membranes were acidified, freeze-dried and kept in a dehumidifier until analysis [46].…”

“…After being removed and carbonated for more than 48 h in steam-containing condensed HCl, the filter membranes used for the POC concentration and δ 13 C analyses were washed three times with deionized water [46]. The filter membranes were acidified, freeze-dried and kept in a dehumidifier until analysis [46]. Using an elemental analysis isotope ratio mass spectrometer (EA Isolink-253 Plus, Thermo Fisher Scientific, Inc., Waltham, MA, USA), the sample filter membranes for the analysis were fully loaded in tin cans [46].…”

“…The filter membranes were acidified, freeze-dried and kept in a dehumidifier until analysis [46]. Using an elemental analysis isotope ratio mass spectrometer (EA Isolink-253 Plus, Thermo Fisher Scientific, Inc., Waltham, MA, USA), the sample filter membranes for the analysis were fully loaded in tin cans [46]. The mean standard deviations of δ 13 C and POC concentrations were ±0.2‰ and ±0.3%, respectively.…”