The coastal front modulates the timing and magnitude of spring phytoplankton bloom in the Yellow Sea

Major seasonal quasi-stationary fronts on shelves play an important role in regulating the spatiotemporal variations in the phytoplankton community. However, knowledge of their effects on the timing and magnitude of spring phytoplankton bloom (SPB) remains limited. Here, based on decadal satellite data (2003-2020), we examine the climatological relationship between the Shandong coastal front (SCF) and SPB in the Yellow Sea. The results show that the onset of SPB occurs either in March (∼56% of the seasons examined) or in April (44%). The peak of SPB most often occurs in April (∼56% of the seasons examined) or is advanced to March (16%) or delayed to May (28%), and that the peak ranges from 1.04 to 2.54 mg Chl-a m^-3. The onset of SPB matches with lower turbulence, particularly when the rate of generation of turbulent kinetic energy (TKE_RT) reaches zero. A higher magnitude of bloom is associated with a greater change in front and a lower TKE_RT. The in situ observations along the SCF transects in the Yellow Sea indicate that weakened SCF in spring associated with a shallower mixing layer enhances the transport of nutrients from the coastal to the shelf waters. Weakened frontal structure and atmospheric forcing in spring can further increase the water stability and decrease turbulence in the upper waters. The variation in hydrodynamic conditions allows shelf phytoplankton to stay longer in the upper waters with sufficient light and nutrients and consequently generate a Chl-a peak. The results suggest that the seasonal changes in front intensity and structure and turbulence are important prerequisites for initiating SPB on the shelf, and that further determines the magnitude of SPB.

Real-time eutrophication status evaluation of coastal waters using support vector machine with grid search algorithm

The development of techniques for real-time monitoring of the eutrophication status of coastal waters is of great importance for realizing potential cost savings in coastal monitoring programs and providing timely advice for marine health management. In this study, a GS optimized SVM was proposed to model relationships between 6 easily measured parameters (DO, Chl-a, C1, C2, C3 and C4) and the TRIX index for rapidly assessing marine eutrophication states of coastal waters. The good predictive performance of the developed method was indicated by the R² between the measured and predicted values (0.92 for the training dataset and 0.91 for the validation dataset) at a 95% confidence level. The classification accuracy of the eutrophication status was 86.5% for the training dataset and 85.6% for the validation dataset. The results indicated that it is feasible to develop an SVM technique for timely evaluation of the eutrophication status by easily measured parameters.

The assessment of the spatial and seasonal variability of chromophoric dissolved organic matter in the Southern Yellow Sea and the East China Sea

Samples of chromophoric dissolved organic matter (CDOM) from the Southern Yellow Sea (SYS) and the East China Sea (ECS) were evaluated by fluorescent Excitation Emission Matrix (EEM) combined with Parallel Factorial Analysis (PARAFAC). Three terrestrial humic-like components (C1, C2 and C3) and one autochthonous protein-like component (C4) were identified. As for seasonal variations, CDOM displayed the following order on the whole: summer>spring>autumn. The C1, C2 and C3 components were mainly dominated by terrestrial inputs and their spatial distributions and temporal variations also can be influenced by primary productivity of phytoplankton, microbial activities and photobleaching. C4 was produced by phytoplankton and microorganisms and consumed by marine bacteria, and besides its distribution was attributed to the influence of riverine inputs. Terrestrial inputs were the dominant sources of CDOM in the SYS and ECS.

Distributions and sea-to-air fluxes of chloroform, trichloroethylene, tetrachloroethylene, chlorodibromomethane and bromoform in the Yellow Sea and the East China Sea during spring

Halocarbons including chloroform (CHCl3), trichloroethylene (C2HCl3), tetrachloroethylene (C2Cl4), chlorodibromomethane (CHBr2Cl) and bromoform (CHBr3) were measured in the Yellow Sea (YS) and the East China Sea (ECS) during spring 2011. The influences of chlorophyll a, salinity and nutrients on the distributions of these gases were examined. Elevated levels of these gases in the coastal waters were attributed to anthropogenic inputs and biological release by phytoplankton. The vertical distributions of these gases in the water column were controlled by different source strengths and water masses. Using atmospheric concentrations measured in spring 2012 and seawater concentrations obtained from this study, the sea-to-air fluxes of these gases were estimated. Our results showed that the emissions of C2HCl3, C2Cl4, CHBr2Cl, and CHBr3 from the study area could account for 16.5%, 10.5%, 14.6%, and 3.5% of global oceanic emissions, respectively, indicating that the coastal shelf may contribute significantly to the global oceanic emissions of these gases.