Seasonal and spring interannual variations in satellite-observed chlorophyll-a in the Yellow and East China Seas: New datasets with reduced interference from high concentration of resuspended sediment (original) (raw)
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Indian Journal of Geo-Marine Sciences
Field-of-view Sensor (SeaWiFS) in 1998 to 2003 are examined to determine whether there have been changes in chlorophyll concentration and suspended sediment as indicated by changes in satellite-derived optical properties during the past two decades in the Yellow and East China Seas (YECS). We compare water-leaving radiance measurements at 443 and 555 nm [the CZCS band is centered at 550 nm, but we consider this comparable to the SeaWiFS 555-nm band] and discuss possible reasons for the changes observed. The shallow coastal areas of the YECS exhibited high water-leaving radiance in the 555nm band (L w 555) during two time periods, indicating that these waters are sediment-dominated Case-2 waters. Between the CZCS era and the SeaWiFS era, L w 443 increased in these areas by 17%−61%, and L w 555 increased by 67−108%. In the deeper waters, L w 443 decreased by 25%−31%, which would indicate an increase in absorbing materials such as chlorophyll and colored dissolved organic matter (CDOM). Between the CZCS and SeaWiFS eras, the average chlorophyll concentration (based on Case-1 algorithms) increased by 15-60% in these offshore deep waters. Periodical in situ measurements from 61 stations in the western coast of Korea from 1978 and 2002 were compared with the trends found in satellite data. The results show that there were increasing trends in temperature and zooplankton biomass, and decreasing trends in salinity and Secchi depth. The satellite data surrounding these stations showed an increase in L w 555 (49 %), a decrease in the L w 443 (−12 %), and an increase in chlorophyll (46 %). From the results, it is inferred that there have been environmental changes in the Yellow Sea during the last two decades from 1979 to 2003.
Physical drivers of chlorophyll variability in the open South China Sea
Journal of Geophysical Research: Oceans, 2016
The variability of chlorophyll a concentration (Chl a) in the open South China Sea (SCS) was examined using observations from two Bio-Argo floats. During the period of September 2014 to August 2015, there was a permanent subsurface Chl a maximum (SCM) in the depth range of 48 to 96 m in the central basin of the SCS. In the northern basin, the SCM disappeared in winter, replaced by enhanced surface layer phytoplankton with high Chl a. The values of the SCM were influenced by the vertical displacement of isotherms. Strong wind forcing and surface cooling were the main physical drivers of high surface Chl a in winter. In the north, stronger wind than in the center, lower sea surface temperature (SST) than in the center, and Kuroshio water intrusion were more favorable for the upward transport of nutrient-rich deep water. A large amount of nitrate could be advected from the Taiwan Strait and shallow continental shelf to the northern basin in winter. A combination of strong wind mixing, surface cooling, Kuroshio water intrusion, and horizontal advection caused the winter surface phytoplankton bloom in the north.
Progress in Oceanography, 2012
Seasonal variability in satellite chlorophyll a concentrations (SCHL) in the Yellow Sea and the East China Sea (YECS) was investigated using 10-year averages of monthly data collected between September 1997 and October 2006. Interannual variations were also assessed to help clarify the influence of Changjiang River discharge (CRD) during summer. The YECS was represented by 12 areas each with different seasonal variability in SCHL. SCHL were overestimated during winter due to re-suspension of sediment near the Changjiang Bank and near coastal areas. Increases of SCHL were observed over large areas of the YECS during spring, as would be expected with the occurrence of spring blooms. The spatial distribution of the summer maximum of SCHL shifted from the Changjiang River mouth to just east of Jeju Island from July to September. An eastward shift of the high SCHL water coincided with the movement of the Changjiang diluted water (CDW), taking approximately 2 months to move from Changjiang River mouth to Jeju Island. Summer SCHL between 1998 and 2006 in this region were positively correlated with CRD with a time lag of 0-2 months, suggesting that the interannual variation of SCHL was controlled by the interannual variation of CRD. SCHL during summer in the Yellow Sea gradually increased over the 10 years, indicating possible eutrophication.
The development of phytoplankton bloom and its association with physical forcing is examined through an interdisciplinary field-work conducted in the vicinity of the central trough of the southern Yellow Sea during March-April 2009, with the aid of a surface Lagrangian drifter deployed at the bloom site. Bloom patches were detected using an empirical value and two of them were traced by the drifter for a period of several days respectively. Both of them appears as thin-layer subsurface chlorophyll a maximum (SCM) throughout the tracing, although their dominant phytoplankton species are not identical at all. The magnitude as well as the onset of these two blooms is different from each other, but both found to be relevant to local oceanic and meteorological conditions. Both of them demonstrate that the changes in the stability of hydrographical structure, especially at layers around the SCM, take a substantial role in triggering or terminating the blooming processes. Those changes in meteorological conditions, like wind speed and directions, solar radiation, are short and cause daily or synoptic scale variations in phytoplankton concentrations, but the frequency of northerly wind events predating the bloom season has a positive effect on the occurrence of spring blooms. The horizontal advection is another contributing factor indicated by the drifter which accounts for the bloom extinction at the station B20. In addition, due to the weak orbital horizontal movement, the bloom above the central trough persists longer and larger.
Journal of Oceanography, 2018
The surface distribution of the phytoplankton community was investigated in July 2009, 2010, 2011, and 2013 on the midshelf of the East China Sea (ECS), which is under the influence of Changjiang River Diluted Water (CDW) and the Kuroshio Current. This study, based on a CHEMTAX analysis of phytoplankton pigments, revealed a predominance of cyanobacteria and prochlorophytes in the eastern ECS, which is perennially under the influence of oligotrophic Kuroshio Surface Water. Towards the west, on the mid-shelf of the ECS, the composition of the phytoplankton community varied from year to year. Diatoms dominated in 2009 and 2013, when dissolved inorganic phosphate (DIP) concentrations were higher than during 2010 and 2011. During the latter two years, characterized as high-nitrate years, a mixed population of cyanobacteria, chlorophytes, and other groups was observed. Cluster analysis based on the phytoplankton community composition together with a PCA of shipboard hydrographic and nutrient data for all four years helped to confirm that the summer phytoplankton community structure of the ECS was regulated by the mixing of water masses and the variability of nutrient ratios within the CDW as it moved offshore. Our results show that elevated DIP concentrations in the CDW favor the growth of diatoms and dinoflagellates. The primary pathway for DIP inputs appears to be the upwelling of high-phosphate subsurface waters along the coast of China.
Japan Geoscience Union, 2018
The surface distribution of the phytoplankton community was investigated in July 2009, 2010, 2011, and 2013 on the midshelf of the East China Sea (ECS), which is under the influence of Changjiang River Diluted Water (CDW) and the Kuroshio Current. This study, based on a CHEMTAX analysis of phytoplankton pigments, revealed a predominance of cyanobacteria and prochlorophytes in the eastern ECS, which is perennially under the influence of oligotrophic Kuroshio Surface Water. Towards the west, on the mid-shelf of the ECS, the composition of the phytoplankton community varied from year to year. Diatoms dominated in 2009 and 2013, when dissolved inorganic phosphate (DIP) concentrations were higher than during 2010 and 2011. During the latter two years, characterized as high-nitrate years, a mixed population of cyanobacteria, chlorophytes, and other groups was observed. Cluster analysis based on the phytoplankton community composition together with a PCA of shipboard hydrographic and nutrient data for all four years helped to confirm that the summer phytoplankton community structure of the ECS was regulated by the mixing of water masses and the variability of nutrient ratios within the CDW as it moved offshore. Our results show that elevated DIP concentrations in the CDW favor the growth of diatoms and dinoflagellates. The primary pathway for DIP inputs appears to be the upwelling of high-phosphate subsurface waters along the coast of China.
Seasonal and interannual variability of chlorophyll in the East China Sea
2010
Monthly chlorophyll-a (Chl-a) concentrations derived from SeaWiFS data for 1997-2005 and chlorophyll measurements from the Atlantic Meridional Transect for 1995-2001 have been analysed to describe seasonal and inter-annual variability of surface Chl-a in the Mauritanian upwelling. There was a moderate to strong correspondence between the seasonal cycles of surface Chl-a and the seasonal cycles of ocean physical and meteorological fields (such as sea-surface temperature, seasurface height, and prevailing wind), with a noticeable exception in 1998 that corresponded to a strong anomalous Chl-a event ($250% increase) in the Mauritanian upwelling. Alongshore wind-stress and wind-stress curl were found to be the most significant factors controlling the variability of Chl-a (jointly explaining more than 50% of total variance). The biological response to the alongshore wind-stress was immediate, but it lagged the wind-stress curl by 1-2 months (each explaining more than 40% of the total Chl-a variability). These observations also demonstrate a link, hitherto unreported, between the Pacific El-Nin˜o Southern Oscillation (ENSO) and anomalous Chl-a field in the Mauritanian upwelling. The multivariate ENSO index was shown to account for a significant part of the variability of the autumn-winter Chl-a anomaly (r ¼ À0.52, po0.01). A cold event, following an intense El Nin˜o in the Pacific during summer, was found to mirror the intensity of wind forcing and phytoplankton concentration in the Mauritanian upwelling a few months later. Therefore, ENSO-related changes in the local atmospheric fields are considered as the preferred candidates for explaining the observed biological changes in the Mauritanian upwelling during 1998Mauritanian upwelling during -1999
Seasonal Variations of Chlorophyll $a$ Concentration in the Northern South China Sea
IEEE Geoscience and Remote Sensing Letters, 2000
Monthly climatology of chlorophyll a concentration (chl a) based on nine years of SeaWiFS data is used to illustrate seasonal variations and spatial structures in the northern South China Sea (SCS). Chl a starts to increase in September at the northern coast of Luzon Island, continues to increase in the autumn, and reaches its maximum in December or January. Maximum chl a is centered in the northern SCS off the northwestern coast of Luzon Island. Chl a starts to decrease gradually in February, and its values become very low from June to August. The region of elevated chl a during the winter bloom season is funnel shaped, with the narrow end at the northern coast of Luzon Island, where the chl a value is highest and opening toward the northwest. The sea surface temperature (SST) in this funnelshaped region is significantly colder than SST in surrounding regions of the same latitude. The present study indicates that the winter blooms indicated by higher chl a and colder SST in the northern SCS are linked strongly to the local winter monsoon. The initial data exploration and analysis presented in this study was carried out using Giovanni, a state-of-the-art Web-based data analysis and visualization tool.