| Typhoon is one of the most serious catastrophic weather events that affect the estuarine and coastal environments in East China. The powerful winds during the typhoon passage strength the vertical mixing of seawater. In addition, the heavy precipitation caused by the typhoon increases the river discharge, in which a great amount of extra terrestrial materials are brought into the near-shore oceans. These two processes greatly modified the physical, chemical, and biological processes in estuarine and coastal areas directly or indirectly affected by the typhoon. The structure and function of the marine ecosystems are also greatly changed after typhoons. Therefore, it is of great importance to study the effects of typhoons on the physical, chemical, and biological environments in estuarine and coastal areas, for the purposes of further evaluating the evolution of marine ecosystem on a long timescale in near-shore oceans that were frequently influenced by typhoons.It is extremely difficult to conduct field sampling during the typhoon periods because of the harsh weather conditions, thus in this study those field data, collected before and after typhoons in the estuarine and coastal areas in East China, are such valuable for studying the influence of the intruding typhoons on the marine environments and ecosystems. In addition, techniques such as remote sensing and numerical model are also utilized in this study together with the field sampling, to investigate the effects of typhoon events on the transport of terrestrial materials, on the changed water structure, and on the distribution and variation of phytoplankton in areas like the upper Minjiang Estuary, the Changjiang Estuary and its adjacent sea, and the northeastern East China Sea (ECS). In particular, the variations of concentration of the surface Chl a after the typhoon events are studied in a great detail, to elucidate the response of the biomass and distribution of surface phytoplankton to the typhoon-induced current changes and heavy precipitation in the ECS. The main conclusions of this study are listed as follows:1. Changing process of terrestrial materials in the upper Minjiang Estuary under the influence of typhoon eventsThe concentrations and fluxes of terrestrial materials in the upper Minjiang Estuary were changed considerably after the two typhoons of Soulik (1307) and Trami (1312). Concentrations of the dissolved inorganic nitrogen (DIN), PO43-, and total suspended matter (TSM) were found to increase by 7.2%,18.2%, and 409.3%, respectively, on the landing day of Soulik, compared to those values measured before this typhoon. On the other hand, the average concentration of Chi a over the same area decreased by 57.7%, possibly owing to the dilution of the increased freshwater discharge. Similar results were also found in this area after the other typhoon of Trami, in which concentrations of DIN, PO43-, and TSM increased by 21.5%,65.4%, and 98.6%, respectively, whereas that of Chl a decreased by 62.2%. In addition, the dissolved organic carbon (DOC) and particulate organic carbon (POC) were also increased by 169.3% and 138.3% , respectively, after the passage of Trami.Concentrations of Chl a were found to decrease on the following days after the landing of Soulik and Trami, and then increased gradually several days later and even exceeded those values measured before the typhoon events, which should be largely ascribed to the increasing nutrient concentrations and the stable hydrodynamic environment at that time. The output fluxes of nutrients and TSM during the two months of July and August could contribute to 35-45% of their total fluxes from Minjiang through the year of 2013, owing to the steep increases of both their concentrations in waters and the freshwater discharge during the typhoon periods.2. Responses of water environment and phytoplankton growth to typhoon passage in the Changjiang Estuary and the adjacent coastal areasDue to the relatively short residing time of this typhoon on the study area, the physical field was found to be more controlled by the expansion of the Changjiang Diluted Water (CDW) and the landward intrusion of offshore waters. The influence of CDW was particularly significant in the upper water columns, seen from the considerably decreased salinity and the increased nutrient concentrations there after this typhoon event. At the same time, the bottom layers after the typhoon were dominated by the intruded high-salinity and low-nutrient offshore waters. The changes in water environment in the middle layer were quite complex because of the multiple effects of the processes described above. Under the influence of the typhoon-induced strong turbulence and the limited solar irradiance, after the typhoon the phytoplankton biomass decreased at the beginning, but recovered rapidly several days later until to values even higher than those measured before the typhoon.3. Variation characteristics of the water environment and primary productivity to the passages of two successive typhoon events in the northeastern ECSThe responses of the water environment and primary productivity to the first typhoon Maemi (0314) were more significant, under the influences of two successive typhoons. The second typhoon Choi-Wan (0315) made the water column even more vertically mixed, and prolonged the duration of the increase in primary productivity. However, the decrease of sea surface temperature (SST) and the increase of primary productivity following the second typhoon were not as considerable as those after the first typhoon, which should be largely owing to the early influence of the first typhoon. In addition to strengthening the vertical mixing, typhoons also changed the advection structure in the upper seawater, through which the distribution of temperature, salinity, nutrients, and phytoplankton were also greatly modified.The redistribution of phytoplankton biomass after the two typhoons in the subsurface layers was also obvious like that at the surface. The subsurface chlorophyll maximum (SCM) layers were destroyed at some stations after the typhoon, and several days later these SCM layers reappeared in which the phytoplankton biomass showed even higher values than those before the typhoon. This study also indicated that some phytoplankton biomass in the SCM could be transferred to the surface during the typhoon period; hence the surface phytoplankton biomass could also be elevated simply by the physical process. Obviously, this increased biomass was of an allochthonous origin and different from that caused by the local productivity simulated by elevated nutrient levels. Thus some previous studies may have overestimated the contribution of the typhoons to the increased primary productivity, because they did not differentiate the phytoplankton biomass with allochthonous and autochthonous origins.4. Varieties of surface phytoplankton growth and distribution after multiple powerful typhoons in the ECSIt is found that the average surface phytoplankton biomass was not always increased after the typhoons, seen from the variations of the Chl a concentrations extracted from the remote sensing data after the passage of the nine typhoons occurring on ECS. The overall Chl a concentrations before the typhoons and the residing time of the typhoon on the sea were the two major factors that affected the Chl a concentrations after the typhoon. In most cases the SST and the precipitation had little effects on the Chl a concentrations after the passage of the typhoons. The surface current changes could also play an important role on the surface phytoplankton biomass after the typhoon, it was found that after one typhoon the low-Chl a waters were continually introduced to the ECS and the surface Chl a concentrations displayed a decreasing trend. |
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