Dissolved organic carbon (DOC) is one of the largest pools of carbon in the ocean, and is of the same size as the carbon dioxide in the atmosphere. Estuaries connecting the land and the ocean are one of the most important DOC sources to the ocean, and play an important role in the global carbon cycle. Because of their complex chemical, physical, geological and biological properties, estuaries have become rich ecological environment. In this study, we investigated the seasonal distributions of DOC in the Taiwan Strait (TS) and Taiwanese rivers, aiming to understand the distributions and variations of DOC in different seasons. The results show that DOC concentrations are generally the highest in the upper estuary, and then decrease downstream due to mixing with the low DOC seawater. The process of river flow constantly accumulates terrestrial material, and the DOC shows positive correlations with Chl. a, CH4 and BOD (Biochemical Oxygen Demand), suggesting that biological activities and pollutions could be sources of DOC in the estuary. The DOC concentrations (salinity<1) varied in dry (Nov.-Apr.) and wet (May-Oct.) seasons with ranges of 42-1185 £gM (mean=245¡Ó254£gM; n=32) and 18-565 £gM (mean=183¡Ó151£gM; n=24), respectively. The total DOC flux of 25 rivers is 87.8 Gg C/yr, which can be translated to the fluxes of all rivers in Taiwan to be 101.9 Gg C/yr. The amount of DOC flux in Taiwan is only about 0.07% of the tropical area, but the per unit area flux (3.92 gC /m2 /yr) is almost twice those of the tropical rivers (2.13 gC /m2 /yr). In Taiwan, the population density and land use are higher than the world average. Consequently, the impacts of the environment by human activities reveal the utmost export of DOC, and need further investigation. Next, in the TS, the DOC shows significant negative correlations with Sigma-T, and the distributions of DOC are mainly controlled by physical mixing in both winter and summer. In the western TS, DOC concentration is relatively high, compared to the eastern part, and is because of low temperature and salinity, but high DOC coastal China current flowing from north to south. DOC concentration decreases with increasing depth owing to the intrusion at depth by the Kuroshio, which contains relatively low DOC. In winter, the import of coastal China current brings more nutrients from north to south, and supports the growth of bacteria which depletes the DOC and oxygen. As the result, DOC decomposition rate is higher in winter than in summer. The TS¡¦s DOC fluxes in summer (northern TS: 3.85¡Ñ1012mol C/yr¡Fsouthern TS: 3.75¡Ñ1012mol C/yr) are higher than in winter (northern TS: 3.69¡Ñ1012mol C/yr¡Fsouthern TS: 2.84¡Ñ1012mol C/yr). Main differences are due to the prevailing southwest monsoon winds in summer transporting more water from the South China Sea to the TS, and the river discharge brings more terrigenous organic matters into the TS. Therefore, the DOC export in summer is higher than in winter.

¡@¡@Sediment loads of Taiwan rivers have increased in recent years because of frequently recurring typhoons. In addition to the dry and wet seasons causing sever erosion and deposition of the riverbed, the long-term records from 1967 to 2009 of suspended sediment concentration and daily discharge observed by the Water Resource Agency in Taiwan Rivers were used in this study. ¡@¡@To understand the sediment deposition or erosion in the rivers, we need to find how the discharge and sediment load interact with each other under the natural and human influences. The hydrological data we used in this study were from Water Resources Agency¡¦s annual reports, including the Danshui River, the Lanyang River, the Zhuoshui River, the Beinan River, and the Gaoping River. ¡@¡@High turbidity happen when sediment concentration exceeds 40 g/L. Judging by this threshold, taking upstream stations of the Zhuoshui River for example, the Nei-Mao-Pu Station (1973-2009) reached this threshold 6 times, Bao-Shih Bridge Station (2004-2009) 19 times, and Yun-Feng Bridge Station (1995-2009) 19 times. But Yen-Ping Bridge Station (2004-2009), which is at the middle part of Zhuoshui River, has never reached the threshold, became the erosion rate and sedimentation rate are balanced here. Chi-Chou Bridge Station (2000-2009) at the downstream reached the threshold for 11 times too. ¡@¡@According to the river graded profile, in the Danshui River, obvious erosion occurred at the Liu-Kwei Bridge Station in 2000, the Po Bridge Station in 2001, and San-Hsia (2) Station in 2004 and 2007. In the Zhuoshui River, significant deposition took place at the Yun-Feng Bridge Station in 2006 and 2010. In the Gaoping River, deposition occurred at the A-Chyi-Ba Bridge Station in 2000, while erosion occurred at Liu-Kwei Station in 2001 and at the A-Chyi-Ba Bridge Station in 2007 and 2008. ¡@¡@Above results show that the major sediments are deposited upstream, forming a temporary sediment storage source region, such as at Li-Lin Bridge Station, Liu-Kwei Station, and Yun-Feng Bridge Station at the Gaoping River, all of which have reached the threshold when a typhoon comes. Investigating daily rainfall data from Taichung Weather Station in 2005, we find that besides typhoon influences, when the rainfall is exceeding 200 mm High turbidity events would occur in the Jhoushuei River. ¡@¡@According to our study, we find that most sediment deposits at the upper reaches, forming temporary sediment storages, so when a typhoon comes, we can see the sediment downstream is often mixed with the sediment coming from upstream storages. That is because the slope in the upper reaches is steeper, and heavy rain makes the temporary deposit easy to erode. On the contrary, the slope in the lower reaches is more gentle so it is easier for sediment to deposit, which is the reason why the stations at downstream often reach the threshold. When the extreme weather occurs, the temporary sediment storages along the entire river course become the source regions to release the sediment out to the sea.