Abstract. The air-sea exchanges of CO 2 in the world's 165 estuaries and 87 continental shelves are evaluated. Generally and in all seasons, upper estuaries with salinities of less than two are strong sources of CO 2 (39 ± 56 mol C m −2 yr −1 , positive flux indicates that the water is losing CO 2 to the atmosphere); mid-estuaries with salinities of between 2 and 25 are moderate sources (17.5 ± 34 mol C m −2 yr −1 ) and lower estuaries with salinities of more than 25 are weak sources . Mixing with low-pCO 2 shelf waters, water temperature, residence time and the complexity of the biogeochemistry are major factors that govern the pCO 2 in estuaries, but wind speed, seldom discussed, is critical to controlling the air-water exchanges of CO 2 . The total annual release of CO 2 from the world's estuaries is now estimated to be 0.10 Pg C yr −1 , which is much lower than published values mainly because of the contribution of a considerable amount of heretofore unpublished or new data from Asia and the Arctic. The Asian data, although indicating high pCO 2 , are low in sea-to-air fluxes because of low wind speeds. Previously determined flux values rely heavily on data from Europe and North America, where pCO 2 is lower but wind speeds are much higher, such that the CO 2 fluxes are higher than in Asia. Newly emerged CO 2 flux data in the Arctic reveal that estuaries there mostly absorb rather than release CO 2 .Most continental shelves, and especially those at high latitude, are undersaturated in terms of CO 2 and absorb CO 2 from the atmosphere in all seasons. Shelves between 0 and 23.5 • S are on average a weak source and have a small flux per unit area of CO 2 to the atmosphere. Water temperature, the spreading of river plumes, upwelling, and biological production seem to be the main factors in determining pCO 2 in the shelves. Wind speed, again, is critical because at high latitudes, the winds tend to be strong. Since the surface water pCO 2 values are low, the air-to-sea fluxes are high in regions above 50 • N and below 50 • S. At low latitudes, the winds tend to be weak, so the sea-to-air CO 2 flux is small. Overall, the world's continental shelves absorb 0.4 Pg C yr −1 from the atmosphere.
Abstract. The eastern China seas are some of the largest marginal seas in the world, where high primary productivity and phytoplankton blooms are often observed. However, little is known about their systematic variation of phytoplankton blooms on large spatial and long temporal scales due to the difficulty of monitoring bloom events by field measurement. In this study, we investigated the seasonal and interannual variability and long-term changes in phytoplankton blooms in the eastern China seas using a 14 yr (1998–2011) time series of satellite ocean colour data. To ensure a proper satellite dataset to figure out the bloom events, we validated and corrected the satellite-derived chlorophyll concentration (chl a) using extensive in situ datasets from two large cruises. The correlation coefficients between the satellite retrieval data and the in situ chl a on the logarithmic scale were 0.85 and 0.72 for the SeaWiFS and Aqua/MODIS data, respectively. Although satellites generally overestimate the chl a, especially in highly turbid waters, both the in situ and satellite data show that the overestimation of satellite-derived chl a has an upper limit value (10 μg L−1), which can be used as a threshold for the identification of phytoplankton blooms to avoid the false blooms resulting from turbid waters. Taking 10 μg L−1 as the threshold, we present the spatial-temporal variability of phytoplankton blooms in the eastern China seas over the past 14 yr. Most blooms occur in the Changjiang Estuary and along the coasts of Zhejiang, with a maximal frequency of 20% (about 73 days per year). The coasts of the northern Yellow Sea and the Bohai Sea also have high-frequency blooms (up to 20%). The blooms show significant seasonal variation, with most occurring in spring (April–June) and summer (July–September). The study also revealed a doubling in bloom intensity in the Yellow Sea and Bohai Sea during the past 14 yr. The nutrient supply in the eastern China seas might be a major controlling factor in bloom variation. The time series in situ nutrient datasets show that both the nitrate and phosphate concentrations increased more than twofold between 1998 and 2005 in the Yellow Sea. This might be the reason for the doubling of the bloom intensity index in the Yellow Sea and Bohai Sea. In contrast, there has been no significant long-term increase or decrease in the Changjiang Estuary, which might be regulated by the Changjiang River discharge. These results offer a foundation for the study of the influence of phytoplankton blooms on the carbon flux estimation and biogeochemical processes in the eastern China seas.
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