Biogeochemical Characteristics Of Dissolved Inorganic Carbon System In The Typical Coastal Waters Of China

As the biggest carbon reservoir on the earth surface, the oceans play a dominant role in the natural regulation of CO₂ in the atmosphere, thus exerting a powerful influence on the climate and marine ecosystem. Dissolved inorganic carbon (DIC) accounts for >85% of the total carbon in seawaters and its variability has significant influence on marine carbon cycling. So, to study DIC in seawaters systematiclly is the basis for a better understanding of the key processes of marine carbon cycling. Two typical marginal seas, namely the Changjiang Estuary and the southern Yellow Sea, were chosen as our study sites. Field observations were carried out in May 2003 and August 2004 in the Changjiang Estuary, and in October 2004 in the southern Yellow Sea. Abundant data of hydrography, chemistry and biology were gained, based on which the behaviour and biogeochemical characteristics of DIC in seawaters of these two areas were investigated. Carbon dioxide fluxes between air-sea interfaces were estimated for the first time throughout the entire salinity gradient in the Changjiang Estuary. Some new knowledge about inorganic carbon chemistry was acquired. 1. In the dry season, the Changjiang Estuary changed gradually from a strong source for atmospheric carbon dioxide in low salinity area to a weak sink in high salinity area. The physical mixing of fresh and salty water had significant influence on this carbon sink/source pattern. In the CO₂ source area, phytoplankton abundance was low and zooplankton mainly belonged to true estuary and estuary marine communities; while in the CO₂ sink area, phytoplankton was high and zooplankton mainly belonged to euryhaline marine communities. In the raniy season, the Changjiang Estuary was a net source for atmospheric CO₂, and the distribution characteristics of parameters related to DIC system were similar to those in fresh and mixed water areas in the raniy season. During the field observation, the Changjiang fresh water mass and the western turbidity maximum were well mixed, while partly mixed in other parts. Spatio-temporal variation characteristics of parameters related to carbon dioxide, i.e. pH, total alkalinity (TA), total carbon dioxide (TCO₂), partial pressure of carbon dioxide (pCO₂) and each of carbon dioxide species (HCO₃^-,CO₃^2- and CO_2(T), were studied. The results indicated that physical mixing of different water masses was the most important factor influencing spatial distribution of TA, TCO₂ and HCO₃^-; while for the other parameters, the influence of biological activities on their spatial distribution was also significant besides the influence of physical mixing of different water masses. For most of the parameters, the main factors that influence their temporal variations were different in different regions. As a whole, p CO₂ decreased from the fresh water to seawater. The region where surface water salinity was below 25 was a source for atmospheric carbon dioxide, while the other region was a sink. Carbon dioxide fluxes between air-sea interface varied from -5.8 to 531.5 mmol m^-2 d^-1. The area between Xuliujing and river mouth emitted (7.0 ± 2.8) × 103 tC per day. Phytoplankton and zooplankton are biological factors that can significantly influence dissolved inorganic carbon in waters, so they were also studied during the investigation. Results indicated that phytoplankton abundance increased seaward, showing a distinct spatial difference, and the dominant species varied with salinity. Correlation between phosphorus and phytoplankton abundance further supported that phosphorus is the controlling factor in phytoplankton growth in the Changjiang Estuary where light is not; correlation between p articulate organic carbon and phytoplankton a bundance further proved the former results that a major fraction of particulate organic carbon in the Changjiang Estuary area was imported by Changjiang freshwater discharge from land. Variation characteristics in zooplankton biomass seemed to relate to tide cycles and in most cases increased during flood...