Non-conservative Behaviors And Biogeochemistry Of Molybdenum In Coastal Waters

We investigated seasonally and spatially biogeochemical behaviors of molybdenum (Mo) in the Pearl River estuary and western Taiwan Strait. The upper Pearl River estuary was characterized with low redox potentials as directly receiving seawage and industrial effluents nearby, while the lower Pearl River estuary and western Taiwan Strait are generally oxic as influenced from coastal currents and seawater intrusion. We observed that dissolved Mo behaved quite differently in these waters in response to coastal currents, algal activities and terrestrial materials. Our laboratory culture experiments of diatom Phaeodactylum triconutum demonstrated that the algal growth and its physiology are sensitive to subtle changes of environmental factors such as Cu, Mo, and CO2levels. Some major findings are listed below.1. Dissolved Mo in the Pearl River Estuary generally increased from the freshwater to seawater endmembers (August:6.2â†'96.2nmol/L; December:19.2â†'103.0nmol/L), as mainly dictated by estuarine mixing process. Particulate Mo was separated into adsorbed and internalized fractions:internalized Mo decreased from the upper parts of the estuary outward (3.7â†'undetectable μg/g); absorbed Mo was also observed with concentrations of as high as1.1μg/g. Similar to particulate Mo, sedimentary Mo decreased outwards (2.9â†'0.2μg/g).Dissolved Mo was generally conservative with salinity. However, non-conservative properties of dissolved Mo were also observed in the estuary. For example, in the upper Pearl River estuary, dissolved Mo received extra contributions inly from suboxic reduction of minerals oxides. In the middle estuary (maximum turbidity zone), dissolved Mo was slightly removed as a result of the adsorption onto oxic particles, especially in August. Such conservative properties could be further reflected via the dynamic behavior of different particulate Mo fractions.2. Dissolved Mo in the Western Taiwan Strait (Range:May,70.6-100.0nmol/L; August,81.0-100.0nmol/L) demonstrated its temporal variations in responses to oceanic currents and algal activities. Generally, the Jiulong River plume was characterized with low levels of dissolved Mo (May,75.6±5.7nmol/L; August,75.6±5.7nmol/L).In addition, we also observed high levels of sedimentary Mo in the western Taiwan Strait (2.0-4.2μg/g), and especially near the plume zone (5.6-12.9μg/g). In May, north east monsoon ceased, followed by a large relaxation of the Min-Zhe coastal current, we observed extremely high algal activities as characterized with high Chl-a (～10μg/L). Accordingly, dissolved Mo was largely removed via adsorption onto particles and organic matter, followed by precipitation into sediments. Flux calculation demonstrated that SGDs and riverine inputs contributed with a relatively small fraction of all dissolved constituents including Mo and P (accounting for <10%of oceanic contributions).3. Laboratory culture experiments of diatom Phaeodactylum triconutum demonstrated that increased levels of Cu could promote the algal growth but decrease physiological states (e.g., ETR and Fv/Fm). CO2addition could promote algal growth and also photosynthetic rates (increased ETR), but depress the respiration (decreased Fv/Fm). Mo addition experiments showed that Mo could promote the agal growth, but inhibit its physiological states. Our results of intracellular and adsorbed metal fractions provide clear evidence that phytoplankton mainly accumulates toxic substances such as high levels of Cu and Mo onto surface, instead of directly uptaking into cells.Overall, this study examined the non-conservative property of dissolved Mo in natural waters, and observed that dissolved Mo could be excessive due to sedimentary releases under reducing conditions, while dissolved Mo could be removed via adsorption onto particles and organic matter with a deficit of as high as10nM from the estuarine mixing line. In particular, algal growth and high levels of organic matter produced could serve as a major carrier removing dissolved Mo out of the water column, and precipitating Mo into sediments.