| Aluminum (Al) is the trace metal in the ocean, can be used as the tracer of terrestrial input and mixing of different water masses with the characteristic of less influences from human activities. The study of marine biogeochemical behavior of Al has got more and more attentions by scientists. Currently, there still exist controversies on the scavenging mechanism of dissolved Al by phytoplankton. Under this background, the distributions, seasonal and annual variations and the influencing factors of dissolved Al in the Changjiang Estuary and its adjacent area are primarily studied in this thesis. The impact of hypoxia on the distributions of dissolved Al were also primary discussed. Aluminum enrichment incubation experiments were carried out in lab to understand the scavenging mechanism by main species of phytoplankton in the marginal seas of China. The results are helpful to understand the biogeochemical cycle of dissolved Al in the ocean.Three cruises were carried out in the Changjiang Estuary and its adjacent area in June, August and October of 2006. The distributions and seasonal variations of dissolved Al are presented in this thesis. The average concentrations of dissolved Al in June, August and October of 2006 were 119±77, 109±80 and 138±73 nmol/L, respectively. A significantly seasonal variation of dissolved Al was observed in this area. With the impact of seasonally variations of Changjiang diluted water, phytoplankton bloom and the wet depositions, the concentrations of dissolved Al in October are highest and are lowest in August. Compared with the results that obtained in the similar seasons of 2002 and 2003, the dissolved Al in August and October of 2006 are relatively higher. The horizontal distributions of dissolved Al were similar in three cruises, with higher concentrations in the coastal area and decreasing with the distance of offshore and opposite with the distribution of salinity, which indicate the characteristic of land-source input. The vertical profiles of dissolved Al shows the characteristics of a minimum at the middle-depth layer and relatively higher in the surface and bottom, which indicates the sources of dissolved Al from the inputs of Changjiang diluted water and atmospheric deposition in the surface and the re-suspension of sediments in the bottom.The vertical profiles of dissolved Al in some stations were similar with dissolved silicate in the cruise of August, shows the nutrient-like vertical profiles. The concentrations of dissolved Al were lowest corresponds to the maximum layer of Chlorophyll a, which indicated the biological mediation of dissolved Al by phytoplankton. There existed apparent hypoxia in the near bottom layer during the cruise of August. The concentrations of dissolved Al in the upper layer of hypoxia region were relatively lower than the other two cruises, which indicated the obvious scavenging processes. These scavenging might come from the absorbion by the phytoplankton and/or adsorption by the biogenic particles. With the sinking and degradation of biogenic particles, Al might dissolved into water column. There didn't exist simple relationship between dissolved Al concentration and hypoxia, and needs further study.Laboratory studies for the scavenging mechanism of dissolved Al by phytoplankton were conducted with incubation experiments, with several diatom species of Skeletonema costatum, Thalassiosira weissflogii and Nitzschia closterium, and two species of Pyrrophyta of Prorocentrum donghaiense and Alexandrium tamatense. Al enrichment incubation experiments were carried out in three groups, which were control group, 300 nmol/L Al enrichment group and the 600 nmol/L Al enrichment group, with duplicate incubations for each group. The results indicated that all of the three diatoms could scavenge the dissolved Al significantly. The scavenged Al by algae existed in both extra-cellular and intra-cullular phase. The percentage of intra-cellular Al in the total phytoplankton Al were 24.64～74.85%, 45.23～81.82% and 15.37～53.96%, respectively, which indicated that the diatoms could absorb the dissolved Al and involve it into the metabolism. The Prorocentrum donghaiense has little effect on the behavior of dissolved Al during the incubation. The Alexandrium tamatense can scavenge the dissolved Al, with the percentage of intra-cellular Al in the total phytoplankton Al of 24～62%. The impact of different phytoplankton blooms on the biogeochemical cycles of dissolved Al are different based on the lab incubation results.The concentrations of intra-cellular Al for Skeletonema costatum (stable stage) in three groups are 0.27±0.01 mg/g, 0.39±0.06 mg/g and 0.23±0.00 mg/g, respectively. The concentration of intra-cellular Al in 300 nmol/L Al enrichment group is highest, which indicates that proper enriched Al can promote Skeletonema costatum to uptake dissolved Al. The concentrations of intra-cellular Al for Thalassiosira weissflogii are 0.09±0.01 mg/g, 0.17±0.07 mg/g and 0.31±0.09 mg/g, respectively. The concentration of intra-cellular Al is increasing with the Al enrichment in the incubation solution. The result evinces that the higher concentration of Al, increase the absorbing capacity of Thalassiosira weissflogii. The concentrations of intra-cellular Al for Nitzschia closterium are 0.30±0.06 mg/g, 1.03±0.17 mg/g and 0.59±0.10 mg/g, respectively, which is similar with Skeletonema costatum. Nitzschia closterium has the maximum intra-cellular Al, followed by Skeletonema costatum and Thalassiosira weissflogii. The detailed scavenging mechanisms needs further study.The concentrations of intra-cellular Al for Alexandrium tamatense are 0.10±0.01 mg/g,0.11±0.03 mg/g and 0.10±0.00 mg/g, respectively. The result indicates that there is no simple relationship between the Al enrichment and the concentrations of intra-cellular Al for Alexandrium tamatense. The concentrations of intra-cellular Al for Alexandrium tamatense are lower than the other three diatoms. |
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