Dissertation Abstract
Biological N2 fixation in the China marginal seas
Zhang, Run 2010 http://mel.xmu.edu.cn/teacherfile.asp?tid=428
Department of Oceanography, Xiamen University, China (China), 250 pp.
N2 fixation in China marginal seas, i.e. the Beibu Gulf, the northern South China Sea, the East China Sea and the southern Yellow Sea was systematically studied using 15N2 tracer assay during 11 cruises. The major results are as follows:
1. Spatial and temporal distribution of biological N2 fixation. In the Beibu Gulf, integrated N2 fixation rate in spring, summer, autumn, and winter was 22 micro-m molN/m2/d, 43 micro-m molN/m2/d, 28 micro-m molN/m2/d, and 19 micro-m molN/m2/d, respectively. Similarly, integrated N2 fixation rate in the northern South China Sea in summer was not high（27 micro-m molN/m2/d）. N2 fixation rate in the East China Sea and the southern Yellow Sea was a little higher, with a value of 59 micro-m molN/m2/d, 50 micro-m molN/m2/d, and 24 micro-m molN/m2/d in summer, autumn and winter, respectively. As to the temporal viariations, higher N2 fixation rates were observed in summer and autumn. As to the spatial viariations, higer N2 fixation rates were observed in the mouth area in the Beibu Gulf, the Kuroshio influenced southern East China Sea area and southern Yellow Sea（35°N）.
2. Contribution to biological production by N2 fixation. N2 fixation could contribute a portion of 1.0%，1.1%, and 0.70% to primary production in the Beibu Gulf annually, northern South China Sea in summer, and the East China Sea and southern Yellow Sea during summer and autumn, respectively. While for the new production, the portion among these sea areas was 3.1%, 7.2% and 5.2%, separately. Thus, fixed nitrogen input from out of the euphotic zone is the most important new nitrogen source supporting biological production in the China marginal seas.
3. Controls on biological N2 fixation. Bioavailability of inorganic nitrogen and phosphorus nutrients was the major factor controlling N2 fixation among these sea areas. Furthermore, in the Beibu Gulf water column, stratification and exchange with South China Sea waters also influenced the distribution of N2 fixation, while in the East China Sea and the southern Yellow Sea water column, stratification and intrusion of Kuroshio waters played an important role.
4. Annual biological N2 fixation flux. The Beibu Gulf, northern South China Sea, East China Sea and southern Yellow Sea would contribute an annual flux of 1.3×10 to the nine power molN/a, 1.7×10 to the ten power molN/a, and 1.5×10 to the ten power molN/a via biological N2 fixation, accounting for 0.02%, 0.24%, and 0.21% of global ocean N2 fixation flux, respectively, indicating that N2 fixation in the China marginal seas is not intense. Consequently, the net sequestration flux of atmospheric CO2 should be 8.8×10 to the nine power molC/a, 1.1×10 to the eleven power molC/a, and 1.0×10 to the eleven power molC/a, separately. N2 fixation was an important exogenous new nitrogen source, however, the contribution (with a portion of 13% and 14% in the northern South China Sea and East China, respectively) was lower than land riverine and atmospheric deposition input.
1. Spatial and temporal distribution of biological N2 fixation. In the Beibu Gulf, integrated N2 fixation rate in spring, summer, autumn, and winter was 22 micro-m molN/m2/d, 43 micro-m molN/m2/d, 28 micro-m molN/m2/d, and 19 micro-m molN/m2/d, respectively. Similarly, integrated N2 fixation rate in the northern South China Sea in summer was not high（27 micro-m molN/m2/d）. N2 fixation rate in the East China Sea and the southern Yellow Sea was a little higher, with a value of 59 micro-m molN/m2/d, 50 micro-m molN/m2/d, and 24 micro-m molN/m2/d in summer, autumn and winter, respectively. As to the temporal viariations, higher N2 fixation rates were observed in summer and autumn. As to the spatial viariations, higer N2 fixation rates were observed in the mouth area in the Beibu Gulf, the Kuroshio influenced southern East China Sea area and southern Yellow Sea（35°N）.
2. Contribution to biological production by N2 fixation. N2 fixation could contribute a portion of 1.0%，1.1%, and 0.70% to primary production in the Beibu Gulf annually, northern South China Sea in summer, and the East China Sea and southern Yellow Sea during summer and autumn, respectively. While for the new production, the portion among these sea areas was 3.1%, 7.2% and 5.2%, separately. Thus, fixed nitrogen input from out of the euphotic zone is the most important new nitrogen source supporting biological production in the China marginal seas.
3. Controls on biological N2 fixation. Bioavailability of inorganic nitrogen and phosphorus nutrients was the major factor controlling N2 fixation among these sea areas. Furthermore, in the Beibu Gulf water column, stratification and exchange with South China Sea waters also influenced the distribution of N2 fixation, while in the East China Sea and the southern Yellow Sea water column, stratification and intrusion of Kuroshio waters played an important role.
4. Annual biological N2 fixation flux. The Beibu Gulf, northern South China Sea, East China Sea and southern Yellow Sea would contribute an annual flux of 1.3×10 to the nine power molN/a, 1.7×10 to the ten power molN/a, and 1.5×10 to the ten power molN/a via biological N2 fixation, accounting for 0.02%, 0.24%, and 0.21% of global ocean N2 fixation flux, respectively, indicating that N2 fixation in the China marginal seas is not intense. Consequently, the net sequestration flux of atmospheric CO2 should be 8.8×10 to the nine power molC/a, 1.1×10 to the eleven power molC/a, and 1.0×10 to the eleven power molC/a, separately. N2 fixation was an important exogenous new nitrogen source, however, the contribution (with a portion of 13% and 14% in the northern South China Sea and East China, respectively) was lower than land riverine and atmospheric deposition input.