Studies On The Relationships Of Solar Ultraviolet Radiation (UVR) And Photosynthetic Carbon Fixation By Phytoplankton Assemblages From The South China Sea

Atmospheric CO2 uptake by the oceans depends on the"marine biological CO2 pump"driven by marine photosynthesis. Solar ultraviolet radiation (UVR, 280-400 nm) is known to affect the photosynthetic processes and carbon fixation of phytoplankton. Therefore, measurements of marine primary production, without UVR being considered as in traditional methods, can result in significant errors. On the other hand, the depletion of the stratospheric ozone layer has caused an increase of solar UVB (280-315 nm) irradiance reaching the Earth's surface, which could influence the marine primary productivity. Based on the above significances of UVR-effects, this thesis was designed to investigate UVR effects on carbon fixation of the phytoplankton assemblages from the typical areas of the South China Sea. The relationships of UVR-related physiological impacts with the changes in environments or taxonomical structure were analyzed, and the responses to UVR of phytoplankton were compared from the coasts to pelagic waters. The main results are as follows:Different features in light transmission of the vessels can lead to the estimation errors on marine photosynthetic carbon fixation as high as 60%. Carbon fixation rates measured with C-14 method using the quartz (UV transmitted), borate glass (partially UV transmitted), polycarbonate (little UV transmitted and little PAR blocked) or common glass vessels (partially UVA transmitted), showed significant differences. Compared with the results from the quartz vessels, the carbon fixation rate could be overestimated by 30% with the common glass vessels, but underestimated by 3% with borate glass or by 30% with polycarbonate vessels, respectively.Phytoplankton carbon fixation could be enhanced at moderate levels of solar radiation by UVA (315-400 nm), though it brought about inhibition at higher levels. Longer wavelengths of UVB (305-315 nm) were also effective in enhancing the photosynthetic carbon fixation. Under UVR alone treatment (PAR, filtered out), UVR could drive the photosynthetic carbon fixation. Therefore, the carbon fixation by phytoplankton assemblages exhibited higher rates in the presence of UVR on cloudy days, but lower rates on sunny days. The capacity of UVR-utilization was related to the size of phytoplankton cells. Micro-cells (>20μm) were able to use UVR more efficiently than nano-cells (5-20μm), and smaller nano- or pico-cells (<5μm) were unable to use UVR for the photosynthetic carbon fixation. Additionally, more UV-absorbing compounds were detected in the cells with UVR-utilized capacities.UVR-induced impacts varied among different seasons and spatially. UVR-induced inhibition was lower in summer than that in winter, although solar radiation was high in the former season when the repair rate of UVR-induced damage was faster in the higher temperature. During the summer period, typhoon's passage influenced the UVR-induced effects on phytoplankton community structure and carbon fixation. The larger cells were dominant in pre-typhoon; while pico-cells were dominant in post-typhoon periods, and UVR decreased the photosynthesis to more extent after typhoon. In winter, the phytoplankton assemblages were dominated by piconano-cells, and more sensitive to UVR. On the other hand, the dominant phytoplankton species changed from micro- to pico-cells from the coastal to off-shore waters, and UVR-induced inhibition on photosynthetic carbon fixation increased with increased distance from the shore or with decreased cell sizes. In the estuaries, physic-chemical environments were very complex because of the influence by the freshwater inputs and the water-exchange with the adjacent open sea; therefore, dramatic changes were found in phytoplankton biomass, taxonomical structure and photosynthetic capacity. Carbon fixation per Chl a or per volume of seawater were lower in the low (<10) or high (>20) salinity zones, but higher in the middle salinity zone (10-20) where more reduction caused by solar UVR was observed on the photosynthetic rates of phytoplankton assemblages.In summary, solar UVR effects on phytoplankton varied spatially and temporally; and also changed according to the changes in community structure. Solar UVB always led to the negative effects on the phytoplankton photosynthesis; while UVA could promote the carbon fixation at low or moderate levels of sunlight on cloudy days or at deeper depths.