Bioavailability Of Dissolved Organic Nitrogen And Its Role During The Outbreak Of Harmful Algal Blooms In Changjiang River Estuary And Its Adjacent Sea

Dissolved organic nitrogen (DON) is the largest of nitrogen pool and its composition and bioavailability play an important influence on the phytoplankton biomass and population structure in the coastal system. The Changjiang River Estuary and its adjacent sea (CJREAS) is one of serious eutrophication area. Recent years, the Harmful Algal Blooms (HABs) has become frequently and occurres the succession of the diatom bloom to the dinoflagellate bloom in the CJREAS. Nitrogen as one of the necessary biogenic elements of phytoplankton growth, its source and translation play an important role in the outbreak of the HAB. However, studies of nitrogen source and the biogeochemical process focused on the dissolved inorganic nitrogen (DIN), and the bioavailability and the role of DON in the outbreak of the HABs is still lack of deeply studies in the CJREAS. In this study, CJREAS was chosen as the target area, and the survey investigation, the degradation experiments of DON and 15N stable isotope techniques were applied to understand the spatial distribution and fluxes of DON including urea, amino acids in this research; Combining the degradation index, the DON degradation experiment and 15N stable isotope experiment to evaluate the potential bioavailability of DON in the different season, especially the period of the succession of diatom bloom to dinoflagellate bloom in the CREAS, and preliminary exploring the mechanism of DON during this period. The research results were as follows:1. Composition, flux and bioavailability of DON in the Changjiang River1) The concentration of DON had a negative correlation with the Changjiang runoff and showed an obvious seasonal variation tendency which was higher in spring and winter, lower in the summer and autumn in Changjiang River at Datong station during 2011 to 2014. Its concentration was between 10.25μmol·L-1 and 90.52μmol·l-1, and accounted for 7.40～50.89% of TDN. Urea, dissolved free amino acids (DFAA) and dissolved combined amino acids (DCAA) concentrations accounted for 19.1、0.6 and 4.2% of DON, respectively.2) The flux of DON was 43.54×104t·yr-1 and accounted for 20% of TDN flux in Changjiang River during 2011 to 2014; Urea, DFAA and DCAA fluxes were 6.62, 0.25 and 1.64×104t·yr-1, respectively, and accounting for 15.2、0.57.3.78% of DON flux, respectively.3) The carbon-normalized of amino acids (TDAA (%DOC) and Degradation Index of amino acids (DI) indicated that DON had high bioavailability and showed higher in spring and summer, lower in autumn and winter in Changjiang River during 2011 to 2014.2. Spatial distribution and bioavailability of DON in the Changjiang River Estuary and its adjacent sea1) Based on the analyses of the survey of 2002 to 2014 in the CJREAS, the results showed the average concentration of DON and DON/TDN showed obvious seasonal variation, which was highest in autumn, followed by summer and spring. The distribution of DON showed a tendency, which was higher in the costal and lower in the offshore because of the influence of runoff and phytoplankton; while DON/TDN showed an opposite tendency, which was higher in the offshore and lower in the coastal.2) Urea and TDAA had the higher bioavailability components in DON pool. In the spring of 2014, their average concentrations were 0.84±0.33 and 0.96±0.74μmol·L-1, respectively, accounted for 26.49±14.14 and 17.73±12.01% of DON concentration, respectively. In the autumn of 2014, their concentrations were 0.98±0.28 and 0.67±0.32umol·L-1, respectively, accounted for 40.42±16.14 and 15.50±8.70% of DON concentration, respectively.3) Based on the DON degradation experiment to confirm the Bio-DON by the value of TDAA (%DON) The results also implied that Bio-DON concentration and Bio-DON/DON showed higher in spring than autumn with the average value 2.76±1.82,1.94±1.08μmol·L-1 and 44.84±26.69,41.43±23.62%, respectively.4) Based on the 15N stable isotope experiment to estimate the uptake rates and turnover time of nitrate, urea, DFAA and DON. The results showed the uptake rates of nitrate, urea, DFAA and bulk DON were 13.05±8.88,15.81±3.00,10.62±4.64 and 45.44±18.32nmol N1-1 h-1 in summer, respectively; and were 16.18±1.89,4.20± 1.44 and 1.94±0.55nmol N 1-1 h-1 in autumn, respectively. The corresponding turnover time of nitrate, urea, DFAA and bulk DON were 228.82±243.45,3.64± 0.49,3.22±1.35 and 13.58±8.10d in summer, respectively; and were 134.88±31.33, 16.79±7.15 and 23.31±6.28d in autumn, respectively.3. Role and mechanism of DON during the outbreak of HABs1) Based on the 15N stable isotope experiment in the lab and flied to confirm the uptake kinetics of nitrate, urea, DFAA and DON in Skeletonema costatum and Prorocentrum donghaiense cultures. The results showed that S. costatum and P. donghaiense could utilize different kinds of nitrogen and exhibite standard Mond-type growth kinetics. It had advantage for P. donghaiense to uptake the nitrate and urea than S. costatum; while it had advantage for S. costatum to uptake the nitrate and urea than P. donghaiense.2) Based on the data of DON degradation experiment indicated that the concentration of Bio-DON decreased by 4.93,7.60 and 6.87μmol·L-1 in section ZA, Z and ZC surface water, respectively; and accounted for above 50% of TDN which had the higher bioavailability. Urea, DFAA and DCAA concentrations decreased obviously by 68.75,27.61 and 23.02%,29.81 (middle layer),47.67 and 37.96%,29.33, 45.41 and 64.61%, respectively.3) Based on the analyses of above, the results indicted that it has advantage for P. donghaiense to developing the bloom than S. costatum in the low nitrate concentration. Meanwhile, the Bio-DON including urea and DFAA could provide nitrogen and promote the development of the P. donghaiense bloom.