Contribution Of Atmospheric Deposition To The Nutrients And Its Ecological Effects In The Bohai

The Bohai Rim,which is one of the most developed regions in China,discharges thousands of tons of contaminants N and P into the Bohai Sea from coastal ports and heavily-polluted rivers,resulting in deterioration of coastal marine eco-environments and degradation of ecological functions.Many investigations indicate that atmospheric nutrient deposition is a critical factor that causes water eutrophication and significantly affects marine ecological environment.Studies have shown that the input of N into marine ecosystem via atmospheric deposition may equal or even surpass the amount from riverine discharge,but rare research has been launched to quantify the fluxes and the influences from atmospheric deposition.We established a3D coupled biogeochemical model,to reasonably quantify the atmospheric deposition flux and to assess its influence on Bohai ecosystem dynamics and nutrients levels.This may deepen the understanding of nitrogen biogeochemical cycling,and be useful for coping with degradation of ecological function and water pollution in the Bohai Sea.In this paper,the spatial distributions and temporal variations of DIN and PO₄-P/DIP in the Bohai Sea were analysed systematically and integrally based on the historical field observations since 1959,which covered almost the entire Bohai Sea.Results showed that the spatial patterns of the DIN and PO₄-P/DIP in the Bohai Sea had no significant change,but had a great fluctuation at time scale from 1980's.The most prominent feature of the DIN and PO₄-P/DIP concentrations is the highest values in the top of the three bays and lowest values in the central region,with moderate contents in other coastal areas.The averaged nitrate nitogen?NO₃-N?,which constitutes most of the DIN,increased from 35.0?g/L to more than 200.0?g/L in the last 3 decades,with the greatest increment near the river estuaries.Though the increase was not that obvious for DIN,PO₄-P concentration still increased in the nearshore regions,especially in the three bays.Meanwhile,significant reduction of PO₄-P/DIP content near the Yellow River estuaries was observed due to remarkable decrease of river discharge and sediment transportation.The averaged PO₄-P/DIP concentration for the entire Bohai Sea had moderate fluctuation compared with DIN in time scale.The sharp decreases in recent 10 years may be caused by the reduction of sewage discharge.Chlorophyll a underwent a slow increase trend with high concentrations in spring and fall,and low values in summer and winter.Chlorophyll a also had a relative uniform vertical distribution in most seasons.Wet and dry atmospheric nitrogen depositions were estimated from 15 stations around the Bohai Sea by using the dataset of precipitation,NOx emission fluxes and NO2 concentrations.The results showed that the southern part of the Bohai Sea,especially in the Bohai Bay region,was the most deposited area.This region can be regarded as a“hot spot”for atmospheric nitrogen deposition across the word.The multi-year averaged wet and dry deposition fluxes was 1.58⁵.09 g N/m²/a and1.44⁶.72 g N/m²/a,respectively.The patterns of wet and dry atmospheric depositions fluxes determined the distribution for DIN and DIP in the Bohai Sea to a certain extent.The nutrient and temperature limitations for phytoplankton growth were analyzed,and the nutrient exchange fluxes of water-sediment interface were.After looking into lab/field or model experiment results within the Bohai Sea region,the optimum temperature for main algae species growth was confirmed to be ca.18-26?.Then a modification to the temperature-limited growth rate for phytoplankton was done,to better depict the algae growth.Based on these works,a 3D coupled biogeochemical model was configured with varied atmospheric deposition fluxes on both spatical and temporal scales.This model was used to carry out research on how and to what extent the atmospheric deposition influences the nutrient distribution and the spatio-temporal variations of the biological variables.Results showed that except for the geographic topography and circulation,the atmospheric depositions played an important role in the spatial distribution of nutrient concentrations in the Bohai Sea.The model showed that the relative contribution of deposited nitrogen to the total DIN in the aquatic ecosystem?from rivers and atmospheric depositions?for the entire Bohai Sea was larger than 54%,causing a ca.56%increase in the phytoplankton biomass on average.These results indicated that the effects of atmospheric N input on nutrient contribution and phytoplankton dynamics varied widely in regional areas due to the uneven spatial distribution of N deposition fluxes and the weak water exchange.