Nitrogen Argon Measurements for the Quantification of Surface Water Nitrogen Fixation in the Baltic Sea (NArrFix )

蓝藻的固氮作用是波罗的海的一种普遍现象。在仲夏（6 - 8月），主要在波罗的海中部和芬兰湾，在没有溶解的无机氮（DIN）的情况下发生。对氮收支的贡献是显著的，并加强了波罗的海的富营养化。目前的输入估计范围在300千吨-吨/年和800千吨-吨/年之间，与河流和机载DIN输入的总和处于同一数量级。不同估计值的巨大范围是由于N2固定的年际变化和与量化N2固定的不同方法（15N孵化；总N预算；二氧化碳分压记录；磷酸盐过量）相关的巨大不确定性以及将当地研究结果外推到整个盆地的结果。我们的方法是基于蓝藻爆发期间地表水N2消耗的大规模记录，辅以Ar测量来解释空气-海洋N2气体交换。N2和Ar浓度将通过空气中N2和Ar的质谱分析（MIMS，膜入口质谱）在连续的地表水流动中平衡来半连续地测定。该测量装置将安装在一艘自愿观测船（VOS，“Finnmaid”）上已建立的全自动测量系统上，用于分析地表水微量气体（CO2, CH4, O2, N2O， CO）。通过这种方法，将获得梅克伦堡湾和芬兰湾之间的N2和Ar浓度，时间分辨率为2 - 3天。将进行两个密集的测量期（2021年和2022年），以量化蓝藻高峰季节（6月至8月）的N2固定。通过这一点，我们的目标是确定触发和可能限制蓝藻生长的因素，如温度，磷的可用性和气象/水文条件。现有测量系统对二氧化碳分压的同步记录将用于蓝藻生物量生产的独立估计，从而用于相关的N2固定。同样，通过与芬兰环境研究所（SYKE）的合作，将获得全氮和全磷的测量，并促进氮预算计算，以便与我们的直接N2固定测量进行一致性测试。为了估计总氮固定，必须对表层得到的速率进行深度积分。这将通过混合层深度的数值模拟（GETM）来实现，混合层深度将由不同的标准定义。将Finnmaid路线获得的N2固定外推到整个盆地是一项更具挑战性的任务。我们将与联邦海事和水文局（BSH）遥感组合作，尝试将遥感数据用于外推程序。

Nitrogen fixation by cyanobacteria is a common phenomenon in the Baltic Sea. It occurs in the absence of dissolved inorganic nitrogen (DIN) during mid-summer (June – August) mainly in the central Baltic Sea and the Gulf of Finland. The contribution to the N budget is significant and enhances the eutrophication of the Baltic Sea. Current input estimates range between 300 kt-N/yr and 800 kt-N/yr and are in the same order of magnitude as the sum of the riverine and airborne DIN input. The huge range of the different estimates is a consequence of both the considerable interannual variability of the N2 fixation and huge uncertainties associated with the different approaches (15N incubation; total N budget; pCO2 records; phosphate excess) for the quantification of the N2 fixation and with extrapolating the results from local studies to entire basins.Our approach is based on large-scale records of the surface water N2 depletion during a cyanobacteria bloom, complemented by Ar measurements to account for the air-sea N2 gas exchange. The N2 and Ar concentrations will be determined semi-continuously by means of mass spectrometric analysis of N2 and Ar in air equilibrated with a continuous flow of surface water (MIMS, Membrane Inlet Mass Spectroscopy). The measurement device will be attached to an established fully automated measurement system for the analysis of surface water trace gases (CO2, CH4, O2, N2O, CO) on a voluntary observation ship (VOS, “Finnmaid”). Through this the N2 and Ar concentration between the Mecklenburg Bight and the Gulf of Finland will be obtained with a temporal resolution of 2 – 3 days.Two intense measurement periods will be performed (2021 and 2022) in order to quantify the N2 fixation during the cyanobacteria high season from June to August. Through this we are aiming to identify the factors which trigger and possibly limit the cyanobacteria growth such as temperature, P availability and meteorological/hydrographic conditions. Concurrent records of the pCO2 by the existing measurement system will be used for independent estimations of the cyanobacteria biomass production and thus of the associated N2 fixation. Likewise, measurements of total N and total P will be available through cooperation with the Finnish Environment Institute (SYKE) and facilitate nitrogen budget calculations for consistency tests with our direct N2 fixation measurements.To estimate the total N2 fixation, the rates obtained for the upper surface layer must be integrated over depth. This will be achieved by numerical modelling (GETM) of the mixed layer depth which will be defined by different criteria. Extrapolation of the N2 fixation obtained along the Finnmaid route to entire basins is a more challenging task. In cooperation with the remote sensing group of the Federal Maritime and Hydrographic Agency (BSH), we will make an attempt to use remote sensing data for an extrapolation procedure.