Nitrate source identification and quantification in the euphotic zone of the Baltic Proper using nitrogen and oxygen stable isotopes

波罗的海的硝酸盐来源将通过氮和氧的稳定同位素进行鉴定和量化。波罗的海受到集水区（面积是海洋本身的4倍）和人类活动的强烈影响。特别是通过河流、点源和大气沉积的硝酸盐和磷的输入导致富营养化，这加剧了底层沃茨的缺氧。另一个来源是真光层中蓝细菌的固氮作用。潜在的硝酸盐来源在很大程度上是已知的。然而，在波罗的海真光层初级生产的数量和相对重要性方面仍然存在很大的不确定性。能够测量硝酸盐的氮和氧同位素的增感器方法将首次在该区域应用。同位素特征使得区分不同的氮源和影响真光层中硝酸盐浓度的过程成为可能。将分析潜在来源（硝酸盐库、大气沉降以及首次出现的溶解有机氮）。进一步的实验研究将有助于确定支持初级生产的不同氮形态（无机和有机）的相对重要性。将建立波罗的海中部真光层的硝酸盐预算。

Nitrate sources of the Baltic Proper will be identified and quantified by means of nitrogen and oxygen stable isotopes. The Baltic Sea is strongly influenced by the catchment (covering 4 times the area of the sea itself) and the anthropogenic activity there. In particular nitrate and phosphorus inputs via rivers, point sources, and atmospheric deposition cause eutrophication which aggravate anoxia in bottom waters. An additional source is nitrogen fixation by cyanobacteria in the euphotic zone. Potential nitrate sources are largely known. However, there are still substantial uncertainties regarding their quantity and relative importance for the primary production in the euphotic zone of the Baltic Proper. The denitrifier method, which enables measurement of both nitrogen and oxygen isotopes of nitrate, will be applied for the first time in this region. The isotope signature makes it possible to distinguish between different nitrogen sources and processes affecting the nitrate concentration in the euphotic layer. Potential sources (nitrate pool, atmospheric deposition plus for the first time dissolved organic nitrogen) will be analyzed. Additional experimental studies will help to determine the relative importance of different nitrogen forms (inorganic and organic) supporting the primary production. A nitrate budget for the euphotic layer in the central Baltic Sea will be established.