Vertical profiles of modelled and measured humic-like and protein-like fluorescence and associated salinity, temperature, potential density anomaly, absorption line height at 676 nm, apparent oxygen utilization and dissolved oxygen concentration in Baltic Sea in 2014- 2020

The aim of this work was to highlight the importance of non-conservative processes and their effect on the dissolved organic matter distribution in the Baltic Sea. This was achieved with the use of an extensive data set and the development of a water masses’ balance model. By modelling dissolved organic matter fluorescence as a function of salinity for the three water masses and at three excitation/emission wavelengths, it was possible to show the strength of non-conservative processes which are manifested through negative (positive) residuals: measurements are smaller (larger) than modelled fluorescence intensities. Negative residuals are due to photochemical and microbial decomposition. Positive residuals are caused by accumulation due to dissolved organic matter extracellular release from phytoplankton, heterotrophic uptake or emission from anoxic sediments. Results confirmed the accumulation of dissolved organic matter in deep layers at all seasons, as well as a strong correlation with apparent oxygen utilization. A generalized concept for the dissolved organic matter cycling in the Baltic was proposed, highlighting photobleaching as the dominating non-linear process that determines the efficiency of the humic-like dissolved organic matter fluorescence removal. The protein-like component was seen to be taken up more efficiently by aerobic prokaryotes at the surface.

本研究旨在阐明非保守过程的重要性，及其对波罗的海溶解有机质（dissolved organic matter, DOM）分布的影响。本研究通过采用大规模数据集并构建水团平衡模型达成了上述研究目标。通过针对三类水团、在三种激发/发射波长条件下，以盐度为自变量对溶解有机质荧光进行建模，得以揭示非保守过程的强度——这类过程通过负（正）残差体现：实测荧光强度低于（高于）模型模拟得到的荧光强度。负残差源于光化学与微生物降解作用，正残差则由浮游植物胞外释放溶解有机质、异养摄取过程或缺氧沉积物排放所引发的有机质累积所导致。研究结果证实，各季节深层海域均存在溶解有机质的累积现象，且其与表观耗氧量（apparent oxygen utilization, AOU）存在显著相关性。本研究提出了波罗的海溶解有机质循环的通用概念框架，明确光漂白是决定类腐殖质溶解有机质荧光降解效率的主导非线性过程。此外，研究发现表层海域的好氧原核生物对类蛋白组分的摄取效率更高。