All dissolved organic matter fluoresces under UV excitation, but only fresh DOM fluoresces under green light, providing a way to track degradation. To investigate how climatic changes impact phytoplankton communities, it is necessary to develop fast new methods to study such communities over large distances. The most important parameters to monitor are the chlorophyll-a concentration and the amount of dissolved organic matter (DOM) produced by phytoplankton cells. It is important to detect not

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... nly total DOM, but also 'new' or 'fresh' DOM, which is produced by living phytoplankton cells and in the initial stage of degradation. Optical remote sensing methods that employ upwelling radiation spectra from the seawater subsurface layer can only be used for measuring total colored DOM (CDOM). 1 Compared with such absorption spectra, DOM fluorescence spectra contain more detailed information, and results 2, 3 suggest that they may be used for DOM composition and source investigations. In the referenced works, radiation in the UV-to-blue wavelength range induced DOM fluorescence, and the researchers did not differentiate between old and new DOM. In our work we investigate the difference between fluorescence of the same DOM induced by radiation at green and UV wavelengths. 4 We assumed that almost all DOM components, both old and fresh, fluoresce under UV or blue excitation. As such, this technique cannot be used to separate fresh DOM from old. On the other hand, green light has insufficient energy to induce the fluorescence of the simpler molecules of old DOM, but the energy is still enough to cause the long-chain molecules of fresh DOM to fluoresce. Thus, we expected that green excitation would allow us to detect fresh DOM fluorescence on the background of total DOM fluorescence.