1. Introduction
The structure of the water masses specific to the Baltic Sea is due to its resembling a quasi-enclosed estuary supplied with huge amounts of fresh water from river runoff and sporadic deep inflows of saline Atlantic water through the Danish Straits. It has therefore been deemed necessary to investigate and monitor the eutrophication of the Baltic (Darecki & Stramski 2004, WoŸniak et al. 2000). Since 1993, lidar measurements of in situ seawater fluorescence spectra have been carried out on board r/v ‘Oceania’ during Baltic cruises as well as during two summer Arctic campaigns (AREX 2001 and 2002) in the Norwegian, Iceland and Greenland Seas. Baltic waters, seriously affected as they are by human activities, have been classified as Case 2 waters; the open waters of the Nordic Seas are Case 1 waters. The difference is due to the quantity and quality of organic matter contained in the upper seawater layer. The present paper analyses the results of the above-mentioned lidar investigations, which were obtained within the framework of a Ph. D. thesis (Drozdowska 2005, unpublished). Our lidar investigations of seawater have made it possible to obtain continuous fluorescence spectra of seawater in the visible light region, in real-time and without disturbing the aquatic medium. The method can thus be applied to detect the main fluorescent constituents of seawater: phytoplankton pigments, humic-type dissolved organic matter, and oils (Piskozub et al. 1998, Drozdowska & Kowalczuk 1999, Drozdowska et al. 2002, 2004, Drozdowska & Darecki 2005, Drozdowska & Król 2005, 2006). These spectra provide information on the concentration of chlorophyll a (Chl a) and coloured dissolved organic matter (CDOM). The concentration of Chl a, for which the fluorescence parameter obtained from the lidarinduced fluorescence spectra is a proxy, is an indicator of phytoplankton abundance (Babichenko et al. 1993, Determann et al. 1994, Fadeev 1999, Barbini et al. 2001). The amount of CDOM is also determined from the CDOM fluorescence parameter calculated from the seawater fluorescence spectrum. The position of the CDOM fluorescence spectral band shifts towards the blue or red wavelengths, depending on the dominant fractions of humic substances (HS). High-molecular-weight HS molecules produce a red shift in the fluorescence and absorption spectra and typically, a lower quantum yield of fluorescence; low-molecular-weight HS molecules give rise to a blue shift in these same spectra and a high quantum yield of fluorescence (Babichenko 2001).
Analyses of seawater fluorescence spectra obtained by the lidar method yield the fluorescence parameters of Chl a and CDOM, which allow seawater masses to be distinguished according to their individual biophysical and fluorescence properties (Barbini et al. 1998). The classification of seawaters with the aid of lidar-induced fluorescence spectra parameters is based on the correlation coefficient (r2) between the fluorescence parameters of Chl a and CDOM. For Case 1 waters these values are large (close to 1), for Case 2 waters, they are low (close to zero). Moreover, according to Salyuk and his co-workers (Pavlov et al. 2000, Salyuk et al. 2003), the positive linear regression coefficients (when r2 is close to 1), a and b, respectively supply information about the rate of CDOM formation from phytoplankton communities and the initial content of organic matter in the aquatic environment. Hence, if the correlations between the Chl a and CDOM fluorescence parameters obtained in a given waters are linear, this means that those waters have similar bio-optical properties.
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