3. Results and discussion
Extensive data were gathered on the occurrence and spatial distribution of natural organic matter – phytoplankton and humic substances – in the upper seawater layer.
In the Baltic Sea, the spatial distributions of Chl a during blooms are quite characteristic, which makes it necessary to use different axis scales during blooming as opposed to non-blooming periods (Fig. 3). During blooming periods, Chl a spatial distribution maps resemble complex, patchy, mosaic-like patterns of small areas with different fluorescence properties. In the other periods, Baltic waters are well mixed and characterised by huge areas with similar, homogeneous fluorescence properties. Similar spatial distribution patterns of the Chl a and CDOM fluorescent parameters were observed in the Nordic (Norwegian, Iceland and Greenland) Seas in 2001 and 2002 (Fig. 4). The largest amount of fluorescent organic matter was observed along the shelf of Spitsbergen and along the Scandinavian coast. Additionally, quite significant amounts of fluorescent organics were recorded in central parts of the seas, except for the area around Bear Island, where the fluorescence parameters were low.
Fig. 5 illustrates the relationships between the fluorescence parameters of Chl a and CDOM obtained during marine experiments carried out in different regions of the Baltic Sea at different seasons. To ensure clarity of the plots, the names of the stations have been removed from the figures. The values presented in Fig. 5a (each point is the average of >50 measurements) were obtained during cruises during blooming (spring) and non-blooming (the other seasons) periods; the stations are marked as circles or triangles, respectively. In general, during algal blooms the fluorescence parameters of Chl a and CDOM obtained in the southern Baltic are subject to considerable variation and represent spatially diverse values. During the non-blooming periods, their values are smaller, and the changes in CDOM fluorescence parameters are much less than those affecting the corresponding parameters of Chl a. Therefore, in the blooming periods a positive correlation (r2 = 0.6) for the Chl a and CDOM fluorescence parameters was obtained for all the stations (number of samples: n=69), regardless of which part of the Baltic was investigated (coastal or open waters). However, in non-blooming periods, the waters of the Gulf of Gdańsk and the Pomeranian Bay differed from the rest of the southern Baltic, the correlation being positive (r2 = 67) only for the results (n=30) obtained at stations located in these two water bodies (Fig. 5b). From this it follows that in algal blooming periods, the activity and concentration of phytoplankton in the southern Baltic are high – hence the homogeneity of the bio-optical and fluorescence properties of the surface waters. On the other hand, the fluorescence properties of southern Baltic waters in autumn and winter are governed mostly by the mixing of the water masses and, with the exception of the Gulf of Gdańsk and the Pomeranian Bay, constitute quite a complex region. Very probably, the homogeneity of these latter waters as regards fluorescence properties is due to river runoffs, which are predominant in these areas and at these times. Similar investigations were carried out in the Nordic Seas; the results are presented in Fig. 6. Each point represents the average of about 20–30 measurements. The unmarked points in Fig. 6 relate to open waters, whereas the points marked by letters refer to stations located in shelf waters and the western part of the Nordic Seas area under scrutiny. In 2001, the lidar investigations covered the central part of the Nordic Seas; during the 2002 campaign they extended as far north as latitude 81◦N. Since maximum values were recorded along the western shore of Spitsbergen in 2002, the Chl a fluorescence factors obtained during these two campaigns differ significantly: 0–4 relative units in 2001 but 0–10 relative units in 2002. The plots (a) and (b) show that the correlations between the Chl a and CDOM fluorescence factors for the open waters were positive (r2 = 0.57 for n=28 and r2 = 0.73 for n=37, respectively). The open Nordic Seas waters are thus described by the same fluorescence properties. Where phytoplankton is the only source of CDOM formation, the relationship between the CDOM fluorescence parameters and the Chl a concentration is linear (Barbini et al. 1998); the open Nordic Sea waters are therefore Case 1 waters. Such a relationship was not, however, observed in the shelf waters off western Spitsbergen and on the border between the Barents and Norwegian Seas owing to the occurrence of additional advected organic matter.