Studies of chlorophyll-a levels in the North Aegean Sea using SeaWiFS data

The hydrographical characteristics of the North Aegean Sea (NAS) have been studied using in-situ data and thermal NOAA AVHRR data. Lately, efforts to simulate the circulation patterns in the NAS have also been performed. Thus, it has been found that the discharge of water of Black Sea origin (BSW) affects extensive areas of the NAS surface waters. Another meso-scale phenomenon is the extensive upwelling south of the mouth region of the strait of the Dardanelles. The knowledge of the water quality situation is, however, small. This paper will describe the use of SeaWiFS ocean color satellite data in order to study the water quality situation in the NAS in terms of chlorophyll-a levels and patterns, especially in relation to the discharge of the BSW, which is rather rich in nutrients and with enhanced levels of chlorophyll. The latter could also act as a "tracer" for the influence of the BSW and for the movement of water masses. The analysis of ocean color satellite data in terms of chl-a is strongly dependent on the atmospheric correction and two different algorithms have been studied, the one in the SeaDAS software package and the MUMM approach. Moreover, two slightly different chlorophyll algorithms have been compared, SeaDAS 3.3 and 4.0 respectively. Results from published data on in-situ measured chl-a levels in the NAS have been compared with MUMM satellite data. It is shown that SeaDAS 3.3, SeaDAS 4.0 and MUMM derived chl-a levels from three locations in the NAS agree fairly well. Conclusive results from the comparison with in-situ chl-a data are difficult to obtain, one reason being possible bio-fouling of the in-situ apparatus. However, it seems as if SeaWiFS MUMM data gives significantly higher chl-a levels during extended periods. Analysis of SeaWiFS data from 1999 clearly shows that enhanced chl-a levels exist during the whole year in the northern part of NAS, approximately north of the island of Limnos with especially high levels during December to April/May (levels of about 0.5-1.1 mg/m(3)). There is a fairly distinct boundary to the water masses to the south with chl-a levels of about 0.2-0.4 mg/m(3) during the whole year. A seasonal transport pattern of the BSW water from the Dardanelles' mouth region is evidenced by the chl-a patterns and is similar to the ones observed using NOAA SST data. A seasonal cycle of the maximum chl-a concentrations at the mouth of the Dardanelles is also found, highest levels (1-3 mg/m(3)) during January-March.