Optical properties of north-eastern Baltic Sea in spring and summer 2007

Underwater irradiation profiles in the north-eastern Baltic Sea near the Estonian northern and north- western coast, near Helsinki, and in the central part of Gulf of Finland were measured in spring and summer 2007. The vertical profiles of downwelling and scalar irradiance in the PAR region were measured in situ using a frame completed with two planar and a spherical PAR sensor. The measuring system allows the calculation of the mean attenuation coefficients of a water column of scalar irradiance (Ko). Optical density of the sea water was varying from 0.13 m^-1 (Tallinn-Helsinki line, May) up to 0.85 m^-1 (Tallinn-Helsinki line, April). The concentrations of optically active substances were highly variable; the chlorophyll a concentration varied from 0.7-21.4 mg m^-3, the suspended particulate matter concentration from 1-7 mg l^-1 and the concentration of dissolved organic matter from 2.1-5.8 mg l^-1. The water transparency was much better in May compared to April 2007. Also, it is seen, that excepting the stations immediately near the port and ship line, the Gulf of Finland was quite clear, especially in May. Correlations between the water depth and optical density were calculated for the shallows near Hiiumaa. Water quality based on measurements conducted in summer 2007 at North-Western Estonian coastal waters indicates that the water quality was on average satisfactory; associating values for diffuse attenuation coefficient was 0.5 m^-1, Secchi depth 3.7 m and chlorophyll a concentration 8 mg m^-3.