PENETRATION OF SOLAR UV IRRADIATION IN COASTAL LAGOONS OF THE SOUTHERN BALTIC SEA AND ITS EFFECT ON PHYTOPLANKTON COMMUNITIES

Abstract Direct measurements of solar UV irradiance at the surface and in the water column were made in several coastal lagoons near the islands of Hiddensee and Rügen in spring and summer 1993 using an internally frosted quartz sphere at the end of a 10 m quartz fiber cable connected to a double monochromator spectroradiometer. The measurements showed a nearly uniform vertical distribution of the absorption coefficient in the whole water column due to effective mixing, shallow depths and little temperature and salinity density stratifications. There was an increase in the absorption coefficient at 310 nm from 5 m⁻¹ in the outer lagoons to 12 m⁻¹ in the open inner lagoons and up to 29 m⁻¹ in the inner lagoons without water exchange with the Baltic Sea as a result of different amounts of UV-absorbing substances such as yellow substance (Gelbstoff) and seston. Depending on the specific concentration of the absorbents, the solar UV-B irradiance decreases to 1% after penetration into the water column between 0.2 m (inner lagoons without water exchange), 0.4 m (inner lagoons with water exchange), and 1 m (outer lagoons), whereas UV-A penetrates on the order of several meters. As phytoplankton populates the top layers of the water column it may be affected by increased solar UV irradiation even in these turbid coastal waters.