CYANOBACTERIAL BLOOMS CAUSE HEATING OF THE SEA-SURFACE

A series of AVHRR (Advanced Very High Resolution Radiometer) satellite images and simultaneous ship transects in July 1992 were used to show that surface accumulations of cyanobacteria (blue-green algae) in the southern Baltic Sea can cause local increases in the satellite-derive d sea surface temperature (SST) by up to 1.5-degrees-C. The warmer SST is attributed to increased absorption of sunlight due to increased ph ytoplankton pigment concentration. The distribution of surface cyanoba cterial accumulations detected as increased reflectance in the visible channel of the AVHRR satellite sensor was correlated with chlorophyll concentration at 5 m depth. Warm SST anomalies ('hot spots') appeared both in accumulations of surface-floating cyanobacteria and in areas of high chlorophyll concentration (detected by shipboard measurements) . The 'hot spots' followed the detailed boundaries of the cyanobacteri al plumes and probably represented a shallow, diurnally heated top lay er that appeared by afternoon in conditions of low wind (2 m s-1) and weak mixing, disappeared during the night due to thermal convection an d were hardly detectable on days with wind speed of 6 to 8 m s-1. The vertical extension of the top diurnally heated layer was probably less than 1 m and definitely less than 5 m, at which depth no temperature increase was detected. It is suggested that the day/night SST differen ce in low-wind conditions may be an indicator of near-surface phytopla nkton pigment concentration.