The spatial variability of radiant energy absorption in seawater can result from the non-homogeneity of the concentration of its optically active components, such as chlorophyll or dissolved organic matter. This non-homogeneity leads to local changes in the radiant heating rate and consequent changes in water temperature. Besides a simple dependence, for instance when a higher phytoplankton concentration is accompanied by a temperature rise, a relation that is more complex may occur. A theoretical analysis suggested that sufficiently strong horizontal changes in seawater absorption may cause, in their close proximity, an additional increase or decrease in the heating rate. To confirm this, we carried out preliminary simultaneous measurements of temperature and the light-beam attenuation coefficient (in the short-wave part of the visible band of the radiation spectrum) in the surface waters of the Gulf of Gdańsk (Southern Baltic) between 2003 and 2005. We determined that additional temperature changes occurred on one side of the boundary between water masses which had significantly different light-beam attenuation coefficients. When solar radiation penetrated through an area with a strong increase in the attenuation coefficient, a local rise in temperature would occur, even by a few tenths of a degree Celsius, often leading to the creation of maxima. In instances of radiation permeating to much more transparent water, the temperature would drop, with the effect being distinctly weaker - the maximum temperature decrease was approximately 0.1°C. These observations corresponded to theoretical predictions of temperature variability resulting from the presence of horizontal optical gradient.