Measurements of chlorophyll α fluorescence accompanied by solar radiation measurements were carried out during two spring cruises in the region of the Gdańsk Basin. Chlorophyll α fluorescence, similarly to that of chlorophyll a concentration in the seawater, showed considerable diurnal variability. The measurements provided statistically significant negative correlation coefficients that indicated that chlorophyll α fluorescence is inhibited as solar PAR and UV radiation intensity increases. The approximate range of radiation intensity that corresponded to a clearly marked decline in chlorophyll α fluorescence and chlorophyll α concentration was determined. It was revealed that the decline in chlorophyll a concentration in the surface water layer resulted from the photodegradation of chlorophyll α as well as phytoplankton grazing. In both cases, the resulting observation was an increase of pheophytine a concentration. The vertical migration of phytoplankton was identified as an additional process induced by high radiation levels.
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