12-hour cycle of matter transformation in the sea surface microlayer in the offshore waters of the Gdańsk Basin (Baltic Sea) during spring

Short-term measurements of nutrient and DOC concentrations and suspended matter (particles, chlorophyll a, phaeophytin, algae and ATP concentrations) carried out in seawater layers of varying thickness (10, 90, 250 μm and the underwater layer – 15 cm depth) in spring form the basis for a discussion of the diurnal fluctuations of nutrient and suspended organic matter concentrations. Quantitative and qualitative differences in the composition of neuston species were recorded in selected sub-layers of the chemically stratified sea surface microlayer. The non-linear regression equation was applied in a mathematical model of the diurnal fluctuations of nutrients and organic matter. Two maxima and two minima were found in the diurnal cycle of nutrient concentrations and organic suspensions in sub-layers of different thickness selected from the sea surface. The first maximum, expressed by the proliferation of phytoneuston, lasted from the very early morning till mid-morning. The second maximum occurred in the afternoon. The chlorophyll a concentration, and ATP and neuston abundance declined markedly around noon, when the biologically-damaging radiation dose increased, compelling the downward migration of organisms. At the same time, the photo-oxidation of dissolved organic matter intensified and the concentrations of inorganic forms of nitrogen and phosphorus rose. A shift (up to 2 h) between the maximum and minimum neuston concentration in these sea surface layers was indicative of phototaxis occurring within the entire surface microlayer and in the underwater layer. After sunset the decline in phytoneuston abundance could be related to zooplankton grazing as well as to respiratory breakdown.