The surface microlayer of water (SML) is a unique ecotone found on all water bodies. It is an interphase for the exchange of matter between the hydrosphere and the atmosphere. This ecotone is capable of accumulating chemicals and microorganisms in amounts as high as 100-fold greater than those observed in the pelagic zone.
Here we report on the accumulation of chemicals and phytoneuston in the SML of the estuarial Łebsko Lake located at the southern coast of the Baltic Sea in the unpolluted region of the World Biosphere Reserve: the Słowínski National Park in Poland. The physicochemical parameters and phytoplankton composition from the SML (thickness of 242± 40μm) were compared with those in subsurface water layers (SUB; 15 cm under the surface). A wide spectrum of chemical and microbiological analyses was performed to investigate the capacity to accumulate substances. Almost all analyzed trace metals (Al, Cr, Mn, Fe, Ni, Cu, Zn, As, Cd, Ba, Pb), biogenic substances such as forms of phosphorus and nitrogen as well as chlorophyll a, pheophytin, and heterotrophic bacteria were detected at higher levels in SML than in SUB. Also, a greater number of taxa and higher abundance and biomass of phytoplankton were found in SML than in SUB. In contrast, some chemical parameters such as salinity components (Cl, SO2– 4, Na+, K+, Mg2+, Ca2+) as well as pH occurred at a comparable level in SML and SUB. Physicochemical factors such as components of salinity in freshwater, brackish, and marine areas determined taxonomic composition, abundance, and biomass of phytoplankton and phytoneuston. Canonical correspondence analysis indicated the most significant chemical factors, such as nutrients and essential metals, affecting phytoplankton composition in the analyzed water layers. The degree of water salinity, chemical components such as metals, nutrients of water, and lotic and lentic environments influenced the qualitative and quantitative parameters of the phytoplankton and phytoneuston; hence, the common occurrence of resistant species such as Desmodesmous communis in the studied estuary and those characteristic of freshwater occurring only in the river stand, e. g., Woronichinia naegeliana and, for example, Chaetoceros decipiens occurring only in the sea.