Abundance of bacteria in beaches of the southern Baltic Sea differing in the level of anthropogenic pressure

The present study focused on the abundance of bacteria in the sand of marine beaches on the southern Baltic Sea coast which differed in the level of anthropogenic pressure. The results showed that the maximum (17.60·10⁶cells · g⁻¹dw.) bacterial abundance was noted on the beach characterized by the highest level of anthropogenic pressure and minimum (10.48·10⁶cells · g⁻¹dw.) on the beach located in the Słowiński National Park. Generally, differences were found in the studied abundance and biomass of bacteria in all investigated beaches in their horizontal profile. The maximum (18.81–23.52·10⁶cells · g⁻¹dw.) density of bacteria was recorded in the dry zones, i.e., dune and the middle part of the beach, and the minimum (9.04–10.52·10⁶cells · g⁻¹dw.) in the wet zones, i.e., under seawater surface and at the waterline. The differences in the abundance of bacteria were found between the surface and subsurface sand layers in the studied beaches. Usually, the highest (19.60·10⁶cells · g⁻¹dw.) number of bacterial cells was determined in the surface sand layer, while with an increasing depth their number decreased. A seasonal variation in bacterial abundance was also documented in the studied beaches. This study resulted in increased knowledge on marine bacteriopsammon abundance, which, as a secondary producer, plays a very important role as the main link between primary producers and higher trophic levels of benthic food webs in marine ecosystems.