Factors affecting the occurrence of algae on the Sopot beach (Baltic Sea)

The occurrence of algae on the Sopot beach was investigated from 2004 to 2006 from the beach management point of view. Various methods were applied in an attempt to understand the mechanisms underlying the accumulation of algae on the shoreline. They included daily observations of the occurrence of macrophyta on the beach, absorption measurements of acetone extracts of the particulate matter in the seawater, the collection of macrophyta and phytoplankton samples for biomass and taxonomic identification, and determination of the degree of decomposition on the basis of chloropigment analyses. The results were related to the environmental conditions: meteorological data and the physicochemical parameters of the seawater. The biomass recorded on the beach consisted mainly of macroalgae and a small proportion of sea grass (Zostera marina). The phytoplankton biomass consisted mainly of dinoflagellates, diatoms, cyanobacteria, euglenoids and cryptophytes. The conclusions to be drawn from this work are that the occurrence of huge amounts of macrophyta amassing on the Sopot beach depends on the combined effect of high solar radiation in spring and summer, high-strength (velocity × frequency) south-westerly winds in May-September, followed by northerly winds, bringing the macrophyta from Puck Bay on to the Sopot beach. At the same time, their abundance along the beach varies according to the shape and height of the shore, the wind strength and the local wind-driven seawater currents. According to estimates, from 2.2–4.4 × 102 tons (dry weight) of macrophyta can be moved on to the Sopot beach in one hour. In October, strong southeasterly winds can also transport huge amounts of decomposing biomass onshore. The phytoplankton content in the total biomass is negligible, even though at low concentrations its biological activity may be considerable. The intensive phytoplankton blooms observed on the Sopot beach in summer are not always caused by cyanobacteria.