A quantitative study of the origin of dreissena polymorpha shell accumulations in Szczecin Bay Baltic Sea

Abundant accumulations of D. polymorpha (Pallas) shells occur on the shores of Szczecin Bay, Baltic Sea (northwest Poland). Studies on the conditions of their formation involved a quantitative analysis of 3 sets: shells of bivalves living in the Bay, shells accumulating on the bottom due to natural mortality of bivalves and shells accumulated by waves in the supralittoral zone of the Bay. Differences in shell-size distribution of the living pelecypods and shells accumulating in the bottom sediments are due to biological and sedimentological processes. Due to the mortality of bivalve populations, shell accumulations are richer in large specimens than the corresponding biocenosis. Formation of supralittoral shell accumulations involves enrichment of the material in small shells, large shells remaining on the basin floor

Stages of the formation of the Łeba barrier-lagoon system on the basis of the geological cross-section near Rąbka (Southern Baltic Coast, Poland)

The ar ti cle pres ents the re sults of a de tailed study of the geo log i cal struc ture of the £eba Bar rier in the R¹bka crosssec tion (South ern Bal tic, Po land). The bar rier sep a rates Lake £ebsko from the Bal tic. Five sed i men tary com plexes were dis tin guished there (M2–M6). The spa tial vari abil ity of the grain-size dis tri bu tion was ex am ined and suc ces sion stages of the mol lusc fauna oc cur ring in the in di vid ual sed i men tary com plexes were dis tin guished. Ra dio car bon dating was used to es tab lish the age of the most im por tant events dur ing the pro cess of for ma tion of the bar rier, which took place in the course of sev eral rel a tive sea-level changes. The first sed i men tary com plex (M2) at R¹bka is connected with the sec ond ingression (i2) of the Bal tic Sea (ca. 6,700–6,000 14C years BP), sea-level sta bi li za tion (6,000–5,500 14C years BP), and at last sea-level low er ing (5,500–5,000 14C years BP) in the re gion of the Gardno-£eba Coastal Plain. The sed i men tary com plex M3 de vel oped in a la goonal en vi ron ment when the bar rier was sit u ated north of its pres ent po si tion (5,000–3,000 14C BP). The next low er ing of the sea-level made the la goon shal lower and caused the emer gence of small but al ready subaerial stretches of bar rier land with a fresh wa ter fauna in the north (4,880±40 14C BP). With the next ingression stage (i3), which took place be tween 4,500 and 3,000 BP, the bar rier shifted to its pres ent-day po si tion and the la goon changed into a fresh wa ter lake. From 3,000 to 1,700 14C BP fos sil soil and peats de vel oped on the bar rier sur face as a re sult of an other sea-level low er ing. The last ingression stages (i4 and i5), youn - ger than 1,700 BP, built up the bar rier, prac ti cally in its to day’s lo ca tion (sed i men tary com plexes M4 and M5). The youn gest sed i men tary com plex (M-6) is rep re sented by pres ent-day beach sands