Seasonal dynamics and decadal changes of benthic foraminiferal assemblages in the western Baltic Sea (NW Europe)

Living benthic foraminiferal assemblages were monitored between January and July 2004 in the Kiel Bight, Baltic Sea. Phytoplankton blooms and the deposition of organic detritus were depicted by fluorometer hydro casts and pigment concentrations in surface sediments. Three depositional pulses of organic matter were identified by high phytoplankton concentrations above the sea floor and elevated pigment concentrations in the surface sediment. The foraminiferal assemblage composition remained rather constant but the population density of Elphidium excavatum clavatum showed a two- to six-fold increase within a few days after organic detritus deposition. The foraminiferal assemblage composition was compared to earlier studies in this area. Elphidium excavatum, Ammotium cassis and E. incertum dominated the living fauna in the 1960s and 1970s. The recent survey revealed a predominance of E. excavatum subspecies (more than 90% of the living assemblage). The average population densities were six times higher than in the 1970s. Data structure and model calculations suggested that E. excavatum clavatum is able to reproduce rapidly, with high offspring numbers at elevated food supply. The diminution of A. cassis has occurred in Kiel Bight during the last ten years and was most likely induced by a period of low deep-water salinities in the early 1990s.