The benthic foraminiferal response to the 2004 spring bloom inthe western Baltic Sea

The diatom spring bloom and response of the benthic foraminifera was monitored between January and July 2004 in the Kiel Bight, western Baltic Sea. Phytoplankton production, the deposition of organic detritus, and feeding of Elphidium excavatum clavatum and Reophax dentaliniformis were monitored by pigment concentrations and pigment composition in water samples, surface sediments, and foraminiferal protoplasm. The population dynamics of the dominant species, Elphidium e. clavatum, was assessed by using their size distributions. The main spring bloom was triggered by a sudden rise in the three-day average insolation to above 170 W m− 2 d− 1 and took place during late February. Chlorophyll a concentrations in the protoplasm of Elphidium e. clavatum followed those in the surface water. The chlorophyll a / fucoxanthin ratio was the same as in bacillariophycean diatoms suggesting a “bloom-feeding” behaviour of Elphidium e. clavatum. Alternatively, the pigment composition in Reophax dentaliniformis only occasionally mirrored that of diatoms, indicating prochlorophycean and cyanophycean algae as this species' preferred food source. The size distributions and the abundances of living Elphidium e. clavatum revealed two reproduction events, which were 17 days apart. Both events coincided with an increase in sedimentary chlorophyll a concentrations. The reproduction was preceded by growth and feeding of adult specimens and it was followed by distinct growth phases of young specimens, which lasted a few days only. One reproductive cycle covered the spring bloom and produced many more offspring than the subsequent cycle. The light carbon isotopic signal from diatoms was only transiently recorded in foraminiferal calcite and with a low offset during the spring bloom. The oxygen isotopic composition of foraminiferal calcite mirrors seasonal δ18O-seawater–salinity relationships rather than temperature.