Abstract

Living (stained) benthic foraminifera were studied in the deepest part (116 m) of the classical marine study area, the Gullmar Fjord on the Swedish west coast. This is a silled fjord with a strongly stratified water column where stagnant conditions temporarily develop in the deeper parts. The aim of this study is to determine how the living foraminiferal assemblage develops with seasonal variations in bottom water hydrography and primary production in the surface water. From August 1993 to December 1994, monthly hydrographic and oxygen measurements were made. Parallel undisturbed sediment cores were taken using a Multiple-corer. Foraminifera from the >63 μm fraction were stained with Rose Bengal to separate living from dead specimens. Monthly values of the surface water chlorophyll a content were also available and give a measure of the primary production. During spring 1994 the dominant foraminiferal species, Stainforthia fusiformis, multiplied its population size by seven times during one month as a consequence of food input from the spring phytoplankton bloom. The abundance of this species correlates positively with the surface water chlorophyll a content. Populations of several other species also increased during the spring of 1994. A deep-water inflow in February 1994 may have triggered reproduction of several foraminiferal species, recognized as a temporary population decline. We also compared our results with Höglund’s (1947) foraminiferal study, performed in the same area in the summer of 1927. We found that a major alteration trending towards a more opportunistic low oxygen tolerant fauna has occurred since then.

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