Seasonal study of δ 18O and δ 13C in living (stained) benthic foraminifera from two Swedish fjords

Abstract

In a previous 16-month seasonal study on living (stained) benthic foraminifera from two fjords on the Swedish west coast, it was reported that foraminifera proliferated in response to phytodetritus input; the strongest response came from the opportunistic species Stainforthia fusiformis. In this study, our objective was to find out if that phytodetritus input resulted in a change in the carbon isotopic composition of the foraminiferal tests. We also wanted to examine if variations in salinity and temperature (due to seasonality or deep-water exchanges) were reflected in the δ 18O values. From S. fusiformis that were obtained from the Havstens Fjord (20 m) and the Gullmar Fjord (119 m) during the 16-month study, we developed a time series of δ 18O and δ 13C. After the spring blooms in the Havstens and the Gullmar Fjord, decreases of about 0.2‰ to 0.3‰ in the foraminiferal δ 13C values were noted; in the Gullmar Fjord after the autumn blooms, decreases of the same order were also noted. Comparing the Havstens and the Gullmar Fjord, we found a 1‰ difference in both δ 13C and δ 18O; we attribute this to hydrographic differences between the two fjords. Using calculated values of δ 18O, together with the measured ones, we noticed that S. fusiformis in the Gullmar Fjord seems to calcify close to equilibrium with respect to the oxygen isotopes. During autumn, water temperatures were relatively high in the Havstens Fjord, and foraminiferal abundance in the fjord was also high after a phytodetritus input; but, the measured δ 18O values do not reflect these higher temperatures. This apparently contradictory combination of results might be explained by a varying δ 18O composition of the water during the year, which counterbalances the temperature effect.