Stable oxygen and carbon isotope information on the establishment of a new, opportunistic foraminiferal fauna in a Swedish Skagerrak fjord basin, in 1979/1980

Significant faunal changes reported from recent, coastal environments, which are not directly influenced by urban and industrial impact, are rarely seen. In Gullmar Fjord on the Swedish west coast, a significant foraminiferal fauna change occurred in connection with severe low-oxygen conditions that evolved in the winter of 1979/1980. A foraminiferal fauna marked by common Skagerrak–Kattegat species, which had previously characterised the deep fjord basin, was replaced by the opportunistic, low-oxygen tolerant species Stainforthia fusiformis (Williamsson).To study this phenomenon further we performed stable oxygen and carbon isotope analyses on the indicator species itself, S. fusiformis, both on specimens from sediment cores representing approximately the last 85 years and on living (stained) individuals taken from a transect across the deep fjord basin. Our purpose was to detail how and why S. fusiformis, came to dominate the fauna.The oxygen isotope results suggest that salinities and temperatures in the deep basin have been relatively constant over the last c. 85 years, while the carbon isotopes show a significant change towards more negative values in association with the faunal shift of 1979/1980. The combined results from both the cores and the surface sediments suggest that S. fusiformis did not inhabit the deep basin until 1980. Before then, almost all specimens of S. fusiformis were small sized and their carbon isotope values suggest they were re-deposited shallow-water specimens that had been transported down to the central, deep basin as part of a suspension load. After a major faunal extinction in 1979–1980, S. fusiformis of all sizes suddenly appeared in large numbers and their carbon isotopic values were similar to the signal from registered in the recent, living fauna within the deep basin. This suggests that the opportunistic S. fusiformis established itself in the deep basin as a consequence of the severe low-oxygen event and the faunal crash of the previously dominating Skagerrak–Kattegat fauna.