A major change in the dinoflagellate cyst flora of Gullmar Fjord, Sweden, at around 1969/1970 and its possible explanation

Abstract

Abstract An ultra high-resolution study of the latest Holocene dinoflagellate cysts from Gullmar Fjord, on the west coast of Sweden, provides evidence for the recognition of at least two major dinoflagellate communities within the fjord over the last 85 years. These communities may result from changes within the North Atlantic Oscillation (NAO) and hydrography of the fjord between the approximate years 1915 and 1999 and/or from the local pollution history. The dinoflagellate cyst populations were compared in detail with hydrographical parameters available from this fjord with its long historical instrumental records. The dinoflagellate cysts fail to demonstrate a convincing ongoing eutrophication for the fjord, although the reduction of Lingulodinium polyedrum partly coincides with the curtailment of activity at a sulphite pulp mill at Munkedal and canning activity at Lysekil, together with a cessation in the influx of untreated sewage from water closets. The significant change in the assemblage composition at about the late 1960s/early 1970s coincides with a change in the NAO from a negative phase to its present-day positive phase. The unravelling of local environmental effects from those associated with regional fluctuations is complex and needs to be approached with caution.