Subrecent changes in the foraminiferal distribution in the Kattegat and the Skagerrak, Scandinavia: anthropogenic influence and natural causes

Foraminifera from surface samples in the Kattegat and the Skagerrak, northwestern Europe, have been analysed to determine the modern foraminiferal distribution. A total of five foraminiferal assemblages are distinguished. These are the Elphidium excavatum, Cassidulina laevigata, Bulimina marginata, Cibicides lobatulus and Trochammina sp. assemblages. Only the first three are found over large areas and these are correlated to either depth, organic carbon content or grain size. At each station a short core was studied to determine whether changes have occurred in the assemblages during the last few hundred years. In some areas no such variations were found, but several of the cores from the Skagerrak and all cores from the Kattegat document changes within this period. The fluctuations in the Skagerrak may be attributed to natural causes, such as species migrations or re-deposition. In the Kattegat a change from a Hyalinea balthica assemblage to the modern B. marginata assemblage always occurs at approximately the same core depth, which presumably represents the biological mixing depth. This change is presumably due to anthropogenic influences, which have caused oxygen depletion in the bottom waters of the Kattegat during the last few decades.