Abstract
The major goal of this study was to identify long term (1998-2022) changes within the macrobenthic communities in the Sound (Öresund), with special emphasis on climate change. Bottom trawling in this area has been banned since 1932. This was compared to changes of the same species at the surrounding Swedish west coast in the north (the Kattegat and the Skagerrak) and the Baltic Sea in the south. Results are also related far back in time in the Sound as the benthic fauna has been unusually well-studied since the 1800s. It is of special interest to study the faunal response to the rise in bottom water temperature which has been recorded in later years. The Sound between Denmark and Sweden is situated between the Baltic Sea, one of the largest brackish waters, and the Kattegat, an extension of the Atlantic. This location provides special conditions for a benthic fauna with a northern touch. Brackish water species typical of the Baltic occur here, but also marine cold-water species typical of the Arctic. Many species therefore live on their edge of distribution in the Sound. The development in abundance 1998-2022 in the Sound with its trawl-ban was similar to heavily trawled areas along the Swedish west coast. Species with a northern distribution were found to decrease parallel with an increase in species with a southern distribution. The sharper reduction in abundance in the Sound compared to surrounding seas may be due to the fact that many species here live at the extreme edge of their range. Indications of disturbed reproduction and submergence in some northern species were also recorded. The most likely explanation of these changes is the increase in temperature which affects many processes that may act in synergy. The transition from a Haploops community to an Amphiura community probably started with hypoxia and high temperatures. Also, temperature and decreasing levels of nitrogen, which control primary production, may have created relative food shortages as Haploops especially seems to benefit from a high trophic level. The change in faunal composition implied a reduction in gamma diversity but also a loss in nutritional value for commercial fish. Increased temperature is likely an explanation why northern species with non-pelagic larvae have decreased and why southern species with pelagic larvae have increased in recent years. This study, where comparisons are difficult because of many complicating factors, verifies that broken time series is an urgent problem for long-term ecological and environmental studies. It is important for the future to preserve long-term series of data collected at the same location and with the same methods.