Substantial changes in the depth distributions of benthic invertebrates in the eastern Kattegat since the 1880s

Bottom trawling and eutrophication are well known for their impacts on the marine benthic environment in the last decades. Evaluating the effects of these pressures is often restricted to contemporary benthic data, limiting the potential to observe change from an earlier (preimpact) state. In this study, we compared benthic species records from 1884 to 1886 by CGJ Petersen with recent data to investigate how benthic invertebrate species in the eastern Kattegat have changed since preimpact time. The study shows that species turnover between old and recent times was high, ca. 50%, and the species richness in the investigation area was either unchanged or higher in recent times, suggesting no net loss of species. Elements of metacommunity structure analysis of datasets from the 1880s, 1990s, and 2000s revealed a clear change in the depth distribution structure since the 1880s. The system changed from a Quasi-nested/Random pattern unrelated to depth in the 1880s with many species depth ranges over a major part of the studied depth interval, to a Clementsian pattern in recent times strongly positively correlated with depth. Around 30% of the 117 species recorded both in old and in recent times, including most trawling-sensitive species, that is large, semiemergent species, showed a decrease in maximal depth of occurrence from the deeper zone fished today to the shallower unfished zone, with on average 20 m. Concurrently, the species category remaining in the fished zone was dominated by species less sensitive to bottom trawling like infauna polychaetes and small-sized Peracarida crustaceans, most likely with short longevity. The depth interval and magnitude of the changes in depth distribution and the changes in species composition indicate impacts from bottom trawling rather than eutrophication. Furthermore, the high similarity of results from the recent datasets 10 years apart suggests chronic impact keeping the system in an altered state.