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Abstract
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.
Highlights
Almost no areas globally are unaffected by humans, and many areas, including the Scandinavian seas, are regarded highly impacted by several pressures (Halpern et al, 2008, 2015)
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
Pearson r correlation was used to test whether latent main environmental gradient was significantly correlated with the measured predictor variable
Summary
Almost no areas globally are unaffected by humans, and many areas, including the Scandinavian seas, are regarded highly impacted by several pressures (Halpern et al, 2008, 2015). We take advantage of a temporal reference, the benthic invertebrate records from the surveys with the canon ship “Hauch,” performed in the 1880s in the Kattegat by CGJ Petersen (1893) This is the earliest comprehensive survey dataset on benthic invertebrate fauna available from the Scandinavian Sea area, from a time before major impact of the mentioned threats. There are reports of decreased Secchi depths in recent times (e.g., Middelboe & Sand-Jensen, 2000), which, at least partly, may be related to eutrophication This together with reports on deeper distribution of eelgrass (Zostera marina) in the late 1800s (Petersen, 1893) than in the second half of the 1900s (e.g., Duarte, 1991) indicates a narrowing of the primary productive euphotic zone, with potential effects on benthic micro-and macroalgal consumers like grazing. We expected trawling-sensitive species (Figure 1) to disappear or occur only in nonfished areas away from the trawled zone if trawling impact was important, leaving richness in trawled areas dominated by trawling-insensitive species
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