In many shelf seas and coastal areas, benthic ecosystems are affected by bottom trawling disturbance and hypoxia (low oxygen concentrations). We developed a methodology to predict benthic community impact from these pressures and used the approach to evaluate the benthic state of the Baltic Sea region.
Both bottom trawling disturbance and hypoxia act on a large spatial scale in the Baltic Sea region. Bottom trawling mainly occurs in the western and southern parts of the Baltic Sea, where otter trawls target demersal fish such as cod, brill, turbot, plaice, and flounder. Hypoxic and anoxic conditions are predominantly observed further north in the offshore waters of the Baltic Sea. The area with low oxygen conditions has greatly expanded over the last decades due to eutrophication.
To estimate benthic community impact from bottom trawling disturbance and hypoxia there is a need to predict the sensitivity of the underlying seafloor and benthic fauna. We expected the sensitivity to both pressures to depend on the longevity of fauna, with impact to be more pronounced in long-lived benthic organisms that typically take longer to recover. Using benthic data from 1558 sampling locations, we found that low salinity in most regions of the Baltic Sea correlates with a benthic community of mostly short-lived species, whereas longer-lived (and more sensitive) fauna dominated the Kattegat.
By combining the pressure layers with the benthic sensitivity layer using a population dynamic model, we were able to predict benthic impact. We found that in 14% (70 000 km2) of the Baltic Sea region benthic biomass is reduced by at least 50% due to the pressures, whereas 8% of the region has reductions of 10–50%. About one quarter of these impacted areas is affected by both pressures, corresponding to 6% (30 000 km2) of the Baltic Sea region with cumulative impacts. The effects of hypoxia cover larger areas and lead to a low habitat state of deep mud and deep mixed sediment. These habitats are most at risk and need to be prioritized for management actions.
The paper can be found here
P D van Denderen, S G Bolam, R Friedland, J G Hiddink, K Norén, A D Rijnsdorp, M Sköld, A Törnroos, E A Virtanen, S Valanko -- Evaluating impacts of bottom trawling and hypoxia on benthic communities at the local, habitat, and regional scale using a modelling approach, ICES Journal of Marine Science, fsz219