European coastal monitoring programmes may fail to identify impacts on benthic macrofauna caused by bottom trawling

The relative effects of bottom trawling, organic enrichment, and natural environmental factors on coastal seabed communities

Coastal regions are under intense and growing pressure from human activities. Here, we examine how human and natural drivers interact with benthic communities, species, and life-history traits across four distinct coastal areas. Sediment organic content was a key driver of seabed community characteristics, with positive (increased benthic biomass) and strongly adverse (depauperate communities) effects observed. Similarly, environmental factors such as bottom currents were highly influential. In contrast, bottom trawling impacts varied by location. Beam trawling in areas of low organic enrichment was most damaging to seabed fauna, even in the presence of strong bottom currents. However, we did not observe any faunal impacts of trawling under chronic enrichment, despite the fishery using relatively heavy gears (e.g. otter trawls) at high intensities. Lastly, we discuss how human and natural factors interact and shape seabed ecosystems under different conditions, and how management measures can be adapted to improve coastal seabed health.

Complementarity and sensitivity of benthic state indicators to bottom-trawl fishing disturbance

Many indicators have been developed to assess the state of benthic communities and identify seabed habitats most at risk from bottom trawling disturbance. However, the large variety of indicators and their development and application under specific geographic areas and management contexts has made it difficult to evaluate their wider utility. We compared the complementarity/uniqueness, sensitivity, and selectivity of 18 benthic indicators to pressure of bottom trawling. Seventeen common datasets with broad regional representation covering a range of pressure gradients from bottom trawling disturbance (n = 14), eutrophication (n = 1), marine pollution (n = 1), and oxygen depletion (n = 1) were used for the comparison. The outcomes of most indicators were correlated to a certain extent with response to bottom trawling disturbance, and two complementary groups of indicators were identified: diversity-based and biological trait-based indicators. Trait-based indicators that quantify the changes in relative abundance of sensitive taxa were most effective in identifying benthic community change in response to bottom trawling disturbance. None of the indicators responded to the trawling pressure gradient in all datasets, and some showed a response that were opposed to the theoretical expectation for some gradients. Indicators that showed clear responses to bottom trawling disturbance also showed clear responses in at least one other pressure gradient, suggesting those indicators are not pressure specific. These results emphasize the importance of selecting several indicators, at least one from each group (diversity and trait-based), to capture the broader signals of change in benthic communities due to bottom trawling activities. Our systematic approach offers the basis from which scientific advisors and/or managers can select suitable combinations of indicators to arrive at a sensitive and comprehensive benthic status assessment.

Baseline Inventory of Benthic Macrofauna in German Marine Protected Areas (2020-2022) before Closure for Bottom-Contact Fishing

The response of benthic habitats and organisms to bottom-contact fishing intensity is investigated in marine protected areas (MPAs) of the German EEZ in the North and Baltic Seas. We examined the current state of macrofauna biodiversity in 2020-2022. Comparative analysis for macrofauna (in- and epifauna) inhabiting nine Natura 2000 MPAs constitutes a baseline to assess the effects of bottom-contact fishing exclusion in the future. Aspects of spatial and temporal variability are briefly summarized and discussed. We provide a species list for each region, including 481 taxa, of which 79 were found in both regions, 183 only in the North Sea, and 219 only in the Baltic Sea. The Baltic Sea dataset surprisingly included higher numbers of taxa and revealed more Red List species. The share of major taxonomic groups (polychaetes, bivalves and amphipods) in species richness showed peculiar commonalities between the two regions. In the North Sea, multivariate analysis of community structure revealed significantly higher within-similarity and stronger separation between the considered MPAs compared to the Baltic MPAs. Salinity, temperature and sediment fractions of sand were responsible for over 60% of the variation in the North Sea macrofauna occurrence data. Salinity, mud fraction and bottom-contact fishing were the most important drivers in the Baltic Sea and, together with other considered environmental drivers, were responsible for 53% of the variation. This study identifies aspects of macrofauna occurrence that may be used to assess (causes of) future changes.

Good-Moderate boundary setting for the environmental status assessment of the macrozoobenthos communities with the Benthic Quality Index (BQI) in the south-western Baltic Sea

The status assessment of the macrofauna community under the European Marine Strategy Framework Directive (MSFD) requires threshold values that mark the transition from good to moderate conditions (G-M boundaries). Using the example of the Benthic Quality Index (BQI) in the south-western Baltic Sea, we demonstrate the possibilities and restrictions of i) defining G-M boundaries using reference areas, historical data and a statistical method and ii) the subsequent evaluation of the resulting G-M boundaries using disturbance data. The historical data from the period 1911 to 1929 proved to be unsuitable for defining G-M boundaries due to their data quality. The G-M boundaries calculated using the statistical method delineated high disturbance values more reliably than those based on reference areas. We conclude that disturbance data are very useful to evaluate G-M boundaries for their suitability, but data on all state conditions are needed.

Impacts of climate change on water quality, benthic mussels, and suspended mussel culture in a shallow, eutrophic estuary

Climate change is a global problem that causes severe local changes to marine biota, ecosystem functioning, and ecosystem services. The Limfjorden is a shallow, eutrophic estuary influenced by episodic summer hypoxia with an important mussel fishery and suspended mussel culture industry. Three future climate change scenarios ranging from low greenhouse gas emissions (SSP1-2.6), to intermediate (SSP2-4.5) and very high emissions (SSP5-8.5) were combined with nutrient load reductions according to the National Water Plans to investigate potential impacts on natural benthic mussel populations and suspended mussel culture for the two periods 2051-2060 and 2090-2099, relative to a reference period from 2009 to 2018. The FlexSem model combined 3D hydrodynamics with a pelagic biogeochemical model, a sediment-benthos model, and a dynamic energy budget - farm scale model for mussel culture. Model results showed that the Limfjorden was sensitive to climate change impacts with the strongest responses of physics and water quality in the worst case SSP5-8.5 scenario with no nutrient reductions. In the two low emissions scenarios, expected improvements of bottom oxygen and Chlorophyll a concentrations due to reduced nutrient loads were counteracted by climate change impacts on water physics (warming, freshening, stronger stratification). Hence, higher nutrient reductions in the Water Plans would be needed to reach a good ecological status under the influence of climate change. Suspended mussel culture was intensified in all scenarios showing a high potential harvest, whereas the benthic mussels suffered from reduced food supply and hypoxia. Provided the environmental changes and trends in social demands, in the future, it is likely that suspended mussel cultivation will become the primary source of mussels for the industry. Model scenarios can be used to inform managers, mussel farmers, fishermen, and the local population on potential future changes in bivalve harvesting and ecosystem health, and to find solutions to mitigate climate change impacts.