Mirjam Snellen (TUD)

Seafloor characterization with acoustic imaging

High resolution maps of sea- and river-floors are valuable to, for example, understand their dynamics, plan engineering projects, and create detailed benthic habitat maps. This presentation addresses the use of the multi-beam echo-sounder (MBES) to create these maps. The MBES is an acoustic system, already in widespread use for mapping bathymetry. It provides a high resolution over extended areas. In addition to the bathymetric measurements, it is very valuable to also record a parameter called backscatter (strength). This parameter is directly related to properties of the sediment and can as such be used to reveal the spatial distribution of the different sediment types. Within DISCLOSE the MBES is used to obtain a high resolution map, revealing the distribution of the different sediment types over large areas. These acoustic results then are used to guide the video surveying and sediment box coring measurements to get more detailed information, concerning both sediment type, but also the epibenthos and macrobenthos in the sediment, respectively. This presentation will focus on the theories behind and the current status of the multi-beam echo-sounder for mapping sediment properties, together with its near-future potential.

Leo Koop (TUD)

High resolution seafloor classification in a sand wave environment

Much of the seafloor of the southern north sea is covered in sand waves of different sizes. It is challenging but valuable to map such seafloors accurately. This talk presents multibeam echosounder backscatter classification results of the Brown Bank area of the North Sea. These results were obtained within DISCLOSE. We apply the so-called Bayesian classification method in a megaripple and sand wave area with significant slopes. Prior to the classification, corrections are implemented to account for the slopes. A trade-off in classification resolutions is found. A higher geo-acoustic resolution is obtained at the price of losing spatial resolution, however, the Bayesian classification method remains robust with respect to these trade-off decisions. The classification results are compared to grab sample particle size analysis and classified video footage. In non-distinctive sedimentary environments, the acoustic classes are not attributed to only the mean grain size of the grab samples but to the full spectrum of the grain sizes. Finally, we show that the Bayesian classification results can be used to characterize the sedimentary composition of megaripples. Coarser sediments were found in the troughs and on the crests, finer sediments on the stoss slopes and a mixture of sediments on the lee slopes.

Ad Stolk (RWS)

Reflection on the presented work in acoustic imaging

Since Johan van Veen used echo sounding to study the bathymetry of the southern North Sea the knowledge of the morphology of the seabed was increasing. But single beam echo sounding needs a lot of interpolation and many details of the seabed were missed. Later, in the early ‘90s multibeam echo sounding revealed a lot of bedforms in their 3D appearance. About ten years later the first results of multibeam backscatter analysis added a full sea floor coverage of sediment classification. The methods, the algorithms behind it and the techniques were improved in later years and now it is even possible to distinguish coarse and fine sands. Because of the limitation in shipping time interpolation between lines is still needed for many surveys. Therefore improvement of interpolation methods is very useful. Another acoustical method to investigate the sea bed is side scan sonar imaging. Also from this technique a sediment classification is possible. The combined use of multibeam backscatter and side scan sonar can help in interpretation of the results, because the two techniques complement each other by e.g. the different penetration into the sea bed. Good sediment sampling during the same survey is needed for all acoustic imaging techniques. Therefore, a good combination of acoustic techniques, geological and biological sampling and, if possible, direct images of the sea bed are necessary to produce sedimentological and ecological maps of the sea floor as a base for policy and management.

Peter Herman (TUD/Deltares)

Long-term dynamics in benthic biodiversity

In this presentation I discuss patterns in the species composition of macrozoobenthos in the Dutch sector of the North Sea, based on long-term monitoring data. Patterns are tightly linked to the landscape features of the sea. Biomass, density and species number are non-randomly distributed over space, but these three variables do not respond in an equal manner to governing physical and chemical factors. Species composition of the benthos allows distinguishing several zones that have a very clear relationship with depth, sediment composition and wave impact. Several hundreds of macrobenthic species have been found in the Dutch North Sea. I discuss the species richness of different zones, taking into account both the local diversity (which is generally higher in deeper and front-dominated zones) and the diversity at the scale of the entire Dutch sector. Shallow dynamic areas, although locally poorer in species, contribute significantly to the total species number, as they have a very distinct fauna with species not occurring elsewhere. Temporal changes at short (yearly) time scales may be governed by variations of natural stress factors at this time scale. I present high-resolution coastal data illustrating this point. The composition of the benthos is also strongly influenced by human activities, which typically vary at a much longer time scale. Physical stability of the seabed is influenced by fisheries disturbance, whereas productivity and food supply are influenced by nutrient run-off. On this longer time scale, the most noticeable change in macrobenthos of the Dutch North Sea has occurred near the Dutch coast, where biomass of shellfish has increased considerably. In off-shore areas, we notice a slight (but not always sustained) increase of species vulnerable to fisheries impact. It remains puzzling what has caused the changes in the coastal zone. Competing hypotheses are that they relate to the decrease of beam trawl intensity, or that they relate to improved food conditions as a result of eutrophication abatement. I present some monitoring data on the latter trends, and discuss different causal mechanisms. Given the high diversity of causal factors affecting the benthos, it remains difficult to distinguish effects of different types of disturbance. A better spatial management of the North Sea, encompassing protected natural areas in different types of habitats, may contribute to a much better appraisal of human influences and consequently to improvement of management of the sea.

Tom Ysebaert (Royal Netherlands Institute for Sea Research & Wageningen Marine Research)

Drivers of small-scale variability in benthic communities

Marine soft sediment habitats are often considered to be homogeneous expanses of sand or mud; however, the opposite is true. For intertidal habitats, such as mudflats and intertidal shoals, we increasingly recognize that benthic communities show variations at different spatial scales, ranging from local (10^-1 m) to regional (10⁴ m) scales. A substantial part of this variation can be explained by the observed environmental variables, but also bio-physical interactions and the presence of (small-scale) biogenic structures play a substantial role in driving this variability. Also in the North Sea, heterogeneous landscapes are present on the seafloor structures at multiple spatial scales, such as sandbanks, sand waves, or (mega)ripples, and these different topographic features most likely shape the diversity of benthic habitats. Regular standard monitoring, however, often neglect this heterogeneity and small-scale variability of the seafloor landscape. Within Disclose we test a multiscale, interdisciplinary approach that combines standard bathymetry, multibeam backscatter classification, video surveying of epibenthos and box coring of sediment macrobenthos to reveal the drivers of small-scale variability in soft-sediment benthic communities. We show that fine-scale seafloor morphology has a large impact on benthic community structure, providing opportunities for spatial upscaling of biological variation through acoustic surveying methods and habitat mapping. We discuss its implications for our understanding of the North Sea benthos biodiversity and make suggestions to improve monitoring schemes.

Gert van Hoey (ILVO)

Reflection on the presented work in benthic biodiversity

Maintaining biodiversity is the essence of nature conservation. Nevertheless, the way it is considered in the major EU directives varies. In the Habitat directive the focus lays more on certain ecological important species within the habitats. In the WFD, they define biodiversity as an essential parameter for the assessment. Whereas, the MSFD focuses more on the general structure and function of the seabed ecosystem, with biodiversity as one component. Assessing this biodiversity, taking into account the variability in space and time, is not straightforward, regardless of the policy framework. Everything starts with the data and how it is collected in space and time. For example, in the Belgian marine waters, our view on the biodiversity of the benthic communities changed, when we included more recent data with a wider spatial coverage (Van Hoey et al., 2004 versus Breine et al., 2018). This all is a plea to develop an integrated, coordinated benthic monitoring strategy on regional scale to fulfill the EU directive assessment needs (Van Hoey et al., 2018).
Two other aspects that will influence future biodiversity assessments, is the need to consider also the functional diversity and the use of DNA-based approaches. The species functions (assessed by traits) is important as it makes a link between the structural aspects of a community and the functional processes and ecosystem services of the seafloor. The DNA-based approaches (e.g. benthic barcoding) are currently investigated to deliver more data in a fast, accurate and cost-effective way.

Han Olff (UG)

Natural and anthropogenic drivers of habitat resilience

The seafloor of the southern North Sea host a range of benthic communities that have evolved under a range of natural disturbance regimes. Shallow communities in exposed locations are subject to frequent disturbances due to heavy bottom shear stress from orbital forces of waves, in combination with forces from tidal currents. I will present an overview of the main spatial gradients in these natural disturbance regimes, and discuss how these interact with sediment texture and seafloor morphology. In addition, the North Sea is characterized by strong spatial heterogeneity in bottom trawling intensity, where not only resilient habitats are frequently affected, but also habitats where communities evolved under low disturbance regimes. I will discuss the consequences of the interplay of these natural and anthropogenic factors, introducing the need for more detailed habitat mapping of physical and biotic drivers of benthic community structure and functioning, better analyses of their resilience to these disturbances. This is needed for better-informed spatial planning, management and conservation of the characteristic seafloor communities of the southern North Sea, especially in the light of the current developments regarding wind farms and discussions on better protection of Natura 2000 areas.

Karin van der Reijden (UG)

Mapping the North Sea habitats

Most maps show the North Sea as a vast blue area in north-western Europe. This is also how the North Sea is commonly perceived by the general public: a mass of grey water with a sandy bottom. However, the opposite is true. Just like on land, the North Sea comprises a large variety of demersal ecotypes that can be determined on factors like seabed morphology, sediment type and water dynamics. On top of that, biologic processes driven by ecosystem engineers that locally modify the environment may also contribute to habitat heterogeneity. Hence, gaining detailed spatial knowledge of habitat variety of the North Sea floor is important for effective management. Here, I will discuss various strategies and considerations for producing such a map. A first approach may be to only consider the spatial distribution of biological communities as the main determinant for distinguishing habitat types. Alternatively, environmental factors such as bathymetry and sediment type may be used as blue prints for demersal habitats. Both approaches have their pros and cons, but the underlying mechanisms are different as are the resulting maps. I will shortly compare both approaches and will give specific examples of small-scale habitat heterogeneity that is rarely taken into account in habitat mapping. I will finally indicate potential management implications of the various habitat mapping approaches.

Olivier Beauchard (Royal Netherlands Institute for Sea Research)

Scale-dependent functional patterns in the marine benthos

Benthology in Dutch marine waters has a long history, and after more than twenty years of regular monitoring, the distributions of the most characteristic benthic macroinvertebrate species are well documented. Now, clear maps of species communities occupying the Dutch seafloors are available. However, the understanding of these species assemblages in their specific environmental conditions remains largely unexplored. Basically, species biological attributes are evolutionary determinants of habitat occupancy, and therefore, combining species distributions with species biology is of crucial necessity to the understanding of species assembly rules. From an applied point of view, investigating the biological functioning of species communities enables comprehensive approaches to explain or predict the effects of human activities on natural habitats, and ultimately, mapping habitat vulnerability. In this work, a description of spatial patterns of benthic species communities through the use of biological traits is presented in a multi-scale context. A first outcome is the prominent effect of hydrodynamisms at the largest scale, which strongly affects organism life strategies in the trade-off opposing offspring to adult survivals. Secondly, and more surprisingly, an increased complexity of the relationships between traits and environmental conditions is observed within habitat under high dynamism. Also, contrasting spatial scale specificities of the relationships between traits and environment are shown; this last point is of critical importance regarding our perception of the sea floor functioning, with major consequences for the implementation of marine protected areas.

Christiaan van Sluis (Stichting De Noordzee)

Effective conservation of the North Sea ecosystem

Truly sustainable use of the North Sea ecosystem depends on accurate knowledge of this ecosystem together with effective implementation of policy measures. Scoring a hundred percent on both factors is a challenge due to ecosystem changes and politics, but should be pursued. The North Sea Foundation aims for a clean and healthy North Sea and contributes to DISCLOSE in the wider communication of results. Our work and the DISCLOSE results show opportunities for more effective integration of scientific knowledge and policy measures.

This presentation discusses the current implementation of marine protected areas (MPAs) in the Dutch part of the North Sea, how policy documents relate to the recent discovery of biogenic reefs and what the relation is between MPAs and the spatial distribution of fisheries and their revenues. It shows that interventions can be scientifically or policy driven and that the unclarity between the two has delayed the implementation of MPAs for more than 20 years. Over the last years we see that the North Sea can harbour rich benthic communities under the right circumstances and that measures can be effective. With that in mind, protection should not be halted by the need to know more. Instead, by implementing high quality MPAs we can learn more about their effectiveness. These areas can help to restore benthic communities and at the same time function as reference for future changes or increasing human use like fishing, offshore wind and dredging.

Tjeerd Bouma (Royal Netherlands Institute for Sea Research)

The need for and potential effects of active habitat restoration – An integrative analysis on restoring marginal seas: the Dutch North Sea as case study

Over the last two centuries, many valuable offshore ecosystems have been lost or degraded in our marginal seas due to anthropogenic impacts. There is a growing awareness that losses of key habitats such as biogenic reefs may have a major impact on ecosystem functions and services. There are many unknowns with respect to our ability to get these lost key systems back. For example, do we have a good historic perspective of the shifting base-line that we can use to set a target? To what extent are species-directed approaches compatible with ecosystem level objectives? And how do the economic initiatives affect overall ecosystem functioning? We aim to discuss these highly multifaceted questions using the Dutch North-Sea as example. We specifically focus on the major change that was induced following the disappearance of the oyster reefs, and which are now target for active reintroduction. Specifically, we will try to highlight current interdisciplinary understanding on 3 key questions:

• What do we know about the change of benthic ecosystems before and since the oyster harvesting around 1850, and does this provide us with a framework for defining restoration objectives.
• What are the key knowledge gaps hampering (oyster-reef) restoration, ranging from questions on habitat suitability, source material and restoration techniques, and what experiments are required to resolve them;
• What are the dominant uncertainties at the ecosystem level, that are related to (oyster-reef) restoration in MPA’s and OWF’s.

With input from: Tjisse van der Heide, Luca van Duren, Peter M.J. Herman, Tim Raaijmakers, Pauline Kamermans, Linda Tonk, Joop W.P. Coolen, Karin Didderen, Wouter Lengkeek, Tom van der Have, Remment ter Hofstede, Mark van Koningsveld, Tony van der Hiele, Anieke van Leeuwen, Jim van Belzen, Johan van der Molen, and Rob Witbaard.

Ton IJlstra (Programme Manager North Sea of the Dutch Ministry of Agriculture, Nature and Food Safety)

Reflection on the presented work in effective conservation – conservation needs in slow motion

Increased interest in North Sea conservation issues is the result of an intensification of various uses of the North Sea. Economic and ecological interests collide and it appears that our actual knowledge of the marine ecosystem of the North Sea falls short of the newly identified conservation needs. More in particular we see that there is a great need for up to date field knowledge and field research. The need to counter climate change puts the focus on the North Sea as an area for the production of sustainable energy. The new spatial claims resulting thereof make it necessary to confront conservation needs with the need for space for windfarms. This confrontation is paralleled by the emergence of and a need to develop new conservation concepts in the North Sea area like the protection of biogenic reefs or the establishment of no-take zones. The paper deals with the question to what extent the emergence of new concepts of conservation can be legally (and policy-wise) based on the existing European or domestic legal and policy instruments. The conclusion is that our actual manner of policy development cannot follow the pace by which conservation needs are identified and need to be protected.