With the growing ambitions of implementing low trophic aquaculture, a framework for cohesive and balanced assessment of effects resulting from seaweed cultivation was performed for a hypothetical case study in 2021. As, traditionally, such assessments tend to focus on the negative impacts, it was recommended to also assess the benefits from seaweed cultivation. The concept of ‘Ecosystem services’ (ES) is a sensible construct to express such benefits, as it focuses on the benefits supplied by the ecosystem. The goal of the present study is investigating how these ecosystem services can be included in the comprehensive framework developed last year for a seaweed cultivation case. In order to achieve this, we specifically looked at the project design process, how to map the project needs and how to select or develop the instruments to meet those needs. Roughly, three phases can be distinguished in the study design process: the ideal design phase, where the project specifications required to fulfil the goal of an assessment are defined to ensure the most comprehensive assessment; the realistic design phase, where one performs a reality check on the ideal design, where practical issues, technical issues and resource availability are considered; and finally an adaptive phase: the phase where modifications are made resulting from new insights. In order to facilitate the study design process we developed and applied a so-called ‘rubric’ tool. This rubric is a questionnaire that scores a wide range of elements that are relevant when evaluating ecosystem services for one or more activities (such as seaweed cultivation). This questionnaire considers and addresses elements of ecosystem services impact assessments grouped in the following three aspects: - Which elements are relevant and considered for the study? -What level of detail is required (for these elements)? -How to quantify ES and process the data?. Each sub-question in the rubric is answered with a score between 0 and 5 (where the scale is arbitrary). In the present study, the questionnaire is applied to the case-study goals. But also two contrasting strategies (i.e. application of top-down versus bottom-up methodologies) are included as examples and are evaluated with the rubric for suitability. The top-down strategy uses an existing framework from the EU project Aquacross, which was also used last year to address seaweed cultivation impacts. This framework uses linear cause-effect chains where effects on the ecosystem components are linked to the capacity to supply ecosystem services. The bottom-up methodology is formulated from scratch. It focuses more on the desired outcome, and more attention is paid to benefits resulting from the activities (rather than impacts). Feedback mechanisms are also considered. It should be noted that both methodologies are only available as concepts and are not yet operational. By applying the rubric to both set case-study goals (including ecosystem services in an assessment of the effects from seaweed cultivation) and the proposed methodologies, both results could be compared. It is shown that neither of the two strategies have a 100% match with the case-study goals. This means that neither methodology is preferable at the moment. Also, this means that the methodologies need to be adjusted or the case-study goals need to be revised. In order to make informed decisions on how to proceed in this process a final piece of the puzzle is still missing, which unfortunately is beyond the scope of the present study. - The missing puzzle piece is ‘consequences’. The rubric approach had helped in structuring the project design phase and support underpinning discussions. It made insightful what the study intentions are, and what the proposed methodologies can and cannot offer. It also shows that there is a mismatch between the ideal design and the realistic design. In order to decide which changes (in either case- study goals or strategies to include ecosystem services) are necessary, it is required to know the consequences of these decisions. Several recommendations are made to generalize the assessment of these consequences by linking case-study goals to requirements and their feasibility. This is not further developed in the present study. In conclusion, the rubric approach can be used to score the capacity and limitations of methodological strategy (such as the top-down and bottom-up strategy evaluated here) on one hand and the improve specifications of requirements for a specific case or project on the other hand. As such the approach can be used to evaluate different methodologies, to determine which strategy is most suitable for a specific research question. The approach can also be used to refine research questions or identifying knowledge and data gaps in an early stage of a project. In addition, the approach can be used iteratively during a project execution to manage and adjust the project requirements (and indirectly stakeholder expectations of the project). As such the approach is suitable in each of the three identified study design phases (targeted design, realistic design, adaptive design). In its current form the rubric approach shows promise, but is not yet able to fully support study design choices for evaluating ecosystem services from the seaweed cultivation case from Tonk et al. (2021). For that purpose the approach needs to be extended such that it addresses consequences of design choices, for which recommendations are made.

European flat oysters (Ostrea edulis), important ecosystem engineers with several important ecosystem services, have near to disappeared from the Dutch North Sea. In order to reintroduce the flat oyster population in the North Sea, the availability of hard substrate for initial settlement is critical. Such substrate is offered in offshore wind farms where further anthropogenic disturbances of the seabed are restricted, making them a promising site for restoration efforts. Nature inclusive designs of scour protection around the base of the monopiles in wind farms could greatly improve the ecosystem contribution of offshore wind farms. However, the best type of substrate to use as scour protection to aid oyster settlement is unknown. In the project EcoScour(Protection), settlement success of oysters on different types of substrates is compared in order to find the best substrate for application in an eco-friendly scour protection in offshore wind farm Borssele V. The substrate types tested are: fresh mussel shell, weathered mussel shell, granite, sandstone, silex, marble, concrete, ECOncrete, roof tile, steel and BESE-elements. Settlement success is compared using three different measures: 1) total numbers of spat settled - in order to compare total spat collected per type of substrate provided in identical baskets, 2) settlement per kg of substrate - to find optimal substrates with regards to transport, and 3) settlement per cm2 of substrate - to compare success between substrates independent of the available space to settle on. The substrates were deployed at three different locations; the saltwater lake Grevelingen in the Netherlands, an oyster spatting pond in New Quay, Ireland and a natural bay in Tralee, Ireland. In the Grevelingen settlement of oyster spat per basket was most successful on weathered mussel shells, while granite was most successful at both locations in Ireland. Per kg fresh mussel shells were the most successful substrate at all locations. In terms of spat collection per cm2, granite (New Quay and Tralee) and marble (lake Grevelingen) were also successful and more so than mussel shells. The final choice of substrate for application in wind farms can depend on several factors. (1) When considering number of spat per mass of substrate, mussel shells are the substrate of choice. (2) When taking into account the cost-effectiveness for application in eco-friendly scour protection, working with construction materials such as granite or (E)concrete or by-products such as silex or marble (both from quarrying) are good options. (3) In view of longevity of the ecosystem and persistence even after decommissioning of the wind farm, substrates that are originally found in the North Sea could be considered. For that purpose, mussel shell or (red) granite gravel would be the best choice of oyster spat collection substrates to apply in eco-friendly scour protection.

To monitor flat oyster reef restoration projects in offshore pilots innovative techniques with a focus on remote monitoring are needed. This will reduce the frequency with which offshore pilot study locations have to be visited. The valve gape monitor can potentially be used as an effective method to monitor survival and behaviour of the European flat oyster Ostrea edulis. This is an in situ device that continuously measures the opening and closing of the shell of live bivalves (valves) equipped with sensors. It has been successfully used in combination with other bivalve species. In this study, eight oysters Ostrea edulis from two different regions (Norway and the Netherlands (Grevelingen)) were fitted with valve gape sensors. Valve gape activity was monitored during different experimental conditions (tide simulation and starvation) under controlled conditions to test the applicability of the valve gape monitor to assist with monitoring of O. edulis reef restoration projects. No significant differences in valve gape activity of Norwegian oysters and oysters from the Grevelingen were detected under controlled conditions. Tide simulation did not appear to affect valve gape activity. However, the oysters were responsive to disturbances such as daily maintenance as well as starvation. Size frequency distribution of gape class clearly shows that fed individuals show longer periods of open valves and percentage closure is significantly higher in the starved oysters. Gape width is larger in fed conditions then in starved conditions. In addition, starved oysters open less wide when they open. These results indicate that food availability affects valve gape activity and the valve gape monitor can provide information on the behaviour and stress response of Ostrea edulis. In situ monitoring of flat oyster gape activity should incorporate environmental parameters such as Chl a, temperature, salinity, current speed and oxygen to provide insight into the environmental stressors on and/or the behavioural patterns of flat oysters.

The cyanobacterium Microcystis can produce microcystins, a family of toxins that are of major concern in water management. In several lakes, the average microcystin content per cell gradually declines from high levels at the onset of Microcystis blooms to low levels at the height of the bloom. Such seasonal dynamics might result from a succession of toxic to nontoxic strains. To investigate this hypothesis, we ran competition experiments with two toxic and two nontoxic Microcystis strains using light-limited chemostats. The population dynamics of these closely related strains were monitored by means of characteristic changes in light absorbance spectra and by PCR amplification of the rRNA internal transcribed spacer region in combination with denaturing gradient gel electrophoresis, which allowed identification and semiquantification of the competing strains. In all experiments, the toxic strains lost competition for light from nontoxic strains. As a consequence, the total microcystin concentrations in the competition experiments gradually declined. We did not find evidence for allelopathic interactions, as nontoxic strains became dominant even when toxic strains were given a major initial advantage. These findings show that, in our experiments, nontoxic strains of Microcystis were better competitors for light than toxic strains. The generality of this finding deserves further investigation with other Microcystis strains. The competitive replacement of toxic by nontoxic strains offers a plausible explanation for the gradual decrease in average toxicity per cell during the development of dense Microcystis blooms.

The upscaling of offshore wind farms in the North Sea creates opportunities for seaweed aquaculture that has the potential to meet part of future resource needs, provided that it is done sustainably. Here a follow-up study of the MIP project in 2018 “Development of Offshore Seaweed Cultivation: food safety, cultivation, ecology and economy” with a focus on ecology and cultivation is presented. In order to ensure a sustainable development of seaweed farming in Dutch offshore and coastal regions in the future, it is essential to collect empirical data on the interaction of seaweed cultivation with marine ecosystems for realistic impact assessments. In subproject 1 “Ecology – Fauna associated with seaweed aquaculture in the North Sea Farm” ecosystem services and impacts of seaweed farming in the North Sea were investigated on the basis of biodiversity, a key parameter for the functioning of ecosystems. Therefore in 2019 the associated fauna on growing seaweed biomass (Saccharina latissima) and cultivation ropes was assessed at the North Sea Farm. A high number of individuals was detected on the seaweeds and cultivation ropes in general (up to 7679 individuals per rope), but species richness was low. Abundance in fauna increased from May to June and all detected species are also known from other hard substrates in the North Sea. Compared to previous assessments of biodiversity with eDNA metabarcoding at the same site, the biodiversity detected in 2019 was very low. However, biodiversity levels may differ from year to year. Moreover, the samples were not taken at the same time points and are therefore not directly comparable and the methodology only included organisms that could be collected by hand (visible to the eye) with a focus on fauna attached to the rope and kelp. It is advised to combine classical morphological biodiversity assessment and eDNA metabarcoding in future assessments to compare results in order to determine the best-suited methodology for biodiversity assessments. Biodiversity in the seaweed farm should be assessed repeatedly every 5 years to check for temporal alterations in fauna composition, especially when cultivation structures, such as anchors, are deployed throughout several years. In subproject 2 “Cultivation” seasonal variation in biomass productivity and chemical composition of kelp was evaluated in order to determine the optimal time point for harvesting in relation to the desired end product. Biomass production at the test-farm was very low in 2019, compared to previous years and a seaweed farm test location near Helgoland in the North Sea. Below 4m environmental conditions for growth were unfavourable (mainly light limitation) for Saccharina latissima. Both in 2018 and 2019 large differences in standing crops over time and depth were observed. Contrary, true protein levels varied only slightly over time. If protein is the target product, final biomass yield of S. latissima will determine the profitability of the mariculture. A combination of economic analyses and growth experiments may assist in determining the optimal cultivation technique. The 2018 experiments performed at the North Sea Farm showed large seasonal variability in the chemical composition of seaweed tissue, and high amounts of nitrogen-containing compounds besides proteins variations. Therefore in 2019, nitrogen, starch and nitrate content in the seaweed tissue were analysed. Nitrate content in S. latissima varied throughout the season and could not fully explain the difference between N measured by Dumas and true protein content in the 2019 samples. Therefore, other seaweed components containing nitrogen must explain this variation, e.g. its accumulation in cellular nitrate pools. As a final note, in order to improve the understanding of environmental conditions in the farm it is recommended that nitrate and phosphate concentrations, two essential macronutrients for growth in seaweeds, should be assessed in the water column at different depths and time points.

In de Nederlandse Exclusieve Economische Zone (EEZ) zijn windparken gepland op diverse locaties. Door de windparken ontstaan nieuwe kansen voor multifunctioneel gebruik zoals maricultuur en niet-bodemberoerende visserij met passieve vistuigen. Deze studie brengt de potentiële - kwantitatieve - productiviteit van het kweken of vissen (passief vistuig) binnen bestaande, geplande en mogelijke toekomstige windparklocaties op de Noordzee in kaart voor een aantal vormen van medegebruik; kweek van bepaalde soorten zeewier en schelpdieren en de niet-bodemberoerende vangst van bepaalde soorten vissen, schaaldieren en inktvissen. Dit project is een vervolg op een voorgaand project waarbij een kwalitatieve beoordeling gegeven is van de kansrijkheid van de gebieden. In de huidige studie is voor een selectie van kansrijke soorten een berekening voor oogst of productie uitgevoerd. Daarnaast is dit rapport bedoeld om op basis van de huidige stand van kennis een inschatting te geven van de orde van grootte van het ruimtebeslag van zeewierproductie binnen toekomstige windparken. Met een aantal berekeningen wordt inzicht gegeven in de orde van grootte van opbrengsten, beschikbare/benodigde oppervlakten en benodigde nutriënten in de Noordzee. Op basis van deze analyse lijkt een areaal van enkele honderden km2 zeewierproductie realistisch, hierbij is uitgegaan van een grove indicatieve berekening, met diverse aannames. Op basis van de beschikbare voedingsstoffen voor mosselen zou tussen ca. 50 en 100 ton drooggewicht mosselen per km2 per jaar kunnen worden geproduceerd, waarbij de bronnen van onzekerheid in acht genomen dienen te worden. Om een indruk te krijgen van het (relatieve) voorkomen van vissen, weekdieren en schaaldieren is gebruik gemaakt van VMS- en logboekgegevens samen met gegevens van twee jaarlijkse surveys (BTS en IBTS). Het is echter niet mogelijk gebleken om voor alle geselecteerde soorten een kwantitatieve inschatting te maken, aangezien er nog weinig bekend is over ruimtelijke verdeling van deze soorten. Daarom is voor deze soorten een overzicht gegeven van wat er in de literatuur over bekend is.