Karen Timmermann

Mussels as a tool for mitigation of nutrients in the marine environment.

Long-line mussel farming has been proposed as a mitigation tool for removal of excess nutrients in eutrophic coastal waters. A full-scale mussel farm optimized for cost efficient nutrient removal was established in the eutrophic Skive Fjord, Denmark where biological and economic parameters related to nutrient removal was monitored throughout a full production cycle (1 yr). The results showed that it was possible to obtain a high area specific biomass of 60 t WW ha(-1) eqvivalent to a nitrogen and phosphorus removal of 0.6-0.9 and 0.03-0.04 t ha(-1)yr, respectively. The analysis of the costs related to establishment, maintenance and harvest revealed that mussel production optimized for mitigation can be carried out at a lower cost compared to mussel production for (human) consumption. The costs for nutrient removal was 14.8 € kg(-1)N making mitigation mussel production a cost-efficient measure compared to the most expensive land-based measures.

Water clarity and eelgrass responses to nitrogen reductions in the eutrophic Skive Fjord, Denmark

Eelgrass depth limits and water clarity in the Skive Fjord estuarine system have not improved despite nutrient input reductions of 30%. Long-term monitoring data (1989–2010) were used to investigate the underlying causes. Dissolved inorganic and organic nitrogen concentrations decreased significantly over time, whereas particulate organic nitrogen concentration, assumed to consist primarily of phytoplankton and phytoplankton detritus and calculated as a proportional factor to chlorophyll a, did not change. Total organic carbon, mostly of autochthonous origin, remained constant despite reduced nitrogen concentrations, resulting in an increasing C:N ratio of the organic material in the water column. Phytoplankton primary production also remained constant suggesting that phytoplankton growth was only limited by nitrogen to a minor degree. Alleviated grazing pressure caused by a reduction in the blue mussel standing stock and a pelagic food web dominated by jellyfish may have contributed to the constantly high phytoplankton levels. Particulate inorganic matter, likely reflecting sediment resuspension, increased over time, most probably in response to removal of blue mussels and declining eelgrass cover. The Skive Fjord estuarine system is affected by multiple pressures—nutrient enrichment, mussel dredging and climate change that must be addressed together for water clarity to improve and eelgrass to recover.

Mussel Production and Water Framework Directive Targets in the Limfjord, Denmark: an Integrated Assessment for Use in System-Based Management

Growth of human activities often conflict with nature conservation requirements and integrated assessments are necessary to build reliable scenarios for management. In the Limfjord, Denmarks largest estuary, nutrient loading reductions are necessary to fulfill EU regulations criteria, such as the Water Framework Directive (WFD). Cuts in nutrient loadings do not necessarily result in corresponding reductions in eutrophication impacts or in improving primary and higher trophic-level production. Similarly, the socioeconomic consequences of a mussel fishery and aquaculture production are complex and hard to predict. This study focuses on the usefulness of a System Approach Framework (SAF) implementation for stakeholder understanding of complex systems and development of sustainable management. Ecological-social-economic (ESE) model simulations clearly demonstrated the potential problems of WFD implementation for mussel fishers and mussel farmers. Simulation of mussel fishery closures resulted in a tenfold increase in the hitherto fishable mussel biomass and a similar decrease in the biomass of shallow-water mussels and medium-sized ones in deep water. A total closure of the mussel fishery could result in an annual profit loss of ~€6.2 million. Scenario simulation of the introduction of one, two, three, and four mussel culture farms of ~19 ha showed that the introduction of line-mussels would decrease the biomass of wild mussels both in shallow and deep waters, affecting the catch and profit of fishers. The SAF, which included consultation with stakeholders at all stages, differs from the traditional public consultation process in that (1) communication was verbal and multilateral, (2) discussion among stakeholders was facilitated, and (3) stakeholder opinions and priorities formed the focus of the ESE assessment.

Development and Use of a Bioeconomic Model for Management of Mussel Fisheries under Different Nutrient Regimes in the Temperate Estuary of the Limfjord, Denmark

Coastal ecosystems worldwide are under pressure from human-induced nutrient inputs, fishing activities, mariculture, construction work, and climate change. Integrated management instruments handling one or more of these problems in combination with socioeconomic issues are therefore necessary to secure a sustainable use of resources. In the Limfjord, a temperate eutrophic estuary in Denmark, nutrient load reductions are necessary to fulfill EU regulations such as the Water Framework Directive (WFD). The expected outcome of these load reductions is an improved water quality, but also reduced production of the abundant stock of filter-feeding blue mussels, Mytilus edulis. This is expected to have significant economic consequences for the million-euro mussel fishing industry taking place in the Limfjord today. We developed a bioeconomic model that can be used to explore the consequences of load reductions for mussel fishery as practiced today, as well as potential management options, to obtain an economically and ecologically sustainable mussel fishery. Model simulations clearly demonstrate a substantial decrease in mussel production after the nutrient load reductions necessary to obtain the targets in the WFD. With todays practice, the mussel fishery in the Limfjord will not be profitable in a future, less eutrophic estuary. However, model simulations also revealed that mussel fishery can be profitable after implementation of the WFD with a reduction in the total fishing quota, fewer fishing vessels, and a higher fishing quota per vessel.

The use of shellfish for eutrophication control

Effects of excess loading of nutrients to the marine environment can be mitigated by mussel cultures, basically through nutrient removal from the marine environment when shellfish are harvested. Shellfish farming also provide other goods and services to the marine environment primarily through the impact on water transparency caused by shellfish filtration. There is an increasing awareness of the mitigation potential in mussel culture in relation to eutrophication, but so far practical examples of culture on full scale devoted to mitigation are few. Further, impact of mussel farming on nutrient cycling, e.g. in the sediments below the culture units, has raised concerns. In this review, we clarify concepts in relation to nutrient mitigation and discuss goods and services delivered by mussel mitigation cultures and their impact on an ecosystem scale based primarily on results from studies in heavily eutrofied estuaries. A multi-criteria approach for site selection is presented based on experiences from Danish waters, and economic aspects of mitigation cultures are analysed in relation to use of the produced mitigation mussels. Future perspectives for extractive cultures are discussed in relation to source of excess nutrients.

Developing a New Management Tool—a Holistic View on the Nitrogen Cycle

New agricultural technologies can reduce the emissions of ammonia associated with e.g. manure spreading. Reduced emissions to the atmosphere have the potential to limit the negative impacts of reactive nitrogen (Nr) on terrestrial ecosystems and human health. But could the new technologies transfer more Nr to the watershed instead and hence lead to increased eutrophication in the aquatic environment? In order to answer questions like this a holistic approach is necessary. Therefore a new management tools is under development at the Danish Center for Energy and Environment (DCE), Aarhus University, where models describing the fate of Nr in the relevant compartments (atmosphere, watershed and aquatic systems) are linked.

Testing a New Holistic Management Tool for Nitrogen—Environmental Impacts of Using Manure Acidification in the Danish Agricultural Sector

The fate of anthropogenic reactive nitrogen (N) is often described as a cascade of different nitrogen forms and effects throughout the environment. In order to describe the fate in detail, a holistic approach covering the flow between the main environmental compartments is needed. Therefore a new management tools has been setup for an area in Denmark. A holistic approach is attempted by linking models for the main compartments (atmosphere, watershed and aquatic) and including a common emission scenario. The scenario describes a new technique for reducing ammonia emissions and at the same time increase N availability for crops using acidification of liquid manure and use of air cleaners in pig and poultry houses. Here the first results from a pilot study in Northern Jutland, Denmark, will be presented.