Mitigation Culture of Mussels: Production and Ecological Impacts

The ecological footprint of modern industrialized society now encompasses the entire biosphere. Interrelated themes of global warming, biodiversity loss, ocean acidification, and eutrophication are defining features of the Anthropocene. The nutritive enrichment of our coastal seas and estuaries has been widely recognized as a core environmental issue that has taken shape in national and international regulation.

Despite the improvements in nutrient load reductions to some coastal waters, ensuing ecological rebounding or otherwise expected improvements have not been realized. In light of internal loading and the manifold negative interactions, there is increasing recognition that multifaceted intervention in coastal ecology is required to rectify the deterioration of coastal ecosystems.

One such intervention is leveraging the intense particle filtration capacity of marine bivalves, where in the western Baltic Sea, the blue mussel (Mytilus edulis) cultivated for the primary purpose of mitigating eutrophication can extract large quantities of nutrients from coastal waters while providing additional ecosystem services.

Several studies have examined bivalve farming practices and their quantitative effects on the environment in relation to seston immobilization and nutrient dynamics. While sharing some features of conventional shellfish production, mitigation mussel cultivation shifts cultivation objectives from product quality to total nutrient content at minimal costs.

Accordingly, modified or new modes of production require optimization procedures in terms of cultivation methods and spatial prioritization. Eutrophic environments are highly dynamic and the interactions of marine mitigation mechanisms with the environment require careful investigation to assess the scale of ecosystem services rendered.

Suspended mussel farms function as large-scale reactors for organic particles, transforming a portion into somatic mass, another into particulate organic wastes deposited on the sea floor, and the remaining as dissolved metabolic wastes. In an organic particle-rich environment, this large-scale filtration mechanism functions to clarify the water column and locally enhance nutrient dynamics, to a degree contingent upon the environmental contexts.

To characterize mitigation production, important ecosystem services and impacts, this PhD thesis encompasses three years of field and model studies across Denmark and the western Baltic Sea. Experimental results from commercial-scale field trials demonstrated that increasing substrate density in the water column or utilization of cultivation technologies with high substrate surface area and nominal requirements for buoyancy maintenance dramatically increase prior estimates of nutrient extractive potential of mitigation mussel farming.

Improved extractive potentials could then be used to evaluate mitigation potentials over the western Baltic Sea by development of a spatial model. Spatial modeling results exhibited several regions where mitigation farms can be prioritized in order to meet water quality goals, while also identifying regions where mussel production may be limited by food depletion.

Therefore, a farm-scale model was developed to evaluate the mechanisms behind food depletion in detail, by exploring scenarios with multiple environmental interactions representative of the varying gradations of conditions across the western Baltic Sea. From model results and field observations of food depletion, relatively minor variability in environmental conditions or farm configuration drove highly differential intensity and extent of depletion signals.

To interpret the complex real-world structure of depletion in a large-scale mitigation mussel farm, field studies were conducted alongside collection and analysis of satellite remote sensing data. Spatial patterns of seston depletion are primarily influenced by hydrodynamic regimes, yet indications of basin-scale feedbacks were suggestive with high biomass loads.

Considering the potential feedbacks of particle immobilization on nutrient dynamics, further field study was conducted to assess the effects of particle deposition at a mitigation mussel farm on biogeochemical processes. Impacts of mitigation farms related to intensive particle immobilization were generally localized within farm areas, and quickly obscured by variability in the existing highly-productive systems.

This thesis and the papers herein establish parameters for optimizing this mitigation instrument and present novel means to evaluate the potential for mitigation in different environmental conditions, including ecological impacts.