Abstract
Worldwide, coastal and marine policies are increasingly aiming for environmental protection, and eutrophication is a global challenge, particularly impairing near-coastal marine water bodies. In this context, mussel mitigation aquaculture is currently considered an effective tool to extract nutrients from such water bodies. Mussel mitigation farming using longline systems with loops of collector material is a well-developed technology and considered promising in the western Baltic Sea. Besides several spatially limited field studies, a suitable spatial model for site-specific implementation is still lacking. In this study, we present a modular spatial model, consisting of a spatial and temporal habitat factor model (Module 1), blue mussel growth model (Module 2), mussel farm model (Module 3), and an avoidance of food limitation model (Module 4). The modules integrate data from in situ monitoring, mussel growth experiments, and eco-physiological modelling for the western Baltic Sea, to estimate spatially explicit nutrient reduction potentials. The model is flexible with respect to farm setups and harvest times and considers natural variability, model uncertainty, and required hydrodynamics. Modelling results proved valid at all scales and modules, and point out key areas for efficient mussel mitigation farms in Danish, German and Swedish areas. Modelled long-term mean mitigation potentials for harvest in November reach up to 0.88 tN/ha and 0.05 tP/ha for a farm setup using 2 m depth-range of the water column and 3.0 tN/ha and 0.17 tP/ha using up to 8 m, respectively. For Danish water bodies, we demonstrate that in efficient areas, mitigation farms (18.8 ha, 90 km collector substrate in loops with 2 m depth-range) required <3.6% of the space to extract the target nitrogen loads for good ecological status. The developed approach could prove valuable for implementing environmental policies in aquatic systems, e.g. in situ nutrient mitigation, aquaculture spatial planning, and habitat suitability mapping.
Highlights
Both European and international coastal and marine policies have the objectives to reach healthy ocean ecosystems, sustained marine ecosystem services, and integrated utilization of marine areas by minimizing conflicts in the marine environment (Ehler and Douvere, 2009; EU, 2014; EU, 2008; EU, 2000; IOC, 2014)
We present a modular spatial model, consisting of a spatial and temporal habitat factor model (Module 1), blue mussel growth model (Module 2), mussel farm model (Module 3), and an avoidance of food limitation model (Module 4)
The presented modular spatial model for nutrient reduction potentials of blue mussel mitigation farms in the Western Baltic Sea proved valid at all scales and modules
Summary
Both European and international coastal and marine policies have the objectives to reach healthy ocean ecosystems, sustained marine ecosystem services, and integrated utilization of marine areas by minimizing conflicts in the marine environment (Ehler and Douvere, 2009; EU, 2014; EU, 2008; EU, 2000; IOC, 2014) In this respect, eutrophication is a global challenge, impairing near-coastal marine water bodies (Smith, 2003). In situ mitigation measures could effectively complement land-based actions to reduce nutrient contents in coastal water bodies (Petersen et al, 2014) This would contribute to minimizing threats of eutrophication to valuable coastal ecosystems, such as the loss of benthic macrophytes (Timmermann et al, 2019), or frequent occurrences of harmful algal blooms (Delegrange et al, 2015). Potential conflicts about marine space with other economic sectors (e.g. fishing and marine transportation) may arise when implementing marine mitigation measures (Maar et al, 2020b)
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