­Blue mussel (Mytilus spp.) cultivation in mesohaline eutrophied inner coastal waters: mitigation potential, threats and cost effectiveness

Abstract

The EU-water framework directive (WFD) focuses on nutrient reductions to return coastal waters to the good ecological status. As of today, many coastal waters have reached a steady state of insufficient water quality due to continuous external nutrient inputs and internal loadings. This study focuses first on the current environmental status of mesohaline inner coastal waters to illustrate their needs of internal measures to reach demanded nutrient reductions and secondly, if mussel cultivation can be a suitable strategy to improve water quality. Therefore, nitrogen, phosphorus, chlorophyll a, and Secchi depth of nine mesohaline inner coastal waters in north east Germany were analyzed from 1990 to 2018. Two pilot mussel farms were used to evaluate their effectiveness as a mitigation measure and to estimate potential environmental risks, including the interactions with pathogenic vibrio bacteria. Further, estimated production and mitigation potential were used to assess economic profitability based on the sale of small sized mussels for animal feed and a compensation for nutrient mitigation. The compensation costs were derived from nutrient removal costs of a waste water treatment plant (WWTP). Results show that currently all nine water bodies do not reach the nutrient thresholds demanded by the WFD. However, coastal waters differ in nutrient pollution, indicating that some can reach the desired threshold values if internal measures are applied. The mitigation potential of mussel cultivation depends on the amount of biomass that is cultivated and harvested. However, since mussel growth is closely coupled to the salinity level, mussel cultivation in low saline environments leads to lower biomass production and inevitably to larger cultivation areas. If 50% of the case study area Greifswald Bay was covered with mussel farms the resulting nitrogen reduction would increase Secchi depth by 7.8 cm. However, high chlorophyll a values can hamper clearance rates (<20 mg m−3 = 0.43 l h−1 dry weight g−1) and therefore the mitigation potential. Also, the risk of mussel stock loss due to high summer water temperatures might affect the mitigation potential. The pilot farms had no significant effect on the total organic content of sediments beneath. However, increased values of Vibrio spp. in bio deposits within the pilot farm (1.43 106 ± 1.10 106CFU 100 ml−1 (reference site: 1.04 106 ± 1.45 106 CFU 100 ml−1) were measured with sediment traps. Hence, mussel farms might act as a sink for Vibrio spp. in systems with already high vibrio concentrations. However, more research is required to investigate the risks of Vibrio occurrence coupled to mussel farming. The economic model showed that mussel cultivation in environments below 12 PSU cannot be economic at current market prices for small size mussels and compensations based on nutrient removal cost of WWTPs.

Perspectives and Scenarios for Coastal Fisheries in a Social-Ecological Context: An Ecosystem Service Assessment Approach in the German Baltic Sea

“…The present benthic aquatic plant diversity is low, with eelgrass (Zostera marina) being the predominant submerged macrophyte [31] and emergent vegetation being rare [32]. Water transparency is less than 2 m and hypoxia can occur occasionally [33,34]. Passive fishing gear is allowed [35].…”

Ecosystem Service Assessments within the EU Water Framework Directive: Marine Mussel Cultivation as a Controversial Measure

“…Within coastal waters, internal mitigation measures, also known as "water-based", "sea-based", or "end-of-pipe" solutions, have come into focus to actively support restoration efforts [8,10,11]. As a consequence, the eight European Baltic Sea littoral states signed a ministerial declaration to intensify measures toward the good ecological status and good environmental status of the Baltic Sea, which includes the promotion of sustainable seabased measures [12].…”

Building with Nature—Ecosystem Service Assessment of Coastal-Protection Scenarios

“…Against the background of sea-level rise and increasing coastal erosion, the maintenance and provision of sandy beaches is of high importance for summer tourism and local economy. Recent studies show that blue mussel farming is a feasible option for the future in the western Baltic Sea [22][23][24], as mussels filter algae and increase water transparency, meet the increasing demand for high-quality proteins and provide sustainably produced feed for increasing organic fish aquaculture. However, presently it hardly exists at all because of a lack of tradition and poor economic perspectives.…”