Spatial Effects of Different Zebra Mussel Farming Strategies in an Eutrophic Baltic Lagoon

Abstract

Finding suitable places to establish a mussel farm is challenging, as many aspects like mussel growth, clearance effect and the risk of low oxygen conditions, have to be considered. We present a tailor-made approach, combining field experiments with a spatially explicit model tool, to support the planning process. A case study was set up in the German part of Szczecin (Oder) Lagoon (Baltic Sea), as it shows all typical eutrophication problems and has a strong need and high potential for nutrient retention measures. Farming zebra mussels (Dreissena polymorpha) is an innovative approach that utilizes a species which is often perceived as a pest. The practical applicability and water quality improvement potential was proven by a pilot farm. Combining the gained knowledge with the simulation model led to a cascade of mussel farm options that differ in purpose, location, and biomass. Placing a mussel farm in an enclosed bay resulted in a remarkable water quality improvement (Secchi Depth increased up to 2 m), but the effect stayed local, the growth was limited and the potential annual nutrient removal reached a threshold of ∼30 t N and 2.8 t P. The same nutrient removal could be reached with much smaller farms in an open sea area, whereas the change of water transparency or bottom oxygen conditions were neglectable. A maximal nutrient removal potential of 1,750 t N and 160 t P per year was estimated, when nearly the entire German part of Szczecin Lagoon with mussel farms was used. This led to a strong reduction of phytoplankton and an increase of Secchi Depth, but also a rising risk of anoxia. Overall, all mussel farm options are only a supportive measure, but not sufficient to reach the Good Environmental Status demanded by the Water Framework Directive. At once, the nutrient export from Szczecin Lagoon to the open Baltic was reduced by up to 3,500 t N and 420 t P per year, making the large-scale mussel farm option also a potential measure within the Marine Strategy Framework Directive.

A trophic cascade facilitates native habitat providers within assemblages of multiple invasive marine species

“…The result from linear regression is shown as a trend line. management of which may be considered using means such as line farming of D. polymorpha (Friedland et al 2019) or native mussels (Buer et al 2020). Another effect to be considered caused by D. polymorpha and that will be impacted by mussel predation is the alteration of benthic habitat heterogeneity.…”

Managing Eutrophication in the Szczecin (Oder) Lagoon-Development, Present State and Future Perspectives

“…The efficiency of mussel farms to locally improve SD and water quality was carried out along Baltic coastal waters in Sweden (Lindahl et al, 2005), Denmark (Nielsen et al, 2016) or Germany (Schröder et al, 2014). With a spatial explicit mussel farm simulation model, Friedland et al (2018) showed that SD could locally be increased even up to 60 cm, but at an increasing risk of anoxia, resulting in a further P-liberation from the sediment. Nevertheless, even in the maximal scenario of Friedland et al (2018), the potential water quality improvement was not enough to reach the GES thresholds of Chl-a.…”

“…With a spatial explicit mussel farm simulation model, Friedland et al (2018) showed that SD could locally be increased even up to 60 cm, but at an increasing risk of anoxia, resulting in a further P-liberation from the sediment. Nevertheless, even in the maximal scenario of Friedland et al (2018), the potential water quality improvement was not enough to reach the GES thresholds of Chl-a. Instead of a maximal mussel farm approach with unpredictable implications, most promising is to establish spatially and temporally restricted farms to support the (re-)occurrence of submerged macrophytes at wisely chosen spots )-presuming that they will grow, if the underwater light conditions get good enough due to the clearance effect of the mussels.…”

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

“…In Germany, the water quality of shallow, enclosed, and semi enclosed water bodies are currently rated unsatisfactory or poor ( UBA, 2017 ). However, semi enclosed systems could still reach the turning point and return to a mesotrophic state, if nutrient loads and internal nutrient concentrations are reduced substantially ( Berthold et al, 2018 ; Friedland et al, 2019a ).…”

“…During recent years, suspended mussel farming has progressed as a novel approach to mitigate eutrophication and support underwater vegetation ( Lindahl & Kollberg, 2008 ; Schernewski, Stybel & Neumann, 2012 ; Friedland et al., 2019a ; Petersen, Loo & Taylor, 2019 ; Timmermann et al, 2019 ).…”