Introduction
The world is facing several crises including climate change, pollution/eutrophication, biodiversity loss and a growing risk of food crisis. Low Trophic Aquaculture (LTA) is seen as a potential solution for multiple crises for two primary reasons; 1) LTA produces healthy food with a very low or negative CO2 footprint, 2) LTA can reduce eutrophication through nutrient removal and create local biodiversity hot spots by attracting epifaunal and mobile species. LTA production is concentrated in the coastal zone due to practical, economic and environmental reasons. As coastal areas are already occupied by many other purposes in densely populated areas such as Europe, the expansion of LTA sectors is challenging, which limits the development of an otherwise sustainable food production. Concurrently, sustainable energy production at sea is expanding rapidly and occupies increasingly larger areas offshore. It is thus obvious to investigate how to co-locate several marine activities e.g., offshore wind and LTA production to solve several problems at the same time and create multi-use spaces. As part of the EU HORIZON project ULTFARMS, we have studied if LTA production of blue mussels (Mytilus edulis) and the seaweed species sugar kelp (Saccharina latissima) and dulse (Palmaria palmata) is biologically and technically possible in two offshore wind farms in the Kattegat, Denmark.
Materials and methods
In Anholt Offshore Wind Farm (OWF) three longlines were established to grow mussels and seaweed in two seasons with different configuration of the lines in different seasons. At Sams�� OWF two tube+net system was established to grow mussels in two seasons. At both locations, marine data loggers were deployed to continuously monitor environmental paramters including temperature, salinity, oxygen, light, Chlorophyll-a, and current velocity. Wind data was retrieved from the farm operators. Seaweed lines were seeded in the laboratory and deployed on site. For mussels, spat collectors (Anholt OWF) or nets (Sams��) were deployed in June. At intervals, and when weather windows allowed, samples were taken from the lines and data loggers were cleaned for fouling and data offloaded.
Results
Physical forcing of ocean currents and wind in both locations were strong. In particular Anholt OWF was challenged by environmental conditions leaving few and narrow operational weather windows due to N-NE currents with mean speeds of 1.36 m/s and many consecutive days of wind speeds above 10 m/s from W-SW. In general, mussel spatfall and growth was successful at both locations, which was corroborated by relatively high chlorophyll-a concentrations in the water. In contrast, growth of both seaweed species was low, likely due to prolonged exposure to low and variable salinity and reduced light availability, as cultivation depths were frequently located within or above a stable halocline/pycnocline despite being placed relatively deep (5 m) to avoid physical forcing.
Discussion
The concept of multi-use of marine space is meaningful in view of the political ambition to minimize Europes dependency on fossil fuels and hence increase offshore wind energy. Here we show that LTA production of species of mussels and seaweeds is biologically possible to some degree in Kattegat, Denmark - an area with existing and planned offshore wind expansions. Compared to more sheltered coastal zones, operating offshore is, however, generally associated with fewer operational weather windows and higher costs. As a result, LTA production is at present only successful in a few locations in Europe. Next steps for a potential broader implementation of multi-use is discussed.
Acknowledgment
THE ULTFARMS project has received from the European Union's Horizon Europe research and innovation programme under Grant Agreement No. 101093888.