Artificial Reef or Ecological Trap
There’s not a lot of data on the underwater ecosystems developing on and under floating wind structures; there’s just not that many of these farms installed yet.
A recent study by researchers in Scottish set out to analyze what data there is and to hypothesize about what the potential positive and negative effects might be for marine life.
It’s potentially important because floating offshore wind is being developed on highly productive shelf seas. They cover approximately 9% of the global ocean area but account for around 16% of global ocean primary production and support 90% of the world’s fish catches.
Wind farm structures can create new habitats at a regional scale and throughout the water column, potentially benefiting a range of pelagic and benthic species. Studies of oil and gas platforms have found artificial reef effects that extend out around 800 meters, and floating structures are known to be used as fish aggregating devices.
Different platform designs offer different amounts of surface available for colonization, different structural complexity and different types of movement. For example, spar-buoy platforms likely provide greater surface area for colonization, while semi-submersible designs offer higher structural complexity, both of which could enhance artificial reef effects and increase fish community development.
But there could be a trap. Ecological traps occur when species are drawn to a site for positive reasons such as feeding, shelter or breeding but end up experiencing negative outcomes. For instance, marine mammals may be attracted to prey on the fish that have been attracted to a floating platform. The mammals may in turn face greater risk of entanglement.
The researchers list three main types of entanglement to consider at floating offshore wind farms:
• primary entanglement: an organism becomes directly entangled with a structure;
• secondary entanglement: debris such as ghost fishing gear becomes caught on a structure and subsequently traps an organism;
• tertiary entanglement: an organism already entangled in debris then becomes caught on the structure.
Mooring lines are typically constructed from chain or synthetic rope with diameters of approximately 120–200mm, and the risk of primary entanglement is considered low. However, secondary or tertiary entanglement risks are potentially higher due to the dynamic movement of moorings and suspended inter-array cabled at floating wind sites. The chance of breaking free from entanglement is considered low due to the high strength of the mooring lines.
The researchers evaluated published studies from two sites with five and one site with 11 turbines and found mixed results regarding the biota developing on them. At Hywind Scotland, for example, ROV video footage showed Atlantic cod, sand eel, ling and whiting close to the structures, but at Hywind Tampen, there was no evidence of reef effects, although the presence of sperm whales and killer whales was detected.
“Results that have emerged so far indicate that structures have been colonized by epibenthic communities, and that structures might impact local increases in primary production, both of which may result in increased biomass levels for certain fish species. However, evidence of consistently increased fish biomass across floating offshore wind sites remains elusive, and obvious reef effects have not been observed in relation to fish communities. If reef effects are present, they may only occur close to (within approximately 100m) of the turbines.”
The researchers note the many gaps in current understanding and the need for more data.
That opportunity for that may be coming. This week, Norway opened a bidding round for three floating wind areas, and California-based Aikido Technologies signed an agreement with Marine Energy Test Centre (METCentre) in Haugesund, Norway, to deploy one of the largest floating wind platforms deployed to date.
The March issue of Marine Technology Reporter looks at how small and agile underwater robots can, among other functions, serve as non-invasive tools for assessing marine life around offshore installations, helping to inform ecosystem-based management approaches.