Certain marine organisms that are sessile (i.e. attached to a substrate) at adult stage produce pelagic larvae (living in the water column). These larvae move passively under the influence of currents before finding a hard substrate to attach to and develop. For these species, offshore wind farms can provide new hard substrate habitats where sessile organisms develop and in turn reproduce. The larvae produced disperse from this new point and can thus reach new rocky areas: this is the so-called stepping stones. In order to illustrate the stepping stones that wind turbines could have at sea, we chose to take a simple example: a rocky reef at a certain distance from other reefs. A wind farm is built between the reef and the rocky areas.
Before farm installation, the organisms on the reefs in ecosystem A produce larvae. The maximum larval dispersal distance means that they cannot reach the rocky reefs in ecosystem B. With the wind farm in place, the organisms on the natural reef in ecosystem A produce larvae. The wind turbine foundations and moorings, and floats in the case of floating wind turbines, act as an artificial reef. Larvae attach themselves to and colonise the wind turbine foundations. Once attached, if conditions are conducive, the organisms will develop and reproduce, in turn releasing pelagic larvae into the water column. Carried by favourable currents, the larvae will reach the rocky reefs of ecosystem B, where the species was initially absent. This is what is referred to as the stepping stones. In this way, the larvae of organisms in ecosystem A, which cannot directly colonise ecosystem B, may possibly do so in the presence of wind turbines.
French offshore wind farms are located close to the French coastline; there is therefore a low risk of
creating a stepping stones between the English and French coasts for example. The stepping stones at the scale of the French coasts would have little impact as many other structures and activities transport species or already act as a relay on this scale (shipping traffic, aquaculture, buoys, etc.). The risk of non-indigenous species spreading through the stepping stones produced by French wind farms appears limited.
In short: the risk of non-indigenous species spreading through the installation of ORE farms in France appears limited but not negligible. The experts recommend developing a protocol for monitoring turbine foundations in order to observe and report the development of non-indigenous species.