The Bard Offshore 1 wind park (BO1) lies just under ninety kilometres northwest of the East Frisian island of Borkum. It comprises a surface of roughly sixty square kilometres. The eighty wind turbines reach beneath the sea to a depth of forty metres. The scale of B01 is enormous. What would happen if there were a massive expansion of offshore wind parks in the German North Sea? And would covering the entire area have an impact on physical and chemical processes in the sea? Jeffrey Carpenter and Lucas Merckelbach, two researchers at the Helmholtz-Zentrum Geesthacht, are pursuing these questions. In order to do so, they have equipped underwater gliders with special sensors. The gliders from Geesthacht now silently roam existing and planned wind park locations at various depths and detect the smallest eddies in the water. Cruising within the wind parks themselves is not permissible, so measurements must be restricted to direct proximity of the wind turbines.
The researchers would like to know what happens beneath the surface of the sea. The turbine shafts, measuring several metres wide, interrupt the natural currents and the large eddies that can easily measure several kilometres in diameter in the North Sea. The research team confronts the question of what “mixing effects” can occur along the shafts as large water masses pass by. And do these new eddies have an impact on the different water layers in the sea? The temperature gradient at approximately twenty metres depth is particularly immense in summer. It is about six degrees Celsius within only a few metres. The more wind turbines that are built, the more small eddies will develop. This could have a measurable impact on water mass stratification when a critical quantity is reached. The gliders from Geesthacht seek out these turbulences.
The following is already clear to the two researchers: if wind parks were built in the entire German "exclusive economic zone – EEZ”, then a significant change in the North Sea’s temperature structure could occur. We can assume that the biogeochemistry of the North Sea would change as a result of the alterations in the stratification and nutrient transport in the surface water. The challenge in research is that these changes in the biological and chemical processes must first be quantified. The same applies to understanding how the physical processes take place. Lucas Merckelbach believes, however, "that only few large-scale changes can be expected at the current extent of wind park construction.” The current scale of offshore wind park planning has actually been reduced. “We therefore expect no wind park impact at present across the entire North Sea," says Merckelbach. That, however, could change again in the near future.
We ask the researchers: "What would happen if there was increased mixing of the warm and cold water layers?"Jeffrey Carpenter sees many processes strongly affected by the existence of stratification in the North Sea. "This includes the transport and suspension of sediment in the water column when the sediment floats by. Another effect could concern the growth of phytoplankton that serves as a vital source of food for many species. In addition, the concentration of dissolved oxygen is important and, of course, so is the water surface temperature," the researcher explains. In his view, these "could lead to a cascade effect for the ecosystem if the food spectrum for other species changes abruptly and they lose their food source." Currently, not much is known within the research field. What is known, however, is that the North Sea is an extremely dynamic and variable system. Depending on how conditions develop over the course of the year, strong differences already exist in the stratification of seawater in the German Bight. One of the enormous tasks is to understand this variability and to grasp it in figures. "At the moment, we are dealing with quantifying the natural structure of small-scale turbulent eddies in German Bight regions that lack wind farms. This forms the basis for better assessing system changes," says Lucas Merckelbach on current state of research.
The original study conducted by the two researchers from the Helmholtz-Zentrum Geesthacht was geared toward feasibility tests. They wanted to examine if water mixing caused by wind farms could have a significant impact on North Sea stratification. They currently use measurements in wind parks and the surroundings as well as computer simulations to better understand and define the mixing processes. They also plan to integrate idealised biological processes so that they can begin to quantify the impact on the North Sea’s ecosystem.
What challenges lie ahead for them? Jeffrey Carpenter sees interdisciplinary cooperation as a significant endeavour. "We think it's important to point out that the impact of offshore wind parks is an extremely complex and varied topic that must be addressed by many scientific disciplines, from physics, chemistry and biology to the social and political sciences," says Carpenter. In regard to the physical and biological consequences, it is still unclear whether this impact is positive or negative – even if the results show that constructing wind parks can have an effect on a large region measuring one hundred square kilometres. In his view, it will "require a lot of work to answer all these questions."
The project is part of the Helmholtz Association’s PACES II Program at the Institute of Coastal Research. This program will presumably run until 2019 or 2020. The first results of the feasibility study were published in 2016. Carpenter and Merckelbach assume they can at least continue until the end of the PACES II Program.
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