The ecological impact of offshore wind farms and sediment dynamics

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The number of offshore wind farms in the North Sea is exponentially increasing. Their construction responds to the urgent need to transition to green energy production and carbon neutrality. The installation of these man-made structures has a notable impact on the marine ecosystem. Some positive impacts include providing a new home for various animals, and offering shelter, food, and breeding grounds. However, there have been observations of black sediment patches in the sampled sediments near the turbine foundations, suggesting a high input of organic matter into the sediment, altering natural biogeochemical processes.


“Understanding how offshore wind farms affect the ocean is crucial. We depend on the balance of marine ecosystems. Any change can lead to unforeseen consequences. New tools like isotopic fingerprinting and amino acids open paths to finding out the ecological implications of man-made structures like the turbines from an offshore wind farm.” – Esther Cepeda Gamella, PhD Student, Institute of Natural Sciences and Ghent University.

To understand what is happening and where this is leading, our team from the Institute of Natural Sciences and University of Ghent is investigating the source of these black sediments through the OUTFLOW project. Previous research suggested that increased excretions, known as fecal pellets, falling into the natural sandy sediments could be the source. The turbines provide new artificial hard surfaces for organisms to attach to, which then feed on particles from the water column. Several organisms, including the blue mussel (Mytilus edulis), a tiny shrimp (Jassa herdmani), and a type of plumose anemone (Metridium senile), produce excretions that fall into the sediment. The substantial number of individuals from these species, thousands per square meter, results in a significant number of excretions, introducing additional carbon that alters biogeochemical processes and enhances anoxic conditions, turning the sediment black.

Part of our research is to develop an isotopic fingerprint to determine if these excretions are the source of the black sediments and to differentiate the excretions from other sources of organic material. Similar to a thumbprint, the unique characteristics, amino acids in this case, present in the excretions build an isotopic fingerprint. Some are exclusively produced by plants or algae but pass to other organisms in the food chain when they are consumed. Each step that takes them further from their origin leaves a mark on some of the amino acids, while others remain the same. These unique differences, marked in the amino acid isotopic signature, are the key to building our fingerprints. By understanding the fingerprints of the different animals in the ecosystem, we can determine the impact of offshore wind farms on the environment.

This fingerprinting technique helps us identify and quantify the contributors to the organic matter pool through the levels of carbon. However, it is still unknown where the carbon added via the faecal pellets ends up. Is it stored and can it be counted as blue carbon, or is it released back into the system? Our team has taken this extra step to fully understand the distribution of the added fecal pellets in terms of carbon addition to the system.

On October 2023, we conducted a pulse-chase experiment on board the RV Belgica. During a ten-day expedition, we were able to collect sediment cores impacted by the offshore wind farm and from an unaffected reference area for comparison. To the collected cores, we added previously prepared and dried 13C-labelled fecal pellets from blue mussels. Our experiment was planned in two parts. Firstly, we performed closed-core incubations after one, three, and five days of fecal pellet addition. The oxygen levels during these incubations were constantly monitored to prevent them from reaching anoxic conditions. The analysis of the oxygen data collected provides an indication of the behavioral activity of the sediment communities in response to the additional carbon present in the fecal pellets. From there, we can compare what is happening in areas with already high levels of carbon, such as offshore wind farm systems, to the reference areas. For the second part, we sampled all available carbon pools: from dissolved inorganic and organic carbon to tiny invertebrate communities and bacteria. We conducted the sampling before, to collect background values, and after adding the labelled fecal pellets. To understand where the carbon goes, we can trace the carbon added to the system by measuring the labelled 13C with respect to the background values. As this experiment constitutes one of the first attempts to fully sample all carbon pools, integrating both parts of the experiment will lead to the creation of a carbon budget for the offshore wind farm system.

We are currently working on preparing publications to share the results and conclusions from both the fingerprinting amino acid technique and the closed-core experiment. We look forward to sharing our findings with both the scientific and non-scientific community.

The OUTFLOW project (Quantifying the cOntribUTion of Fouling fauna to the Local carbon budget of an Offshore Wind farm) is supported by BELSPO and INSITE North Sea.