Abstract
Subsea power cables transport offshore generated energy to shore. Generally buried, they protrude from the seabed when crossing an existing pipeline or cable. Rock is placed on top of the cable to protect it, forming a cable crossing of ~0.2 Ha. This hard substrate in a predominantly sandy environment allows reef formation adding to the biodiversity of an area. TenneT will install another 90 cable crossings before 2032. Optimising the technical design to facilitate reef development could stimulate species biodiversity. In this study, annual field surveys over three years post-installation were conducted to assess the influence of rock size (D90 80 mm 'sprinkle layer' and D90 400 mm 'armour layer') and material (granite and marble) on species biodiversity. These surveys employed dropcam and eDNA methods to monitor differences in macrobenthic and fish abundance and diversity. Our findings indicate that the addition of calcareous marble did not significantly increase the species biodiversity in terms of fish or macrobenthos. Species diversity increased over the three survey years. Lastly, only one invasive species was found on both marble and granite rock. Our findings show that cable crossings can contribute to the species biodiversity, where the addition of marble rock does not result in a higher species count. Larger, more stable rocks should be used as the outer layer of a cable crossing in order to promote reef development. The observed species composition did not suggest that an ecological climax stage was reached at year three. Additional surveys in years 5 and 10 are recommended to determine if these findings remain consistent over longer periods.