Jon Dickson1, 2,
1Royal Netherlands Institute for Sea Research, Netherlands, 2University of Groningen, Netherlands
Hard substrates play an important role in global marine systems as settlement surface for sessile reef-forming species such as corals, seaweeds, and shellfish. In soft-sediment systems, natural hard substrates such as stones, bedrock and driftwood are essential as they support diverse assemblages of reef-associated species. However, availability of these hard substrates has been declining in many estuaries and shallow seas worldwide due to human impacts. This is also the case in the Dutch Wadden Sea, where natural hard substrates have gradually disappeared due to burial by sand and/or active removal by humans. In addition, driftwood that was historically imported from rivers has been nullified by upstream logging and coastal damming of estuaries. To investigate the historic ecological role of wood presence in the Wadden Sea as settlement substrate and fish habitat, we constructed three meter high artificial reefs made of felled pear trees. Results demonstrate that these reefs rapidly developed into hotspots of biodiversity. Within six months, the tree-reefs were colonized by sessile hard substrate associated species, providing both shelter and food for fish. Six fish species were observed on the reefs, while only two species were caught on sandy control sites; moreover, the abundance of fish on the reefs was five times higher. Within 16 months, cuttlefish eggs and juvenile Pholis gunnelus were noted on the reefs, showing that the tree reefs are not merely fish attraction devices, but also providing spawning and nursery habitat. Individuals of the most commonly caught species, the five-bearded rockling Ciliata mustela, were larger on the reef; these patterns also hold true for common prawn, Palaemon serratus, which were also larger and ten times more numerous on the reefs. The rapid colonization of tree reefs by both sessile and mobile species shows that rapid ecological restoration may be possible in soft-bottomed systems.
Biography
Prior to arriving in the marine restoration sector where he currently works at finishing his PhD, Dickson worked as a forest fire fighter in Canada as well as a polar guide in Antarctica and the Arctic. When he was not fighting fires or driving boats between icebergs, he enjoyed backpacking along remote Canadian and Icelandic coastlines where he marvelled at the prevalence of driftwood in all shapes and sizes. This lead him to invent a new type of reef using historically present wood.