Long and short-term impacts of marine sand mining on shallow sandy habitats (German Bight, SE North Sea)

Finn.Mielck [ at ] awi.de


With regard to current and projected sea-level rise, sandy shorelines are under increasingly strong pressure. In order to compensate the loss of material at threatened coastlines, soft coastal protection measures such as beach nourishment are considered to be environmentally-friendly and sustainable way, all over the world. This results in a higher demand of nourishing material, which is frequently extracted from the near-shore seafloor. In order to predict long- and short-term impacts of such mining on the seafloor habitats, as well as the potential for natural regeneration, we investigated the largest extraction area in the German Bight the (Westerland Dredging Area, WDA). Here, sand mining began in 1984. We conducted several measurement campaigns between the years 2008 and 2016 using a set of high-resolution hydroacoustic techniques including multibeam echo-sounder and sidescan sonar. For ground-truthing, sediment samples and underwater videos were taken. The measurements show that up to approximately 20 m deep pits with a diameter of particularly more than 1 km were dredged into the seafloor. The depressions caused by this sand mining are still detectable more than 30 years later. The formerly steep slopes at the fresh dredging pits flattened out due to slope failures and spilt sand after only a few months. Grain-size analyses revealed that mainly fine sand entered the pits in this early phase. However, after approximately one year, muddy sediments, most likely of terrigenous origin, dominate the deposition. Since the sedimentation rates of this muddy material seem to be relatively slow, a complete refill of the post-dredging pits is likely to take many decades. A natural regeneration towards the former seafloor conditions is only visible at the edges of the very oldest dredging pits. Underwater video recordings and samples show that new habitats for e.g. brittle stars were established on the muddy seafloor. These organisms are not normally found in such sand-dominated coastal areas in high abundances. The same applies to stone deposits lying on the sandy seafloor at the edges of the dredging zones. These were separated from the sand during the extraction process. Stones on the seafloor provide protection for many species and might form important hotspots for biodiversity in this area.

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Conference (Poster)
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GEOHAB 2018, 07 May 2018 - 11 May 2018, Santa Barbara, California.
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Mielck, F. , Hass, C. , Michaelis, R. , Sander, L. , Papenmeier, S. , Galvez, D. and Wiltshire, K. H. (2018): Long and short-term impacts of marine sand mining on shallow sandy habitats (German Bight, SE North Sea) , GEOHAB 2018, Santa Barbara, California, 7 May 2018 - 11 May 2018 .

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