Sedimentary and faunistic effects of medium-deep sand mining along the Dutch Coast

Abstract

This paper presents the results of a study into the ecological long-term effects of medium-deep sand extraction along the Dutch coast. A comparison was made of the benthic fauna in and outside medium deep extraction pits of various ages, ranging between 2 and 12 years. The pits and reference areas were also compared in terms of water depth, median grain size, silt content and percentage of organic matter.

The water depth of all sand extraction pits was on average 4 m larger than the surrounding reference areas. The average silt content in the extraction pits had increased by a factor 4 compared to the surrounding reference areas. The average silt content in the pits was 9.5 %. This increase differed between pits (0 and 46 %). The percentage of organic matter had almost doubled from 0.87 % in the reference areas to 1.50 % in the pits.

The medium-deep sand extraction pits are faunistically characterized by a higher average macro-benthic biomass and density but the number of species shows no clear difference with the reference areas. Faunistic heterogeneity in the pits is lower than in the surrounding reference areas. The macro-fauna in the pits is characterized by the greater abundance of deposit feeders and interface feeders while the benthic fauna in the reference areas is characterized by bivalves i.e. filter feeders. Small demersal fish did not show clear trends although some species, such as sandeel, showed a difference in abundance between pit and reference area. The effect of age of the mining pit on faunistic composition could not be separated from differences related to geographical position.

The results of this study show that recolonization of a newly mined medium deep sand extraction pit is fast but that the complete recovery and return of the original benthic fauna in these pits can be a very long process. The sedimentary differences between the pits suggests that recovery depends on the abiotic environment. The data suggest that by moving from shallow mining pits to medium deep pits a critical depth is passed beyond which hydrographical changes alter the settlement of fines and determine the composition of the macrofaunal community.