Gas extraction under intertidal mudflats is associated with declines in sediment grain size and minor changes in macrozoobenthic community composition

Abstract

O_LIIn intertidal environments, land subsidence may change the local flooding regime and sediment composition, two main drivers of the macrozoobenthic community structure. In the Dutch Wadden Sea, a UNESCO world heritage site, gas extraction has resulted in an average subsidence of up to 2 mm y-1 of intertidal mudflats. These mudflats support a highly productive macrozoobenthic community, which offers important resources for birds and fishes. To what extent land subsidence due to gas extraction affects sediment and macrozoobenthos remains unknown and increasingly important given sea level rise. C_LIO_LITaking advantage of a monitoring program, we evaluated the effect of anthropogenically caused land subsidence on sediment composition and intertidal macrozoobenthos. Nearly 4600 points were sampled yearly (2008-2020) across the Dutch Wadden Sea, allowing us to compare sediment composition and macrozoobenthos biomass within and outside the subsidence area while controlling for the main drivers of these variables. We also compared population trends within and outside the subsidence area for 31 species with different habitat use in terms of depth and sediment composition. C_LIO_LISediment median grain size decreased in the subsided area at 1 {micro}m y-1 while on average remaining constant in other intertidal mudflats. Mud fraction was 3% higher within the subsided area throughout the studied period. This had no effect on the total biomass of macrozoobenthos. The biomass of species that use deeper areas increased within the subsidence area compared to outside, and the opposite was true for species using shallower places, but comparable patterns were also found in an area not affected by subsidence. C_LIO_LIChanges in median grain size could be happening, and minor changes in macrozoobenthic community composition. For a successful implementation of the "hand on the tap" principle in the Wadden Sea, it is necessary to define beforehand the relevant variables that represent the natural values, implement proper monitoring, and define threshold values above which effects are not acceptable. We propose median grain size, mud fraction and macrozoobenthic biomass as good measures of the natural values of the Wadden Sea, and the methods used here as a way for identifying anthropogenic effects on them. C_LI