Macrobenthos richness and biomass preferentially geared towards one half of asymmetrical sand waves

Sand waves are dynamic, sinusoidal bedforms that have been thoroughly studied in the context of the physical and hydrodynamical processes dominating these environments. However, information about the ecological and biogeochemical characteristics within these bedform habitats have been far fewer in comparison. To address this knowledge gap, a field campaign was undertaken in the summer of 2017 to investigate the biogeomorphology of asymmetrical sand waves in the Dutch North Sea, near island Texel. The goal was specifically to address both the macrofaunal community composition and the associated biogeochemistry along the different sections of these sand waves.  Using a combination of several field sampling techniques and lab incubations on board the NIOZ RV-Pelagia, we collected a comprehensive dataset covering the macrofauna assemblage, nutrient flux, oxygen consumption, sediment grain size and permeability, as well as physical and environmental data, within a transect line (< 1 km) that covered several sand waves. Here, we show considerable variability in the species abundance, composition and biomass, which were all significantly higher on the steeper sides of the sand waves; the multivariate statistical analyses on the datasets showed a significant influence of the sand wave position on benthic composition. Correspondingly, measurements from the steep slopes also exhibited a higher concentration of chl-a and organic matter, higher O₂ consumption, more fine particles and lower sediment permeability. Despite the overall homogeneity (e.g., sandy sediment) of a well-developed bedform environment such as a sand wave field, it is clearly possible to find significant variations in the benthic community composition and biogeochemical activity on a small spatial scale.  Oftentimes, studies look at larger spatial scales to maximize the characterization of an entire region. However, given the diverse environmental gradients within the North Sea, our observations may not be sufficiently captured or even missed altogether when superimposed upon such large spatial scales.  Thus, a close examination of the interrelated parameters such as biology, biogeochemistry, sedimentology and morphology should also be considered, at a high resolution, over a small local scale for such seemingly uniform habitats.  We hope our results will contribute valuable insight into small-scale patterns of variability in dynamic bedform environments.