Comparing grab and dredge sampling for shoreface benthos using ten years of monitoring data from the Sand Motor mega nourishment

Epifauna and infauna are often sampled by different types of sampling devices including grabs, and dredges, depending on various factors such as local environmental conditions, research questions and available budget. Because grabs and dredges have a different species-specific selectivity, both methods could be complementary but for some research questions they could give comparable results. In this study, a comparison is made between macrobenthos (epifauna and infauna) sampled simultaneously by a Van Veen grab and a quantitative benthic dredge, based on a large dataset that was collected from 2010 to 2019 as part of an extensive monitoring program to study the impact of the Sand Motor, a mega beach nourishment that was implemented in 2011 on the sandy North Sea coast in the Netherlands. Because of the larger mesh size of the sieve in the benthic dredge (5 mm) compared to the Van Veen (1 mm), only larger mollusks, echinoderms and crustaceans are identified in the dredge samples. This resulted in a lower number of taxa and average densities in the 676 benthic dredge samples (42 taxa and 98.7 ind. m⁻², respectively) compared to the 636 Van Veen samples (228 taxa and 1380.9 ind. m⁻², respectively). Due to the larger sampling area of the benthic dredge (10–15 m²) compared to the Van Veen (0.1 m²) the chance to encounter a species is higher in a benthic dredge than in a Van Veen grab sample. Although the benthic dredge only samples a subset of the benthic community that is sampled by the Van Veen grab, multivariate (nMDS) analysis of the data showed remarkable similarities between Van Veen and benthic dredge samples with major gradients correlated to water depth and temporal changes in environmental conditions due to the presence of the Sand Motor. This suggests that both techniques are indicative for the spatial variation and development of the macrobenthic community.