Meiofaunal diversity of Belgian sandy beaches: historical baselines and a new generation of ecosystem monitoring

Sandy beaches are highly dynamic ecosystems that face increasing pressures from climate change and human activities, yet much of their biodiversity remains uncharacterised and absent from routine monitoring. This is particularly true for meiofauna: the heterogenous assemblage of small organisms inhabiting the small spaces between sand grains. Soft-bodied taxa (e.g. platyhelminths, acoels) are especially underrepresented, despite being highly speciose and contributing substantially to biomass. Hence, current biodiversity assessments provide an incomplete picture of sandy beach ecosystems, limiting our ability to assess their health, and the threats they face.

We present a unique, historical dataset on the meiofauna of six sandy beaches along the Belgian coast, sampled bimonthly in 1985–1986. These beaches represent contrasting environmental conditions, particularly in terms of wave exposure, an important factor shaping meiofaunal communities. At each locality and sampling moment, two sediment cores (3.6 cm diameter) were collected at mid-tide to 40 cm depth: one for meiofaunal and one for granulometric analyses. Diversity, densities, and biomasses of all major meiofaunal taxa were assessed, including soft-bodied taxa. Meiofaunal densities varied strongly, from approximately 180 to more than 4,500 individuals per 10 cm². Nematodes dominated numerical abundance, particularly at sheltered fine-sand localities, where they comprised 80–90% of total meiofaunal abundance. In contrast, turbellarian flatworms became proportionally more important at more exposed beaches, contributing up to 24% of total abundance and 43% of total meiofaunal biomass.

Building on this baseline, a newly funded research programme will establish a contemporary, coast-wide framework for beach health monitoring leveraging these meiofaunal communities. Historical beaches will be revisited within a broader sampling design, now spanning both environmental and anthropogenic pressure gradients (e.g., tourism, beach nourishments). Alpha taxonomy and barcoding will be combined to develop a curated reference database for Belgian interstitial meiofauna. Metabarcoding will subsequently characterise communities across differing conditions, and resulting data will be used to explore biodiversity patterns and identify their underlying ecological drivers.

By placing contemporary biodiversity assessments in the context of this historical baseline, we will provide a unique temporal perspective on changes in meiofaunal diversity and community structure along the Belgian coast. Together, the historical baseline and new molecular framework will provide a foundation for tracking this overlooked component of marine biodiversity through time, and enable its scalable integration into future sandy-beach ecosystem monitoring.