A multifaceted approach towards understanding biotic and abiotic processes structuring sandy beach meiofauna

In order to understand the importance of abiotic and biological processes in structuring the nematode distribution in sandy beaches this PhD was developed. To accomplish this, a dual approach of field observations and ex-situ experiments was used. For this study two sandy beaches located in different climatic areas and under different tidal regimes: temperate and macrotidal vs tropical and microtidal were sampled. A microcosm approach was applied to investigate the role of the dominant polychaete Scolelepis squamata in the colonisation process of defaunated temperate sediments. Nematodes colonised randomly treatments with/without the polychaete, but diversity was higher in S. squamata treatment during the second week of the experiment whereas the initially most successful colonising species decreased in densities. It suggested that the polychaete seems to facilitate the earlier establishment of non-predatory nematode species. Contrary to the previous results, the colonisation in the presence of the polychaete did not show any effect on the nematode community of tropical sandy beach. The divergence of these results may be attributed to the dissimilar nematode composition. A field study concern the vertical distribution of nematodes over a tidal cycle was performed combining the assessment of physical variables and biological interactions. The results indicated that smaller deposit feeder nematodes were dominant at the subsurface whereas large nematodes were concentrated at surface. Moisture explained the community structure, whereas biological interactions drove the vertical segregation. A pulse-chase experiment was also performed, in which benthic and planktonic diatoms were added to the benthic community. As results, all meiofaunal and two macrofaunal species consumed benthic diatoms. Then, this study evidenced the importance of benthic carbon sources in sandy beach food webs. Finally, an across-shore sampling design evaluated the role of microhabitat in determining nematode zonation at De Panne. Samples from sandbars and runnels were taken during low tide. Multivariate analyses showed that a horizontal zonation was evidently found in the sandbar, while it was less prominent in the runnel microhabitat. This different between microhabitats in this temperate sandy beach, compared to the patterns from a subtropical one, indicates that a general pattern is unlikely to exist in macrotidal sandy beaches