Assessing the Effects of Macroalgae Beds on Coastal Sedimentary Nematodes in the South Caspian Sea

In this study, the null hypothesis is that sediment and overlying water parameters, biological and functional diversities of benthic nematodes, do not differ between macroalgal beds and adjacent unvegetated sediments. Samples were collected from unvegetated sediments and adjacent sediments beneath the macroalgae beds at three stations along the south coast of the Caspian Sea. Two species of macroalgae, Cladophora glomerata and Ulva intestinalis, were identified. Sediments under the macroalgal bed had a higher content of microplastics and total organic matter as well as a higher dissolved oxygen compared to unvegetated sediments. A total of 16 nematode species belonging to 14 genera were identified, with Daptonema, represented by three species, being the numerically dominant genus. Both total and vertical nematode densities, as well as diversity indices, were higher in vegetated sediments at each station. Analysis of community structure revealed notable differences between the two habitats, due mostly to the presence or absence of certain nematode species. These variations in nematode community characteristics may be linked to the higher dissolved oxygen levels in the overlying water and the increased total organic matter and food in vegetated areas. Macroalgae in coastal areas can serve as biotic retention zones for microplastics, enhance nematode densities and diversity, and alter functional diversity and community structure. The retreat of the Caspian Sea will lead to the loss of this habitat, which may result in declines of several meio- and macrofauna taxa closely associated with macroalgal beds.