Structural features of the polychaete community at rocky-boulder substrates in nearshore waters in Protected Areas of Crimea and Caucasus

Studies of the benthos in nearshore water areas within Protected Areas along the Black Sea are an important ecological task. In this connection, protected habitats can be recognised as intact natural areas, where the entire natural complex is preserved with its typical taxonomic structure and biodiversity features. This study is based on the obtained quantitative data on the biodiversity of benthic assemblages in coastal Protected Areas. It aims to study the structure of polychaete communities as inhabitants of fouling areas at sublittoral (depth 0–3 m) rocky/boulder substrates in four Protected Areas along the Black Sea (Cape Martyan State Nature Reserve, Karadag State Nature Reserve, Opukskiy State Nature Reserve, and Utrish State Nature Reserve). The studied sites differ in their geographic location, orographic shape of the coastal strip, features of underwater substrates, the pattern of spatial coverage and species composition of macroalgae foulings and other environmental parameters. The purpose of this study was also to make a prognostic estimation of the expected species richness (Sexp) and comparative analysis of the structural organisation of polychaete complexes on rocky substrates in the selected coastal Protected Areas. The prognostic modelling of polychaete species richness was based on the known extrapolation algorithms (estimators Chao-2, Jackknife-2, and bootstrap). The results show that the expected level of species richness (Sexp) exceeds the actually detected (Sobs) number of species in each of the four Protected Areas by 20–95%, depending on the estimator and total number of collected samples. In all sublittoral water areas, it was found that with an increase in the number of samples the accumulation curves do not reach a horizontal asymptote. This indicates an underestimated species richness of polychaetes. At the rocky sublittoral zone of the Cape Martyan State Nature Reserve, the fastest increase in Sexp values was revealed with an increase in the number of samples taken. This relationship can be determined by heterogeneity of microhabitats in this area, where the increased number of polychaete species is also formed. Considering the other Protected Areas, the extrapolative curves of species richness have a flatter mode and a smaller expected number of species (Sexp), despite the larger number of samples analysed. According to the results of the clustering and ordination analyses, based on the species composition and abundance of pol