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 polychaetes, almost all samples taken were grouped into separate clusters corresponding to each of the four studied Protected Areas. The reliability of structural differences between polychaete complexes was statistically confirmed by pairwise comparison of three areas (Utrish, Karadag, and Martyan). The comparison of sites conditionally named Utrish and Opukskiy demonstrated that structural differences between polychaete complexes were statistically insignificant probably due to the affinity of the geomorphologic pattern of hard-bottoms and composition of fouling vegetation in sublittoral zones of both Protected Areas. The specific polychaete complexes have been formed on rocky/boulder substrates within each of the investigated Protected Areas, being different in their taxonomic structure and quantitative characteristics of the certain species. For each polychaete complex, the most significant species were identified, which determine both the intracomplex taxonomic structure (indicating species) and structural differences between the complexes (discriminating species). In the polychaete complex of the rocky sublittoral in the Karadag State Nature Reserve such indicating species were Nereis zonata, Platynereis dumerilii, and Syllis gracilis. In the Cape Martyan State Nature Reserve and Utrish State Nature Reserve, the indicating taxa were Polyophthalmus pictus, Nereis sp., and Pholoe inornata. Finally, in the Opukskiy State Nature Reserve, Notomastus latericeus and Pholoe inornata were recognised as indicating species. Most of these species may also be considered as discriminating species. Parameters of their quantitative development reflect the structural features of polychaete complexes formed under the influence of specific environmental factors in the sublittoral zone of each of the four studied coastal Protected Areas. The obtained results of the structural-faunistic analysis of polychaete complexes can be applied for comparative assessment of the biodiversity features of benthic communities during the comprehensive ecological monitoring of the coastal Protected Areas.