Diversity and structure of epibenthic communities of the red algae zone in the White Sea

At high latitudes, the marine transitional zone between the highly productive kelp communities and the devoid of vegetation deep-waters is occupied mainly by number of red algae species, both encrusting and cespitose, which associate with rich and diverse fauna. However, zoologists and botanists never scrutinized these communities in equal manner up to present. We investigated the transition red algae zone in the White Sea in summer 2016 and 2017 using scuba equipment. The photic depth is limited to 18–23 m at different locations. Sessile species prevail among 315 epibenthic organisms in species number ( ~ 80%) and biomass ( ~ 90%). Communities of the red algae zone are rich in species (from 54 to 177). The most distinct differences were observed between the communities of Kandalaksha and Onega Bays. Vertical heterogeneity of the red algae zone was revealed; at most locations, Rhodophyta predominate in the upper subzone and Cirripedia predominate in the lower parts of the red algae zone. At the investigated depths, shallow-water and deep-water sets of species were defined. Interspecific spatial interactions between sessile organisms are revealed: 72.2 ± 3.5% of them were recorded as epibionts, and 29.1 ± 2.5% were registered as basibionts. These interactions provide evidence of the increase of species diversity: 32.7 ± 6.8% of sessile taxa were recorded exclusively as epibionts. Algae Phycodrys rubens, Odonthalia dentata, and Coccotylus truncatus, ascidian Styela rustica, barnacles Balanus crenatus, Verruca stroemia, and polychaete Pista maculata are foundation species, each supporting multi-level suites (up to four levels) of dependent species.