Long-Term Dynamics of Subarctic Intertidal Macrofauna: Common Trends and the Role of Local Environment

The role of local vs regional drivers of long-term community dynamics is still poorly explored. Intertidal macrofauna was studied in two small bights of the White Sea, the Medvezhya (MB, steeply sloping sandy beach, 66° 21′ N, 33° 36′ E) and Seldyanaya (SB, sheltered silty flat, 66° 20′ N, 33° 37′ E). Macrofauna was collected annually in 1987–2017 four times a year (in winter, spring, summer, and autumn) at four stations arranged across the beaches. Both communities were similar in species composition, spatial distribution, and functional structure but substantially differed in their dynamics. Epifauna was more abundant in MB and infauna in SB. The total biomass in MB showed a long-term upward trend with 4- to 5-year quasi-periodic oscillations without clear seasonal variations. In SB, biomass showed no long-term trend but seasonal and interannual variations, dropping sharply after severe ice-scouring events and only partially recovering afterwards. Species richness was relatively constant in MB but decreased in SB, dropping noticeably after scouring impact. Species structure in MB showed two relatively stable states, with a transition period in 1993–1998, when the structure gradually changed. In SB, no clear long-term trends in structure were observed, but rapid and abrupt fluctuations occurred in years of severe ice scour, particularly in winter/spring. The community dynamics is suggested to be largely determined by local topography and disturbance regime, resulting in relative stability of the MB assemblage but a gradual degradation of that in SB, without any correlation with climatic variables (sea surface temperature and Arctic Oscillation Index).