Biodiversity and abundance patterns of rock encrusting fauna in a temperate fjord

• Encrusting fauna on rocks and the factors driving changes along the length of the fjord and changing depths was established.
• No trend in species composition, number, diversity and number of individuals along the length of the fjord was observed.
• Three separate assemblages: intertidal, shallow and deep subtidal were revealed with depth change.
• Environmental parameters related to depth are the driving forces in shaping encrusting assemblages in Trondheimsfjord.

Fjords are semi-enclosed systems often with usually strong physical and chemical gradients. These gradients provide the opportunity to test the influence of various physical and chemical factors on biodiversity. However study area of this investigation, Trondheimsfjord, is a large water body where especially salinity gradient along the fjord is not well pronounced. The goal of this study was to establish within a temperate fjord a baseline identifying encrusting fauna on rocks and determine the factors driving changes along the length of the fjord and changing depths. There was no trend in species composition change and increase or decrease in number of species, diversity and number of individuals along the fjord. This was likely due to the relative homogeneity of both substrate (rocks) and environmental parameters. Nevertheless, the influence of fresh water inflow in the vicinity of the river mouth was apparent by the presence of characteristic brackish-water species at these locations. Multidimensional scaling analysis revealed three separate assemblages: intertidal, shallow and deep subtidal (below 50 m). Intertidal assemblages were species poor (one to 11 species) but relatively abundant (six to 2374 indiv./m2 of rocks). Number of individuals and biomass was highest in the shallow subtidal (2059–13,587 indiv./m2 of rocks). Overall the highest species number (45) was recorded at 50 m depth which is probably result of low competition pressure yet still relatively high nutrient concentration in comparison to shallower locations. Environmental parameters (i.e., tidal currents, wave action, salinity) change more drastically with depth than along the fjord and these changes are the major driving forces in shaping encrusting assemblages in Trondheimsfjord.