Undercover interactions – how beneficial are seaweed canopies at high latitude?Reef-associated Biodiversity

Markus Molis¹, Julius Ellrich², Marta Prieto¹

¹The Arctic University of Norway, Norway ²Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research, Germany

Seaweed canopies are considered beneficial for understory organisms by ameliorating environmental stress. Their role has, however, not been tested in field experiments at high latitudes. On mid-intertidal rocky shores in Northern Norway, we observed lower barnacle (Semibalanus balanoides) but higher limpet (Patella vulgata) abundances under than next to individual seaweed (Ascophyllum nodosum) canopies. The objective of this study was to estimate the separate and combined effects of canopies (whiplash and shading) and limpets (bulldozing and accidental consumption) on barnacle settlement (cyprid density) and recruitment (recruit density) success. We hypothesized synergistic detrimental effects of canopies and limpets on barnacle abundance due to stress amelioration by shading but no whiplash effect of canopy on limpets. In March 2022, a 7-month field experiment (randomized blocks) started prior to peak barnacle settlement, in which we removed (i) neither limpets nor canopy (= control), either (ii) limpets or (iii) canopy, and (iv) both (n = 12). In experimental plots, number of cyprids and recruits was counted and limpet foraging activity quantified as area of radular scratch marks on monthly deployed wax plates. We found lowest cyprid and recruit densities in control plots and highest in plots where either canopy or both, canopy and limpets were removed. Limpet and canopy presence reduced cyprid density, on average, 21 and 81%, respectively, as well as recruit density on average, 1.5- and 6-fold, respectively. Limpet foraging activity was, on average, 2.2-times higher where canopies were removed. Contrarily to our hypothesis, canopy and limpet removal had additive effects on cyprid and recruit density. Canopy negatively affected cyprid and recruit density and limpet foraging activity, suggesting canopies may function for understory organisms as disturbance (whiplash) rather than benefactor on rocky shores at high latitudes, where thermal stress is less prevalent.