Intertidal populations of Ulva spp. and Undaria pinnatifida are good habitat providers for invertebrates but not for fish

Vegetated habitats play a key role in the structure and function of ecosystems by modifying local environmental conditions and providing food and habitat to a great range of organisms. The aim of this study is to assess the relative importance of two widely distributed macroalgae species in providing ecosystem functions in coastal areas where they are prevalent (Patagonia, Argentina, Lat 43.73°, Lon 64.92°) in comparison to low-vegetated areas without these two species. In October 2018 and May 2019, we tested whether Ulva spp. and Undaria pinnatifida perform a series of ecosystem functions commonly associated with vegetated coastal environments. Our study suggests that Ulva spp. decreases water current and provides habitat for epifauna, as the adjacent low-vegetated areas without Ulva spp. present higher gypsum block dissolution rates, lower biomass and richness of total epifauna, and lower grazer biomass than areas dominated by Ulva spp. U. pinnatifida, on the other hand, only showed a positive effect on epifauna biomass relative to low-vegetated areas, and no effect on the rest of the variables measured, despite providing similar levels of coverage to Ulva spp., and having a larger and more complex structure. None of the two macroalgae influence the habitat use by fishes. Despite smaller size and a labile structure, Ulva spp. presence had an effect on more variables than U. pinnatifida, and the function provided by U. pinnatifida was also provided by Ulva spp., namely U. pinnatifida did not provide new functions to the system.