Combined effects of wrack identity and solar radiation on associated beach macrofaunal assemblages

The ecological role of wrack as a habitat-forming species for beach macrofauna was investigated by experimental manipulation of 2 macroalgal species: Laminaria ochroleuca Bachelot de la Pylaie, 1824, and Sargassum muticum (Yendo) Fensholt. Both species are abundant in rocky bed communities, and commonly found shaping wrack patches on Atlantic sandy beaches in NW Spain. In beach ecosystems, this resource subsidy provides macroinvertebrates with accessible food, refuge and nursery in an otherwise inhospitable environment. Our analysis indicated a substantial shift in the dominant macrofauna, from wrack-dependent consumers in L. ochroleuca patches to terrestrial larval species in S. muticum patches. In addition, we examined in situ for the first time the direct impacts of changes in ambient solar radiation on the macroalgal biochemical content, and the consequent effects on wrack-associated macrofauna. During a 5 d experiment, cut-off filters were applied to generate treatments: P: photosynthetically active radiation (PAR) (400-700 nm), PA: PAR + UVA (320-700 nm), PAB (procedural control): PAR + UVA + UVB (280-700 nm), and C: control with no filter. Lack of natural radiation affected the biochemical content of the wrack and promoted differences in the associated macrofaunal composition; taxa assemblages among wrack patches exposed to C and PAB treatments were more similar compared to P or PA exposed patches. Our results indicated that changes in wrack nutritional quality and phenols caused by a moderate manipulation of the UVR had consequent effects on beach macrofauna looking for shelter, food or both. We suggest that in a scenario of global impacts, changes in the identity and fundamental biochemical traits of wrack can affect beach macrofaunal assemblages with unpredictable implications for coastal food-webs and ecosystem functioning.