Aim: To understand spatial-temporal changes (beta-diversity) in coastal communities and their drivers in the context of climate change. Coastal ecosystems are extremely exposed and dynamic, where changes in seaweed assemblages have been associated with changing water temperatures. However, at the local scale, the effects of changes in the upwelling events and related stressors seek further exploration.

Location: Galicia rías, North West of the Iberian Peninsula

Methods: Using data collected in 42 sampling localities in Galicia rias and over two time periods (1998 and 2014), we analyzed changes in the seaweed community’s composition through time and space. We calculated the temporal beta diversity index and spatial beta diversity as the pairwise composition differences between sampling localities. We use generalized dissimilarity models, to identify local environmental drivers of spatial and temporal beta-diversity.

Results: We found a significant change in seaweed communities of Galicia rias, between 1998 and 2014 (temporal beta-diversity). They were mostly related to species loss rather than to species replacement. The dissimilarity among localities (spatial beta diversity) was significantly higher in 2014 than 1998. Nitrate concentration was consistently predicted as the main driver of both temporal and spatial beta-diversity patterns.

Main conclusions: Unlike other studies in marine ecosystems, our results suggest that observed changes in the structure of perennial seaweed assemblages in Galicia Rias might lead to a local biotic heterogenization, indirectly linked to climate change through changes in nutrients availability and the upwelling intensity. Changes in Galicia seaweed communities call scientific attention to the importance of local stressors in climate change studies.

Presence/absence (0/1) for 39 perennial seaweeds at 42 rocky intertidal localities along NW Spain (coded S1-S42). The same set of sites was visited in 1998-9 and resurveyed in 2014 with exactly the same transect sampling methodology. Seaweeds named with a six-letter code with the first three letter of the seaweed’s genus named and the first three letters of the seaweeds species name.