Twenty years of observed and predicted changes in subtidal red seaweed assemblages along a biogeographical transition zone: inferring potential causes from environmental data

Abstract

International audienceAim:To assess environmental changes within a marine biogeographical transition zone and how they have affected seaweed assemblages and distributions over the past two decades.Location:Brittany (western France, Europe) – a biogeographical transition zone between cold-temperate and warm-temperate regions.Methods:We assessed spatio-temporal variation for three environmental parameters [sea-surface temperature (SST), suspended inorganic matter and chlorophyll a] between 1992 and 2012 in five adjoining regions using generalized linear models. To investigate changes in assemblages and distributional patterns of red seaweeds based on sampling surveys conducted during two separate periods (1992–1998 and 2010–2012), we used two complementary approaches, multivariate data analysis and species distribution models (SDMs) with a set of modelling procedures.Results:Coastal water temperature in Brittany has increased by 0.7 °C on average over the past two decades (0.35 °C per decade). At a finer scale, changes in SST showed that Brittany constitutes a mosaic of contrasting conditions, with the western and north-western regions being colder and less affected by climate change than the other three regions. Our results suggest that increasing SST caused significant changes in subtidal red seaweed assemblages over the 20-year period. Between the two periods, SDMs predicted significant species shifts for seven out of ten representative species, and reductions in the distribution ranges of most species.Main conclusions:Our study confirmed important differences across the different regions of the studied biogeographical transition zone. Changes in abiotic parameters and red seaweed assemblages are expected to occur at varying extremes across these regions, with western and north-western Brittany representing the most stable zones that might constitute a potential refuge for certain species when responding to global changes

Status, trends and drivers of kelp forests in Europe: an expert assessment

“…Brittany constitutes a mosaic of contrasting conditions, with the western and north-western regions being colder and less affected by climate change than the other three regions (DerrienCourtel et al 2013;Gallon et al 2014). The highest abundance of L. digitata and L. hyperborea is found in these two colder regions and correlates with higher genetic diversity, although a trend of decrease in these specieś abundance is revealed for some parts of the central European coasts for small isolated marginal populations (Billot et al 2003;Valero et al 2011;Couceiro et al 2013;Robuchon et al 2014).…”

Climatic and environmental factors influencing occurrence and distribution of macroalgae — a fjord gradient revisited

“…Northwards migration of warm temperate macroalgae, or extinction of cold temperate and Arctic species in the southern part of their distribution ranges, will probably require increased sea water temperatures over a longer period of time. In Brittany, Gallon et al (2014) observed a complex pattern of red algae responses to a warming over 2 decades, with declines in the occurrence rates of many warm-water red algae and increases in cold-water species. On the other hand, the results of the present study show that a local and episodic temperature increase of 3−5 °C will have a clear, positive effect especially on many red algal species present in the cold temperate Atlantic zone.…”

Cross-taxon congruence in the rarity of subtidal rocky marine assemblages: No taxonomic shortcut for conservation monitoring

“…This peculiar pattern may be explained by the seemingly more 399 stable temperatures in north-western Brittany than elsewhere (Gallon et al, 2014). North-400 western Brittany is also characterised by a widespread and continuous rocky bottom, separated 401 from the more fragmented rocky bottom of the other regions of Brittany by large sandy beaches 402 (Cabioch, 1968;Méléder et al, 2010;Raffin, 2003;Retière, 1979).…”

“…Nevertheless, since (i) these changes have been rather 491 slowly occurring, (ii) western and north-western Brittany have been identified as the most 492 stable zones for other phyla (red seaweeds) (Gallon et al, 2014) and (iii) our metric is averaged 493 at the community level, we assume that these changes would not have major impacts on our 494 findings. We nonetheless recognize that, given the accelerating environmental changes, the 495 temporal aspect is a major challenge for such understudied and costly-to-sample habitats, and 496 this challenge should be rapidly tackled in future studies.…”

“…Yet, changes have been shown to occur in subtidal algal communities over the temporal 487 span of our database samplings (Gallon et al, 2014). Among changes, it can be expected that 488 some rare species may have become more common (such as warm-water species at their 489 northern range limits), and common species may have become rare (such as cold-water species 490…”