Shallow subtidal macroalgae in the North-eastern Atlantic archipelagos (Macaronesian region): a spatial approach to community structure

Abstract

Shallow subtidal macroalgal communities in the North-eastern Atlantic archipelagos (Azores, Madeira, Canaries and Cape Verde) were studied in order to identify their spatial organization patterns and the main drivers of change. Fifteen islands and 145 sites across 15º of latitude and 2850 km were sampled. We found high spatial variability across the scales considered (archipelago, island and site). The structure of macroalgal communities differed among archipelagos, except between Madeira and the Canaries, which were similar. Across a latitudinal gradient, macroalgal communities in the Azores were clearly separated from the other archipelagos; communities in Madeira and the Canaries occupied an intermediate position, while those in Cape Verde appeared at the opposite end of the gradient. In the Azores, species with warm-temperate affinities dominated communities. Cape Verde communities were, in contrast, dominated by tropical taxa, whereas in the subtropical Canaries and Madeira there was a mixture of species with colder and warmer affinities. Apart from crustose coralline algae, the Dictyotales were the group with greatest cover; larger and longer-lived species were progressively replaced by short-lived species along a latitudinal gradient from north to south. The perennial species Zonaria tournefortii dominated the sea-bottom in the Azores, the semiperennial Lophophora variegata in the Canaries, the filamentous algae in Madeira and the ephemeral Dictyota dichotoma in Cape Verde. We hypothesized that the differences among archipelagos could be explained by synergies between temperature and herbivory, which increased in diversity southwards, especially in Cape Verde. This was supported by the predominance of noncrustose macroalgae in the Azores and of crustose macroalgae in Cape Verde, as would be predicted from the greater herbivore activity. At the scale of islands and sites, the same set of environmental variables drove differences in macroalgal community structure across all the Macaronesian archipelagos.

Lobophora (Dictyotales) Species Richness, Ecology and Biogeography Across the North‐Eastern Atlantic Archipelagos and Description of Two New Species1

“…Lobophora delicata is a photophilic species that grows from the intertidal to shallow subtidal, although in the Azores it has a scarce cover and other dictyotaleans such as Zonaria tournefortii outnumber it in abundance (Sangil et al. ). In the central archipelagos of Madeira and Canaries, Lobophora species are the potential dominant species in rocky bottom communities.…”

“…Although in this archipelago D. africanum does not present high‐density populations, there are great populations of herbivore fishes, such as surgeons, scarids and damselfishes, which control the development of any noncrustose macroalgae including the species of Lobophora (Sangil et al. ).…”

“…Seaweed assemblages vary drastically along this latitudinal gradient, with communities dominated by cold‐temperate taxa in the northernmost archipelago of the Azores, a mixture of species with colder and warmer affinities in Madeira and the Canaries, and a dominance of tropical taxa in the southernmost Cape Verde Islands (Sangil et al. ). The Dictyotales Bory de Saint‐Vincent (Phaeophyceae), apart from the crustose coralline algae, are the dominant macroalgae in Macaronesia.…”

“…Lobophora is one of the most ecologically important genera, though important differences in abundance exist between and within archipelagos (Sangil et al. ). Lobophora populations colonize rocky substrates from the intertidal to deep subtidal, at least to 80 m depth (C. Sangil, pers.…”

Novel approach to enhance coastal habitat and biotope mapping with drone aerial imagery analysis

“…To assess the composition and distribution of conspicuous sessile organism assemblages within the study area, scientific divers conducted underwater surveys over rocky bottoms. Considering the depth-related zonation present in rocky reef 58 , 59 and the limitations in assessing depth and bottom nature from aerial imagery 8 , 29 , 31 , 60 , underwater photoquadrat surveys targeting sessile organisms were conducted in two shallow depth ranges: 4–6 m and 9–12 m. During each survey a buoyant GPS logger was deployed and anchored to the bottom for georeferencing the sampling location where six haphazardly placed photoquadrats were collected (in an estimated 5-m radius from the anchoring point). Photographs were taken parallel to the quadrat frame with an Olympus OMD10 equipped with a Panasonic Lumix G 8 mm f/3.5 lens inside a Nauticam NA-ED10 housing and with S&S YS-01 strobes.…”