Seasonality in the canopy structure of the endangered brown macroalga Cystoseira abies-marina at Gran Canaria Island (Canary Islands, eastern Atlantic)

Abstract

Cystoseira abies-marina is a canopy-forming brown seaweed distributed along the western Mediterranean and the adjacent Atlantic coasts, which has suffered massive declines in recent decades, particularly in the Canary Islands. Here, we describe seasonal variation in the canopy structure of this alga, addressing the role of environmental drivers. Four sites around the island of Gran Canaria were investigated monthly during an entire annual cycle. Annually, the non-fertile stage made up the majority of populations, in terms of frond density, while the fertile stage was comparatively sparser. This fertile stage, however, had the largest biomass and reached the longest lengths, showing significant seasonality. Best fitted GAM models included wave action, PAR and seawater temperature, but only accounted for a moderate variation in the seasonal frond structure of this alga. Total frond biomass, mostly fertile fronds, showed a bimodal pattern, with a peak in spring and a less accentuated peak in late summer-early autumn. This pattern was particularly obvious at sites with a wide annual variation in wave action, with lower biomass at times of high wave action. The frond size-structure was dominated, at all sites and times, by small fronds. The high frond density seems to promote intraspecific facilitation throughout the year. These results provide fundamental knowledge to improve the conservation and potential restoration actions for endangered populations of this alga.