Wave height vs. elevation effect on macroalgal dominated shores: an intercommunity study

Abstract

Sheltered intertidal environments are characterized by highly diversified macroalgal communities which vary over short spatial scales depending on various environmental pressures. In this study, relations between wave heights and vertical elevation were analyzed in order to identify the respective roles of these factors on the structure of macroalgal communities. We measured both wave heights and elevation on six macroalgal communities of three Northeast Atlantic rocky shores. The effects of these structuring factors on seaweed communities were seasonally evaluated in early summer and late autumn using pressure recorders and digital terrain model data. Results reveal that waves were higher during late autumn than in early summer. At shore-scale, elevation was the main factor determining both the structure and the species composition of the macroalgal communities (13.7% of total variance explained), while hydrodynamics played a lower, although significant role (< 1% of the total variance). Based on the cover of canopy-forming species, intertidal macroalgal communities can be grouped into three clusters, corresponding to three height levels on the shore (higher, middle, and lower shore). Canopy composition and structure were mainly explained by the effect of elevation (45%) in the higher level of the shore and by both elevation (42%) and wave height (2.5%) in the middle shore. On the contrary, low shore communities were affected by a more balanced effect of elevation (explaining 14% of the total variance and wave height associated with site effect explaining 15% of such effect). This study gave new insights of how hydrodynamics and elevation condition the extension of macroalgal communities.