Abstract Assessment of ecosystem natural variability is a major goal in ecology, not only to better understand fluctuations, but also as a basis for distinguishing such natural variability from changes caused by human activities. We document here spatial variability of several Cymodocea nodosa attributes relevant to ecosystem functioning and indicative of ecosystem health at three localities near its upper and lower latitudinal distribution limits. At each locality, we chose three healthy and unaltered meadows, and, at each meadow, three sampling areas. The attributes measured were shoot density, biomass allocation to leaves and roots, shoot size, carbohydrate reserves, element content (C, N, P, S) and stable isotope ratios (δ13C, δ15N and δ34S). Geographical differences (among localities) accounted for between 65 and 90% of the total variability, while variability within localities (both between meadows and within meadows) accounted for substantial but lower contributions in most attributes. This suggests a relatively important degree of geographic plasticity in the attributes examined. Results presented here contribute to our basic knowledge of C. nodosa and its variability over large areas, which can be especially relevant for coastal monitoring and management programs.