Reconciling vertical and horizontal variability in Sargassum populations for improved environmental monitoring

The genus Sargassum C. Agardh (Phaeophyceae) and other fucoids are important focuses of worldwide research owing to their ecological role and value as indicators of environmental quality. Sargassum species are among the most common habitat-forming macroalgae in shallow subtidal rocky habitats along the Brazilian coast. Understanding patterns of variation in Sargassum populations at multiple scales is essential to determine monitoring designs. The aim of this study was to answer three main questions, using Ilha Grande Bay as a case study: (1) Which are the relevant scales of horizontal variability in cover and density of Sargassum populations? (2) Are horizontal patterns consistent across depth?, and (3) Should small depth differences be included in monitoring designs for shallow Sargassum populations? These issues were addressed through a sampling program including five horizontal scales: area (2000–6500 m apart), island (500–1000 m apart), site (100–350 m apart), point (3–10 m apart), and quadrat (0.4–3 m apart). The same approach was repeated at two depths (1.0–2.0 m and 3.5–4.5 m). Quadrat, point, and site were identified as relevant scales of variability for all variables, although patterns of horizontal variability were not consistent through depth. These findings suggest that both small-scale vertical and small to intermediate-scale horizontal variations (up to 10 m and ~ 300 m, respectively) should be included in monitoring programs of Sargassum populations. Ultimately, this has technical and economic implications for management and conservation initiatives focused on such important habitat formers.