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

Natural and anthropogenic factors as possible drivers of variability in rocky shore assemblages at multiple spatial scales

Spatial and temporal variability in rocky shore assemblages at multiple scales can be driven by natural factors and anthropogenic disturbances, operating either separately to each other or in complex combinations. Using the shallow subtidal rocky habitat in Ilha Grande Bay (southeastern Brazil) as a model system, this study analyzed five natural and five anthropogenic variables as possible drivers of variability of cover in rocky shore assemblages, focusing on the following questions: (i) Which are the relevant horizontal scales of variation in assemblage structure?; (ii) Are patterns of horizontal variation consistent across a small depth range?; (iii) Are there significant relationships between spatial patterns of macroalgae and macroinvertebrates, and their potential natural and anthropogenic drivers? These issues were addressed through a hierarchical sampling strategy including five horizontal scales, namely quadrat (0.4–3 m), point (3–10 m), site (100–350 m), island (500–1000 m) and area (2000–6500 m), repeated at two depths (1–2 m and 3.5–4.5 m). The correlations of cover data of 21 groups of organisms and five natural and five anthropogenic variables were analyzed. Assemblages showed relevant variation among quadrats, points and sites, at both depths, while variation at the area scale was relevant only at 3.5–4.5 m depth, indicating that patterns of horizontal variation were not fully consistent between depths. Substrate rugosity, rocky shore extension, water movement, percentage of deforested land area and distance to urban center were the environmental variables that most explained spatial patterns of the assemblages. Our study highlights the potential role of such variables as drivers of variation in rocky subtidal assemblages at multiple spatial scales. For environmental protection programs, relevant scales of variation of target populations and assemblages need to be considered, to effectively capture their dynamics and underlying processes