Spatial variability in the epiphytic algal assemblages of Zostera marina seagrass beds

Abstract

Within-bed variation of epiphytic algal assemblages was studied in 3 Zostera marina beds around Plymouth Sound (Devon, United Kingdom) in order to determine if distance from edge of bed affected the assemblage composition. Replicate seagrass samples were taken at 0, 1, 3 and 6 m along a transect running from the edge of each bed. Individual epiphytes were enumerated and placed into 6 'functional' groups: filamentous, foliose, corticated filamentous, corticated foliose, saccate and coralline. The abundance of each functional group did not vary with distance into each bed, nor was it related to measures of seagrass architecture or potential grazing pressure from gastropods. A positive relationship between amphipod abundance and total epiphytes suggest these crustaceans are influenced by epiphytes as an available substrate rather than exerting a grazing effect. Differences amongst beds, however, were evident for filamentous, corticated filamentous and coralline functional groups. These groups comprised 99% of all epiphytes, resulting in the total algal abundance being different amongst beds. Similarly, the assemblage composition of the epiphytes was consistent within each bed, but differed amongst all 3 beds. Overall, therefore, epiphytic abundance and composition was not affected by distance from the edge of the bed, seagrass architecture or grazing pressure within the bed, but existence of within-bed variation cannot be completely ruled out due to the comparatively low power of the analysis. The only significant variation occurred at the amongstbed level, probably due to variations in large-scale factors.