Phytoplankton metacommunity structure in subarctic rock pools

Rock pools are ephemeral aquatic systems that provide important ecosystem functions as stepping stones for sustaining aquatic biodiversity within an inhabitable terrestrial matrix. We investigated the fine-scale (<60 m) spatial variation of phytoplankton communities in 30 pristine subarctic rock pools in Finland within the metacommunity context. We differentiated between straight-line overland and watercourse distances among the rock pools, as the latter may reflect actual dispersal pathways more realistically. Environmental variables, such as total phosphorus, varied by an order of magnitude among the pools, but showed a rather random and patchy distribution instead of a distinct spatial gradient. Bray-Curtis dissimilarities and taxon-accumulation curves showed that the community composition differed considerably among the pools. Redundancy analysis suggested that such among-pool differences in communities may partly be driven by the variation in conductivity and water temperature. Variation partitioning further showed that the pure spatial component was considerably larger and residual variation lower when watercourse distances were used instead of overland distances. This indicates that watercourse distances better represented the spatial structure of the phytoplankton metacom_munity than overland distances. These results further suggest that occasional flooding during snow melt or heavy rainfall as well as random faunal fertilization probably shaped the community composition and that phytoplankters were mainly dispersed via watercourses in this rock pool system.