Disturbance from pond management obscures local and regional drivers of assemblages of primary producers

Ponds have significant conservation value due to their potentially high contribution to local and regional diversity. However, most ponds are located in anthropogenically influenced areas, and their biodiversity is constantly threatened by human activities. Thus, knowledge of the effect of pond management on biodiversity is essential for designing effective conservation strategies. Here, we study the main drivers of diversity of three functional groups of primary producers (phytoplankton, filamentous green algae and submerged macrophytes) in 87 ponds distributed across a large region (c. 90000km2) in Southern Spain. We hypothesised that spatial effects would increase with increasing propagule size. However, given the regional extent of the study, we anticipated that both spatial and environmental controls would be significant. We determined -, - and -components of biodiversity for each functional group and pond type (embankment ponds, excavated ponds and artificial ponds) and assessed the influence of environmental and spatial drivers on diversity with generalised additive models (GAMs). Redundancy analyses (RDAs) with variation partitioning were used to determine the relative contribution of environmental and spatial predictors of the community assembly. Spatial variables were calculated by applying distance-based Moran's eigenvector maps (db-MEM). Both - and -diversities of phytoplankton and filamentous green algae varied significantly with pond type. Generally, environmental predictors of diversity were more important than spatial variables. Assemblage structure was controlled by water chemistry and eutrophication, with a marked influence of pond type. Spatial variables included broad-scale variation for the three groups of primary producers, which were also strongly influenced by the management regime. Limited management activities, as occurs at embankment ponds, promoted the local richness of phytoplankton and contributed importantly to the regional diversity of macrophytes. Moreover, the relative contribution of environmental and spatial variables was similar between embankment ponds and natural ecosystems, that is, dispersal limitation increasing with propagule size. Excavated and artificial ponds also contributed importantly to regional diversity by enhancing phytoplankton and filamentous green algal -diversity. However, spatial patterns in the latter pond type did not meet our expectations, most likely due to the intensive disturbance from pond management hampering the development of macrophytes.