Controls on diatom biogeography on South Carolina (USA) barrier island beaches

Abstract

Benthic microalgae are of great significance to coastal productivity and nutrient cycling, yet determinants of their biogeography and community assembly remain unclear. We identified and compared spatial and environmental mechanisms that shape beach diatom biogeography and diversity at regional scales. We sampled at various spatial scales (from 10 cm to 120 km) from 4 barrier island beaches in South Carolina, USA, and characterized diatom assemblages using DNA metabarcoding. Twelve environmental variables from sediments, adjacent waters, and atmosphere were recorded. Despite attempts to sample from similar habitat types, environmental factors varied among islands. Correspondingly, biomass and α-diversity metrics likewise varied and were positively related to sediment nutrient levels. Community composition also varied, with each island dominated by different diatom taxa. Redundancy analysis revealed that spatial influences, in addition to environmental factors (water temperature and porewater nutrients), were significantly related to community dissimilarity (β-diversity). Variation partitioning attributed only ~5% of community variation to geographic distance alone, whereas 22% was explained by combined spatial/environmental factors. A high proportion (73%) of community dissimilarity remained unexplained, which likely implicates stochastic processes, although unmeasured physico-chemical or biological factors cannot be discounted. Weak spatial effects suggest that dispersal limitation was relatively unimportant, likely due to extensive sediment transport between beaches. Instead, spatially structured environmental variables appeared to condition beach diatom biogeography. These findings broaden our understanding of microbial biogeography to less well-studied microorganisms and marine habitats. More practically, results suggest some degree of predictability in diatom community changes in the face of anthropogenic challenges.