Marine Microphytobenthic Assemblage Shift along a Natural Shallow-Water CO₂ Gradient Subjected to Multiple Environmental Stressors

Predicting the effects of anthropogenic CO₂ emissions on coastal ecosystems requires an understanding of the responses of algae, since these are a vital functional component of shallow-water habitats. We investigated microphytobenthic assemblages on rock and sandy habitats along a shallow subtidal pCO₂ gradient near volcanic seeps in the Mediterranean Sea. Field studies of natural pCO₂ gradients help us understand the likely effects of ocean acidification because entire communities are subjected to a realistic suite of environmental stressors such as over-fishing and coastal pollution. Temperature, total alkalinity, salinity, light levels and sediment properties were similar at our study sites. On sand and on rock, benthic diatom abundance and the photosynthetic standing crop of biofilms increased significantly with increasing pCO₂. There were also marked shifts in diatom community composition as pCO₂ levels increased. Cyanobacterial abundance was only elevated at extremely high levels of pCO₂ (>1400 μatm). This is the first demonstration of the tolerance of natural marine benthic microalgae assemblages to elevated CO₂ in an ecosystem subjected to multiple environmental stressors. Our observations indicate that Mediterranean coastal systems will alter as pCO₂ levels continue to rise, with increased photosynthetic standing crop and taxonomic shifts in microalgal assemblages.