Contrasting resistance of prokaryotic plankton biomass and community composition to experimental nutrient perturbations in a coastal upwelling system (NW Spain)

Coastal prokaryote plankton communities can be responsive to nutrient additions and may show variations in their resistance to those disturbances. For testing the response of prokaryote plankton to nutrient additions at two different coastal locations with different exposure to environmental disturbances, three nutrient addition experiments were carried out at the ocean shelf outside the Ría de Vigo, NW Spain, in spring, summer, and autumn 2014, and the data set was combined with that from comparable previous experiments performed inside the ría. Inorganic, organic, and a mixture of both organic and inorganic nutrients were added to natural surface prokaryote communities, and subsequently taxonomic community composition and biomass changes were assessed. The addition of a mix of inorganic and organic nutrients led to the greatest changes in composition. Vibrionales was the taxon showing the greatest changes in the mixed and organic amendments, whereas Alteromonadales and Oceanospirillales responded to inorganic nutrient additions. The Euclidean distance among samples was used to estimate compositional resistance to disturbances derived from experimental nutrient additions. Changes in prokaryote biomass were used to evaluate functional resistance. In general, prokaryote communities were more resistant to inorganic than to organic nutrient additions. Communities sampled inside the ría, which are presumably exposed to greater environmental perturbations, showed less pronounced short-term variability in the non-amended control than those from outside the ría. By contrast, communities from outside the ría, where the ambient organic matter concentrations were higher, were more resistant to organic and mixed nutrient additions than those from the ría. Our data suggest that the perturbation history appears to be related to the resistance of microbial communities to changes in nutrient inputs into the coastal ocean.