Changes in the metabolic balance of contrasting microbial food webs after nutrient enrichment

Two sets of inorganic (nitrate, ammonium, phosphate) and/or organic (glucose, amino acids) nutrient addition experiments were performed in contrasting planktonic ecosystems, including both oligotrophic central Atlantic waters and mesotrophic coastal waters off the NW Iberian Peninsula, to assess the direction and magnitude of the shifts in the metabolic balance, defined as the primary production to community respiration ratio (P/R), of different microbial plankton communities. Large (>2 μm) phytoplankton accounted for most of the primary production response to nutrient additions in all the microbial plankton communities studied. Microbial plankton metabolic shifts after inorganic or organic nutrient additions appeared to be dependent on both the production and biomass size structure of phytoplankton communities. The strongest tendencies towards short-term autotrophy (after inorganic additions) and heterotrophy (after organic additions) were registered in microbial communities initially dominated by large phytoplankton cells (i.e. herbivore food webs) with high and low relative contribution of large cells to primary production, respectively, suggesting that the metabolic balance of herbivore food webs is less resistant to nutrient additions than that of microbial food webs (dominated by picophytoplankton). By contrast, regardless of the trophic structure of phytoplankton populations, the short-term response of the microbial plankton metabolic balance to the combined inorganic and organic nutrient additions appeared to be mainly related to the initialP/Rratios, which implies that the more heterotrophic the microbial community, the stronger the tendency towards heterotrophy after mixed additions.