Nutrient-specific responses of a phytoplankton community: a case study of the North Atlantic Gyre, Azores

Nutrient concentrations are unevenly distributed in the oceans, influencing the abundance and composition of phytoplankton communities. Even so, the dominant driving factors responsible for variability between phytoplankton communities are still unclear. In the North Atlantic Gyre, the Azores present a good opportunity to study phytoplankton communities of oligotrophic areas that experience nutrient pulses. We followed the development of an enclosed natural phytoplankton community occurring off the coast of Terceira (Azores) and tested the effects of single (nitrate, phosphate, silicate and a mix of the trace metals Fe, Co, Cu, Mo, Zn and Mn) and combined nutrient enrichments on phytoplankton abundance, particulate organic matter (POM) build-up, nutrient drawdown and community composition. Towards the end of the microcosm-based incubation, biomass developed dramatically (430-fold) when all the nutrients considered were added simultaneously. Importantly, the community composition at the end of the incubation was dependent on the combination of nutrients supplied, with diatoms dominating most of the treatments; coccolithophores under Phosphate + Trace Metals; and organisms with characteristics of a nitrogen fixer such as low delta N-15 under full nutrient enrichment. These results indicate group-specific nutrient requirements and limitations occurring near the Azores with a few taxa dominating the groups' response to nutrient pulses.