The development of a marine phytoplankton community was studied in a series of mesocosm tanks exposed to different levels of nutrient inputs. Key ecosystem variables such as phytoplankton species development, ecosystem net production (NEP), pH and bacteria production were measured. The overall aim was to mimic the consequences of extreme weather events by applying nutrients in either repeated (pulse treatment) versus a single inputs (full treatment). Regardless of treatment type, pH increased steadily, until nutrients became exhausted. During the experiment, potentially nuisance dinoflagellates developed and became dominant whereas diatoms became rare as compared to the parallel controls. At pH > 9, a shift from the presence of the potential nuisance Alexandrium pseudogonyaulax towards high pH tolerant Prorocentrum species was observed. Diatoms disappeared when A. pseudogonyaulax became dominant. The NEP was significantly higher (ca. 2500 µg C L- 1 d- 1) in the nutrient amended tanks than in the control tanks (ca. 500 µg C L- 1 d- 1). NEP dropped substantially as nutrients became depleted whereas bacterial communities maintained a high production likely driven by easy access to carbohydrates, turning the ecosystem net heterotrophic. This study suggests that a single nutrient dose drives pH higher than multiple smaller nutrient doses injected albeit the total amount of nutrient injected to the treatments was similar. Such changes affect pH, species composition and rates of pelagic production ultimately uncoupling high bacterial production from primary production, leading to severe oxygen consumption following ephemeral blooms