Phytoplankton response to nutrient pulses in an upwelling system assessed through a microcosm experiment (Algarrobo Bay, Chile)

Nutrient enrichment in coastal areas can lead to severe disturbances in marine ecosystems with implications on ecosystem functioning. The primary goal of this study was to understand the response of phytoplankton, from a region with intense upwelling events, to pulsed nutrient enrichments. A microcosm experiment using natural assemblages was conducted, following the addition of inorganic nitrogen (N) and phosphorus (P) and considering two different nutrient limitation conditions (N:P<16 and N:P>16). Laboratory work included the analysis of nutrients, as well as phytoplankton cell abundances. A fast response to the pulsed nutrient enrichments were observed in both treatments as biomass increased. Although higher biomass values were found under higher N concentrations, the community's composition was similar in both treatments. Centric diatoms, particularly Chaetoceros spp., strengthened their dominance, suggesting that, under these conditions, these may have physiological advantages that promote their growth over other groups. Throughout the experiment, phytoflagellates and pennate diatoms were common, while dinoflagellates were scarce. This study contributes to understanding how phytoplankton communities' growth and composition relate to nutrient pulses. These results provide relevant data for environmental quality assessment and management of marine environments.