Biological oceanography, biogeochemical cycles, and pelagic ecosystem functioning of the east-central South Pacific Gyre: focus on Easter Island and Salas y Gómez Island

International audienceThe Exclusive Economic Zone of Chile defined by Easter Island and Salas y Gómez Island is in the South Pacific Subtropical Gyre (SPSG), putting it at the center of the most oligotrophic and biomass poor waters in the world. Only 10 biological oceanographic expeditions have entered this zone in 105 years (1905-2010). We review key aspects of the plankton ecosystem and biogeochemical function relevant for the understanding of and conservation planning for marine environments. Plankton production is limited by lack of dissolved inorganic fixed nitrogen, not phosphorous. Higher organic nitrogen levels might be biologically unavailable. Short-term experiments suggested iron is not limiting, yet iron still likely limits nitrogen fixation, and thus production, at longer time scales, as the presence of nitrogen-fixers is exceptionally low compared to other ocean gyres. Plankton function is dominated by the smallest unicellular organisms, picoplankton (<3 µm in diameter). The SPSG represents a center of high biodiversity for picoplankton, as well as heterotrophic organisms such as tinntinids, siphonophores, and possibly amphipods, although data for key zooplankton, such as copepods, are lacking. Many groups exhibit negative relationships between diversity and total plankton biomass. High diversity might result from dispersal from a very large metacommunity and minimal competition within functional groups. Whether an island-mass effect causes a real or apparent increase in plankton biomass around Easter Island must be confirmed by high-resolution sampling in situ. Long-term threats to the planktonic ecosystem may include climate change-enhanced ocean stratification and plastic marine debris accumulation. Finally, priorities for future research are highlighted. Oceanografía biológica, ciclos biogeoquímicos y funcionamiento del ecosistema pelágico de la región centro-este del Giro del Pacífico Sur: centrado en Isla de Pascua e Isla Salas y Gómez RESUMEN. La Zona Económica Exclusiva definida para Isla de Pascua e Isla Salas y Gómez está en el Giro Subtropical del Pacífico Sur (GSPS), en el centro de las aguas más oligotróficas y con menor biomasa del mundo. Sólo 10 expediciones oceanográfico-biológicas han entrado en esta zona en 105 años (1905-2010). Revisamos los aspectos clave del ecosistema planctónico y la función biogeoquímica relevante para su comprensión y planificación de su conservación en ambientes marinos. La producción de plancton está limitada por el nitrógeno inorgánico disuelto y no por el fósforo. Los niveles más altos de nitrógeno orgánico pueden no estar biodisponibles. Experimentos sugirieron que el hierro no es limitante, sin embargo, probablemente limita la fijación de nitrógeno y así la producción a escalas de tiempo mayores, ya que los fijadores de nitrógeno son excepcionalmente escasos en comparación con otros giros oceánicos. El plancton está dominado por picoplancton (