Potential effects of shoaling of oxygen minimum zone on the population of Acartia (Acanthacartia) tonsa in the Humboldt Current Upwelling System (23°S)

Abstract

Expansion and shoaling of the Oxygen Minimum Zone (OMZ) in coastal upwelling systems due to climate change can have dire consequences on the ecosystem structure and productivity. The OMZ restricts the habitat range of most zooplankton, and species with low tolerance to hypoxia may suffer physiological stress and mortality as a result. During 2013 and 2014, the copepod Acartia tonsa collected off northern Chile was used to evaluate how hypoxia may affect its survival and vital rates. Exposure of gravid females to severe hypoxia (∼0.2 mL L−1 dissolved oxygen) resulted in total mortality. Hypoxia at ∼0.5 mL L−1 decreased female survival to ∼20% and egg production to nil. Hypoxia at ∼0.8 mL L−1 decreased survival to ∼50% but no change in egg production rate compared to the normoxia treatment. Egg hatching success was ∼68% and naupliar development time was ∼36% lower in hypoxia than normoxia. The critical and lethal oxygen levels for the species were estimated to be 1.78 and 0.73 mL L−1, respectively. Based on modeled historical data, the 1.5 mL L−1 isopleth has been moving upward at a rate of 1.3–2.0 m per year in northern Chile, barring interruptions during El Niño events. The long-term, gradual shoaling of the OMZ is expected to have significant impacts on the zooplankton community and productivity in the region.