Extreme El Niño southern oscillation conditions have contrasting effects on the body condition of five euphausiid species around the northern Antarctic Peninsula during winter

Abstract

El Niño southern oscillation (ENSO) events drive profound global impacts on marine environments. These events may result in contrasting conditions in the Southern Ocean, with differing effects on euphausiid species because of their diverse life histories, habitats, and feeding ecologies. We conducted oceanographic surveys during winter (2012–2016) around the northern Antarctic Peninsula and examined the dietary carbon sources, trophic position, and body condition of five euphausiid species (Euphausia crystallorophias, E. frigida, E. superba post-larvae and larvae, E. triacantha, and Thysanoessa macrura) in relation to environmental conditions each year. In addition to general patterns among taxa, we focused on how contrasting conditions during an ENSO-neutral year (2014) and an ENSO-positive year (2016) affected the type, quality, and distribution of food resources each year, as well as the body condition of each species. We observed high chlorophyll-a, low salinity, and shallow upper mixed-layer depths in 2014, and low chlorophyll-a, high salinity, and deep upper mixed-layer depths in 2016. Carbon sources varied among years, with most species enriched in δ13C when ENSO conditions were dominant. Trophic position and body condition also varied among years, with different responses among species depending on conditions; inter-annual variation in δ15N was minimal, while E. triacantha was the only species with notably lower body condition in 2016. We conclude that ENSO conditions around the northern Antarctic Peninsula may result in a more favorable feeding environment for all euphausiid species except E. triacantha, which may be the most negatively impacted by the predicted increase in ENSO conditions.