Climate variability and spatiotemporal dynamics of five Southern Ocean krill species

Understanding the ecological response of marine organisms to future climate change will benefit from quantifying spatiotemporal aspects of their distribution and abundance as well as the influence of ocean–atmospheric climate modes on their population cycles. Our study provides a synthesis of 18years of data (1992–2009) for 5 krill (euphausiid) species monitored near the North Antarctic Peninsula (NAP) during austral summer. Distribution and abundance data are presented for postlarval stages of Euphausia crystallorophias, E. frigida, E. superba, E. triacantha and Thysanoessa macrura and larval E. superba and T. macrura. Intraseasonal, interannual and longer-term distribution and abundance patterns are quantified relative to climate modes driving ecosystem variability off the Antarctic Peninsula: El Niño-Southern Oscillation (ENSO), Southern Annual Mode (SAM) and associated zonal and meridional winds. Interannual abundance variations of all 5 species are significantly correlated with seasonally averaged ENSO indices and, with the exception of E. triacantha, elevated population sizes are associated with the higher productivity La Niña phase. Time-lagged responses of each species to ENSO indices approximate their generation times and suggest evolution of their life histories and reproductive efforts in accordance with the ENSO cycle. Postlarval E. crystallorophias and E. frigida and larval T. macrura demonstrate significant abundance increases after 1998 associated with a shift from an El Niño dominated period to predominantly La Niña and “Nino-neutral” conditions. Seasonal changes in species distributions and co-occurrence indicate portions of the southernmost E. frigida, E. triacantha and T. macrura populations move poleward with E. superba during late-summer, suggesting that environmental conditions associated with sea ice development (e.g., food, retention) may be more favorable than within the Antarctic Circumpolar Current during low productivity seasons. Spatial distributions of larval and postlarval T. macrura suggest 2 separate spawning populations within oceanic and coastal waters. Lastly, mean euphausiid species concentrations and abundance relationships encountered during 1992–2009 are remarkably similar to those reported for the Antarctic Peninsula during the 1928–1935 Discovery Investigations. Circumpolar observations of E. superba “superswarms” associated with a major climatically related ecosystem change in the late 1970s may have represented an episodic period of anomalous peak abundance. Subsequent abundance estimates compared to these anomalies would indicate a significant decrease, but should not be taken to reflect the impact of climate warming.