Publication Abstract

Despite known ENSO (El Niño Southern Oscillation) effects that propagate into the southwest Atlantic, knowledge of how these anomalies result in ecological impacts is poorly understood. Two 3600 km cruise tracks from the Falkland Islands to Tristan da Cunha were repeated seven months into a La Niña in March 2018 and subsequently seven months into an El Niño event in March 2019. Wind strength was higher in 2019 than 2018, which was associated with greater mixed layer depth and lower sea surface temperature, but higher surface chlorophyll a. Underway acoustics showed that pelagic biomass (0-200 m) was shallower and more concentrated in 2018 than 2019. Bird diversity and abundance was higher in 2018, with a dominance of smaller prions and petrel species (that tend to surface feed on smaller prey), as opposed to a dominance of larger bird species in 2019, which feed on larger prey. A dramatic difference was also seen in marine mammals, with 33% filter feeding Mysticeti recorded in 2018, but 97% Odontoceti in 2019. This study documented an ocean basin scale shift in the southwest Atlantic pelagic ecosystem associated with variability in regional winds, which correlated, with a time lag of 1 to 2 month, with the Bivariate ENSO Index (BEST). The predicted climate change increase in intensity of ENSO events is therefore likely to lead to changes in pelagic assemblages, their predators and their prey, with the potential to alter the ecosystem structure of the southwest Atlantic.

Simon A. Morley Fabio Campanella*, Al Baylis, Dave KA Barnes, James B. Bell*, Ashley Bennison, Martin A Collins, Trevor Glass, Stephanie M. Martin, Paul Whomersley*, Emma F. Young, Andy Schofield