Spatial distributions of Southern Ocean mesozooplankton communities have been resilient to long‐term surface warming

Abstract

The biogeographic response of oceanic planktonic communities to climatic change has a large influence on the future stability of marine food webs and the functioning of global biogeochemical cycles. Temperature plays a pivotal role in determining the distribution of these communities and ocean warming has the potential to cause major distributional shifts, particularly in polar regions where the thermal envelope is narrow. We considered the impact of long-term ocean warming on the spatial distribution of Southern Ocean mesozooplankton communities through examining plankton abundance in relation to sea surface temperature between two distinct periods, separated by around 60 years. Analyses considered 16 dominant mesozooplankton taxa (in terms of biomass and abundance) in the southwest Atlantic sector of the Southern Ocean, from net samples and in situ temperature records collected during the Discovery Investigations (1926)(1927)(1928)(1929)(1930)(1931)(1932)(1933)(1934)(1935)(1936)(1937)(1938) and contemporary campaigns (1996)(1997)(1998)(1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008)(2009)(2010)(2011)(2012)(2013). Sea surface temperature was found to have increased significantly by 0.74°C between the two eras. The corresponding sea surface temperature at which community abundance peaked was also significantly higher in contemporary times, by 0.98°C. Spatial projections indicated that the geographical location of community peak abundance had remained the same between the two eras despite the poleward advance of sea surface isotherms. If the community had remained within the same thermal envelope as in the 1920s-1930s, community peak abundance would be 500 km further south in the contemporary era. Studies in the northern hemisphere have found that dominant taxa, such as calanoid copepods, have conserved their thermal niches and tracked surface isotherms polewards. The fact that this has not occurred in the Southern Ocean suggests that other selective pressures, particularly food availability and the properties of underlying water masses, place greater constraints on spatial distributions in this region. It further demonstrates that this community is thermally resilient to present levels of sea surface warming.

Predicting future distributions of lanternfish, a significant ecological resource within the Southern Ocean

“…Rapid adaptations have been recorded for some species, such as the observation by Irwin, Finkel, Muller‐Karger, and Ghinaglia () that some phytoplankton species have adapted to certain aspects of their environmental niche, with spatial distributions tracking changes in temperature and irradiance. Tarling, Ward, and Thorpe () have also demonstrated that the distribution of the South Atlantic copepod community has remained largely unchanged over the past 80 years despite a 1°C warming in surface temperatures, which may be explained by thermal acclimation in biomass‐dominant species, as well as other constraints to species distributions such as food availability. Nevertheless, past evidence suggests that, for most species, particularly marine organisms, the dominant response to climate change is shifting distributions rather than evolutionary changes (Parmesan, Root, & Willig, ).…”

“…We assume that no genetic adaptation or evolutionary processes will take place by the end of our study's timeframe (2100) that some phytoplankton species have adapted to certain aspects of their environmental niche, with spatial distributions tracking changes in temperature and irradiance. Tarling, Ward, and Thorpe (2017) have also demonstrated that the distribution of the South Atlantic copepod community has remained largely unchanged over the past 80 years despite a 1°C warming in surface temperatures, which may be explained by thermal acclimation in biomass-dominant species, as well as other constraints to species distributions such as food availability.…”

Copepod species abundance from the Southern Ocean and other regions (1980–2005) – a legacy

“…In the Southern Ocean, copepods are the most important zooplankton organisms next to Antarctic krill and salps, both in abundance and biomass (e.g. Pakhomov et al, 2000;Shreeve et al, 2005;Smetacek and Nicol, 2005;Ward et al, 2014;Tarling et al, 2017). In the Southern Ocean, copepods are also the most diverse zooplankton taxon, accounting for more than 300 species (Kouwenberg et al, 2014).…”

“…and species of the family Oncaeidae (e.g. Hopkins, 1985;Atkinson, 1998;SchnackSchiel, 2001;Tarling et al, 2017). Together these taxa can comprise up to 95 % of the total abundance and up to 80 % of the total biomass of copepods (Schnack-Schiel et al, 1998).…”

“…Boysen-Ennen and Piatkowski, 1988;Hopkins and Torres, 1988;Boysen-Ennen et al, 1991;Pakhomov et al, 2000;Dubischar et al, 2002;Ward et al, 2014;Tarling et al, 2017). A major zooplankton monitoring programme in the Southern Ocean is the Continuous Plankton Recorder survey (SO-CPR), providing a large-scale coverage of surface Antarctic zooplankton species distribution abundances over the last 25 years (Hosie et al, 2003;McLeod et al, 2010).…”