A mesoplankton biomass decline in the Central Atlantic coupled with an increase of surface temperature and an expansion of low‐productive zones

Abstract

AbstractAimClimate‐driven trends in environmental processes and their effect on marine ecosystems remain poorly understood despite an obvious decrease in marine biological resources which undergo changes in stock size mediated by phyto‐ and mesoplankton. A mesoplankton multiannual dataset is one of the best tools to elucidate general trends in marine ecosystems.LocationOligo‐ and mesotrophic waters of the Central Atlantic.Time Period1950–2016.Major Taxa StudiedChaetognatha, Cnidaria, Crustacea, Mollusca.MethodsWet mesoplankton biomass (B) values were estimated in 1159 mesoplankton (animals 0.5–30.0 mm long) samples collected in the open ocean epipelagic (0–200 m) in 1950–2016. We ran Generalized Linear Mixed Models and analysed general trends in B, in satellite‐derived surface chlorophyll (Chl, as a proxy of phytoplankton biomass in 1998–2015), and surface temperature (SST, NCEP/NCAR reanalysis, 1948–2015). We further compared the multiannual trends in B, SST, Chl and in space of the oligotrophic and mesotrophic Chl contours in selected regions and in the whole Central Atlantic.ResultsWe showed a statistically significant decline of B within the whole Central Atlantic (9‐fold during 1950–2016) and within each of the three selected regions. These trends were always coupled with other statistically significant effects: increase of SST, decline of Chl, expansion of Chl‐depleted areas and shrinkage of Chl‐enriched areas.Main ConclusionsThe results unveil the following causal relationship in the Central Atlantic: SST increase → strengthening of stratification → Chl decline—expansion of low‐productive areas → B decrease, which, along with existing prognostic climate models, suggests future mesoplankton decline in the Central Tropical Atlantic. The decline in Chl and B is expected to amplify decrease of higher trophic levels biomass and thus to result in changes in the marine food webs. Observed changes in biomass and production will imply direct repercussions for world fisheries and other services provided by Ocean.