Three‐dimensional distribution of mesoplankton assemblages in the Central Atlantic

Abstract

Aim

Mesoplankton distribution depends on many factors including depth, hydrological parameters, trophic conditions, etc. Recent studies show a link between mesoplankton biomass and trophic factors in the deep sea, whereas relationships between deep‐sea mesoplankton structure/composition and trophic factors remain unknown.

Location

Central Atlantic, between 32°S and 42°N, 14°W and 46°W.

Time period

September–November 2012, 2013, 2016.

Major taxa studied

All mesoplankton taxa.

Methods

We linked composition of mesoplankton assemblages to trophic conditions: surface chlorophyll a (Chl) concentration and particulate organic carbon (POC) flux at different depths—the epipelagic (0–200 m), mesopelagic (200–800/1000 m), upper (800/1000–1500 m), and lower (1500–3000 m) bathypelagic. We collected 146 samples at 40 stations from corresponding depths with Bogorov–Rass plankton net (opening 1.0 m ², mesh 500 µm). The effect of trophic conditions on mesoplankton composition was analysed in horizontal and vertical dimensions using linear mixed‐effects models and canonical correspondence analyses (CCAs). Non‐transformed and transformed data matrices were processed using the Bray–Curtis similarity index, permutational multivariate analysis of variance (PERMANOVA), similarity profiles permutation (SIMPROF) and Kruskal–Wallis tests, and Dunn’s post‐hoc comparisons. Diversity was estimated using the Shannon–Wiener and Hurlbert rarefaction indices.

Results

We identified 118 zooplankton taxa including 90 copepods. The depth effect (vertical dimension of the trophic factor) was greater than the horizontal effect of Chl and POC distribution. The epi‐, meso‐, upper, and lower bathypelagic mesoplankton were further analysed separately. Five assemblages linked to Chl and POC were identified; two of them (an oligotrophic one in the Southern or Northern Gyre and a mesotrophic one in the Equatorial Atlantic) were recorded at all depths.

Main conclusions

We found a link between structure/composition of mesoplankton assemblages and trophic factors enabling projections of mesoplankton taxonomic structure throughout the water column using Chl and POC maps, respectively. These results will contribute to the development of ecological research methodology and to better understanding of carbon transport in the deep sea.