Localised mixing and heterogeneity in the plankton food web in a frontal region of the Sargasso Sea: implications for eel early life history?

Previous studies have demonstrated that patches of eel larvae are found in the frontal region of the Subtropical Convergence Zone (STCZ), but to date no clear evidence of why this region might confer advantage to the larvae has been presented. This study demonstrates that there may be localized patches within a frontal region in the STCZ in the Sargasso Sea that experience elevated vertical mixing and an associated vertical flux of nutrients. This localized vertical mixing was suggested by a group of stations within the frontal region that exhibited a greater similarity (Jaccard index) between the diatom communities at 10 m and >100 m (in the deep chlorophyll maximum, DCM) than in other parts of the frontal region. Thorpe displacements supported the hypothesis of elevated mixing intensities around these stations, as did vertical mixing rates inferred from stratification and vertical current shear calculated from acoustic Doppler current profiler (ADCP) measurements. Combining these mixing estimates with vertical nutrient gradients suggests that nutrient fluxes to the euphotic zone at these mixing sites may be an order of magnitude greater than elsewhere in the frontal region. This mixing may influence the plankton food web, as indicated by elevated values/concentrations of (1) primary production, (2) variable fluorescence (Fv/Fm) and (3) total seston. In addition, the fraction of the total biomass of both copepods and nauplii found closest to the DCM in the frontal region correlated with the stratification (Brunt-Väisälä frequency), with the greatest fractions found below 75 m at the most weakly stratified stations. While this study cannot directly link these observations to eel larvae ecology, Munk et al. (2010; Proc R Soc B 277:3593–3599) reported that eel larvae were most abundant at locations where we found evidence for elevated vertical mixing.