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

Abstract

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 (F v /F m ) 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.

Species composition and diversity of fish larvae in the Subtropical Convergence Zone of the Sargasso Sea from morphology and DNA barcoding

“…Richardson et al . () reported localized patches of highly elevated vertical mixing and associated nutrient enrichment of the euphotic zone in the same areas with high leptocephali abundances. This would probably stimulate the planktonic food web productivity and the presence of marine snow, which may be an important food source for leptocephali (Miyazaki et al ., ; Miller et al ., ).…”

“…The prominent fronts in the STCZ study area were observed via satellite imagery and in situ hydrographic measurements (Munk et al, 2010). Average water depth at the eight stations was 5400 m. Hydrographic data were collected at each station with a SBE 9/11 CTD (Sea-Bird Electronic, Bellevue, WA, USA) prior to larval fish sampling and details on hydrographic characteristics have been reported elsewhere (Munk et al, 2010;Andersen et al, 2011;Riemann et al, 2011;Richardson et al, 2014).…”

“…2a-c). Further hydrographic characteristics from the 2007 expedition have been published elsewhere (Munk et al, 2010;Andersen et al, 2011;Riemann et al, 2011;Richardson et al, 2014), and in other studies (Wegner, 1982;Kleckner and McCleave, 1988.…”

“…In order to assess species composition and spatial variability of fish larvae in relation to areas of localized environmental heterogeneity in open-ocean areas, we examined fish larvae collected in the early spring near prominent thermal fronts during a larval fish survey in the Subtropical Convergence Zone (STCZ) in the Sargasso Sea. This survey was part of the Danish circumglobal 'Galathea 3' expedition from 2006 to 2007 (http://www.galathea3.dk), which in this part of the expedition primarily focused on the early life of European and American eels in the STCZ of the Sargasso Sea (Munk et al, 2010), and the biological characteristics of the area (Andersen et al, 2011;Riemann et al, 2011;Richardson et al, 2014). This study investigated species composition and distributions of all larval fish collected at eight stations located in close proximity to frontal zones in the STCZ using a combination of DNA barcoding and morphological identification techniques.…”

Vertical distribution of phytoplankton and primary production in relation to nutricline depth in the open ocean

“…In all regions of the world's oceans where water column stratification occurs, the maximum chlorophyll concentration in the water column is often found well below the surface. In some cases, these deep chlorophyll maxima (DCMs) have been shown to contribute significantly to total water column PP (arctic: Richardson et al 2005, Martin et al 2013temperate: Weston et al 2005, Hickman et al 2012, Fernand et al 2013, Lyngsgaard et al 2014subtropical: Fawcett et al 2014, Richardson et al 2014. DCMs are often, but not always, positioned in or near the nutricline, and this has often been interpreted as the DCM being vertically positioned so that access to both light and nutrients is maximised (Klausmeier & Litchman 2001, Fennel & Boss 2003, Cullen 2015.…”