Front‐Eddy Influence on Water Column Properties, Phytoplankton Community Structure, and Cross‐Shelf Exchange of Diatom Taxa in the Shelf‐Slope Area off Concepción (∼36–37°S)

Abstract

In eastern boundary current systems (EBCSs), submesoscale to mesocale variability contributes to cross‐shore exchanges of water properties, nutrients, and plankton. Data from a short‐term summer survey and satellite time series (January–February 2014) were used to characterize submesoscale variability in oceanographic conditions and phytoplankton distribution across the coastal upwelling and coastal transition zones north of Punta Lavapié, and to explore cross‐shelf exchanges of diatom taxa. A thermohaline front (FRN‐1) flanked by a mesoscale anticyclonic intrathermocline eddy (ITE‐1), or mode‐water eddy, persisted during the time series and the survey was undertaken during a wind relaxation event. At the survey time, ITE‐1 contributed to an onshore intrusion of warm oceanic waters (southern section) and an offshore advection of cold coastal waters (northern section), with the latter forming a cold, high chlorophyll‐a filament. In situ phytoplankton and diatom biomasses were highest at the surface in FRN‐1 and at the subsurface in ITE‐1, whereas values in the coastal zone were lower and dominated by smaller cells. Diatom species typical of the coastal zone and species dominant in oceanic waters were both found in the FRN‐1 and ITE‐1 interaction area, suggesting that this mixture was the result of both offshore and onshore advection. Overall, front‐eddy interactions in EBCSs could enhance cross‐shelf exchanges of coastal and oceanic plankton, as well as sustain phytoplankton growth in the slope area through localized upward injections of nutrients in the frontal zone, combined with ITE‐induced advection and vertical nutrient inputs to the surface layer.

Phytoplankton Size Structure in Association with Mesoscale Eddies off Central-Southern Chile: The Satellite Application of a Phytoplankton Size-Class Model

“…The first one is that the satellite-derived spatial distribution of the PSC in CZ and CTZ in the study region is consistent with previous reports using in situ data from the same region; that is, nanoplankton dominates in the CTZ, whereas microplankton does so in the CZ, e.g., [30,60,81,84]. The second one is that previous in situ studies on PSC associated with mesoscale eddies have shown that an important fraction of the coastal microplankton in this region appears to be advected by these features during their early stages of development [32,33].…”

“…In the region off central-southern Chile, waters with high nitrate and silicate concentrations (~10 µM) have been associated with the upwelling of the Equatorial Subsurface Waters (ESSW) in the CZ, favoring the dominance of the microplankton fraction (e.g., diatoms) [85,86]. In contrast, waters in the CTZ are often lower in silicate concentration leading to changes in the nitrate:silicate ratios [33]. Ratios close to 1:1 have favored the growth of large phytoplankton cells, whilst higher values (>3:1) can produce shifts in the size of diatoms and/or changes towards other small functional groups [33,87,88].…”

“…Two of the anticyclones were found to be subsurface-intensified anticyclonic eddies, one of which interacted with an oceanic seamount at some point, as revealed by satellite and CTD (conductivity, temperature and depth) data collected during the FIP Seamount cruise [61], while the other interacted with an upwelling front, as observed in CTD data during the PHYTOFRONT cruise [33]. Specifically, the studied eddies are composed of (i) a subsurface anticyclone (ssAC1), which was born in the coastal region south of Point Lavapié …”

“…For sAC, this could be explained as a response to the physical dynamics of this eddy type, usually characterized by a downward displacement of isopycnals, which forces a flush of nutrients below the euphotic zone and the consequent decrease in primary production [3,[9][10][11]. In the case of ssAC2, Morales et al [33] have previously described that this eddy was interacting with a coastal front during a relaxation phase of upwelling, time at which the nanoplankton made the largest contributions to total Chl-a.…”

“…Studies in EBCS have reported an exchange of chemical and biological properties between coastal and oceanic waters associated with the propagation of mesoscale eddies, e.g., [28][29][30][31]. Most of these studies are based on total Chl-a, and only a few have analyzed phytoplankton community structure within mesoscale eddies or other submesoscale features, e.g., [32][33][34][35]. The later, however, are restricted in space and time, mainly due to the difficulty in obtaining high-resolution, in situ data associated with the complete trajectory of eddies.…”