Abstract
AbstractMesoscale and submesoscale processes that contribute to localized increases in nutrients in the sunlit layer can stimulate phytoplankton growth and community changes, but the mechanisms involved remain sparsely documented with in situ data in the case of Eastern Boundary Upwelling Systems (EBUSs) and of most ocean regions. The role of diapycnal mixing in providing nutrients to the upper layer and in influencing phytoplankton size structure was analyzed in an area of intense mesoscale and submesoscale activity during the coastal upwelling season off Concepción (~36–37°S), the Humboldt Current EBUS. Diapycnal nutrient fluxes based on conductivity, temperature, and depth vertical eddy diffusivity (Kz) values (the Thorpe scale method) and on nutrient gradients were assessed in association with size‐fractionated chlorophyll‐a and microdiatom abundance derived from in situ sampling in an area including a mesoscale intrathermocline eddy (ITE) adjacent to a coastal upwelling front (CUF). The indirect estimates of Kz values spanned between 0.01 and 4 × 10−4 m2 s−1, and maxima in diapycnal nitrate flux per station ranged between 0.08 and 19.1 mmol m−2 day−1. Maxima in the upward fluxes were detected at the subsurface (15–40 m depth) in the CUF and ITE areas, coinciding with maxima in the micro‐ and nano‐chlorophyll‐a fractions and in microdiatom abundance. These results suggest that ITE and CUF features, as well as their interaction, can generate intense diapycnal mixing and, thereby, contribute to increasing nutrient availability below the mixed layer. In turn, these processes enhance the contribution of larger phytoplankton cells in the coastal transition zone of EBUSs.
Highlights
Mesoscale and submesoscale physical processes in the ocean have been identified as important contributors to the transport and mixing of tracers through diverse mechanisms, thereby influencing directly biological communities and biogeochemical cycles
Satellite data indicated that the sampling took place in a region of colder waters (17 °C) in the coastal transition zone (CTZ), separated by a strong SST gradient denoted by the 16–17 °C isotherms (Stations 6–7 and 16–18, ~70–100 km from the coast) (Figure 1a)
Eastern Boundary Upwelling Systems (EBUSs) are regions of intense mesoscale and submesoscale activity, such as coastal upwelling front (CUF), filaments, surface eddies and intrathermocline eddy (ITE), and all these features have an important impact on phytoplankton distribution, community structure, and ecosystem dynamics
Summary
Mesoscale and submesoscale physical processes in the ocean have been identified as important contributors to the transport and mixing of tracers (e.g., nutrients, oxygen, and plankton) through diverse mechanisms, thereby influencing directly biological communities and biogeochemical cycles. Mesoscale to submesoscale dynamics can lead to changes in nutrient availability and, thereby, in phytoplankton growth rates and community composition (see reviews in Lévy et al, 2018; Mahadevan, 2016; McGillicuddy, 2016). EBUS coastal upwelling leads to high levels of phytoplankton biomass and primary production; patches of moderate biomass levels are detected in the CTZ and, in association with mesoscale and submesoscale dynamics, contribute to an offshore advection of waters nearest to the CZ and/or through local growth in the CTZ, mediated by vertical injections of nutrients to the photic layer (Callbeck et al, 2017; Chenillat et al, 2015; Correa‐Ramirez et al, 2007; Pietri et al, 2013)
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