Abstract
Understanding the influence of mesoscale and submesoscale features on the structure of phytoplankton is a key aspect in the assessment of their influence on marine biogeochemical cycling and cross-shore exchanges of plankton in Eastern Boundary Current Systems (EBCS). In this study, the spatio-temporal evolution of phytoplankton size classes (PSC) in surface waters associated with mesoscale eddies in the EBCS off central-southern Chile was analyzed. Chlorophyll-a (Chl-a) size-fractionated filtration (SFF) data from in situ samplings in coastal and coastal transition waters were used to tune a three-component (micro-, nano-, and pico-phytoplankton) model, which was then applied to total Chl-a satellite data (ESA OC-CCI product) in order to retrieve the Chl-a concentration of each PSC. A sea surface, height-based eddy-tracking algorithm was used to identify and track one cyclonic (sC) and three anticyclonic (ssAC1, ssAC2, sAC) mesoscale eddies between January 2014 and October 2015. Satellite estimates of PSC and in situ SFF Chl-a data were highly correlated (0.64 < r < 0.87), although uncertainty values for the microplankton fraction were moderate to high (50 to 100% depending on the metric used). The largest changes in size structure took place during the early life of eddies (~2 months), and no major differences in PSC between eddy center and periphery were found. The contribution of the microplankton fraction was ~50% (~30%) in sC and ssAC1 (ssAC2 and sAC) eddies when they were located close to the coast, while nanoplankton was dominant (~60–70%) and picoplankton almost constant (<20%) throughout the lifetime of eddies. These results suggest that the three-component model, which has been mostly applied in oceanic waters, is also applicable to highly productive coastal upwelling systems. Additionally, the PSC changes within mesoscale eddies obtained by this satellite approach are in agreement with results on phytoplankton size distribution in mesoscale and submesoscale features in this region, and are most likely triggered by variations in nutrient concentrations and/or ratios during the eddies’ lifetimes.
Highlights
Physical dynamics in oceans promote processes of change and adaptation in biological systems at global and regional scales [1,2]
In situ data of total and size-fractionated Chl-a concentrations analyzed in this study were mainly obtained in the coastal zone (CZ) (77%), with total Chl-a values ranging between 0.20 and 17.76 mg m−3, while the remaining (23%) correspond to the coastal transition zone (CTZ), with values between 0.15 and 1.66 mg m−3 (Figure 1)
The general trends of these relationships were captured by the model based on the calculated regional parameters (Table 1), according to which the nanoand picoplankton reached asymptotic values of CNPm ~2.12 mg m−3 and CPm ~0.19 mg m−3, while Chl-a concentrations higher than these were only achieved by the micro-phytoplankton (Figure 2a–d)
Summary
Physical dynamics in oceans promote processes of change and adaptation in biological systems at global and regional scales [1,2]. There is a particular type of anticyclonic eddy, called intrathermocline (ITE) or subsurface mode water eddy, which represents 30–55% of the anticyclonic eddy population in Eastern Boundary Current Systems (EBCS) [13]. These eddies (ITEs) are characterized by a typical radius of ~20–60 km and have a vertical extent of ~500 m, together with dome-shaped isopleths in the upper layers and a bowl shape in the lower layers, a minimum in oxygen, and high rates of phytoplankton productivity [14,15,16,17]
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