Oceanic Fine-Scale Circulation and Nutricline: Unveiling Uncertainty and Variability.

Interactions between physical and biogeochemical processes have traditionally been studied at ocean basin scales or in in regions with by large mesoscale features. At finer scales, such as fronts and small eddies, modeling studies have offered valuable insights into how physical features influence biogeochemistry. However, these interactions remain understudied using empirical data due to the challenges of identifying and sampling these dynamic structures. In oligotrophic regions, the vertical distribution of nutrients plays a crutial role in shaping phytoplankton diversity. Nutrient profiles typically exhibit near-zero concentrations in the upper water column and higher concentrations at depth, separated by the nutricline – a transitional zone marked by sharp or gradual changes in nutrient concentrations. The depth of the nutricline (defined as its upper limit) and its strength (reflected in the associated concentration gradient) are closely linked to nutrient fluxes into the photic layer, which are critical for sustaining new primary production.

 

In spring 2023, the BioSWOT-Med campaign (doi.org/10.17600/18002392) investigated the influence of fine-scale circulation on biogeochemical processes and phytoplankton biodiversity in the North Balearic Front (Western Mediterranean Sea). Coordinated with the initial CalVal phase of the SWOT (Surface Water and Ocean Topography) satellite mission, the campaign leveraged the high spatial resolution of SWOT, capable of detecting circulation features as small as 7–10 km and used an adaptive Lagrangian sampling strategy. Three distinct fine-scale features within a region approximately 50 kilometers wide were targeted: a frontal zone separating an anticyclonic eddy from a cyclonic eddy, encompassing contrasting water masses. A comprehensive dataset of nitrate and phosphate concentrations was collected using a Niskin bottle carousel (discrete profiles down to 500 m), a high-resolution pumping system (sampling every 2–4 m down to 50 m) and one BGC-Argo float (sampling of nitrates down to 400 m).

 

Estimating nutricline depths and concentration gradients at this unprecedented scale was constrained by uncertainties associated with near-zero phosphate concentrations in the upper water column and the discrete sampling methods. To address these challenges, innovative data processing techniques were employed. Statistical approaches to reconstruct continuous nutrient profiles enabled more precise estimates of nutricline depths and gradients, while facilitating the application of functional data analysis. Significant variability across the front appeared: concentration gradients (nitracline depths) were highest (shallowest) in the cyclonic feature and lowest (deepest) in the anticyclonic feature, emphasizing the link between fine-scale oceanic structures and distinct vertical nutrient distributions. The underlying processes driving the observed variability remain to be elucidated. This study opens interesting perspectives on nutrient supply to the photic layer driven by fine-scale oceanic circulation in oligotrophic regions, and their role in shaping phytoplankton community dynamics.