Abstract
Abstract. Nutrient-rich water upwells offshore of northwest Africa and is subsequently advected westwards. There it forms eddies and filaments with a rich spatial structure of physical and biological/biogeochemical properties. Here we present a high-resolution (2.5 km) section through upwelling filaments and an eddy obtained in May 2018 with a TRIAXUS towed vehicle equipped with various oceanographic sensors. Physical processes at the mesoscale and submesoscale such as symmetric instability, trapping of fluid in eddies, and subduction of low potential vorticity (which we use as a water mass tracer) water can explain the observed distribution of biological production and export. We found a nitrate excess (higher concentrations of nitrate than expected from oxygen values if only influenced by production and remineralization processes) core of an anticyclonic mode water eddy. We also found a high nitrate concentration region of ≈5 km width in the mixed layer where symmetric instability appears to have injected nutrients from below into the euphotic zone. Similarly, further south high chlorophyll-a concentrations suggest that nutrients had been injected there a few days earlier. Considering that such interactions of physics and biology are ubiquitous in the upwelling regions of the world, we assume that they strongly influence the productivity of such systems and their role in CO2 uptake. The intricate interplay of different parameters at the kilometer scale needs to be taken into account when interpreting single-profile and/or bottle data in dynamically active regions of the ocean.
Highlights
In the euphotic zone of a stratified ocean, productivity tends to be limited by nutrients as vertical exchange is suppressed in geostrophically balanced flows
Physical processes at the mesoscale and submesoscale such as symmetric instability, trapping of fluid in eddies, and subduction of low potential vorticity water can explain the observed distribution of biological production and export
Here we ask the question: what aspects of highly resolved observed biological structures can be explained by hydrography or physical dynamics versus active biological processes? Here we present a high-resolution section through a filament and an eddy of upwelled water offshore of northwest Africa that reveals many of the above-discussed processes in a hitherto unavailable detail
Summary
In the euphotic zone of a stratified ocean, productivity tends to be limited by nutrients as vertical exchange is suppressed in geostrophically balanced flows. In addition to the continuous input of nutrients from below the mixed layer, nitrogen regeneration may support new production in the filaments (Clark et al, 2016) Such fronts and filaments are ubiquitous over Earth’s oceans, especially in regions that have processes which generate lateral density gradients, but their overall impact on vertical nutrient supply remains understudied (Mahadevan, 2016). Sea surface temperature (SST) showed that upwelling of cold and nutrient-rich water close to the coast produced a strong bloom This water was subsequently advected offshore and formed a filament that moved out into the ocean towards the west. This exchange can be an efficient way to bring water from below the mixed layer into the mixed layer associated with small-scale mixed layer eddies and filaments characterized by Ro 1 and outcropping isopycnals At such locations, Hosegood et al (2017) found higher phytoplankton concentrations (as shown by chlorophyll-a fluorescence). In addition to the TRIAXUS, we used vesselmounted sensor data, analyzed underway samples, and interpreted satellite data
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