High-resolution physical--biogeochemical structure of a filament and an eddy of upwelled water off northwest Africa

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 nitrate concentrations than would be expected from oxygen values if only influenced by production and remineralization processes) core of an anti-cyclonic 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. A similar region a little further south had high chlorophyll-a concentrations suggesting that nutrients had been injected there a few days earlier. Considering that such interactions of physics and biology are ubiquitous in the world's upwelling regions, we assume that they have strong influences on the productivity of such systems and their role in CO2 uptake. The intricate interplay of different parameters at kilometer scale needs to be taken into account when interpreting single profile and/or bottle data in dynamically active regions of the ocean

Fluxes of nutrients in a three-dimensional meander structure of the Antarctic Polar Front

The horizontal and vertical advection and the vertical diffusion of two plant nutrients (nitrate and silicate) are estimated at the Antarctic Polar Front (APF) in the Atlantic sector using quasi-synoptic, high-resolution physical and chemical measurements. Our results suggest that the routes of nutrient supply are more complex than indicated by existing large-scale views. The vertical advection associated with mesoscale upwelling events is shown to be between two and three orders of magnitude larger than the diffusion, and to potentially amount to the phytoplankton uptake rate locally. Averaged over the survey area, however, the vertical nutrient transport is downward and concords with the front acting as a barrier to the northward export of surface nutrients by the Ekman drift. This poses significant constraints on the global cycles of nutrients and may have an impact in the sediment record

A new way to look at mesoscale zooplankton distributions: an application at the Antarctic Polar Front

During the austral summer of 1996 an intensive multidisciplinary survey was carried out in the Antarctic Polar Front. In this paper, we examine the relationship between hydrographic features and the acoustic backscatter signal from a vessel-mounted acoustic Doppler current profiler. We introduce two new analysis methods: mapping of depth-integrated scattering cross section and a model of the vertical zooplankton distribution through idealised population dynamics. Three distinct layers are characterised by a different balance between diel migration behaviour and direct/indirect forcing by mesoscale physical processes in the ocean

A 3-D mesoscale map of primary production at the Antarctic Polar Front: results of a diagnostic model

A diagnostic model is established to estimate synoptically the mesoscale distribution of primary production at the Antarctic Polar Front (APF). The model domain is a 3-D box, centred at roughly 50 S and 10 E, of about 1 degree latitude and 2 degree longitude horizontal extent, and of 300 m depth. The box was surveyed in high resolution during austral summer 1995/1996 with a towed undulating vehicle and by complementary ship-based measurements. Measurements of global solar radiation, of the underwater light field, and of the chlorophyll concentration from the survey are used as input variables for the model. The model is based on photosynthesis-light relationships, with parameters taken from in vitro incubations performed during the survey. The model results show mesoscale patches of elevated primary production along a meander of the APF, and lowest production in a cold cyclonic eddy south of the front. Production is confined to a shallower depth range in the front than outside, due to self-shading effects from generally higher mixed-layer chlorophyll concentrations. Self-shading effects account for variations of the percent light depths, and of the saturation light depth, by a factor of 2 within the survey area. Primary production at the surface varies horizontally between 7 and 56 mg C m^-3 d^-1, with a mean of 26 mg C m^-3 d^-1, and vertically integrated production ranges from 295 to 975 mg C m^-2 d^-1, with an areal mean of 585 mg C m^-2 d^-1. Changes by a factor of 2 in integrated production occur on horizontal scales as small as 10 km. Production rates also differ significantly between days as a result of changes in global solar radiation

Significance of the Polar Frontal Zone for large-sized diatoms and new production during summer in the Atlantic sector of the Southern Ocean

The chlorophyll a (chl a) biomass and primary production of three phytoplankton size fractions were estimated in the Atlantic sector of the Southern Ocean from 12 December 1995 to 20 January 1996. Elevated concentrations of chl a, primary production, and contribution of microplankton (&gt;20 ?m) coincided with dominance by large or long-chained diatoms (Thalassiothrix spp., Pseudonitzschia lineola, and Chaetoceros spp.) in the Polar Frontal Zone (PFZ, 49.5°S–52°S). Vertically resolved assimilation numbers (i.e. primary production normalized to chl a) and intrinsic growth rates of microplankton were much lower at high-biomass stations of the Northern Polar Frontal Zone (NPFZ) than in adjacent waters. Silicic acid appeared to be the proximal factor limiting the growth and yield of resident diatoms in the NPFZ. A carbon budget showed that, during the sampling period, diatom dominance at high-biomass sites was associated with near-steady-state conditions. This system exhibited a low POC sinking flux relative to total primary production, despite strong dominance by microplankton. South of 52°S, total chl a and primary production were generally low, but increased sharply in a mixed Phaeocystis-diatom bloom that extended from 61°S to 65°S in the Seasonal Ice Zone (SIZ). We estimated that the PFZ contributes from 37% to 67% of the total open-water new production in the Southern Ocean near the Greenwich meridian, depending on whether blooms occur or not in the SIZ.<br/

Mesoscale frontal dynamics: shaping the environment of primary production in the Antarctic Circumpolar Current

The frontal regions of the Antarctic Circumpolar Current (ACC) differ from other parts of the ACC due to higher phytoplankton concentrations and primary production rates. We hypothesise that the enhancement of primary production results from the mesoscale frontal dynamics, in particular the cross-front circulation related to baroclinic instability.The hypothesis is corroborated by data collected in austral summer 1995/1996 at the Antarctic Polar Front between 1 W and 12 E during quasi-synoptic surveys with a measuring system combining towed and vessel-mounted instruments. Further confirmation is obtained from moored current meters and an earlier section worked across the front in austral winter 1992.The quasi-synoptic surveys cover a meander structure of the front and a cold cyclonic eddy located to its south. The highest chlorophyll concentrations, correlating with enhanced primary production rates, are found in a band of mesoscale patches aligned with the front and in a tongue extending southward from the front along the leading edge of a meander ridge. The increased chlorophyll concentrations at the meander edge are explained by confluence of surface water, which carries with it the phytoplankton grown in favourable light conditions. While mesoscale surface confluence structures the synoptic chlorophyll distribution pattern, the mean enhancement of primary production at the front can be attributed to the influence of cross-front circulation on stratification. Ageostrophic cross-front circulation related to mesoscale upwelling and downwelling was identified at a site of eddy/front interaction. Consistent with the principle of potential vorticity conservation, upwelling was found to occur on the anticyclonic, equatorward side of the jet and downwelling on the cyclonic, poleward side in the frontogenetic situation. The associated cross-front circulation is characterised by poleward motion of light water at the surface and a reversed flow of dense water at greater depth; thus it contributes to stratification and thereby to a more favourable photic environment for the phytoplankton growing in the shallower mixed layer. While the cross-front circulation varies on horizontal scales of &lt;10 to &gt;100 km and time scales of days to weeks, it is constrained to sites of available potential energy, i.e. fronts marked by sloping isopycnals

Thick-shelled, grazer-protected diatoms decouple ocean carbon and silicon cycles in the iron-limited Antarctic Circumpolar Current

Diatoms of the iron-replete continental margins and North Atlantic are key exporters of organic carbon. In contrast, diatoms of the iron-limited Antarctic Circumpolar Current sequester silicon, but comparatively little carbon, in the underlying deep ocean and sediments. Because the Southern Ocean is the major hub of oceanic nutrient distribution, selective silicon sequestration there limits diatom blooms elsewhere and consequently the biotic carbon sequestration potential of the entire ocean. We investigated this paradox in an in situ iron fertilization experiment by comparing accumulation and sinking of diatom populations inside and outside the iron-fertilized patch over 5 wk. A bloom comprising various thin- and thick-shelled diatom species developed inside the patch despite the presence of large grazer populations. After the third week, most of the thinner-shelled diatom species underwent mass mortality, formed large, mucous aggregates, and sank out en masse (carbon sinkers). In contrast, thicker-shelled species, in particular Fragilariopsis kerguelensis, persisted in the surface layers, sank mainly empty shells continuously, and reduced silicate concentrations to similar levels both inside and outside the patch (silica sinkers). These patterns imply that thick-shelled, hence grazer-protected, diatom species evolved in response to heavy copepod grazing pressure in the presence of an abundant silicate supply. The ecology of these silica-sinking species decouples silicon and carbon cycles in the iron-limited Southern Ocean, whereas carbon-sinking species, when stimulated by iron fertilization, export more carbon per silicon. Our results suggest that large-scale iron fertilization of the silicate-rich Southern Ocean will not change silicon sequestration but will add carbon to the sinking silica flux

Particulate organic carbon export across the Antarctic Circumpolar Current at 10°E: Differences between north and south of the Antarctic Polar Front

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