An observational study of oceanic eddy generation mechanisms by tall deep-water islands (Gran Canaria)

[EN]Oceanic eddy generation by tall deep-water islands is common phenomenon. It is recognized that these eddies may have a significant impact on the marine system and related biogeochemical fluxes. Hence, it is important to establish favourable conditions for their generation. With this objective, we present an observational study on eddy generation mechanisms by tall deep-water islands, using as a case study the island of Gran Canaria. Observations show that the main generation mechanism is topographic forcing, which leads to eddy generation when the incident oceanic flow is sufficiently intense. Wind shear at the island wake may acts only as an additional eddy-generation trigger mechanism when the impinging oceanic flow is not sufficiently intense. For the case of the island of Gran Canaria we have observed a mean of ten generated cyclonic eddies per year. Eddies are more frequently generated in summer coinciding with intense Trade winds and Canary Current.

Lagrangian evolution of a mid ocean anticyclonic eddy

[EN]The lagrangian evolution of 4 months old anticyclonic intrathermocline eddy of the Canary Eddy Corridor is investigated from the trajectories of 5 satellites tracked drifting buoys. Buoys were drogued below and above the Ekman depth at 15 m and 100 m, respectively. One buoy remained inside the eddy during almost 4 months being thus a long lived coherent feature with a life span of at least 8 months. The eddy consisted in a central core rotating in solid body rotation with a rather constant periodicity of 4 days and in an outer ring rotating much more slowly with periodicities between 8 and 12 days…

Horizontal distribution of invertebrate larvae around the oceanic island of Gran Canaria: the effect of mesoscale variability

In October 1991, the horizontal distribution of invertebrate larvae was studied in the waters surrounding the island of Gran Canaria (Canary Islands). The cruise was typified by the presence of three recurrent mesoscale hydrographic structures: a cyclonic eddy southwest of the island, a warm lee region downstream of the island and the offshore boundary of an upwelling filament from the African coast reaching the southeast of the island. Decapod larvae were the most abundant group. In general, a rather high spatial variability was found. The horizontal distribution of the invertebrate larvae groups showed that the highest values of abundance occurred in an elongated zone around the island oriented in the overall direction of flow, leeward and windward of the island, while the lowest values occurred off the eastern and western flanks of the islands. On the other hand, Stomatopoda and Mollusca larvae showed a distribution associated with the boundary of the upwelling filament and decapod larvae of pelagic species were distributed around the eddy structure. Our results suggest specific retention mechanisms for the larvae of neritic invertebrate populations that are related to the particular physical oceanography around Gran Canaria. Se estudió la distribución horizontal de las larvas de invertebrados alrededor de la isla de Gran Canaria (Islas Canarias) en octubre de 1991. La campaña se caracterizó por la presencia de tres fenómenos oceanográficos de mesoescala recurrentes: un remolino ciclónico al suroeste de la isla, la estela cálida a sotavento de la misma y el borde de un filamento proveniente del afloramiento africano situado al sureste de Gran Canaria. Las larvas de crustáceos decápodos fueron las más abundantes. En general, las larvas de los diferentes taxa estudiados mostraron una gran variabilidad en cuanto a su distribución espacial. La distribución horizontal de las larvas de los diferentes grupos de invertebrados mostró que las densidades más altas se registraban alrededor de la isla en la banda cercana y orientada hacia el suroeste, en la dirección del flujo, mientras que los valores más bajos se encontraron en el norte y en los flancos este y oeste de la isla. Por otro lado, las larvas de estomatópodos y moluscos presentaron una distribución asociada al frente del filamento, mientras que las larvas de decápodos pelágicos se distribuyeron en torno al remolino ciclónico. Los resultados obtenidos sugieren la existencia de mecanismos de retención específicos para el mantenimiento de las poblaciones insulares, relacionados con los procesos físicos de mesoescala predominantes

Regional and temporal variability of sinking organic matter in the subtropical northeast Atlantic Ocean: a biomarker diagnosis

Sinking particles through the pelagic ocean have been traditionally considered the most important vehicle by which the biological pump sequesters carbon in the ocean interior. Nevertheless, regional scale variability in particle flux is a major outstanding issue in oceanography. 5 Here, we have studied the regional and temporal variability of total particulate organic matter fluxes, as well as chloropigment and total hydrolyzed amino acid (THAA) compositions and fluxes in the Canary Current region, between 20–30 N, during two contrasting periods: August 2006, characterized by warm and stratified waters, but also intense winds which enhanced eddy development south of the Canary Islands, 10 and February 2007, characterized by colder waters, less stratification and higher productivity. We found that the eddy-field generated south of the Canary Islands enhanced by >2 times particulate organic carbon (POC) export with respect to stations (FF; farfield) outside the eddy-field influence. We also observed flux increases of one order of magnitude in chloropigment and 70% in THAA in the eddy-field relative to FF stations. 15 Principal Components Analysis (PCA) was performed to assess changes in particulate organic matter composition between stations. At eddy-field stations, higher chlorophyll enrichment reflected “fresher” material, while at FF stations a higher proportion of pheophytin indicated greater degradation due to microbes and microzooplankton. PCA also suggests that phytoplankton community structure, particularly the dominance of 20 diatoms versus carbonate-rich plankton, is the major factor influencing the POC export within the eddy field. In February, POC export fluxes were the highest ever reported for this area, reaching values of 15 mmolCm−2 d−1 at 200m depth. Compositional changes in pigments and THAA indicate that the source of sinking particles varies zonally and meridionally and suggest that sinking particles were more degraded at 25 near-coastal stations relative to open ocean stations.

Mesoscale eddies: Hotspots of prokaryotic activity and differential community structure in the ocean

[EN] To investigate the effects of mesoscale eddies on prokaryotic assemblage structure and activity, we sampled two cyclonic eddies (CEs) and two anticyclonic eddies (AEs) in the permanent eddy-field downstream the Canary Islands. The eddy stations were compared with two far-field (FF) stations located also in the Canary Current, but outside the influence of the eddy field. The distribution of prokaryotic abundance (PA), bulk prokaryotic heterotrophic activity (PHA), various indicators of single-cell activity (such as nucleic acid content, proportion of live cells, and fraction of cells actively incorporating leucine), as well as bacterial and archaeal community structure were determined from the surface to 2000m depth. In the upper epipelagic layer (0?200 m), the effect of eddies on the prokaryotic community was more apparent, as indicated by the higher PA, PHA, fraction of living cells, and percentage of active cells incorporating leucine within eddies than at FF stations. Prokaryotic community composition differed also between eddy and FF stations in the epipelagic layer. In the mesopelagic layer (200?1000 m), there were also significant differences in PA and PHA between eddy and FF stations, although in general, there were no clear differences in community composition or single-cell activity. The effects on prokaryotic activity and community structure were stronger in AE than CE, decreasing with depth in both types of eddies. Overall, both types of eddies show distinct community compositions (as compared with FF in the epipelagic), and represent oceanic ?hotspots? of prokaryotic activity (in the epi- and mesopelagic realms).

Regional and temporal variability of sinking organic matter in the subtropical northeast Atlantic Ocean: A biomarker diagnosis

[EN] Sinking particles through the pelagic ocean have been traditionally considered the most important vehicle by which the biological pump sequesters carbon in the ocean interior. Nevertheless, regional scale variability in particle flux is a major outstanding issue in oceanography. Here, we have studied the regional and temporal variability of total particulate organic matter fluxes, as well as chloropigment and total hydrolyzed amino acid (THAA) compositions and fluxes in the Canary Current region, between 20?30_ N, during two contrasting periods: August 2006, characterized by warm and stratified waters, but also intense winds which enhanced eddy development south of the Canary Islands, and February 2007, characterized by colder waters, less stratification and higher productivity. We found that the eddyfield generated south of the Canary Islands enhanced by >2 times particulate organic carbon (POC) export with respect to stations (FF; far-field) outside the eddy-field influence. We also observed flux increases of one order of magnitude in chloropigment and 2 times in THAA in the eddy-field relative to FF stations. Principal Components Analysis (PCA) was performed to assess changes in particulate organic matter composition between stations. At eddy-field stations, higher chlorophyll enrichment reflected ?fresher? material, while at FF stations a higher proportion of pheophytin indicated greater degradation due to microbes and microzooplankton. PCA also suggests that phytoplankton community structure, particularly the dominance of diatoms versus carbonate-rich plankton, is the major factor influencing the POC export within the eddy field. In February, POC export POC export within the eddy field. In February, POC export fluxes were the highest ever reported for this area, reaching values of _15 mmolCm?2 d?1 at 200m depth. Compositional changes in pigments and THAA indicate that the source of sinking particles varies zonally and meridionally and suggest that sinking particles were more degraded at near-coastal stations relative to open ocean stations.

Towards the construction of a validated numerical system to study the mesoscale dynamics of the North East Atlantic (2003-2006)

Máster en Oceanografía

Coupling between upper ocean layer variability and size-fractionated phytoplankton in a non-nutrient-limited environment

[EN] We describe the coupling between upper ocean layer variability and size-fractionated phytoplankton distribution in the non-nutrient-limited Bransfield Strait region (BS) of Antarctica. For this purpose we use hydrographic and size-fractionated chlorophyll a data from a transect that crossed 2 fronts and an eddy, together with data from 3 stations located in a deeply mixed region, the Antarctic Sound (AS). In the BS transect, small phytoplankton (<20 μm equivalent spherical diameter [ESD]) accounted for 80% of total chl a and their distribution appeared to be linked to cross-frontal variability. On the deepening upper mixed layer (UML) sides of both fronts we observed a deep subducting column-like structure of small phytoplankton biomass. On the shoaling UML sides of both fronts, where there were signs of restratification, we observed a local shallow maximum of small phytoplankton biomass. We propose that this observed phytoplankton distribution may be a response to the development of frontal vertical circulation cells. In the deep, turbulent environment of the AS, larger phytoplankton (>20 μm ESD) accounted for 80% of total chl a. The proportion of large phytoplankton increases as the depth of the upper mixed layer (ZUML), and the corresponding rate of vertical mixing, increases. We hypothesize that this change in phytoplankton composition with varying ZUML is related to the competition for light, and results from modification of the light regime caused by vertical mixing.