African Coastal Upwelling Research Articles

A 50-year (1971–2021) mesozooplankton biomass data collection in the Canary Current System: Base line, gaps, trends, and future prospect

Mesozooplankton have been widely used as a bioindicator of marine ecosystems due to their key position in ocean food webs, rapid response to environmental changes, and ubiquity. Here, we show mesozooplankton biomass values in the Canary Current System from 1971 to 2021 in three different areas in relation to mesoscale activity: (1) scarcely affected by mesoscales structures (North of the Canary Islands), (2) affected by mesoscale activity and the presence of the islands (South and around the islands), and (3) close to the Northwest African coastal upwelling system (Upwelling influenced). A Generalized Additive Mixed Model (GAMM) was used to analyze the general mesozooplankton biomass trend throughout the studied period discriminating differences in biomass between the areas, annual cycle, and day-nighttime periods. The GAMM showed a significant negative biomass tendency North of the Canary Islands over the 50-year time-series compared to the South and around the islands, and significant differences between day and nighttime periods (p < 0.001) and the annual cycle (p < 0.0001). Linear regression analyses showed different tendencies depending on the area, season, and period. When comparing biomass data of the most oligotrophic zone (north of the islands) with other tropical-subtropical time-series stations in Hawaii (HOTS) and Bermuda (BATS), we obtained increasing biomass tendencies for both fixed time stations but decreasing tendency for our time-series.

Sustainability Assessment of the Societal Costs of Fishing Activities in a Deliberative Perspective

Assessing the social cost of fisheries is generally seen as a matter of how to monetize the components of fisheries. This paper presents an assessment of the societal cost of fishing activities, seen as a social process that is expected to contribute to the better management of aquatic resources, affecting sustainable development in coastal areas around the world. The originality of this article lies in considering the sustainability assessment from a deliberative perspective. It aims at defining the types of guiding concepts, frameworks, and information sets that might be appropriate for decision support, as we enlarge our scope of concern from fisheries to the ecosystems of eco-regions in the long term. In defining the societal cost of fisheries, through interviews, the objective is, first of all, to identify the social effects (positive and negative) of fishing métiers. By comparing fishing activities in a multi-criteria and multi-actor analysis, this evaluation is intended as a means for the actors to express in different ways (scientific indicators, institutional objectives, etc.) their judgment regarding the sustainability of the fishing profession. This analysis is the basis for defining the methods of monetizing these effects in different eco-regions (West African coastal upwelling and the deltas of Southeast Asia).

CO2 fluxes in the Northeast Atlantic Ocean based on measurements from a surface ocean observation platform

The seasonal and spatial variability of the CO2 system parameters and CO2 air-sea exchange were studied in the Northeast Atlantic Ocean between the northwest African coastal upwelling and the oligotrophic open-ocean waters of the North Atlantic subtropical gyre. Data was collected aboard a volunteer observing ship from February 2019 to February 2020. The seasonal and spatial variability of CO2 fugacity in seawater (fCO2,sw) was strongly driven by the seasonal temperature variation, which increased with latitude and was lower throughout the year in coastal regions where the upwelling and offshore transport was more intense. The thermal to biological effect ratio (T/B) was approximately 2, with minimum values along the African coastline related to higher biological activity in the upwelled waters. The fCO2,sw increased from winter to summer by 11.84 ± 0.28 μatm°C−1 on the inter-island routes and by 11.71 ± 0.25 μatm°C−1 along the northwest African continental shelf. The seasonality of total inorganic carbon normalized to constant salinity of 36.7 (NCT) was studied throughout the region. The effect of biological processes and calcification/dissolution on NCT between February and October represented >90% of the reduction of inorganic carbon while air-sea exchange described <6%. The seasonality of air-sea CO2 exchange was controlled by temperature. The surface waters of the entire region acted as a CO2 sink during the cold months and as a CO2 source during the warm months. The Canary basin acted as a net sink of −0.26 ± 0.04 molC m−2 yr−1. The northwest African continental shelf behaved as a stronger sink at −0.48 ± 0.09 molC m−2 yr−1. The calculated average CO2 flux for the entire area was −2.65 ± 0.44 TgCO2 yr−1 (−0.72 ± 0.12 TgC yr−1).

The gastropod Osilinus atrata as a bioindicator of Cd, Cu, Pb and Zn contamination in the coastal waters of the Canary Islands

The aim of this study was to assess the use of Osilinus atrata (Gastropoda, Mollusca) as a biomonitor of heavy metals (Cd, Cu, Pb and Zn) within the coastal zone of the Canary Islands. In general, this top-shell snail showed high intraspecific variability in its tissue metal concentrations, which decreased according to the following sequence: Cu≥Zn>Cd≥Pb. In particular, Cd and Zn concentrations were higher in males than in females, whereas Cu concentrations increased slightly with weight, and Pb concentrations decreased. Cd and Cu concentrations showed notable spatial variability; Cd concentrations were higher in the eastern islands (Alegranza, La Graciosa, Lanzarote and Fuerteventura) and La Palma, whereas Cu levels were highest in some western islands (Gran Canaria, La Gomera and El Hierro). The arrival of waters from the African coastal upwelling, the pollution of coastal waters with agricultural fertilisers and the competition for uptake between metals appear to be the likely causes within these patterns of accumulation. In conclusion, O. atrata presents a great potential as a biomonitor of heavy metals, however, more information and further studies are necessary.

Spatial variation patterns of subtidal seaweed assemblages along a subtropical oceanic archipelago: Thermal gradient vs herbivore pressure

The structure and composition of subtidal rocky seaweed assemblages were studied at 69 sites on the Canary Islands (northeastern Atlantic). This group of islands are situated at the southern boundary of the warm temperate region and adjacent to the cold waters from the northwest African coastal upwelling, which creates a difference of almost 2 °C in surface seawater temperature from the eastern to the western islands. This thermal variation allows an examination of the transition between the warm temperate and the tropical regions along this longitudinal gradient together with the hypothesised Fucales-dominated assemblages towards the eastern islands in contrast to the Dictyotales-dominated assemblages towards the western ones. Environmental and biological parameters were considered in order to investigate which were the main factors explaining spatial variation along the gradient in a multi-scaled approach. Although seventy-nine macroalgae were identified, 87.63% of the total mean cover was due to six taxa ( Lobophora variegata, nongeniculate corallines, Canistrocarpus cervicornis, Jania adhaerens, Cystoseira abies-marina and Pseudolithoderma adriaticum). At a large scale, sea urchin density explained the highest variation in seaweed assemblages (26.94%), and its pattern of distribution across the islands. The expected pattern of distribution according to the upwelling distance only occurred in restricted areas of the Canarian Archipelago in absence of herbivore pressure and habitat degradation. Spatial variations within islands (medium scale) were mainly related to wave exposure, while at a small scale these were mostly due to the degree of sedimentation.

Seasonal variability of the Canary Current: A numerical study

[1] A high-resolution numerical model study of the Canary Basin in the northeast subtropical Atlantic Ocean is presented. A long-term climatological solution from the Regional Oceanic Modeling System (ROMS) reveals mesoscale variability associated with the Azores and Canary Current systems, the northwest African coastal upwelling, and the Canary Island archipelago. The primary result concerns the Canary Current (CanC) which, in the solution, transports ∼3 Sv southward in line with observations. The simulated CanC has a well-defined path with pronounced seasonal variability. This variability is shown to be mediated by the westward passage of two large annually excited counterrotating anomalous structures that originate at the African coast. The anomalies have a sea surface expression, permitting their validation using altimetry and travel at the phase speed of baroclinic planetary (Rossby) waves. The role of nearshore wind stress curl variability as a generating mechanism for the anomalies is confirmed through a sensitivity experiment forced by low-resolution winds. The resulting circulation is weak in comparison to the base run, but the propagating anomalies are still discernible, so we cannot discount a further role in their generation being played by annual reversals of the large-scale boundary flow that are known to occur along the African margin. An additional sensitivity experiment, where the Azores Current is removed by closing the Strait of Gibraltar presents the same anomalies and CanC behavior as the base run, suggesting that the CanC is rather insensitive to upstream variability from the Azores Current.

Interpretation of the paleo-primary production record in the NW African coastal upwelling system as potentially biased by sea level change

Using a regional circulation-ecosystem model subject to a set of boundary conditions that reflect present-day (PD) and Last Glacial Maximum (LGM) conditions, we investigate how changes in preformed subsurface nutrient concentrations as opposed to a local drop in sea level or increase in wind stress influence the biological productivity off the northwest African coastal upwelling area. A reduction of subsurface nutrient concentration at LGM reduced the zonally averaged primary production (PP). The sea level change modifies the shelf morphology, the upwelling circulation, and the productivity and displaces the high-productivity zone. According to our model results, the sedimentary record of PP is best explained by a simulation considering a doubled wind stress over NW Africa. We conclude that production regionally along the coast of NW Africa during the LGM may have been lower than today, despite the fact that most of the sediment cores north of 22°N show local increases in PP during the last glacial period. This paradox may be due to the sea level change and the associated eastward shift of the coastline and upwelling system.

The ichthyoplankton assemblage of the Canaries–African Coastal Transition Zone: A review

In this paper we review information on the ichthyoplankton assemblage of the Canaries–African Coastal Transition Zone (C–ACTZ). This CTZ shows the singularity that the Canary Archipelago interrupts the main flow of the Canary Current and Trade Winds, introducing large mesoscale variability, in the form of island warm wakes and cyclonic and anticyclonic eddies downstream of the islands. Besides, upwelling filaments stretch towards the archipelago from the African coastal upwelling, transporting phytoplankton, zooplankton and fish larvae. They also interact with eddies shed from the islands to exchange water properties and biogenic material. All these mesoscale features influence the composition, structure, abundance and distribution of the larval fish community (LFC) of the region. The Canary Current (CC) and eddies shed from the islands drag larvae of island neritic fish species into the oceanic region and contribute, along warm wakes, to the horizontal distribution of fish larvae. Upwelling and upwelling filaments transport larvae of African neritic species into the oceanic region. These larvae dominate the LFC and account for the relatively high average larval fish abundance found in the C–ACTZ during the summer upwelling season. Filaments originated in the region of Cape Juby–Cape Bojador are entrained around a quasi-permanent cyclonic eddy, trapped between Gran Canaria Island and the African coast, forming a system through which most of the African neritic larvae may return to the African shelf. However, some larvae reach the eastern islands of the Canary archipelago and they may be spread all over the neritic region of the archipelago by eddies shed from the islands. Also in summer, the distribution of the LFC of the C–ACTZ is vertically stratified and fish larvae seem to carry out little or not diel vertical migration. Overall, this study highlights the strong relationship between mesoscale oceanographic processes and the LFC in the C–ACTZ.

Air-sea fluxes of biogenic bromine from the tropical and North Atlantic Ocean

Abstract. Air-sea fluxes and bulk seawater and atmospheric concentrations of bromoform (CHBr3) and dibromomethane (CH2Br2) were measured during two research cruises in the northeast Atlantic (53–59° N, June–July 2006) and tropical eastern Atlantic Ocean including over the African coastal upwelling system (16–35° N May–June 2007). Saturations and sea-air fluxes of these compounds generally decreased in the order coastal &gt; upwelling &gt; shelf &gt; open ocean, and outside of coastal regions, a broad trend of elevated surface seawater concentrations with high chlorophyll-a was observed. We show that upwelling regions (coastal and equatorial) represent regional hot spots of bromocarbons, but are probably not of major significance globally, contributing at most a few percent of the total global emissions of CHBr3 and CH2Br2. From limited data from eastern Atlantic coastlines, we tentatively suggest that globally, coastal oceans (depth &lt;180 m) together contribute ~2.5 (1.4–3.5) Gmol Br yr−1 of CHBr3, excluding influences from anthropogenic sources such as coastal power stations. This flux estimate is close to current estimates of the total open ocean source. We also show that the concentration ratio of CH2Br2/CHBr3 in seawater is a strong function of concentration (and location), with a lower CH2Br2/CHBr3 ratio found in coastal regions near to macroalgal sources.

Mesoscale distribution of clupeoid larvae in an upwelling filament trapped by a quasi-permanent cyclonic eddy off Northwest Africa

The distribution of fish larvae in relation to a filament shed from the Northwest African coastal upwelling was studied in February 2001. During the cruise, the filament was located between 27°N and 28°N, extending from the African coastal upwelling zone to the south of Fuerteventura Island (Canary Islands). This filament was trapped and remained over the quasi-permanent cyclonic eddy observed in previous studies. Almost all Sardina pilchardus, Engraulis encrasicolus and Sardinella aurita larvae caught during the cruise were associated with upwelled waters and filament structures. The sampled larval fish assemblage was composed by 12.6% of clupeoid larvae. These were distributed as follows: 73.9% were S. pilchardus, 20.7% were E. encrasicolus and 5.4% were S. aurita. Their distribution suggested that the coastal upwelling filament is a mechanism of transport from the upwelling area to oceanic waters, but its junction with the generated cyclonic eddy may not always work as a retention structure for those transported larvae, as described in previous studies. Physiological studies based on gut fluorescence and ETS activity of clupeoid larvae, as proxies for grazing and respiration, respectively, denoted a switch from pigmented food near the upwelling zone to unpigmented food toward the ocean. This pattern agrees with observed mesozooplankton feeding along an upwelling filament in previous studies. Therefore, this work confirms the close relationship between clupeoids distribution and mesoscale circulation, as well as constitutes the first assessment of the metabolic activity of those larvae in the region.

Modelling an alkenone-like proxy record in the NW African upwelling

Abstract. A regional biogeochemical model is applied to the NW African coastal upwelling between 19° N and 27° N to investigate how a water temperature proxy, alkenones, are produced at the sea surface and recorded in the slope sediments. The biogeochemical model has two phytoplankton groups: an alkenone producer group, considered to be coccolithophores, and a group comprising other phytoplankton. The Regional Ocean Modelling System (ROMS) is used to simulate the ocean circulation and takes advantage of the Adaptive Grid Refinement in Fortran (AGRIF) package to set up an embedded griding system. In the simulations the alkenone temperature records in the sediments are between 1.1 and 2.3°C colder than the annual mean SSTs. Despite the seasonality of the coccolithophore production, this temperature difference is not mainly due to a seasonal bias, nor to the lateral advection of phytoplankton and phytodetritus seaward from the cold near-shore waters, but to the production depth of the coccolithophores. If coretop alkenone temperatures are effectively recording the annual mean SSTs, the amount of alkenone produced must vary among the coccolithophores in the water column and depend on physiological factors (e.g. growth rate, nutrient stress).

Annual cycle of clupeiform larvae around Gran Canaria Island, Canary Islands

AbstractThe distribution and abundance of fish larvae was studied along the eastern and southern shelf of Gran Canaria Island (Canary Islands) from July 2000 to June 2001. Oblique bongo hauls were carried out fortnightly during the daytime, coinciding with days of full and new moon. During February, the area was sampled every 2–5 days. About 17.3% of the ichthyoplanktonic community was composed of clupeiform larvae: 92.9% of these larvae were Sardinella aurita, whereas 4.7% and 2.4% were respectively Engraulis encrasicolus and Sardina pilchardus. Sardinella aurita larvae appeared during the whole year with two periods of maximum abundance: June to September and December to February. During the full moon their abundance was on average 38.5% (±6.8%) of their numbers during the new moon, showing a clear lunar periodicity. Engraulis encrasicolus larvae appeared from November to March, also coinciding with the new moon. Sardina pilchardus larvae only appeared during two short periods, both coinciding with filaments shed from the African coastal upwelling which reached the island. This fact confirms the transport of fish larvae from the upwelling area off northwest Africa to the Canary Islands, promoting a genetic flow among both sites.

Vertical distribution of fish larvae in the Canaries-African coastal transition zone in summer

This study reports the vertical distribution of fish larvae during the 1999 summer upwelling season in the Canaries-African Coastal Transition Zone (the Canaries-ACTZ). The transition between the African coastal upwelling and the typical subtropical offshore conditions is a region of intense mesoscale activity that supports a larval fish population dominated by African neritic species. During the study, the thermal stratification extended almost to the surface everywhere, and the surface mixed layer was typically shallow or non-existent. Upwelling occurred on the African shelf in a limited coastal sub-area of our sampling. The vertical distributions of the entire larval fish population, as well as of individual species, were independent of the seasonal thermocline. Fish larvae and mesozooplankton were concentrated at intermediate depths regardless of the thermocline position, probably because of its weak signature and spatial and temporal variability. Day/night vertical distributions suggest that some species did not perform diel vertical migration (DVM), whereas others showed either type I DVM or type II DVM. The opposing DVM patterns of different species compensate for each other resulting in no net DVM for the larval fish population as a whole.

Steady-state DCM dynamics in Canaries waters

This paper concerns the deep chlorophyll maximum (DCM) dynamics in a steady state, using primarily data from the Canary Islands Area of Filament and Eddy eXchange obtained in August 1999 during a cruise between oligotrophic waters west of La Palma and the north-west African coastal upwelling. CTD-fluorometer observations of the deep fluorescence maximum (DFM) were confirmed by water samples from which chlorophyll was extracted. The DFM–DCM was perturbed at many stations by island-generated eddies and similar features, and the paper focuses on unperturbed stations, which were identified by the occurrence of the DFM close to the 26.4 kg m −3 isopycnal. The DFM at these stations occurred at the top of the nitracline, in the presence of 0.3–2 μM nitrate+nitrite, and at 24 h mean isolumes of 10–20 μE m −2 s −1 . The classical compensation depth model predicts the occurrence of the DFM–DCM at less illumination and hence at too great a depth, making it necessary to take into account additional losses, especially those due to the respiration of microheterotrophs in biomass equilibrium with phytoplankton. The depth of the steady-state DCM is compatible with the predictions of a microplankton model, given (i) a ratio of 0.3–0.6 of microheterotroph to total microplankton biomass, and (ii) other losses (due to meszooplankton grazing and vertical mixing) of about 0.1 d −1 .

Coastal upwelling and associated retention indices derived from satellite SST. Application to Octopus vulgaris recruitment

The thermal infrared data from the satellites of the European Meteosat series are used to characterize the dynamics of the West African coastal upwelling for the period 1984–1998. Sea surface temperature (SST) images are produced from satellite data with a spatial resolution of 5 km and a time-step of 5 days. These are used to characterize the spatial structure of the upwelling by automatic localisation of the SST minima at each coast location and to derive a normalised upwelling intensity index, based on SST differences. A new index for retention potential is also proposed. The strong interannual variability observed in these indices is quantified, mainly in terms of intensity and seasonal lag. An objective method of space partitioning, based on the variability of the proposed indices is introduced. Objective analysis applied to three characteristic regions shows that the spawning areas of the coastal pelagic species may be clearly delineated in terms of spatial and temporal optimisation of enrichment and retention processes. The importance of the retention processes is shown for the recruitment of Sardinella aurita in Senegal and for Octopus vulgaris in Mauritania. According to the concept of Bakun's triad for upwelling areas, a precise determination of the upwellings intensity and associated spatial processes is essential for evaluating the balance between enrichment and its optimal use in an ecological sense. A Java-based software package is produced as part of this study to visualise long series of pre-processed images and to extract the above indices. © 2000 Ifremer/CNRS/IRD/Éditions scientifiques et médicales Elsevier SAS coastal upwelling / sea surface temperature / upwelling index / coastal retention process / recruitment variability

Mesoscale variability in phytoplankton biomass distribution and photosynthetic parameters in the Canary-NW African coastal transition zone

ABSTRAff: Pigment distribution and photosynthesis versus irradiante (P-E) responses in the rnixed layer (ML) and in the chlorophyil maximurn (CM] were examined in the coastal transition zone (CTZ) between the NW African coastal upwelling and the Canary Islands during August 1993. The sampling included 2 island-generated eddies in the lee of the archipelago and an upwelling filament from the Afncan continental shelf, entrained around a cyclonic eddy about 100 km width. Chiorophyii a and P-E pararneters (a, P,) showed reqonal differences refleding changes in the water column structure and phytoplankton species composition. The deep CM shoaied from -100 to -10 m as the African shelf was approached, and there was a clear offshore-onshore transition in the verticaiiy integrated chlorophyli distribution and P-E responses related to the upwelling front. CM oceanic samples presented high a (0.020 to 0.042 mg C mg-' chl h-' [pmol m-' S-']-') and photomhibition. indicating adaptation to low irradmnce. Differences between CM and ML in P-E responses decreased in the vicinity of the upwelling untu they eventuaiiy became indistinguishable in a well-rnixed station on the continental sheif. Island-generated eddies affected mainly the productivity and chlorophyli distributions at the deep CM. Nutrient input in the eddy center resulted in an increase of a in the CM to the leve1 of the upwellmg samples (-0.053 mg C mg-' chl he' [wol m-' S-']-'). On the basis of the mesoscaie variability in the physical structure of the water column and the vertical distribution of chlorophyil, the CTZ was divided into 4 subregions with characteristic photosynthetic pararneters. Average integrated production in these areas vaned from -100 mg C m-' d-' m the most oligotrophic subregion to more than 500:: mg m-2 d-l .be upwe--ly zuiie. T;ie raiige vaA*'iioii &le p~otosymi'ne~c pararneiers in this CTZ was of the sarne magnitude as ranges observed in basin-scale studies of the Atlantic. Integrated produdion estimates are also in the lower and higher extremes of the observed values in the ocean. Our results highlight the importante of addressing the variability of the photosynthetic parameters in coastal upweiiing-open ocean transition zones in order to model primary production at regional scales.

The influence of island-generated eddies on chlorophyll distribution: a study of mesoscale variation around Gran Canaria

This study reports hydrographic and biological observations from three cruises where cyclonic and anticyclonic eddies were observed downstream of Gran Canaria island. Based on field data and remote sensing images (AVHRR and CZCS), two mechanisms associated with island-generated eddies, largely responsible for the formation and distribution of chlorophyll around the Canary Islands, are proposed. First, nutrient pumping and vertical uplifting of the deep chlorophyll maximum by cyclonic eddies might represent important sources of primary production in the oligotrophic waters of the Canary region. Second, eddies are responsible for the horizontal transport and distribution of chlorophyll originating near the islands or off the African coast. Water with high chlorophyll content, resulting from island stirring or local upwelling at the flanks of the islands, is incorporated into cyclonic eddies in their development and subsequently transported downstream. On the other hand, anticyclonic eddies can also entrain water rich in chlorophyll when interacting with the offshore boundary of the African coastal upwelling. This chlorophyll will be advected southward as the eddy drifts. The recurrence of cyclonic and anticyclonic eddies, together with the presence of upwelling filaments throughout the year, must have important biological consequences in the formation and transport of organic matter in the Canary region. reserved

Phytoplankton pigment patterns in the Canary Islands area as determined using Coastal Zone Colour Scanner data

Data from the Nimbus-7 Coastal Zone Colour Scanner (CZCS) were analyzed to determine the phytoplankton pigment patterns in the ocean surrounding the Canary Islands. Two different pattterns were observed in this region. One pattern was associated with the influence of the northwest African coastal upwelling system on the ocean surrounding the Canary Islands and the other was associated with individual islands, probably due to the disruptive effect of the Canary Islands on the flow of the Canary Current

Systems analysis and simulation of the NW African marine upwelling region

Abstract A temporal and spatial marine upwelling system model is developed for analysis of prominent biological and physical system interactions which result in obserbed nutrient distribution patterns in the northwest African coastal upwelling system near 21° N. The model is composed of a physical submodel describing a two-dimensional circulation pattern, and a series of biological submodels describing nutrient regeneration. Model results were qualitatively consistent with nutrient distributions observed during an upwelling event in the 1974 JOINT-I study area. Simulated silicate distribution patterns suggest that silica dissolution parameter estimates reported for the northwest African region may represent local conditions, but are likely overestimates at a system level of resolution. Sensitivity analysis results identify grazing threshold level as the primary regulator of model grazing stress. Paucity of herbivorous nekton grazing suggests that primarily phytophagous grazing habit for clupeoid fishes is unlikely in the northwest African upwelling area.