Coastal Upwelling System Research Articles

Mechanisms and drivers controlling spatio-temporal evolution of pCO2 and air-sea CO2 fluxes in the southern Java coastal upwelling system

Upwelling systems are known for their complex dynamic processes spanning a wide variety of spatio-temporal scales, due to strong coupling between the ocean and atmosphere. One such upwelling system is found at the eastern boundary of the Indian Ocean along the southern coast of Java during the southeast monsoon (SM). This study examines the mechanisms and drivers involved in the spatial and temporal evolutions of surface pCO2 in this upwelling system over seasonal to inter-annual timescales using a coupled, high-resolution regional model. A decomposition analysis was used to quantify the changes in pCO2 in response to changes in surface-temperature (T), surface-salinity (S), dissolved inorganic carbon (DIC), and total alkalinity (ALK). The upwelling of deeper, cooler waters decreases the surface pCO2 by 50 ± 1.48 μatm from June to September. However, the presence of DIC-rich waters at significantly shallower depths increases the surface pCO2 by 27 ± 0.9 μatm. The surface pCO2 increases by 5 ± 0.50 μatm due to changes in salinity, which exerts less control than T-driven changes. Biological changes increase the surface pCO2 by 17.54 ± 2.74 μatm. The maximum seasonal amplitudes of the variations in pCO2 caused by seawater solubility and biology are in counterbalance during the SM. Therefore, the upwelling-driven combined effect of physical mechanisms dominates the effect of biological mechanisms in reducing surface pCO2 under these conditions. During the SM, the southern coast of Java acts as a seasonal CO2 sink (−1.36 ± 6.48 g C m−2 year−1), although consideration of the mean flux over 2006–2017 shows that the region acts as a source of CO2 (1.31 ± 5.03 g C m−2 year−1). Furthermore, following a negative Indian Ocean Dipole (IOD) year, a strong sink of atmospheric CO2 is observed during the SM.

Seasonality of coastal upwelling trends in the Mauritania-Senegalese region under RCP8.5 climate change scenario

The Mauritania-Senegalese upwelling region (MSUR), the southernmost region of the Canary current upwelling system, is well-known for its coastal productivity and the key role it plays in enriching the oligotrophic open ocean through the offshore transport of the upwelled coastal waters. The great ecological and socio-economic importance makes it necessary to evaluate the impact of climate change on this region. Hence, our main objective is to examine the climate change signal over the MSUR with a high resolution regional climate system model (RCSM) forced by the Earth system model MPI-ESM-LR under RCP8.5 scenario. This RCSM has a regional atmosphere model (REMO) coupled to a global ocean model (MPIOM) with high-resolution in the MSUR, which allows us to evaluate the wind pattern, the ocean stratification, as well as the upwelling source water depth, while maintaining an ocean global domain. Under RCP8.5 scenario, our results show that the upwelling favourable winds of the northern MSUR are year-round intensified, while the southern MSUR presents a strengthening in winter and a weakening in March–April. Along with changes in the wind pattern, we found increased ocean stratification in the spring months. In those months southern MSUR presents a shallowing of the upwelling source water depth associated to changes in both mechanisms. However, in winter the whole MSUR shows a deepening of the upwelling source water depth due to the intensification of the upwelling favourable winds, with the increased ocean stratification playing a secondary role. Our results demonstrate the need to evaluate the future evolution of coastal upwelling systems taking into account their latitudinal and seasonal variability and the joint contribution of both mechanisms.

Harmful algae diversity from a coastal upwelling system detected by high-throughput sequencing

IntroductionIn recent years, environmental DNA (eDNA) amplicon sequencing has been used to unveil plankton diversity in the field. Nevertheless, molecular methods, such as this, are rarely used in harmful algal bloom (HAB) monitoring programs, which mainly rely on morphological identification by conventional light microscopy.MethodsThe present study focused on a shallow marine environment (Ría de Vigo, Northwest Spain), where sediment and plankton samples were collected from 2016 to 2018.ResultsThe application of eDNA amplicon sequencing allowed us to simultaneously detect 25 potential harmful species (mainly diatoms and dinoflagellates) included in the IOC-UNESCO Taxonomic Reference List of Harmful Microalgae. Among these, causative agents of amnesic shellfish poisoning (Pseudo-nitzschia spp.) paralytic shellfish poisoning (Gymnodinium catenatum and Alexandrium minutum), azaspiracid producers (Azadinium poporum) and ichthyotoxic haptophytes (Chrysochromulina leadbeateri), were identified. Some toxic microalgae were better represented in sediment (e.g., Pseudo-nitzschia pungens, Gymnodinium catenatum) or planktonic fractions (e.g., Pseudo-nitzschia, Gymnodinium smaydae), confirming the importance of including both sediment and plankton fractions in eDNA monitoring studies. Despite the limitations of sequencing short amplicons, it was possible to discern in this study six Pseudo-nitzschia species and associate each of them with each seasonal peak produced in summer periods. Furthermore, several species previously unreported in Ría de Vigo (Pseudo-nitzschia turgidula, Chrysochromulina leadbeateri, Azadinium poporum) could be detected.DiscusionThese results point out the application of eDNA amplicon sequencing to expand our knowledge about harmful species in HAB monitoring programs and early warning systems for low abundant and rare taxa.

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.

Physical processes and biological productivity in the upwelling regions of the tropical Atlantic

Abstract. In this paper, we review observational and modelling results on the upwelling in the tropical Atlantic between 10∘ N and 20∘ S. We focus on the physical processes that drive the seasonal variability of surface cooling and the upward nutrient flux required to explain the seasonality of biological productivity. We separately consider the equatorial upwelling system, the coastal upwelling system of the Gulf of Guinea and the tropical Angolan upwelling system. All three tropical Atlantic upwelling systems have in common a strong seasonal cycle, with peak biological productivity during boreal summer. However, the physical processes driving the upwelling vary between the three systems. For the equatorial regime, we discuss the wind forcing of upwelling velocity and turbulent mixing, as well as the underlying dynamics responsible for thermocline movements and current structure. The coastal upwelling system in the Gulf of Guinea is located along its northern boundary and is driven by both local and remote forcing. Particular emphasis is placed on the Guinea Current, its separation from the coast and the shape of the coastline. For the tropical Angolan upwelling, we show that this system is not driven by local winds but instead results from the combined effect of coastally trapped waves, surface heat and freshwater fluxes, and turbulent mixing. Finally, we review recent changes in the upwelling systems associated with climate variability and global warming and address possible responses of upwelling systems in future scenarios.

Phytoplankton community changes in a coastal upwelling system during the last century

Phytoplankton community changes in a coastal upwelling system during the last century

Response of phytoplankton to coastal upwelling: The importance of temporal and spatial scales

AbstractThe Western Iberian Coast (WIC) is characterized by a dynamic coastal upwelling system with interactions between oceanographic features and continental river run‐off. The present study investigates the spatio‐temporal variability in the concentrations of Chlorophyll a (Chl a) and turbidity, and their relationships with physical and other biogeochemical properties along the entire coast of Portugal, as well as the response of phytoplankton to coastal upwelling pulses. In situ data were obtained during four oceanographic campaigns conducted in spring (April–May 2019), autumn (October 2018 and 2019), and early spring (February–March 2020). Upwelling conditions were evaluated through an upwelling index and auxiliary data such as sea surface temperature and height. In the WIC, the in situ surface temperature and salinity have a clear spatial pattern, both increasing from North to South. In autumn 2018 the highest temperatures coincided with the weakening of spring–summer upwelling conditions. Overall, high Chl a was associated with lower temperatures and higher dissolved oxygen concentrations (e.g., spring 2019). During spring 2019, an increase in Chl a, especially in the northern area, was associated with previous upwelling conditions. In early spring 2020, ongoing upwelling conditions in the northern area were associated with high turbidity and nutrients close to the shoreline with no immediate biological responses. An increase in Chl a was visible in satellite imagery a few days later. These results highlight the importance of understanding the complexity of coastal processes in the WIC.

Extracellular enzyme activity in the coastal upwelling system off Peru: a mesocosm experiment

Abstract. The Peruvian upwelling system is a highly productive ecosystem with a large oxygen minimum zone (OMZ) close to the surface. In this work, we carried out a mesocosm experiment off Callao, Peru, with the addition of water masses from the regional OMZ collected at two different sites simulating two different upwelling scenarios. Here, we focus on the pelagic remineralization of organic matter by the extracellular enzyme activity of leucine aminopeptidase (LAP) and alkaline phosphatase activity (APA). After the addition of the OMZ water, dissolved inorganic nitrogen (N) was depleted, but the standing stock of phytoplankton was relatively high, even after N depletion (mostly &gt; 4 µg chlorophyll a L−1). During the initial phase of the experiment, APA was 0.6 nmol L−1 h−1 even though the PO43- concentration was &gt; 0.5 µmol L−1. Initially, the dissolved organic phosphorus (DOP) decreased, coinciding with an increase in the PO43- concentration that was probably linked to the APA. The LAP activity was very high, with most of the measurements in the range of 200–800 nmol L−1 h−1. This enzyme hydrolyzes terminal amino acids from larger molecules (e.g., peptides or proteins), and these high values are probably linked to the highly productive but N-limited coastal ecosystem. Moreover, the experiment took place during a rare coastal El Niño event with higher than normal surface temperatures, which could have affected enzyme activity. Using a nonparametric multidimensional scaling analysis (NMDS) with a generalized additive model (GAM), we found that biogeochemical variables (e.g., nutrient and chlorophyll-a concentrations) and phytoplankton and bacterial communities explained up to 64 % of the variability in APA. The bacterial community best explained the variability (34 %) in LAP. The high hydrolysis rates for this enzyme suggest that pelagic N remineralization, likely driven by the bacterial community, supported the high standing stock of primary producers in the mesocosms after N depletion.

The niche of a stress-tolerant specialist, Dinophysis acuminata, in a coastal upwelling system

Dinophysis acuminata, the main cause of shellfish harvesting bans in Europe, blooms in the Galician Rías (NW Spain) throughout the upwelling season (ca. March to September). Here we illustrate rapid changes in vertical and across ría-shelf distributions of diatoms and dinoflagellates (including D. acuminata vegetative and small cells) in Ría de Pontevedra (RP) and Ría de Vigo (RV) during transitions from spin-down to spin-up phases of upwelling cycles. A subniche approach based on a Within Outlying Mean Index (WitOMI) showed that under the transient environmental conditions met during the cruise, both vegetative and small cells of D. acuminata colonized the Ria and Mid-shelf subniches, exhibiting good tolerance and extremely high marginality, in particular the small cells. Bottom-up (abiotic) control overwhelmed biological constraints, and shelf waters became a more favourable environment than the Rías. Contrasting higher biotic constraints inside the Rías were found for the small cells, with a subniche possibly controlled by unsuitable physiological status (notwithstanding the higher density) of the vegetative cell population. Results here on behaviour (vertical positioning) and physiological traits (high tolerance but very specialized niche) of D. acuminata give new insights into the ability of this species to remain in the upwelling circulation system. Higher shelf-ría exchanges in the Ría (RP) with more dense and persistent D. acuminata blooms reveal the relevance of transient event-scales and species- and site-specific characteristics to the fate of these blooms. Earlier statements about simple linear relationships between average upwelling intensities and the recurrence of Harmful algae bloom (HAB) events in the Galician Rías Baixas are questioned.

Potential effects of shoaling of oxygen minimum zone on the population of Acartia (Acanthacartia) tonsa in the Humboldt Current Upwelling System (23°S)

Expansion and shoaling of the Oxygen Minimum Zone (OMZ) in coastal upwelling systems due to climate change can have dire consequences on the ecosystem structure and productivity. The OMZ restricts the habitat range of most zooplankton, and species with low tolerance to hypoxia may suffer physiological stress and mortality as a result. During 2013 and 2014, the copepod Acartia tonsa collected off northern Chile was used to evaluate how hypoxia may affect its survival and vital rates. Exposure of gravid females to severe hypoxia (∼0.2 mL L−1 dissolved oxygen) resulted in total mortality. Hypoxia at ∼0.5 mL L−1 decreased female survival to ∼20% and egg production to nil. Hypoxia at ∼0.8 mL L−1 decreased survival to ∼50% but no change in egg production rate compared to the normoxia treatment. Egg hatching success was ∼68% and naupliar development time was ∼36% lower in hypoxia than normoxia. The critical and lethal oxygen levels for the species were estimated to be 1.78 and 0.73 mL L−1, respectively. Based on modeled historical data, the 1.5 mL L−1 isopleth has been moving upward at a rate of 1.3–2.0 m per year in northern Chile, barring interruptions during El Niño events. The long-term, gradual shoaling of the OMZ is expected to have significant impacts on the zooplankton community and productivity in the region.

Spatio-temporal variations in upwelling indices in Arabian Sea coastal upwelling systems and associated biological productivity using remote sensing observations

ABSTRACT In this paper, the spatio-temporal variations of Yemen, Oman and west coast of India upwelling are studied using wind and Sea Surface Temperature (SST) based upwelling indices. The relative contributions of offshore Ekman transport and Ekman pumping are compared in these upwelling regions. It has been found that offshore Ekman transport is the more dominant mechanism in coastal areas of Oman from 17°N to 21°N with upwelling indices reaching up to 3–4 m2/s during June-July while for the Yemen coast (13–16.5°N), Ekman pumping is stronger. The west coast of India has much weaker upwelling during the southwest monsoon. Offshore Ekman transport is the main mechanism for upwelling activity during the southwest monsoon for the west coast of India. A combined upwelling index was developed, adding the contributions of Ekman transport and integrated Ekman pumping. Chlorophyll data from 1998 to 2017 showed an increase in chlorophyll concentration during southwest monsoon season for all three upwelling regions. The maximum increase in southwest monsoon chlorophyll was observed for northern Yemen (15–16.5°N) with an increase of 10–20 times. This is followed by coastal region of Oman (17–19°N) and southern part of west coast of India (8–10°N).

Phytoplankton transport out of the euphotic zone by frontal subduction and gravitational sinking in the Santa Barbara Channel, CA, USA

Production of particulate organic carbon (POC) in nutrient-rich coastal waters over continental shelves, its export to depth, and its transport to deeper ocean waters is a poorly quantified component of the global carbon cycle. A critical step in quantifying this vertical transport is identifying shelf processes that export phytoplankton out of the euphotic zone. During cruises of the Santa Barbara Coastal Long Term Ecological Research project, we discovered substantial chlorophyll a (chl a) below the euphotic zone in the Santa Barbara Channel, a part of the southern California Current System. Observations from towed, undulating vehicles revealed deep chlorophyll layers near fronts where upwelled waters from central California converged with lower-density waters from the Southern California Bight. The mean fraction ± 1 standard deviation (SD) of chlorophyll biomass below the euphotic zone spanning the entire Santa Barbara Channel was ~7 ± 9% during 13 cruises averaged across all seasons. In one spring cruise, the fraction was ~30%, and in other cruises the layers were absent. Phytoplankton export out of the euphotic zone by subduction was indicated by spatial coherence between chl a and sloping density surfaces. Vertical plumes of chl a crossing density surfaces indicated enhanced gravitational export within cyclonic eddies. Chl a in water samples below the euphotic zone, away from fronts and cyclonic flows, suggested additional phytoplankton export. Our results emphasize the importance of subduction in the export of phytoplankton and POC out of the euphotic zone in coastal upwelling systems.

Conservative Surface Wave Effects on a Wind-Driven Coastal Upwelling System

Abstract Upwelling brings deep, cold, and nutrient-rich water to the euphotic zone, enhancing biological primary productivity. Coastal upwelling is affected by various factors, such as winds, topography, and tides. However, it remains unclear how the upwelling is affected by surface waves, particularly the Stokes drift and its related forces, that is, conservative wave effects. Here using a coupled wave–circulation model, we examined how conservative wave effects impact the wind-driven coastal upwelling system over an idealized continental shelf. We showed that conservative wave effects reduce upwelling but enhance downwelling; consequently, the amount of deep cold water brought up to the surface by upwelling is reduced with waves, leading to a weaker upwelling front than that without waves. Conservative wave effects also change the potential vorticity (PV) fluxes across the sea surface/bottom and alter the thickness of surface/bottom negative-PV layers. In addition, conservative wave effects modify the turbulent thermal wind (TTW) associated with the upwelling front, forming a Stokes–TTW balance. Further, we studied sensitivities of the upwelling and downwelling magnitudes to four parameters: wave height, wind stress, shelf slope, and wave incident angle. We combined these parameters into a single nondimensional number that can indicate when conservative wave effects need to be included in the upwelling and downwelling. Significance Statement Upwelling is important to the marine ecosystem because it enhances biological primary productivity by bringing nutrient-rich water to the euphotic zone from depths. However, it remains unclear how the upwelling is affected by ubiquitous surface waves. Here using numerical simulations, we showed that Stokes drift and its related forces due to surface waves reduce upwelling but enhance downwelling. It implies that there could be a substantial bias in the estimation of upwelling and downwelling if surface waves are not considered. Further, we proposed a nondimensional number to indicate when surface waves need to be considered in the upwelling and downwelling.

New Insights about Upwelling Trends off the Portuguese Coast: An ERA5 Dataset Analysis

In recent decades, several studies have highlighted the importance of the temporal and spatial structure of upwelling in defining the high levels of productivity of coastal upwelling systems. This work intends to assess the temporal and spatial trends of upwelling along the west and south Portuguese coasts from 1979 to 2020, comparing the patterns between these regions. Two different methodologies to calculate the upwelling indexes (UI), based on wind and sea surface temperature (SST) data, were applied to relate the wind-induced upwelling-favourable conditions (UIET) with the expected response on superficial waters, as indicated by the SST patterns (UISST). The upwelling-favourable conditions are quite consistent and more frequent and intense on the west coast than on the south coast. Spatially, it was verified from the UIET that upwelling-favourable conditions are more intense in association with the main west coast capes and that there is an intensification of favourable winds towards Cape São Vicente, both on the west and south coasts. Seasonally, upwelling-favourable UIET was found to be more consistent in the summer on both coasts. However, it also exists in the winter months. In terms of interannual variations, it should be highlighted that between 1992 and 2005 more intense favourable conditions and an apparent change in the seasonality after 2015 were found. Although some of the results derived from the UIET are corroborated by the UISST (namely, the main spatial trends and interannual variations in the upwelling intensity), several uncertainties are associated with the last index that interfere with its interpretation. For future works, it is advisable to develop a more robust SST-based index that can circumvent the uncertainties pointed out in the present study.

Habitat compression indices for monitoring ocean conditions and ecosystem impacts within coastal upwelling systems

Upwelling ecosystems are characterized by intense seasonal productivity that supports highly dynamic species populations, high diversity of mid and upper trophic levels, and a myriad of important fisheries. Climate variability and long-term change will impact upwelling intensity, timing and persistence, thereby potentially threatening resilience of coastal food webs and stability of ecosystem services. The spatial footprint of cool upwelled waters in the surface mixed-layer supports trophic transfer and ultimately the productivity of fisheries. The spatial area of upwelled water in eastern boundary upwelling systems can vary dramatically in response to both local and remote atmospheric forcing. These variations contribute to dynamic habitats that impact the structure, function, and spatial characteristics of marine ecosystems. We quantified the variability in “cool-water thermal habitat” area as a new ecological indicator, the Habitat Compression Index (HCI), for the nearshore (within 150 km of the coast) waters of the California Current Large Marine Ecosystem (CCLME). The HCI can be easily updated from ocean model products and satellite observations of sea surface temperature. We describe standardization of the HCI, regional variability, and evaluate HCI relationships with other indicators commonly used to inform ecosystem context within the CCLME. Importantly, our approach to calculating the HCI is easily extendable to other upwelling ecosystems. Further, we discuss the management context of the HCI such as assessing risk of whale entanglement in a highly profitable fishery, and implications for monitoring ecosystem shifts in coastal upwelling systems and the fisheries they support.

A 17-year time-series of diatom populations‘ flux and composition in the Mauritanian coastal upwelling

Understanding seasonal and multiyear variability of primary producers’ populations in the Mauritanian coastal upwelling system along the northwestern African margin may help to predict future impact of climate change (e.g., nutrient availability, productivity, and phyto- and zooplankton dynamics). For this, continuous, long time-series are required. A major challenge in obtaining these time-series is the logistics associated with the uninterrupted, in-situ sampling over several years. Sediment traps represent a reliable alternative. In this study, we assess the variations of the diatom community in samples almost continuously collected between June 2003 and March 2020 with 17 sediment traps deployed at site CBeu (=Cape Blanc eutrophic), located at c. 20°N-18°45’W, offshore Mauritania in the Canary Current Eastern Boundary Upwelling Ecosystems (CC-EBUE). In addition to describing the multiyear dynamics of the total diatom flux and major shifts in the species-specific composition of the populations, our study addresses questions such as (i) how constant is the intrannual pattern of populations’ occurrence, (ii) what the amplitude of annual changes is, and (iii) how populations’ shifts relate to physical setting dynamics. Matching the occurrence of most intense seasonal upwelling, highest diatom flux maxima mainly occur in spring and summer between 2003 and 2020. The diverse diatom community (e.g., benthic, coastal upwelling, coastal planktonic, and open-ocean diatoms) closely follows the annual cycle of atmospheric and hydrologic conditions. Benthic diatoms dominate during spring and summer (e.g., upwelling season), while open-ocean diatoms contribute the most in fall and winter when the upper water column stratifies. As no persistent –either decreasing or increasing trend of diatom productivity over the 17 sampled years, our results are at odds with Bakun’s hypothesis of upwelling intensification. Anchoring temporal changes of diatoms in a wider environmental frame allows for insights into the complex dynamics of the Mauritanian upwelling ecosystem and the populations’ response to climate forcing. This helps in establishing the scientific basis for modeling future states of the CC-EBUE and/or comparable environments.

Variability of surface and subsurface phytoplankton blooms in a seasonal coastal upwelling system

Understanding how phytoplankton respond to their physical environment is key to predicting shifts in bloom dynamics under a changing climate. This research investigates phytoplankton dynamics in a seasonal coastal upwelling system of the eastern Great Australian Bight (GAB), South Australia, using >10 years of in-situ mooring and CTD observations together with >18 years of satellite and wind data. Earlier studies concluded that upwelling in the region is driven by interactions between upwelling-favourable winds associated with synoptic weather events during austral summer months (Jan–March) and the formation of a pool of nutrient-rich shelf-bottom water on the Lincoln Shelf, known as the Kangaroo Island (KI) Pool. Surface phytoplankton blooms in the upwelling centre, which form off the southern tip of the Eyre Peninsula, are characterized by chlorophyll-a levels of ∼1–2 mg/m3. Our analysis of satellite chl-a data indicates the absence of surface phytoplankton blooms in the upwelling centre in more than a third of all upwelling seasons, despite the existence of statistically similar upwelling-favourable winds. The KI Pool is located ∼100 km upstream from the upwelling centre in a marine region that generally displays very low surface phytoplankton levels. Analysis of water-column data indicates strong interannual variability the KI Pool that is correlated with the intensity of surface phytoplankton blooms in the upwelling centre. The analysis also reveals the existence of substantial (∼1 mg/m3 chl-a) subsurface phytoplankton blooms at depths between ∼20 and 70 m in shallower waters of the Lincoln Shelf, where nutrient-rich shelf-bottom water of the KI Pool reaches into the euphotic zone. Our data suggest that subsurface phytoplankton blooms, forming at depths that cannot be detected by satellites, represent a major driver of the upwelling ecosystem of the eastern GAB.

Automated detection of coastal upwelling in the Western Indian Ocean: Towards an operational “Upwelling Watch” system

Coastal upwelling is an oceanographic process that brings cold, nutrient-rich waters to the ocean surface from depth. These nutrient-rich waters help drive primary productivity which forms the foundation of ecological systems and the fisheries dependent on them. Although coastal upwelling systems of the Western Indian Ocean (WIO) are seasonal (i.e., only present for part of the year) with large variability driving strong fluctuations in fish catch, they sustain food security and livelihoods for millions of people via small-scale (subsistence and artisanal) fisheries. Due to the socio-economic importance of these systems, an "Upwelling Watch" analysis is proposed, for producing updates/alerts on upwelling presence and extremes. We propose a methodology for the detection of coastal upwelling using remotely-sensed daily chlorophyll-a and Sea Surface Temperature (SST) data. An unsupervised machine learning approach, K-means clustering, is used to detect upwelling areas off the Somali coast (WIO), where the Somali upwelling – regarded as the largest in the WIO and the fifth most important upwelling system globally – takes place. This automatic detection approach successfully delineates the upwelling core and surrounds, as well as non-upwelling ocean regions. The technique is shown to be robust with accurate classification of out-of-sample data (i.e., data not used for training the detection model). Once upwelling regions have been identified, the classification of extreme upwelling events was performed using confidence intervals derived from the full remote sensing record. This work has shown promise within the Somali upwelling system with aims to expand it to the rest of the WIO upwellings. This upwelling detection and classification method can aid fisheries management and also provide broader scientific insights into the functioning of these important oceanographic features.

Rapid bacterioplankton transcription cascades regulate organic matter utilization during phytoplankton bloom progression in a coastal upwelling system

Coastal upwelling zones are hotspots of oceanic productivity, driven by phytoplankton photosynthesis. Bacteria, in turn, grow on and are the principal remineralizers of dissolved organic matter (DOM) produced in aquatic ecosystems. However, the molecular processes that key bacterial taxa employ to regulate the turnover of phytoplankton-derived DOM are not well understood. We therefore carried out comparative time-series metatranscriptome analyses of bacterioplankton in the Northwest Iberian upwelling system, using parallel sampling of seawater and mesocosms with in situ-like conditions. The mesocosm experiment uncovered a taxon-specific progression of transcriptional responses from bloom development (characterized by a diverse set of taxa in the orders Cellvibrionales, Rhodobacterales, and Pelagibacterales), over early decay (mainly taxa in the Alteromonadales and Flavobacteriales), to senescence phases (Flavobacteriales and Saprospirales taxa). Pronounced order-specific differences in the transcription of glycoside hydrolases, peptidases, and transporters were found, supporting that functional resource partitioning is dynamically structured by temporal changes in available DOM. In addition, comparative analysis of mesocosm and field samples revealed a high degree of metabolic plasticity in the degradation and uptake of carbohydrates and nitrogen-rich compounds, suggesting these gene systems critically contribute to modulating the stoichiometry of the labile DOM pool. Our findings suggest that cascades of transcriptional responses in gene systems for the utilization of organic matter and nutrients largely shape the fate of organic matter on the time scales typical of upwelling-driven phytoplankton blooms.

The influence of the land–sea breeze on coastal upwelling systems: locally forced vs internal wave vertical mixing and implications for thermal fronts

Land–sea breeze forcing near a land boundary drives both a locally forced response and an associated offshore propagating internal wave response, the effects of which can be difficult to separate. These processes enhance vertical mixing near the critical latitude for diurnal-inertial resonance (30° N/S), and are a feature of all four major eastern boundary upwelling systems. Here, we employ 1D- and 2D-vertical model configurations forced by a land–sea breeze to quantify the relative contributions of the locally forced and internal wave responses to surface currents and vertical mixing, and test sensitivity to latitude and bottom slope. We further include a sub-inertial alongshore wind to consider the role of the land–sea breeze in the context of upwelling systems. At the critical latitude, the internal waves generated via thermocline pumping near the land boundary are evanescent (in agreement with theory) and largely absent ∼50 km offshore. The internal waves are shown to contribute to vertical mixing, which can be ∼20% greater than that due to the forced response alone, further deepening the surface Ekman boundary layer. This deepening reduces the sub-inertial offshore advection of surface waters, thereby retaining the upwelling front closer to the land boundary and driving a net warming of the nearshore surface waters. Cross-shore horizontal oscillations of the upwelling front generated by the land–sea breeze drive strong diurnal variability in sea surface temperature, in agreement with observations from a cross-shore mooring array in the southern Benguela (∼32.3° S).