Nutrient-rich Deep Water Research Articles

Ocean iron cycle feedbacks decouple atmospheric CO2 from meridional overturning circulation changes

The ocean’s Meridional Overturning Circulation (MOC) brings carbon- and nutrient-rich deep waters to the surface around Antarctica. Limited by light and dissolved iron, photosynthetic microbes incompletely consume these nutrients, the extent of which governs the escape of inorganic carbon into the atmosphere. Changes in MOC upwelling may have regulated Southern Ocean outgassing, resulting in glacial-interglacial atmospheric CO2 oscillations. However, numerical models that explore this positive relationship do not typically include a feedback between biological activity and abundance of organic chelating ligands that control dissolved iron availability. Here, I show that incorporating a dynamic ligand parameterization inverts the modelled MOC-atmospheric CO2 relationship: reduced MOC nutrient upwelling decreases biological activity, resulting in scant ligand production, enhanced iron limitation, incomplete nutrient usage, and ocean carbon outgassing, and vice versa. This first-order response suggests iron cycle feedbacks may be a critical driver of the ocean’s response to climate changes, independent of external iron supply.

Development Of Novel Processed Tilapia Products For The Group Of Female Fish Farmers In The Village Of Ranu Klakah Lumajang

Floatingfish cages are utilized in fish farming systems using public water bodies such as lakes and seas, with tilapia being a widely cultivated fish in Indonesia due to its economic and nutritional value. Ranu Klakah, a volcanic lake in East Java, is explored as a site for tilapia cultivation. Upwelling, the process of nutrient-rich deep water rising to the surface due to wind-induced currents, is examined for its effects on water quality and fish productivity. The activity focuses on the impact of upwelling on tilapia, including reduced water quality, scarcity of natural fish food, physiological stress, disease susceptibility, and decreased fish production. To address the challenges posed by upwelling, it suggests processing tilapia into value-added products, like fish nuggets and skin chips, to enhance market value. Community service activities involving training for making these processed fish products are described, aiming to empower women in the fish farming sector and improve family income. We concludewith insights into the potential benefits of these activities and their positive impact on the local community.

The Southern Ocean Hub for Nutrients, Micronutrients, and Their Isotopes in the Global Ocean

The sustenance of marine primary productivity depends on the supply of macro- and micronutrients to photosynthesizers in the ocean’s sunlit surface. Without supply from the deep, sinking particles would deplete the upper ocean of these vital elements within decades. Over the last 20 years, it has been recognized that the Southern Ocean, where nutrient-rich deep waters are brought to the surface and the water masses that fill much of the upper ocean are formed, plays a pivotal role in replenishing upper-ocean nutrients. Photosynthesizers that grow and take up nutrients within the Southern Ocean circulation “hub” thus have an outsize influence on global-​scale distributions of macronutrients and many micronutrients. The GEOTRACES program has contributed observations of the concentration and stable isotope composition of “nutrient-​type” metals like zinc, cadmium, and nickel, within the Southern Ocean and beyond it, that are driving a sea change in our understanding of their marine cycles. Simultaneously, our understanding of Southern Ocean circulation has been refined, with recognition of the importance of longitudinal variability and subtropical overturning. Here, we aim to bring together these two strands of progress, review insights gained into marine micronutrient cycling, and consider the questions that remain to be resolved.

Effects of Tropical Cyclone (TC) Hellen on the north-westward movement of chlorophyll in the northern Mozambique Channel.

An intense tropical cyclone (TC), TC Hellen, occurred in the northern Mozambique Channel on March 27, 2014, and moved from the east coast of the African continent to the northern Madagascar island. TC Hellen dramatically altered the marine environment in the northern Mozambique Channel, resulting in a significant chlorophyll-a (Chl-a) bloom. A giant surface Chl-a northwest-ward movement from the northwest coast of Madagascar Island was first observed after the passage of TC Hellen in the northern Mozambique Channel. The dynamic mechanisms of these phenomenon were studied by satellite remote sensing, multisource reanalysis data, and Argo float data. The results show that transient northwestward-moving eddies, upwelling, and winds had important effects on the Chl-a bloom and its northwestward movement. Ekman transport driven by coastal southeasterly winds entrained waters with high Chl-a concentrations to the northwest, while TC Hellen enhanced cyclonic eddy upwelling and uplifted nutrient-rich deep water to the upper ocean. This vertical mixing and upwelling in turn triggered the Chl-a bloom in the offshore surface layer. This study provides insight into the reflection of phytoplankton dynamics by TCs in the northern Mozambique Channel.

Glacial expansion of carbon-rich deep waters into the Southwestern Indian Ocean over the last 630 kyr

Oceanic carbon storage is one of the main sinks for atmospheric CO2, and thought to be the major contributing factor for CO2 drawdown during past glacial periods. Both physical and biogeochemical processes control the capacity of carbon storage in the ocean. During glacial periods of the Pleistocene the larger volume of deep-water masses of Southern Hemisphere origin in the Atlantic has been shown to promote carbon storage in the Southern Ocean. However, the latitudinal extension of this water mass in the Indian Ocean has been scarcely studied. In this study, we combine foraminiferal εNd and benthic δ13C of two sediment cores in the southwest Indian Ocean (MD96–2077, 33°S, 3781 m water depth; MD96–2052, 19°S, 2627 m water depth), to reconstruct the spatial and temporal evolution of glacial carbon-rich deep waters in the SW Indian over the last 630 kyr. The combined use of foraminiferal εNd and benthic δ13C allows to distinguish δ13C changes related to water mass mixing and from respired carbon accumulation within the water masses. Nutrient-rich deep waters, which cannot be explained by the enhanced proportion of southern-sourced waters, were present at core sites deeper than 2700 m during glacial periods and extended at least until 33°S into the SW Indian Ocean. From Marine Isotope Stage (MIS) 14 to MIS 10, glacial carbon storage increased gradually until reaching its highest capacity during the extreme glacial periods MIS 12 and 10. Orbital forcing (100-kyr eccentricity, 41-kyr obliquity), restricted air-sea exchange and enhanced ocean stratification, fostered higher carbon storage during periods of relatively lower eccentricity and obliquity. Furthermore, after MIS 10, a progressive transition was observed from 100-kyr eccentricity to 41-kyr obliquity cycles in benthic δ13C and δ18O records of core MD96–2077 and sea-ice cover changes derived from ice-rafted debris of the Agulhas Plateau composite core site.

Historical population changes of Adélie penguins in the Ross Sea region, Antarctica, and its climatic forcings

The atmospheric-oceanic circulation patterns, especially for the Southern Annular Mode (SAM) and El Niño-Southern Oscillation (ENSO), two major atmospheric circulation patterns in the Ross Sea region, have been reported to greatly affect climate and marine ecosystems. However, from a historical perspective, the influence of atmospheric-oceanic circulation patterns on penguin populations remains unclear in this region. Here, we analyzed two lacustrine sediment cores collected from abandoned penguin colonies at Inexpressible Island, Ross Sea, Antarctica, and by applying alternative geochemical indices, successfully reconstructed the populations of the Adélie penguins (Pygoscelis adeliae) over the past ∼1500 years. We found that penguin population peaked during 750–1350 AD at Inexpressible Island, potentially due to habitat expansion in the warmer climate. After comparing with historical records of penguin populations at Cape Bird, Dunlop Island, and Cape Adare, all were found to have a common increase during the 750-1350 AD period in the Ross Sea. The population trend also coincided with extreme activities of El Niño and SAM (+). We inferred that the SAM-ENSO might promote influxes of Circumpolar and Modified Circumpolar Deep Waters into the Ross Sea. The enhanced influx of nutrient-rich deep water, together with a warmer climate may jointly enhance polynya efficiency and population increase of Adélie penguins. Our study indicates the potentially significant roles of ENSO and SAM in the regulation of Antarctic ecosystems.

Artificial Upwelling—A Refined Narrative

AbstractThe current narrative of artificial upwelling (AU) is to translocate nutrient rich deep water to the ocean surface, thereby stimulating the biological carbon pump (BCP). Our refined narrative takes the response of the solubility pump and the CO2 emission scenario into account. Using global ocean‐atmosphere model experiments we show that the effectiveness of a hypothetical maximum AU deployment in all ocean areas where AU is predicted to lower surface pCO2, the draw down of CO2 from the atmosphere during years 2020–2100 depends strongly on the CO2 emission scenario and ranges from 1.01 Pg C/year (3.70 Pg CO2/year) under RCP 8.5 to 0.32 Pg C/year (1.17 Pg CO2/year) under RCP 2.6. The solubility pump becomes equally effective compared to the BCP under the highest emission scenario (RCP 8.5), but responds with CO2 outgassing under low CO2 emission scenarios.

Controls on Organic Matter Accumulation from an Upper Slope Section on the Early Cambrian Yangtze Platform, South China

The early Cambrian witnessed profound environmental changes and biological evolution in Earth’ history. During this period, organic-rich shales were widely distributed over almost the entire Yangtze Block. However, the dominant factor that drove the significant accumulation of organic matter (OM) remains controversial and is still debated. Here, we analyzed TOC, organic carbon isotopes, iron speciation, major and trace elements for the lower Cambrian Niutitang Formation in the upper slope Meiziwan section, to investigate the dominant factor controlling OM accumulation. High contents of TOC and Baxs reveal an OM-enriched feature of the Niutitang Formation, and the coupled relationship between them suggest a strong production control on OM accumulation at Meiziwan. Meanwhile, negative relationships between TOC and chemical index of alteration (CIA) values as well as Al contents suggest that influence of chemical weathering and terrestrial input on OM accumulation were limited. Fairly low CoEF × MnEF values provide strong evidence that the deposition of organic-rich shales was under the control of oceanic upwelling event. The upwelling event would bring nutrient-rich deep waters into surface water, stimulating phytoplankton bloom and primary productivity in surface water and facilitating OM enrichment. Meanwhile, enhanced accumulation of OM would have promoted subsequent bacterial sulfate reduction, leading to the occurrence of occasional euxinia (evidenced by iron speciation and redox-sensitive trace element data) and promoting preservation of OM. Taken together, our results shed light on the critical role of oceanic upwelling on the marine primary productivity on the earliest Cambrian Yangtze Platform.

Geostrophic flows control future changes of oceanic eastern boundary upwelling

Equatorward alongshore winds over major eastern boundary upwelling systems (EBUSs) drive intense upwelling via Ekman dynamics, surfacing nutrient-rich deep waters and promoting marine primary production and fisheries. It is generally thought, dating back to Bakun’s hypothesis, that greenhouse warming should enhance upwelling in EBUSs by intensifying upwelling-favourable winds; yet this has not been tested. Here, using an ensemble of high-resolution climate simulations with improved EBUS representation, we show that long-term upwelling changes in EBUSs differ substantially, under a high-emission scenario, from those inferred by the wind-based upwelling index. Specifically, weakened or unchanged upwelling can coincide with intensified upwelling-favourable winds. These differences are linked to long-term changes of geostrophic flows that dominate upwelling changes in the Canary and Benguela currents and strongly offset wind-driven changes in the California and Humboldt currents. Our results highlight the controlling role of geostrophic flows in upwelling trends in EBUSs under greenhouse warming, which Bakun’s hypothesis overlooked.

Nutrient composition (Si:N) as driver of plankton communities during artificial upwelling

Artificial upwelling brings nutrient-rich deep water to the sun-lit surface to boost fisheries or carbon sequestration. Deep water sources under consideration range widely in inorganic silicon (Si) relative to nitrogen (N). Yet, little is known about how such differences in nutrient composition may influence the effectiveness of the fertilization. Si is essential primarily for diatoms that may increase food web and export efficiency via their large size and ballasting mineral shells, respectively. With a month-long mesocosm study in the subtropical North Atlantic, we tested the biological response to artificial upwelling with varying Si:N ratios (0.07-1.33). Community biomass increased 10-fold across all mesocosms, indicating that basic bloom dynamics were upheld despite the wide range in nutrient composition. Key properties of these blooms, however, were influenced by Si. Photosynthetic capacity and nutrient-use efficiency doubled from Si-poor to Si-rich upwelling, leading to C:N ratios as high as 17, well beyond Redfield. Si-rich upwelling also resulted in 6-fold higher diatom abundance and mineralized Si and a corresponding shift from smaller towards larger phytoplankton. The pronounced change in both plankton quantity (biomass) and quality (C:N ratio, size and mineral ballast) for trophic transfer and export underlines the pivotal role of Si in shaping the response of oligotrophic regions to upwelled nutrients. Our findings indicate a benefit of active Si management during artificial upwelling with the potential to optimize fisheries production and CO2 removal.

The importance of the dissolved organic matter pool for the carbon sequestration potential of artificial upwelling

In the face of climate change there is a need to reduce atmospheric CO2 concentrations. Artificial upwelling of nutrient-rich deep waters has been proposed as a method to enhance the biological carbon pump in oligotrophic oceanic regions in order to increase carbon sequestration. Here we examine the effect of different artificial upwelling intensities and modes (single pulse versus recurring pulses) on the dynamics of the dissolved organic matter pool (DOM). We introduced nutrient-rich deep water to large scale mesocosms (~44 m3) in the oligotrophic subtropical North Atlantic and found that artificial upwelling strongly increased DOM concentrations and changed its characteristics. The magnitude of the observed changes was related to the upwelling intensity: more intense treatments led to higher accumulation of dissolved organic carbon (>70 μM of excess DOC over ambient waters for the most intense) and to comparatively stronger changes in DOM characteristics (increased proportions of chromophoric DOM (CDOM) and humic-like fluorescent DOM), suggesting a transformation of the DOM pool at the molecular level. Moreover, the single upwelling pulse resulted in higher CDOM quantities with higher molecular weight than the recurring upwelling mode. Together, our results indicate that under artificial upwelling, large DOM pools may accumulate in the surface ocean without being remineralized in the short-term. Possible reasons for this persistence could be a combination of the molecular diversification of DOM due to microbial reworking, nutrient limitation and reduced metabolic capabilities of the prokaryotic communities within the mesocosms. Our study demonstrates the importance of the DOC pool when assessing the carbon sequestration potential of artificial upwelling.

Plankton Community Respiration and Particulate Organic Carbon in the Kuroshio East of Taiwan.

Biological organic carbon production and consumption play a fundamental role in the understanding of organic carbon cycling in oceans. However, studies on them in the Kuroshio, the western boundary current in the North Pacific Ocean, are scarce. To better understand the variations of plankton community respiration (CR) and particulate organic carbon (POC), eight cruises. which covered four seasons over a 2-year period, were surveyed across the Kuroshio at the KTV1 transect east of Taiwan. Spatially, a coastal uplift of isotherms (i.e., onshore lifting and offshore deepening) was observed along the KTV1 transect. During the uplift, the cold and nutrient-rich deep waters shoal to shallow water and enhance phytoplankton growth, resulting in higher values of phytoplankton, POC, and plankton CR on the onshore side. In this study, phytoplankton was dominated by picophytoplankton including Prochlorococcus, Synechococcus, and picoeukaryotes. Plankton CR was low, and its mean depth-normalized integrated rate (the upper 100 m water depth) ranged from 7.07 to 22.27 mg C m-3 d-1, to which the picophytoplankton and heterotrophic bacteria contributed the most. The mean depth-normalized integrated value of POC ranged from 12.7 to 21.6 μg C L-1. POC is mainly associated with phytoplankton biomass with a mean carbon ratio of chlorophyll a/POC ≈ 1.03. All results suggest that plankton CR and POC variations may be associated with picoplankton dynamics in the Kuroshio.

Progressive expansion of seafloor anoxia in the Middle to Late Ordovician Yangtze Sea: Implications for concurrent decline of invertebrate diversity

The Great Ordovician Biodiversification Event (GOBE) achieved its peak during the Middle Ordovician, likely in association with climatic cooling and a rise of atmospheric O2. However, unstable redox states developed widely in contemporaneous epeiric seas, challenging previous assumptions about sustained oceanic oxygenation driven by deep-ocean ventilation in the aftermath of Ordovician cooling. Here, we investigate two Middle-Upper Ordovician shale-dominated successions from intra-shelf basin and slope settings of the Yangtze Sea, South China. Negative shifts in bulk δ15N and productivity proxies reveal temporal changes in the pattern of nutrient cycling under moderate-to-high productivity conditions, which were likely sustained by increased upwelling of cold and nutrient-rich deep waters in sync with contemporaneous climatic cooling. As a result, marine anoxia likely expanded in deep waters of the Yangtze Sea, as reflected in enrichments of highly reactive iron and redox-sensitive metals (Mo, U). Although moderate Mo abundances (25-50 ppm) and δ98/95Mo values (+0.4 to +0.8‰) within the euxinic interval suggest that bottom waters had low sulfide concentrations, the δ98/95Mo values are comparable with those of coeval euxinic facies (∼+0.6‰). Furthermore, our modeling results show that low δ98/95Mo can be produced when sulfidic sinks dominate Mo removal, suggesting that Middle-Late Ordovician oceans may have been less oxygenated than previously assumed, at least for the continental seas of South China, Baltica and Laurentia. These environmental changes may provide a new explanation for the ∼50% decline of invertebrate species following the peak of GOBE. Our findings thus link the progressive expansion of marine anoxia and climatic cooling to a profound biotic change during the Mid-Late Ordovician Period.

Island Mass Effect: A Review of Oceanic Physical Processes

Increased Net Primary Productivity (NPP) around small islands have been documented worldwide. Despite having been known for decades, the interactions between physical and biogeochemical processes behind this phenomenon – that takes the name of Island Mass Effect –remain unclear. In this paper we review the physical processes involved while proposing a method to identify the prevailing mechanisms by analyzing their imprint on NPP and Sea Surface Temperature (SST). These mechanisms can be quite different, but all enhance vertical exchanges, increasing the input of nutrients in the euphotic layer and favoring biological productivity. Nutrient-rich deeper waters are brought up to the surface through upwelling and mixing, leaving a cold imprint on the SST as well. Here we analyze satellite data of SST and NPP around small islands and archipelagos to catalog the physical mechanisms that favor the Island Mass Effect, with the aid of oceanic and atmospheric reanalysis. The multiplicity of these processes and the convolution of their interactions highlight the complexity of the physical forcing on the biomass production and the uniqueness of each island. However, analysis from 19 small islands throughout the tropics shows that two kinds of SST patterns emerge, depending on the size and altitude of the island. Around islands with considerable elevation and greatest diameters, cold/warm anomalies, most likely corresponding to upwelling/downwelling zones, emerge. This signal can be mainly ascribed to oceanic and atmospheric forcing. Around small islands, on the other hand, warm anomalies do not appear and only local cooling, associated with current-island interactions, is found. In the vicinity of a single island, more than one process responsible for the increased nutrient input into the euphotic layer might coexist, the prevailing one varying along the year and depending on the strength and direction of the incoming atmospheric and oceanic flow.

Distribution and biomass of gelatinous zooplankton in relation to an oxygen minimum zone and a shallow seamount in the Eastern Tropical North Atlantic Ocean

Physical and topographic characteristics can structure pelagic habitats and affect the plankton community composition. For example, oxygen minimum zones (OMZs) are expected to lead to a habitat compression for species with a high oxygen demand, while upwelling of nutrient-rich deep water at seamounts can locally increase productivity, especially in oligotrophic oceanic waters. Here we investigate the response of the gelatinous zooplankton (GZ) assemblage and biomass to differing oxygen conditions and to a seamount in the Eastern Tropical North Atlantic (ETNA) around the Cape Verde archipelago. A total of 16 GZ taxa (>1100 specimens) were found in the upper 1000 m with distinct species-specific differences, such as the absence of deep-living species Atolla wyvillei and Periphylla periphylla above the shallow seamount summit. Statistical analyses considering the most prominent groups, present at all stations, namely Beroe spp., hydromedusae (including Zygocanna vagans, Halicreas minimum, Colobonema sericeum, Solmissus spp.) and total GZ, showed a strong positive correlation of abundance with temperature for all groups, whereas oxygen had a weak negative correlation only with abundances of Beroe spp. and hydromedusae. To account for size differences between species, we established length-weight regressions and investigated total GZ biomass changes in relation to physical (OMZ) and topographic characteristics. The highest GZ biomass was observed at depths of lowest oxygen concentrations and deepest depth strata at the southeastern flank of the seamount and at two stations south of the Cape Verde archipelago. Our data suggest that, irrespective of their patchy distribution, GZ organisms are ubiquitous food web members of the ETNA, and their habitat includes waters of low oxygen content.

Atlantic inflow is the primary driver of remotely sensed autumn blooms in the Barents Sea

Arctic shelf seas have historically hosted a single spring bloom, contrasting with temperate seas, where additional smaller autumn blooms occur regularly, caused by storm systems mixing nutrient-rich deep waters towards the surface ocean. Post millennium, autumn blooms have increased in frequency in Arctic shelf seas. Delayed sea-ice formation, stronger autumn winds and greater inflows of nutrient-rich temperate waters have all been suggested to support growing annual net primary production and an increasing incidence of autumn blooms. Here, we investigated data sets of remotely sensed September chlorophyll a, sea surface temperature, current and wind speeds. We explored mechanistic drivers of autumn blooms in the Barents Sea, one of the most productive Arctic shelf seas, to better understand the role of strong winds and the ingression of Atlantic waters in the dynamics of autumn blooms. We performed geographically resolved regressions between remotely sensed September chlorophyll a and environmental conditions in the Barents Sea, demonstrating a strong dependency of autumn bloom intensity on Atlantic inflow. This result highlights the importance of increased study of autumn phytoplankton bloom dynamics on Arctic shelf seas, especially the further collection and dissemination of in situ cell count and nutrient data to determine the significance of autumn blooms for wider ecosystem function.

Multiple controls on the paleoenvironment of the early Cambrian black shale-chert in the northwest Tarim Basin, NW China: Trace element, iron speciation and Mo isotopic evidence

Research on early Cambrian sedimentary successions of different continental margins are important in better understanding the redox heterogeneity of the early Cambrian ocean, and further the evolutionary relationship between the early animals and their environment. In the present study, we carried out trace element, iron speciation, and Mo isotope analysis of the basal Cambrian Yurtus black shales and cherts in the inner ramp Shiairike section of the northwest Tarim Basin. Our integrated data, combined with previously published data of correlative units in this region, indicate strong basin restriction during deposition in the inner ramp area, with more severe restriction for the upper part of the studied unit. Our investigation supports strong upwelling of nutrient-rich deep water and high primary productivity, with predominately anoxic bottom waters during deposition of the rocks. The anoxia in bottom waters and preservation of organic carbon were likely maintained by both high primary productivity and bottom water stagnancy. Despite the overall anoxic bottom water condition, intermittent oxic intervals may have existed; and the variabilities in redox conditions and related Mn and/or Fe cycling in combination with low and variable [H2S]aq in bottom waters may have attributed to the highly variable δ98Mo values in the studied unit. Furthermore, a high δ98Mo value of 2.14‰ documented in the lower part of the unit is indicative of high δ98Mo of contemporary seawater, pointing to a relatively oxygenated ocean with limited seafloor anoxia.

The world’s largest heliothermal lake newly formed in the Aral Sea basin

The Aral Sea desiccation is one of the worst aquatic ecological disasters of the last century, important for understanding the worldwide trends to degradation of arid lakes under water use and climate change. Formerly the fourth largest lake worldwide, the Aral Sea has lost ∼90% of its water since the early 1960s due to irrigation in its drainage basin. Our survey on the seasonal thermal and mixing regime in Chernyshev—a semi-isolated hypersaline part of the Aral Sea—revealed a newly formed two-layered structure with strong gradients of salinity and water transparency at mid-depths. As a result, the Chernyshev effectively accumulates solar energy, creating a temperature maximum at the water depth of ∼5 m with temperatures up to 37 °C. Herewith, this part of the Aral Sea has evolved to an unprecedently large (∼80 km2) heliothermal lake akin to artificial solar ponds used for ‘green energy’ production. The newly formed heliothermal lake, with transparent and freshened layer on top of the hypersaline and nutrient-rich deep water, acts as a solar energy trap and facilitates intense biogeochemical processes. The latter reveal themselves in practically 100% opacity of the deep layer to the solar light, permanent deep anoxia, and growing methane concentrations. The recent emergence of the Chernyshev as a heliothermal lake provides an opportunity for tracing the biogeochemical and ecological response of aquatic ecosystems to suddenly changed environmental conditions.

Effect of Intensity and Mode of Artificial Upwelling on Particle Flux and Carbon Export

Reduction of anthropogenic CO2 emissions alone will not sufficiently restrict global warming and enable the 1.5°C goal of the Paris agreement to be met. To effectively counteract climate change, measures to actively remove carbon dioxide from the atmosphere are required. Artificial upwelling has been proposed as one such carbon dioxide removal technique. By fueling primary productivity in the surface ocean with nutrient-rich deep water, it could potentially enhance downward fluxes of particulate organic carbon (POC) and carbon sequestration. In this study we investigated the effect of different intensities of artificial upwelling combined with two upwelling modes (recurring additions vs. one singular addition) on POC export, sinking matter stoichiometry and remineralization depth. We carried out a 39 day-long mesocosm experiment in the subtropical North Atlantic, where we fertilized oligotrophic surface waters with different amounts of deep water. The total nutrient inputs ranged from 1.6 to 11.0 μmol NO3– L–1. We found that on the one hand POC export under artificial upwelling more than doubled, and the molar C:N ratios of sinking organic matter increased from values around Redfield (6.6) to ∼8–13, which is beneficial for potential carbon dioxide removal. On the other hand, sinking matter was remineralized at faster rates and showed lower sinking velocities, which led to shallower remineralization depths. Particle properties were more favorable for deep carbon export in the recurring upwelling mode, while in the singular mode the C:N increase of sinking matter was more pronounced. In both upwelling modes roughly half of the produced organic carbon was retained in the water column until the end of the experiment. This suggests that the plankton communities were still in the process of adjustment, possibly due to the different response times of producers and consumers. There is thus a need for studies with longer experimental durations to quantify the responses of fully adjusted communities. Finally, our results revealed that artificial upwelling affects a variety of sinking particle properties, and that the intensity and mode with which it is applied control the strength of the effects.

Nitrogen loss processes in response to upwelling in a Peruvian coastal setting dominated by denitrification – a mesocosm approach

Abstract. Upwelling of nutrient-rich deep waters make eastern boundary upwelling systems (EBUSs), such as the Humboldt Current system, hot spots of marine productivity. Associated settling of organic matter to depth and consecutive aerobic decomposition results in large subsurface water volumes being oxygen depleted. Under these circumstances, organic matter remineralisation can continue via denitrification, which represents a major loss pathway for bioavailable nitrogen. Additionally, anaerobic ammonium oxidation can remove significant amounts of nitrogen in these areas. Here we assess the interplay of suboxic water upwelling and nitrogen cycling in a manipulative offshore mesocosm experiment. Measured denitrification rates in incubations with water from the oxygen-depleted bottom layer of the mesocosms (via 15N label incubations) mostly ranged between 5.5 and 20 nmol N2 L−1 h−1 (interquartile range), reaching up to 80 nmol N2 L−1 h−1. However, actual in situ rates in the mesocosms, estimated via Michaelis–Menten kinetic scaling, did most likely not exceed 0.2–4.2 nmol N2 L−1 h−1 (interquartile range) due to substrate limitation. In the surrounding Pacific, measured denitrification rates were similar, although indications of substrate limitation were detected only once. In contrast, anammox (anaerobic ammonium oxidation) made only a minor contribution to the overall nitrogen loss when encountered in both the mesocosms and the Pacific Ocean. This was potentially related to organic matter C / N stoichiometry and/or process-specific oxygen and hydrogen sulfide sensitivities. Over the first 38 d of the experiment, total nitrogen loss calculated from in situ rates of denitrification and anammox was comparable to estimates from a full nitrogen budget in the mesocosms and ranged between ∼ 1 and 5.5 µmol N L−1. This represents up to ∼ 20 % of the initially bioavailable inorganic and organic nitrogen standing stocks. Interestingly, this loss is comparable to the total amount of particulate organic nitrogen that was exported into the sediment traps at the bottom of the mesocosms at about 20 m depth. Altogether, this suggests that a significant portion, if not the majority of nitrogen that could be exported to depth, is already lost, i.e. converted to N2 in a relatively shallow layer of the surface ocean, provided that there are oxygen-deficient conditions like those during coastal upwelling in our study. Published data for primary productivity and nitrogen loss in all EBUSs reinforce such conclusion.