Periodic Reporting for period 1 - NOCEANIC (Key factors driving particulate organic matter fluxes and related nitrogen losses in the main anoxic oxygen minimum zones of the world oceans)
Periodo di rendicontazione: 2019-07-01 al 2021-06-30
The release of dinitrogen (N2) fuelled by sinking particulate organic matter (POM) is a key mechanism that drives biogeochemical cycles in anoxic oxygen minimum zones (OMZs). However, little is known about what controls this mechanism because of the challenges involved in observing it. This novel information is important because OMZ expansion is affecting the oceans’ role in mitigating the Earth’s climate and the size of fish stocks, ultimately inflicting consequences on European and world society.
Noceanic is an innovative interdisciplinary project that aims to shed light regarding the interlinks between sinking POM and release of N2 by exploiting – for the first time – time-series of optical proxies of POM and physicochemical data collected by autonomous profiling robots (i.e. “biogeochemical (BGC) Argo floats).
Noceanic is an innovative interdisciplinary project that aims to shed light regarding the interlinks between sinking POM and release of N2 by exploiting – for the first time – time-series of optical proxies of POM and physicochemical data collected by autonomous profiling robots (i.e. “biogeochemical (BGC) Argo floats).
Three objectives were proposed to achieve the main goal of Noceanic: (1) estimating POM fluxes in anoxic OMZs and the Black Sea; (2) linking variability in POM fluxes to externally-forced primary productivity and the secondary source of POM; and (3) quantifying N release rates in anoxic OMZs and the Black Sea.
The work performed to achieve each objective is summarised below.
Objective 1. Data collected by BGC-Argo floats were quality controlled and exploited to estimate fluxes of POM of different sizes in OMZs over time series ranging between 1-4 years. For the latter task, I used three independent novel approaches based on optical properties and images of POM. I also developed a new metric to estimate the attenuation rates of POM fluxes within water layers. These outputs allowed me to assess how the fluxes of POM linked to a given range of size are reinforced and/or attenuated within water layers of OMZs. This information is fundamental to understand if the OMZs indeed reinforce the efficiency of the oceans in sequestering atmospheric CO2 by exporting surface POM to the abyss (1000 m).
Objective 2. I used quality controlled data recorded by BGC-Argo floats to model the production of POM in the surface (1-100 m) and sub-surface (>100-180 m) in the eastern tropical north Pacific OMZ. In addition, I exploited satellite data to assess how atmospheric forcing (i.e. wind and rain) can partially modulate the production and accumulation of POM mainly in the surface. For the latter purpose, I also developed a metric to assess how atmospheric forcing can in part foster the formation and accumulation of POM of a given range of sizes. These findings allowed me to partially identify and quantify the predominant mechanisms that drive the formation, accumulation, and eventual export of surface-POM to the abyss over a given period of the year.
Objective 3. N2 release rates have been indirectly estimated from the attenuation rates of the POM fluxes and the coefficients of the reactions involved. In addition, (1) I developed a novel combined approach based on oxygen (O2) and optical backscattering (bbp) recorded by the float’s sensors to better delineate the section with maxima release rates of N2, and (2) I am developing an innovative method to model N2 release rates from NO3- measurements recorded by the floats. The novel insights provided by these methods are allowing me to validate the intrinsic relationship between POM fluxes and N2 release rates, to explain this correlation in terms of temporal changes in the injection of POM from the two POM sources to the anoxic section (100-500 m, O2 ≤ 1 µM), and to better constrain N2 release rates and potential uncertainties in the current estimations.
Noceanic impacted a myriad of groups that include the general public, kids and international scientific communities because its outputs were disseminated by multiple ways. These multiple ways included: (1) disseminating the results and associated field activities by social media (i.e. twitter), (2) communicating the activities and basic knowledge acquired to young people through the educational project "adopt a float initiative" led by the Laboratoire d’Oceanographie de Villefranche, (3) delivering formal seminars in the LOV, Europe, and USA (most of them were conducted by video conferences), and (4) publishing in high-impact scientific journals (i.e. Global Biogeochemical Cycles, and Biogeosciences) and guaranteeing open access to articles to maximise their diffusion.
The work performed to achieve each objective is summarised below.
Objective 1. Data collected by BGC-Argo floats were quality controlled and exploited to estimate fluxes of POM of different sizes in OMZs over time series ranging between 1-4 years. For the latter task, I used three independent novel approaches based on optical properties and images of POM. I also developed a new metric to estimate the attenuation rates of POM fluxes within water layers. These outputs allowed me to assess how the fluxes of POM linked to a given range of size are reinforced and/or attenuated within water layers of OMZs. This information is fundamental to understand if the OMZs indeed reinforce the efficiency of the oceans in sequestering atmospheric CO2 by exporting surface POM to the abyss (1000 m).
Objective 2. I used quality controlled data recorded by BGC-Argo floats to model the production of POM in the surface (1-100 m) and sub-surface (>100-180 m) in the eastern tropical north Pacific OMZ. In addition, I exploited satellite data to assess how atmospheric forcing (i.e. wind and rain) can partially modulate the production and accumulation of POM mainly in the surface. For the latter purpose, I also developed a metric to assess how atmospheric forcing can in part foster the formation and accumulation of POM of a given range of sizes. These findings allowed me to partially identify and quantify the predominant mechanisms that drive the formation, accumulation, and eventual export of surface-POM to the abyss over a given period of the year.
Objective 3. N2 release rates have been indirectly estimated from the attenuation rates of the POM fluxes and the coefficients of the reactions involved. In addition, (1) I developed a novel combined approach based on oxygen (O2) and optical backscattering (bbp) recorded by the float’s sensors to better delineate the section with maxima release rates of N2, and (2) I am developing an innovative method to model N2 release rates from NO3- measurements recorded by the floats. The novel insights provided by these methods are allowing me to validate the intrinsic relationship between POM fluxes and N2 release rates, to explain this correlation in terms of temporal changes in the injection of POM from the two POM sources to the anoxic section (100-500 m, O2 ≤ 1 µM), and to better constrain N2 release rates and potential uncertainties in the current estimations.
Noceanic impacted a myriad of groups that include the general public, kids and international scientific communities because its outputs were disseminated by multiple ways. These multiple ways included: (1) disseminating the results and associated field activities by social media (i.e. twitter), (2) communicating the activities and basic knowledge acquired to young people through the educational project "adopt a float initiative" led by the Laboratoire d’Oceanographie de Villefranche, (3) delivering formal seminars in the LOV, Europe, and USA (most of them were conducted by video conferences), and (4) publishing in high-impact scientific journals (i.e. Global Biogeochemical Cycles, and Biogeosciences) and guaranteeing open access to articles to maximise their diffusion.
Originality and innovative aspects of the research programme.
Nitrogen release rates fuelled by sinking POM is a key mechanism that drives biogeochemical cycles in anoxic OMZs of the world oceans. However, little is known about how this mechanism varies over time and across anoxic regions because of the challenges involved in observing what controls the timing and magnitude of POM injection inside the anoxic layer and how it fuels N2 production. This mechanism has been studied by incubation experiments and satellite-based POM fluxes both of which show limitations. The first method provides information with limited spatio-temporal resolution and can significantly overestimate N production. The second method is based on the principle that POM fluxes from the surface to the anoxic layer can be predicted from a power-law; this principle however cannot be applied in anoxic OMZs due to the existence of a secondary source of POM at the top of the anoxic layer. Therefore, an “eye below the surface” is needed to obtain a vertically-resolved view of POM fluxes and improve the estimation of N release rates. This information gap can be filled by exploiting time series of data collected by BGC-Argo floats. As outcomes of the proposed project, I expect to develop new knowledge about the key mechanisms that drive temporal changes in N release rates, to contribute to reducing uncertainties in the mechanistic prediction of N2 release and overall, to deepen understanding of the biogeochemical response of oceans to ongoing OMZ expansion.
Noceanic is original and innovative because:
(1) I developed and applied a novel method to estimate – for the first time – POM-based N2 release rates with unprecedented vertical and temporal resolution in the largest anoxic OMZs of the world oceans. This information is important because little is known about how N2 release rates vary over time and across anoxic OMZs.
(2) This is a multidisciplinary project which harness the combined benefits of advanced technology (BGC-Argo floats and remote sensing) to understand the key factors driving N2 release rates in the most important anoxic OMZs and the Black Sea. This information is critical because it is unknown how the secondary source of POM drives N2 production and potentially affects the N budget of OMZs.
Societal implications of the project
An expected outcome of Noceanic is its impact on the European strategy for global ocean observations by promoting the inclusion of OMZs – a critical shift because OMZ expansion is affecting the oceans’ role in mitigating the Earth’s climate and the size of fish stocks, ultimately inflicting consequences on European society. I therefore anticipate that Noceanic’s insights and data delivered (e.g. dissolved oxygen) can be used in the future as vital references to contribute to policies to protect tropical ecosystems prone to deoxygenation – a critical step that has a direct socio-economic impact.
Nitrogen release rates fuelled by sinking POM is a key mechanism that drives biogeochemical cycles in anoxic OMZs of the world oceans. However, little is known about how this mechanism varies over time and across anoxic regions because of the challenges involved in observing what controls the timing and magnitude of POM injection inside the anoxic layer and how it fuels N2 production. This mechanism has been studied by incubation experiments and satellite-based POM fluxes both of which show limitations. The first method provides information with limited spatio-temporal resolution and can significantly overestimate N production. The second method is based on the principle that POM fluxes from the surface to the anoxic layer can be predicted from a power-law; this principle however cannot be applied in anoxic OMZs due to the existence of a secondary source of POM at the top of the anoxic layer. Therefore, an “eye below the surface” is needed to obtain a vertically-resolved view of POM fluxes and improve the estimation of N release rates. This information gap can be filled by exploiting time series of data collected by BGC-Argo floats. As outcomes of the proposed project, I expect to develop new knowledge about the key mechanisms that drive temporal changes in N release rates, to contribute to reducing uncertainties in the mechanistic prediction of N2 release and overall, to deepen understanding of the biogeochemical response of oceans to ongoing OMZ expansion.
Noceanic is original and innovative because:
(1) I developed and applied a novel method to estimate – for the first time – POM-based N2 release rates with unprecedented vertical and temporal resolution in the largest anoxic OMZs of the world oceans. This information is important because little is known about how N2 release rates vary over time and across anoxic OMZs.
(2) This is a multidisciplinary project which harness the combined benefits of advanced technology (BGC-Argo floats and remote sensing) to understand the key factors driving N2 release rates in the most important anoxic OMZs and the Black Sea. This information is critical because it is unknown how the secondary source of POM drives N2 production and potentially affects the N budget of OMZs.
Societal implications of the project
An expected outcome of Noceanic is its impact on the European strategy for global ocean observations by promoting the inclusion of OMZs – a critical shift because OMZ expansion is affecting the oceans’ role in mitigating the Earth’s climate and the size of fish stocks, ultimately inflicting consequences on European society. I therefore anticipate that Noceanic’s insights and data delivered (e.g. dissolved oxygen) can be used in the future as vital references to contribute to policies to protect tropical ecosystems prone to deoxygenation – a critical step that has a direct socio-economic impact.