Alfonso Mucci

This paper presents the Gulf of St. Lawrence and Estuary Dataset (GOSLED), a quality-controlled compilation of biogeochemical observations collected during 21 research cruises in the St. Lawrence Estuary, Gulf of St. Lawrence, and Saguenay Fjord between 2003 and 2023. This dataset integrates hydrographic measurements and a broad suite of discrete biogeochemical variables into a single, standardized compilation suitable for reuse, synthesis, and long-term analysis. GOSLED includes discrete measurements of dissolved oxygen, carbonate-system parameters, macronutrients, dissolved organic carbon, selected biogeochemical gases, stable isotope ratios of carbon and water, and transient and deliberate tracers. Data were compiled from multiple independent research cruises and laboratory archives (2003–2020), including contributions from the Marine Environmental Observation, Prediction and Response Network (MEOPAR) – Réseau Québec maritime (RQM) Gulf of St. Lawrence Tracer Release Experiment (TReX; 2021–2023), RQM Odyssée Saint Laurent program (2018–2023), and the Fisheries and Oceans Canada (DFO) Atlantic Zone Monitoring Program (AZMP; fall 2022). Sampling was conducted predominantly during the ice-free season, resulting in limited winter coverage across much of the system. All data were harmonized and processed following primary quality-control procedures adapted from GLODAP and CODAP-NA standards. Secondary crossover analysis was not possible due to a lack of deep-water (>1500 m) sampling. This paper documents the data provenance, quality-control procedures, known limitations, and recommended considerations for dataset usage. GOSLED is archived at the Canadian Integrated Ocean Observing System – St. Lawrence Global Observatory (CIOOS-SLGO) and is publicly accessible at https://doi.org/10.26071/d6f3fdfc-788d-48ff (Nesbitt et al., 2026).

Organic carbon (OC) burial in sediments is partly controlled by catabolic biodegradation and remineralization by heterotrophic bacteria, but long‐term records of these bacterially‐mediated processes have not been available. Here, we use stable hydrogen isotope ratio of the saturated C ₁₆ fatty acid (δ ² H _C16:0 ) as a proxy of the prevalence of heterotrophic bacterial activity to reconstruct the history of Holocene OC biodegradation and remineralization from a sediment core recovered on the southwest Greenland Shelf. A positive shift in the δ ² H _C16:0 during the Holocene Thermal Maximum indicates increased inputs of heterotrophic bacterial biomass to sediments. This implies efficient biodegradation and remineralization of OC during this warm period that are linked to low sediment accumulation rates (SARs) and elevated primary productivity. We propose that enhanced bacterial catabolism of OC in the absence of increased SARs will reduce OC burial efficiency in warmer, high‐latitude oceans, thereby providing positive feedback to climate warming.

This paper presents the Gulf of St. Lawrence and Estuary Dataset (GOSLED), a quality-controlled compilation of biogeochemical observations collected during 21 research cruises in the St. Lawrence Estuary, Gulf of St. Lawrence, and Saguenay Fjord between 2003 and 2023. This dataset integrates hydrographic measurements and a broad suite of discrete biogeochemical variables into a single, standardized compilation suitable for reuse, synthesis, and long-term analysis. GOSLED includes discrete measurements of dissolved oxygen, carbonate-system parameters, macronutrients, dissolved organic carbon, selected biogeochemical gases, stable isotope ratios of carbon and water, and transient and deliberate tracers. Data were compiled from multiple independent research cruises and laboratory archives (2003–2020), including contributions from the Marine Environmental Observation, Prediction and Response Network (MEOPAR) – Réseau Quëbec maritime (RQM) Gulf of St. Lawrence Tracer Release Experiment (TReX; 2021-2023), RQM Odyssée Saint Laurent program (2018-2023), and the Fisheries and Oceans Canada (DFO) Atlantic Zone Monitoring Program (AZMP; fall 2022). Sampling was conducted predominantly during the ice-free season, resulting in limited winter coverage across much of the system. All data were harmonized and processed following primary quality-control procedures adapted from GLODAP and CODAP-NA standards. Secondary crossover analysis was not possible due to a lack of deep-water sampling. This paper documents the data provenance, quality-control procedures, known limitations, and recommended considerations for dataset usage. GOSLED is archived at the Canadian Integrated Ocean Observing System - St. Lawrence Global Observatory (CIOOS-SLGO) and is publicly accessible at https://doi.org/10.26071/d6f3fdfc-788d-48ff (Nesbitt et al., 2026).

The Gulf and Lower St. Lawrence Estuary have experienced major environmental change over the past century, including the development of hypoxic bottom waters and their simultaneous warming and acidification. Here, we use biogeochemical observations collected during the 2021–2023 TReX project as well as historical data, combined with a tracer-calibrated 1D Advection-Diffusion model with variable boundary conditions to represent dissolved oxygen (DO) and dissolved inorganic carbon (DIC) dynamics within the core of the oxygen minimum zone (27.15–27.3 kg m^-3 isopycnals) of the Laurentian Channel. The rate of in-channel oxygen utilization in the deep layer was nearly invariant from 2003 to 2023 at 21.1 ± 2.5 µmol kg^-1 yr^-1 and the DIC accumulation rate was estimated to be 18.3 ± 2.5 μmol kg^-1 yr^-1. Using δ¹³C_DIC data, we assess the effect of microbial organic matter remineralization processes and dilution of the ¹³C_DIC pool (−6.6×10^-3 ‰ per μmol of added metabolic DIC). These data and the use of a tracer-calibrated model to resolve advection and mixing dynamics reconcile differences in prior estimates of biogeochemical transformation rates. Finally, we apply the model to the mitigation scenario proposed by Wallace et al. (2023) for artificial re-oxygenation of the Laurentian Channel bottom waters using pure oxygen. We estimate that the injection of ~8.3 × 10⁵ tonnes yr^-1 of oxygen, equivalent to an additional 55 μmol kg^-1 relative to the 2023 boundary concentration proximal to the Cabot Strait, would be required to achieve and maintain above hypoxic levels (>62.5 μmol kg^-1) at the head of the Laurentian Channel. Using the model, we estimate the time required to re-establish steady-state along-channel distributions of DO and DIC following a change in offshore boundary conditions to be about 10 years, or twice the along-channel transit time.

The Gulf and Lower St. Lawrence Estuary have experienced major environmental change over the past century, including the development of hypoxic bottom waters and their simultaneous warming and acidification. Here, we use biogeochemical observations collected during the 2021–2023 TReX project as well as historical data, combined with a tracer-calibrated 1D Advection-Diffusion model with variable boundary conditions to represent dissolved oxygen (DO) and dissolved inorganic carbon (DIC) dynamics within the core of the oxygen minimum zone (27.15–27.3 kg m^-3 isopycnals) of the Laurentian Channel. The rate of in-channel oxygen utilization in the deep layer was nearly invariant from 2003 to 2023 at 21.1 ± 2.5 µmol kg^-1 yr^-1 and the DIC accumulation rate was estimated to be 18.3 ± 2.5 μmol kg^-1 yr^-1. Using δ¹³C_DIC data, we assess the effect of microbial organic matter remineralization processes and dilution of the ¹³C_DIC pool (−6.6×10^-3 ‰ per μmol of added metabolic DIC). These data and the use of a tracer-calibrated model to resolve advection and mixing dynamics reconcile differences in prior estimates of biogeochemical transformation rates. Finally, we apply the model to the mitigation scenario proposed by Wallace et al. (2023) for artificial re-oxygenation of the Laurentian Channel bottom waters using pure oxygen. We estimate that the injection of ~8.3 × 10⁵ tonnes yr^-1 of oxygen, equivalent to an additional 55 μmol kg^-1 relative to the 2023 boundary concentration proximal to the Cabot Strait, would be required to achieve and maintain above hypoxic levels (>62.5 μmol kg^-1) at the head of the Laurentian Channel. Using the model, we estimate the time required to re-establish steady-state along-channel distributions of DO and DIC following a change in offshore boundary conditions to be about 10 years, or twice the along-channel transit time.

The Gulf and Lower St. Lawrence Estuary have experienced major environmental change over the past century, including the development of hypoxic bottom waters and their simultaneous warming and acidification. Here, we use biogeochemical observations collected during the 2021–2023 TReX project as well as historical data, combined with a tracer-calibrated 1D Advection-Diffusion model with variable boundary conditions to represent dissolved oxygen (DO) and dissolved inorganic carbon (DIC) dynamics within the core of the oxygen minimum zone (27.15–27.3 kg m^-3 isopycnals) of the Laurentian Channel. The rate of in-channel oxygen utilization in the deep layer was nearly invariant from 2003 to 2023 at 21.1 ± 2.5 µmol kg^-1 yr^-1 and the DIC accumulation rate was estimated to be 18.3 ± 2.5 μmol kg^-1 yr^-1. Using δ¹³C_DIC data, we assess the effect of microbial organic matter remineralization processes and dilution of the ¹³C_DIC pool (−6.6×10^-3 ‰ per μmol of added metabolic DIC). These data and the use of a tracer-calibrated model to resolve advection and mixing dynamics reconcile differences in prior estimates of biogeochemical transformation rates. Finally, we apply the model to the mitigation scenario proposed by Wallace et al. (2023) for artificial re-oxygenation of the Laurentian Channel bottom waters using pure oxygen. We estimate that the injection of ~8.3 × 10⁵ tonnes yr^-1 of oxygen, equivalent to an additional 55 μmol kg^-1 relative to the 2023 boundary concentration proximal to the Cabot Strait, would be required to achieve and maintain above hypoxic levels (>62.5 μmol kg^-1) at the head of the Laurentian Channel. Using the model, we estimate the time required to re-establish steady-state along-channel distributions of DO and DIC following a change in offshore boundary conditions to be about 10 years, or twice the along-channel transit time.