Clarifying Water Column Respiration and Sedimentary Oxygen Respiration Under Oxygen Depletion Off the Changjiang Estuary and Adjacent East China Sea

The Changjiang Estuary and its adjacent East China Sea are among the largest coastal hypoxic sites in the world. The oxygen depletion in the near-bottom waters (e.g., meters above the seabed) off the Changjiang Estuary is caused by water column respiration (WCR) and sedimentary oxygen respiration (SOR). It is essential to quantify the contributions of WCR and SOR to total apparent oxygen utilization (AOU) to understand the occurrence of hypoxia off the Changjiang Estuary. In this work, we analyzed the δ18O and O2/Ar values of marine dissolved gas samples collected during a field investigation in July 2018. We observed that the δ18O values of dissolved oxygen in near-bottom waters ranged from 1.039 to 8.457‰ (vs. air), generally higher than those of surface waters (−5.366 to 2.336‰). For all the sub-pycnocline samples, the δ18O values were negatively related to O2 concentrations (r2 = 0.97), indicating apparent fractionation of δ18O during oxygen depletion in the water column. Based on two independent isotope fractionation models that quantified the isotopic distillation of dissolved oxygen concentration and its δ18O, the mean contributions of WCR and SOR to total near-bottom AOU were calculated as 53 and 47%, respectively. Beneath the pycnocline, the WCR contribution to the total AOU varied from 24 to 69%, and the SOR contribution varied from 31 to 76%. The pooled samples beneath both the pycnocline and upper mixed layer indicated that WCR contributions (%) to total AOU increased with increasing AOU (μmol/L), whereas SOR% – AOU had the reverse trend. We propose that the WCR% and SOR% contributions to the total AOU of the sub-pycnocline waters are dynamic, not stationary, with changes in ambient environmental factors. Under hypoxic conditions, we observed that up to 70% of the total AOU was contributed by WCR, indicating that WCR is the major oxygen consumption mechanism under hypoxia; that is, WCR plays a vital role in driving the dissolved oxygen to become hypoxic off the Changjiang Estuary.

Peruvian upwelling plankton respiration: calculations of carbon flux, nutrient retention efficiency, and heterotrophic energy production

Oceanic depth profiles of plankton respiration are described by a power function, RCO2 = (RCO2)0 (z/z0)b, similar to the vertical carbon flux profile. Furthermore, because both ocean processes are closely related, conceptually and mathematically, each can be calculated from the other. The exponent b, always negative, defines the maximum curvature of the respiration–depth profile and controls the carbon flux. When |b| is large, the carbon flux (FC) from the epipelagic ocean is low and the nutrient retention efficiency (NRE) is high, allowing these waters to maintain high productivity. The opposite occurs when |b| is small. This means that the attenuation of respiration in ocean water columns is critical in understanding and predicting both vertical FC as well as the capacity of epipelagic ecosystems to retain their nutrients. The ratio of seawater RCO2 to incoming FC is the NRE, a new metric that represents nutrient regeneration in a seawater layer in reference to the nutrients introduced into that layer via FC. A depth profile of FC is the integral of water column respiration. This relationship facilitates calculating ocean sections of FC from water column respiration. In an FC section and in a NRE section across the Peruvian upwelling system we found an FC maximum and a NRE minimum extending down to 400 m, 50 km off the Peruvian coast over the upper part of the continental slope. Finally, considering the coupling between respiratory electron transport system activity and heterotrophic oxidative phosphorylation promoted the calculation of an ocean section of heterotrophic energy production (HEP). It ranged from 250 to 500 J d−1 m−3 in the euphotic zone to less than 5 J d−1 m−3 below 200 m on this ocean section.

Numerical analysis of the primary processes controlling oxygen dynamics on the Louisiana shelf

The Louisiana shelf, in the northern Gulf of Mexico, receives large amounts of freshwater and nutrients from the Mississippi–Atchafalaya river system. These river inputs contribute to widespread bottom-water hypoxia every summer. In this study, we use a physical–biogeochemical model that explicitly simulates oxygen sources and sinks on the Louisiana shelf to identify the key mechanisms controlling hypoxia development. First, we validate the model simulation against observed dissolved oxygen concentrations, primary production, water column respiration, and sediment oxygen consumption. In the model simulation, heterotrophy is prevalent in shelf waters throughout the year, except near the mouths of the Mississippi and Atchafalaya rivers, where primary production exceeds respiratory oxygen consumption during June and July. During this time, efflux of oxygen to the atmosphere, driven by photosynthesis and surface warming, becomes a significant oxygen sink. A substantial fraction of primary production occurs below the pycnocline in summer. We investigate whether this primary production below the pycnocline is mitigating the development of hypoxic conditions with the help of a sensitivity experiment where we disable biological processes in the water column (i.e., primary production and water column respiration). With this experiment we show that below-pycnocline primary production reduces the spatial extent of hypoxic bottom waters only slightly. Our results suggest that the combination of physical processes (advection and vertical diffusion) and sediment oxygen consumption largely determine the spatial extent and dynamics of hypoxia on the Louisiana shelf.

Peru upwelling plankton respiration: calculations of carbon flux, nutrient retention efficiency and heterotrophic energy production

Oceanic depth profiles of plankton respiration are described by a power function, RCO2 = (RCO2)0(z/z0)b similar to the vertical carbon flux profile. Furthermore, because both ocean processes are closely related, conceptually and mathematically, each can be calculated from the other. The exponent (b), always negative, defines the maximum curvature of the respiration depth-profile and controls the carbon flux. When b is large, the C flux (FC) from the epipelagic ocean is low and the nutrient retention efficiency (NRE) is high allowing these waters to maintain high productivity. The opposite occurs when b is small. This means that the attenuation of respiration in ocean water columns is critical in understanding and predicting both vertical FC as well as the capacity of epipelagic ecosystems to retain their nutrients. The NRE is a new metric defined as the ratio of nutrient regeneration in a seawater layer to the nutrients introduced into that layer via FC. A depth-profile of FC is the integral of water column respiration. This relationship facilitates calculating ocean sections of FC from water column respiration. In a FC section across the Peru upwelling system we found a FC maximum extending down to 400 m, 50 km off the Peru coast. Finally, coupling respiratory electron transport system activity to heterotrophic oxidative phosphorylation promoted the calculation of an ocean section of heterotrophic energy production (HEP). It ranged from 250 to 500 J d−1 m−3 in the euphotic zone, to less than 5 J d−1 m−3 below 200 m on this ocean section.

Numerical analysis of the primary processes controlling oxygen dynamics on the Louisiana Shelf

The Louisiana shelf in the northern Gulf of Mexico receives large amounts of freshwater and nutrients from the Mississippi/Atchafalaya River system. These river inputs contribute to widespread bottom-water hypoxia every summer. In this study, we use a physical-biogeochemical model that explicitly simulates oxygen sources and sinks on the Louisiana shelf to identify the key mechanisms controlling hypoxia development. First, we validate the model simulation against observed dissolved oxygen concentrations, primary production, water column respiration, and sediment oxygen consumption. In the model simulation, heterotrophy is prevalent in shelf waters throughout the year except near the mouths of the Mississippi and Atchafalaya Rivers where primary production exceeds respiratory oxygen consumption during June and July. During this time, efflux of oxygen to the atmosphere, driven by photosynthesis and surface warming, becomes a significant oxygen sink while the well-developed pycnocline isolates autotrophic surface waters from the heterotrophic and hypoxic waters below. A substantial fraction of primary production occurs below the pycnocline in summer. We investigate whether this primary production below the pycnocline is mitigating the development of hypoxic conditions with the help of a sensitivity experiment where we disable biological processes in the water column (i.e. primary production and water column respiration). In this experiment below-pycnocline primary production reduces the spatial extent of hypoxic bottom waters only slightly. Our results suggest that the combination of physical processes and sediment oxygen consumption largely determine the spatial extent and dynamics of hypoxia on the Louisiana shelf.

Landscape heterogeneity influences carbon dioxide production in a young boreal reservoir

Surface carbon dioxide (CO2) emissions exhibit a high degree of spatial heterogeneity in the young boreal Eastmain-1 hydroelectric reservoir, located in northern Quebec, Canada. Estimates of the individual components of net CO2 production within the reservoir (benthic respiration, water column respiration, and primary production) furthermore provide a link between the heterogeneity in surface CO2 emissions and the flooded landscapes below. Specifically, the preflood carbon stock and soil–sediment respiration rates of flooded landscapes were found to influence benthic CO2 production, the rate of decline of hypolimnetic dissolved organic carbon (DOC), and the estimated rate at which flooded landscapes release DOC, further influencing water column respiration rates. Estimates of the individual components of net CO2 production in Eastmain-1 are supported by a positive relationship (t test, r2 = 0.64, P < 0.01) between measured surface CO2 emissions (mean ± SE = 1540 ± 145.4 mg C·m–2·day–1) and independently derived estimates of total net CO2 production (mean ± SE = 1230 ± 162.4 mg C·m–2·day–1). Our findings emphasize the utility of fundamental landscape characterization prior to construction in predicting reservoir greenhouse gas emissions.