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1. Early Diagenesis in the Hypoxic and Acidified Zone of the Northern Gulf of Mexico: Is Organic Matter Recycling in Sediments Disconnected From the Water Column?

   https://doi.org/10.3389/fmars.2021.604330  

   Rabouille, Christophe; Lansard, Bruno; Owings, Shannon M.; Rabalais, Nancy N.; Bombled, Bruno; Metzger, Edouard; Richirt, Julien; Eitel, Eryn M.; Boever, Anthony D.; Beckler, Jordon S.; et al (March 2021, Frontiers in Marine Science)

   null (Ed.)

   Hypoxia and associated acidification are growing concerns for ecosystems and biogeochemical cycles in the coastal zone. The northern Gulf of Mexico (nGoM) has experienced large seasonal hypoxia for decades linked to the eutrophication of the continental shelf fueled by the Mississippi River nutrient discharge. Sediments play a key role in maintaining hypoxic and acidified bottom waters, but this role is still not completely understood. In the summer 2017, when the surface area of the hypoxic zone in the nGoM was the largest ever recorded, we investigated four stations on the continental shelf differentially influenced by river inputs of the Mississippi-Atchafalaya River System and seasonal hypoxia. We investigated diagenetic processes under normoxic, hypoxic, and nearly anoxic bottom waters by coupling amperometric, potentiometric, and voltammetric microprofiling with high-resolution diffusive equilibrium in thin-films (DET) profiles and porewater analyses. In addition, we used a time-series of bottom-water dissolved oxygen from May to November 2017, which indicated intense O 2 consumption in bottom waters related to organic carbon recycling. At the sediment-water interface (SWI), we found that oxygen consumption linked to organic matter recycling was large with diffusive oxygen uptake (DOU) of 8 and 14 mmol m –2 d –1 , except when the oxygen concentration was near anoxia (5 mmol m –2 d –1 ). Except at the station located near the Mississippi river outlet, the downcore pore water sulfate concentration decrease was limited, with little increase in alkalinity, dissolved inorganic carbon (DIC), ammonium, and phosphate suggesting that low oxygen conditions did not promote anoxic diagenesis as anticipated. We attributed the low anoxic diagenesis intensity to a limitation in organic substrate supply, possibly linked to the reduction of bioturbation during the hypoxic spring and summer. 

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2. Unprecedented summer hypoxia in southern Cape Cod Bay: an ecological response to regional climate change?

   https://doi.org/10.5194/bg-19-3523-2022  

   Scully, Malcolm E.; Geyer, W. Rockwell; Borkman, David; Pugh, Tracy L.; Costa, Amy; Nichols, Owen C. (January 2022, Biogeosciences)

   Abstract. In late summer 2019 and 2020 bottom waters in southern Cape Cod Bay (CCB) became depleted of dissolved oxygen (DO), with documented benthicmortality in both years. Hypoxic conditions formed in relatively shallow water where the strong seasonal thermocline intersected the sea floor, bothlimiting vertical mixing and concentrating biological oxygen demand (BOD) over a very thin bottom boundary layer. In both 2019 and 2020, anomalouslyhigh sub-surface phytoplankton blooms were observed, and the biomass from these blooms provided the fuel to deplete sub-pycnocline waters of DO. Theincreased chlorophyll fluorescence was accompanied by a corresponding decrease in sub-pycnocline nutrients, suggesting that prior to 2019 physicalconditions were unfavorable for the utilization of these deep nutrients by the late-summer phytoplankton community. It is hypothesized thatsignificant alteration of physical conditions in CCB during late summer, which is the result of regional climate change, has favored the recentincrease in sub-surface phytoplankton production. These changes include rapidly warming waters and significant shifts in summer wind direction, bothof which impact the intensity and vertical distribution of thermal stratification and vertical mixing within the water column. These changes inwater column structure are not only more susceptible to hypoxia but also have significant implications for phytoplankton dynamics, potentiallyallowing for intense late-summer blooms of Karenia mikimotoi, a species new to the area. K. mikimotoi had not been detected in CCBor adjacent waters prior to 2017; however, increasing cell densities have been reported in subsequent years, consistent with a rapidly changingecosystem. 

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3. The role of seasonal hypoxia and benthic boundary layer exchange on iron redox cycling on the Oregon shelf

   https://doi.org/10.1002/lno.12476  

   Evans, Natalya; Floback, Alexis E.; Gaffney, Justin; Chace, Peter J.; Luna, Zachary; Knoery, Joël; Reimers, Clare E.; Moffett, James W. (December 2023, Limnology and Oceanography)

   David Hambright (Ed.)

   Abstract Widespread hypoxia occurs seasonally across the Oregon continental shelf, and the duration, intensity, and frequency of hypoxic events have increased in recent years. In hypoxic regions, iron reduction can liberate dissolved Fe(II) from continental shelf sediments. Fe(II) was measured in the water column across the continental shelf and slope on the Oregon coast during summer 2022 using both a trace metal clean rosette and a high‐resolution benthic gradient sampler. In the summer, Fe(II) concentrations were exceptionally high (40–60 nM) within bottom waters and ubiquitous across the Oregon shelf, reflecting the low oxygen condition (40–70 μM) at that time. The observed inverse correlation between Fe(II) and bottom water oxygen concentrations is in agreement with expectations based on previous work that demonstrates oxygen is a major determinant of benthic Fe fluxes. Rapid attenuation of Fe(II) from the benthic boundary layer (within 1 m of the seafloor) probably reflects efficient cross‐shelf advection. One region, centered around Heceta Bank (~ 44°N) acts a hotspot for Fe release on the Oregon continental shelf, likely due to its semi‐retentive nature and high percent mud content in sediment. The results suggest that hypoxia is an important determinant of the inventory of iron is Oregon shelf waters and thus ultimately controls the importance of continental margin‐derived iron to the interior of the North Pacific Basin. 

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4. The role of local winds and a semi-persistent coastal front on the diurnal heat budget in an upwelling bay

   https://doi.org/10.1016/j.csr.2026.105702  

   Thibodeau, April P; Walter, Ryan K; Mazzini, Piero LF; Connolly, Thomas P; Edwards, Christopher A; Robbins, Ian (August 2026, Continental Shelf Research)

   Coastal embayments in nearshore upwelling systems (“upwelling bays”) play a disproportionately large role in regional oceanography. In these systems, local diurnal wind forcing and thermal gradients associated with an upwelling shadow front that forms between warmer sheltered waters inside the bay and colder recently upwelled waters outside the bay strongly influence local temperature dynamics. Despite their importance to local ecosystems, there are no long-term studies that assess the heat budget inside upwelling bays. In this study, we analyzed approximately two years of water temperature throughout the water column using an autonomous profiler in a small upwelling bay in central California (San Luis Obispo Bay). We coupled these measurements with local meteorological data and in-situ temperature measurements made outside the bay to investigate how local diurnal wind forcing and the presence of the upwelling shadow front modify the diurnal heat budget inside the bay. Over the study period, strong seasonality in regional upwelling coincided with changes in local wind forcing, front strength (bulk temperature difference between inside and outside the bay), and temperature structure, from which we identified various forcing regimes to quantify the diurnal heat budget. During the non-upwelling season when the front was largely absent, the heat budget was primarily a balance between changes in heat content and surface heat fluxes, regardless of local wind forcing strength. This was also the primary balance during the upwelling season when local winds were weak. In contrast, during the upwelling season when the upwelling shadow front was persistent and the local winds were strong, increasing front strength led to increasingly large residual heat fluxes, which were interpreted to be due to advection (advective fluxes not calculated directly). These results highlight the importance of both local wind forcing and frontal intensity on the heat budget inside the upwelling bay, with significant implications for residence time, temperature dynamics, and local biogeochemistry in similar systems that are ubiquitous globally. 

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5. Effects of hypoxia and acidification on Calanus pacificus: behavioral changes in response to stressful environments

   https://doi.org/10.3354/meps14142  

   Wyeth, AC; Grünbaum, D; Keister, JE (September 2022, Marine Ecology Progress Series)

   Copepods, which play major roles in marine food webs and biogeochemical cycling, frequently undergo diel vertical migration (DVM), swimming downwards during the day to avoid visual predation and upwards at night to feed. Natural water columns that are stratified with chemical stressors at depth, such as hypoxia and acidification, are increasing with climate change. Understanding behavioral responses of copepods to these stresses—in particular, whether copepods alter their natural migration—is important to anticipating impacts of climate change on marine ecosystems. We conducted laboratory experiments using stratified water columns to measure the effects of bottom water hypoxia and pH on mortality, distribution, and swimming behaviors of the calanoid copepodCalanuspacificus. When exposed to hypoxic (0.65 mg O₂l^-1) bottom waters, the height ofC.pacificusfrom the bottom increased 20% within hypoxic columns, and swimming speed decreased 46% at the bottom of hypoxic columns and increased 12% above hypoxic waters. When exposed to low pH (7.48) bottom waters, swimming speeds decreased by 8 and 9% at the base of the tanks and above acidic waters, respectively. Additionally, we found a 118% increase in ‘moribund’ (immobile on the bottom) copepods when exposed to hypoxic, but not acidic, bottom waters. Some swimming statistics differed between copepods collected from sites with versus without historical hypoxia and acidity. Observed responses suggest potential mechanisms underlyinginsituchanges in copepod population distributions when exposed to chemical stressors at depth. 

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