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Quantifying the contributions of riverine vs. oceanic nitrogen to hypoxia in the East China Sea

Abstract.

"In the East China Sea, hypoxia (oxygen ≤ 62.5 mmol m−3) is frequently observed off the Changjiang (or Yangtze River) estuary covering up to about 15 000 km2. The Changjiang is a major contributor to hypoxia formation because it discharges large amounts of freshwater and nutrients into the region. However, modeling and observational studies have suggested that intrusions of nutrient-rich oceanic water from the Kuroshio Current also contribute to hypoxia formation.  [...]"

Source: Biogeosciences
Authors: Fabian Große et al.
DOI: 10.5194/bg-17-2701-2020

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Reconstructing N2-fixing cyanobacterial blooms in the Baltic Sea

beyond observations using 6- and 7-methylheptadecane in sediments as specific biomarkers

Abstract.

"Summer cyanobacterial blooms represent a threat to the Baltic Sea ecosystem, causing deoxygenation of the bottom water and the spread of the so-called dead zones. The history of the Baltic Sea cyanobacterial blooms is known from in situ and satellite observations since the early 1980s but is still not well understood. [...]"

Source: Biogeosciences
Authors: Jérôme Kaiser et al.
DOI: 10.5194/bg-17-2579-2020

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Chesapeake Bay water quality declines by four percentage points

"An estimated 38% of the Chesapeake Bay and its tidal tributaries met clean water standards for clarity, oxygen and algae growth between 2016 and 2018. This score is lower than the record high 42% from the previous reporting period, but is still the fifth highest estimate of water quality standards attainment since 1985. This four percentage point decrease is due in large part to a decline in dissolved oxygen in the open waters of the Bay, those areas beyond the shoreline and shallows. Dissolved oxygen is necessary for the survival of the Bay’s aquatic species, and is a factor in the annual dead zone. [...]"

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Changing perspectives in marine nitrogen fixation

Abstract.

"Biological dinitrogen (N2) fixation, the reduction of atmospheric N2 to ammonia, is important for maintaining the fertility of the oceans by providing biologically useful nitrogen to support primary organic matter production (i.e., carbon dioxide fixation). N2 fixation offsets the removal of combined nitrogen by microbial denitrification and anaerobic ammonium oxidation (anammox) and export to the deep sea. For several decades, there has been a lack of consensus as to whether losses of N through microbial removal pathways are balanced by biological nitrogen fixation, along with other inputs such as atmospheric nitrogen deposition and terrestrial runoff. [...]"

Source: Science
Authors: Jonathan P. Zehr1 and Douglas G. Capone
DOI: 10.1126/science.aay9514

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No nitrogen fixation in the Bay of Bengal?

Abstract.

"The Bay of Bengal (BoB) has long stood as a biogeochemical enigma, with subsurface waters containing extremely low, but persistent, concentrations of oxygen in the nanomolar range which – for some, yet unconstrained, reason – are prevented from becoming anoxic. One reason for this may be the low productivity of the BoB waters due to nutrient limitation and the resulting lack of respiration of organic material at intermediate waters. [...]"

Source: Biogeosciences
Authors: Carolin R. Löscher et al.
DOI: 10.5194/bg-17-851-2020

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Shedding New Light on the Nitrogen Cycle in the Dark Ocean

"Every year, the Mississippi River dumps around 1.4 million metric tons of nitrogen into the Gulf of Mexico, much of it runoff from agricultural fertilizer. This nitrogen can lead to algal blooms, which in turn deplete oxygen concentrations in the water, creating hypoxic dead zones. The nitrogen cycle is a phenomenon environmental scientists would really like to understand better. “As humans, we do put a lot of reactive nitrogen compounds into the ocean, especially in coastal regions, by…river runoff,” said Katharina Kitzinger of the Max Planck Institute for Marine Microbiology in Bremen, Germany. “It’s really crucial to understand how microbes turn over this excess nitrogen that we put into the environment. [...]”"

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Macroalgal metabolism and lateral carbon flows can create significant carbon sinks

Abstract.

"Macroalgal beds have drawn attention as one of the vegetated coastal ecosystems that act as atmospheric CO2 sinks. Although macroalgal metabolism as well as inorganic and organic carbon flows are important pathways for CO2 uptake by macroalgal beds, the relationships between macroalgal metabolism and associated carbon flows are still poorly understood. In the present study, we investigated carbon flows, including air–water CO2 exchange and budgets of dissolved inorganic carbon, total alkalinity, and dissolved organic carbon (DOC), in a temperate macroalgal bed during the productive months of the year. [...]"

Source: Biogeosciences
Auhtors: Kenta Watanabe et al.
DOI: 10.5194/bg-17-2425-2020

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Abundant nitrite-oxidizing metalloenzymes in the mesopelagic zone of the tropical Pacific Ocean

Abstract.

"Numerous biogeochemical reactions occur within the oceans’ major oxygen minimum zones, but less attention has been paid to the open ocean extremities of these zones. Here we report measurements on oxygen minimum zone waters from the Eastern to the Central Tropical North Pacific, which we analysed using metaproteomic techniques to discern the microbial functions present and their influence on biogeochemical cycling. [...]"

Source: Nature Geoscience
Authors: Mak A. Saito et al.
DOI: 10.1038/s41561-020-0565-6

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Cretaceous oceanic anoxic events prolonged by phosphorus cycle feedbacks

Abstract.

"Oceanic anoxic events (OAEs) document major perturbations of the global carbon cycle with repercussions for the Earth's climate and ocean circulation that are relevant to understanding future climate trends. Here, we compare the onset and development of Cretaceous OAE1a and OAE2 in two drill cores with unusually high sedimentation rates from the Vocontian Basin (southern France) and Tarfaya Basin (southern Morocco). OAE1a and OAE2 exhibit remarkable similarities in the evolution of their carbon isotope (δ13C) records, with long-lasting negative excursions preceding the onset of the main positive excursions, supporting the view that both OAEs were triggered by massive emissions of volcanic CO2 into the atmosphere. However, there are substantial differences, notably in the durations of individual phases within the δ13C positive excursions of both OAEs. [...]"

Source: Climate of the Past
Authors: Sebastian Beil et al.
DOI: 10.5194/cp-16-757-2020

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Response of the western proto-North Atlantic margin to the early Aptian Oceanic Anoxic Event (OAE) 1a: an example from the Cupido platform margin

-Gulf of Mexico, NE Mexico

Abstract.

"Integrated microfacies and geochemical analyses conducted on five stratigraphic sections in northeastern Mexico (ancentral western margin of the proto-North Atlantic) reveal major paleoenvironmental changes in shallow water and pelagic environments in the prelude and run-up of the early Aptian oceanic anoxic event (OAE) 1a. During the Barremian–Aptian transition, the replacement of photozoan rudist-coral by mesotrophic/eutrophic orbitolinid-miliolid communities in the Cupido platform occurred in association with increased nutrient input. [...]"

Source: Cretaceous Research
Authors: Fernando Núñez-Useche et al.
DOI: 10.1016/j.cretres.2020.104488

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