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1. Metabolism of a subtidal rocky mussel reef in a high-temperate setting: pathways of organic C flow

   https://doi.org/10.3354/meps13372  

   Attard, KM ; Rodil, IF ; Berg, P ; Mogg, AOM ; Westerbom, M ; Norkko, A ; Glud, RN ( July 2020 , Marine Ecology Progress Series)

   null (Ed.)

   Mytilid mussels form abundant, species-rich reefs on rocky substrates, but the role of this key habitat in carbon (C) cycling remains poorly understood. We performed a seasonal study on a 5 m deep photic Mytilus trossulus reef in the Central Baltic Sea to investigate pathways and rates of organic C flow. Reef gross primary production (GPP) and respiration ( R ) were estimated seasonally using underwater O 2 eddy covariance on hourly and daily timescales. Photogrammetry and biotic sampling were used to quantify reef rugosity and mussel coverage, and to derive mussel filtration and biodeposition. Mussels were highly abundant, reaching ~50000 ind. m -2 , and the reef structure increased the seabed surface area by 44%. GPP hourly was up to 20 mmol O 2 m -2 h -1 and GPP daily was up to 107 mmol O 2 m -2 d -1 , comparable to a nearby seagrass canopy. Hourly eddy fluxes responded linearly to light intensity and flow velocity, with higher velocities enhancing reef O 2 uptake at night. Reef R daily exceeded GPP daily on 12 of 13 measurement days, and R annual (29 mol O 2 m -2 yr -1 ) was 3-fold larger than GPP annual . The reef sustained a productive community of microbes and fauna whose activities accounted for ~50% of R annual . Horizontal water advection promoted food supply to the reef and likely facilitated substantial lateral C export of mussel biodeposits. Our analyses suggest that a reduction in mussel reef extent due to ongoing environmental change will have major implications for the transport and transformation of C and nutrients within the coastal Baltic Sea. 

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2. Nutrient Controls on Export Production in the Southern Ocean

   https://doi.org/10.1029/2019GB006236  

   Arteaga, Lionel A. ; Pahlow, Markus ; Bushinsky, Seth M. ; Sarmiento, Jorge L. ( August 2019 , Global Biogeochemical Cycles)

   We use observations from novel biogeochemical profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program to estimate annual net community production (ANCP; associated with carbon export) from the seasonal drawdown of mesopelagic oxygen and surface nitrate in the Southern Ocean. Our estimates agree with previous observations in showing an increase in ANCP in the vicinity of the polar front (∼3 mol C m⁻² y⁻¹), compared to lower rates in the subtropical zone (≤ 1 mol C m⁻² y⁻¹) and the seasonal ice zone (<2 mol C m⁻² y⁻¹). Paradoxically, the increase in ANCP south of the subtropical front is associated with elevated surface nitrate and silicate concentrations, but decreasing surface iron. We hypothesize that iron limitation promotes silicification in diatoms, which is evidenced by the low silicate to nitrate ratio of surface waters around the Antarctic polar front. High diatom silicification increases the ballasting effect of particulate organic carbon and overall ANCP in this region. A model‐based assessment of our methods shows a good agreement between ANCP estimates based on oxygen and nitrate drawdown and the modeled downward organic carbon flux at 100 m. This agreement supports the presumption that net biological consumption is the dominant process affecting the drawdown of these chemical tracers and that, given sufficient data, ANCP can be inferred from observations of oxygen and/or nitrate drawdown in the Southern Ocean.

    

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3. Pond methane dynamics, from microbial communities to ecosystem budget, during summer in Alaska

   https://doi.org/10.1002/lno.12003  

   Vizza, Carmella ; Jones, Stuart E. ; Hart, Julia A. ; West, William E. ; Lamberti, Gary A. ( December 2021 , Limnology and Oceanography)

   Ponds play a larger role in the global freshwater methane (CH₄) budget than predicted from surface area alone. To improve our understanding of pond CH₄dynamics, we measured summer CH₄production, concentrations, and emissions to the atmosphere in nine Alaskan wetland ponds along with potential physical, chemical, and biological regulators. Pond CH₄production (0.64, 0.086–1.3 mmol m⁻²d⁻¹; median, interquartile range), as assessed with slurry incubations, was positively related to water‐column temperature and chlorophylla(Chla), negatively influenced by oxygen levels, and varied with microbial community structure. Average water‐column CH₄concentrations (0.39, 0.21–0.87 μmol L⁻¹) were lower in deeper ponds and at higher oxygen levels, and as expected, they were correlated with diffusive emissions (0.055, 0.024–0.20 mmol m⁻²d⁻¹) assessed with flux chambers. Based on a mass balance approach, 39–99% of CH₄produced in ponds was oxidized. Pond ebullition (3.7, 0.60–24 mmol m⁻²d⁻¹) was higher and more variable than diffusive emissions. Additionally, pond ebullition rates were better correlated with production rates from the previous month. We also systematically compared the ratio of ebullition to diffusive CH₄emissions in our ponds and other northern lakes, which was negatively related to water depth (n = 71), but positively related to Chla (n = 28). Our study sheds light on the factors that influence pond CH₄dynamics and demonstrates that pond ebullition is a significant CH₄source worthy of continued study.

    

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4. Summer Carbonate Chemistry in the Dalton Polynya, East Antarctica

   https://doi.org/10.1029/2018JC014882  

   Arroyo, M. C. ; Shadwick, E. H. ; Tilbrook, B. ( August 2019 , Journal of Geophysical Research: Oceans)

   The carbonate chemistry in the Dalton Polynya in East Antarctica (115°–123°E) was investigated in summer 2014/2015 using high‐frequency underway measurements of CO₂fugacity (fCO₂) and discrete water column measurements of total dissolved inorganic carbon (TCO₂) and total alkalinity. Air‐sea CO₂fluxes indicate this region was a weak net source of CO₂to the atmosphere (0.7 ± 0.9 mmol C m⁻²day⁻¹) during the period of observation, with the largest degree of surface water supersaturation (ΔfCO₂= +45 μatm) in ice‐covered waters near the Totten Ice Shelf (TIS) as compared to the ice‐free surface waters in the Dalton Polynya. The seasonal depletion of mixed‐layer TCO₂(6 to 51 μmol/kg) in ice‐free regions was primarily driven by sea ice melt and biological CO₂uptake. Estimates of net community production (NCP) reveal net autotrophy in the ice‐free Dalton Polynya (NCP = 5–20 mmol C m⁻²day⁻¹) and weakly heterotrophic waters near the ice‐covered TIS (NCP = −4–0 mmol C m⁻²day⁻¹). Satellite‐derived estimates of chlorophylla(Chla) and sea ice coverage suggest that the early summer season in 2014/2015 was anomalous relative to the long‐term (1997–2017) record, with lower surface Chlaconcentrations and a greater degree of sea ice cover during the period of observation; the implications for seasonal primary production and air‐sea CO₂exchange are discussed. This study highlights the importance of both physical and biological processes in controlling air‐sea CO₂fluxes and the significant interannual variability of the CO₂system in Antarctic coastal regions.

    

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5. Oxygen Seasonality, Utilization Rate, and Impacts of Vertical Mixing in the Eighteen Degree Water Region of the Sargasso Sea as Observed by Profiling Biogeochemical Floats

   https://doi.org/10.1029/2020GB006824  

   J. Billheimer, Samuel ; Talley, Lynne D. ; Martz, Todd R. ( March 2021 , Global Biogeochemical Cycles)

   Seasonal oxygen structure and utilization in the Sargasso Sea are characterized using nine profiling floats with oxygen 2021 sensors (years 2005–2008), deployed in an Eighteen Degree Water (EDW) experiment (CLIMODE). During autumn‐winter when the mixed layer is deepening, oxygen increases from the surface to the base of the EDW at 400 m. During spring‐summer, oxygen decreases except between the seasonal pycnocline and compensation depth, creating the seasonal shallow oxygen maximum layer (SOMax) with oxygen production of 0.04 μmol kg⁻¹·day⁻¹. In the underlying seasonal oxygen minimum (SOMin), the oxygen utilization rate (OUR) is 0.10 μmol kg⁻¹·day⁻¹, decreasing with depth to 0.04 μmol kg⁻¹·day⁻¹in the EDW. Remineralization in May to August is double that of August to November. The Sargasso Sea is a net carbon producer; estimated annual export production from the top 100–250 m is 2.9 mol C m⁻²and from the top 400 m is 4.2 mol C m⁻². Below the EDW, oxygen decreases seasonally at the same time as in the EDW, indicating remineralization down to 700 m. However, on isopycnals in this deeper layer, oxygen increases during May to September, likely due to lateral advection from nonlocal surface outcrops. Summer shoaling of these isopycnals creates this paradox. The complex vertical oxygen structure in the upper 200 m enables important vertical diffusive flux that modifies the OUR calculated from oxygen change. Ignoring mixing underestimates maximum remineralization by 19% and underestimates maximum net production by 88%. However, vertical mixing is negligible in the deeper layers, so the associated total integrated remineralization error is 5%–9%.

    

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