Riverine nitrogen supply to the global ocean and its limited impact on global marine primary production: a feedback study using an Earth system model

A common notion is that negative feedbacks stabilize the natural marine nitrogen inventory. Recent modeling studies have shown, however, some potential for localized positive feedbacks leading to substantial nitrogen losses in regions where nitrogen fixation and denitrification occur in proximity to each other. Here we include dissolved nitrogen from river discharge in a global 3-D ocean biogeochemistry model and study the effects on near-coastal and remote-open-ocean biogeochemistry. We find that at a steady state the biogeochemical feedbacks in the marine nitrogen cycle, nitrogen input from biological N2 fixation, and nitrogen loss via denitrification mostly compensate for the imposed yearly addition of 22.8 to 45.6 Tg of riverine nitrogen and limit the impact on global marine productivity to < 2 %. Global experiments that regionally isolate river nutrient input show that the sign and strength of the feedbacks depend on the location of the river discharge and the oxygen status of the receiving marine environment. Marine productivity generally increases in proximity to the nitrogen input, but we also find a decline in productivity in the modeled Bay of Bengal and near the mouth of the Amazon River. While most of the changes are located in shelf and near-coastal oceans, nitrogen supply from the rivers can impact the open ocean, due to feedbacks or knock-on effects.

Earth, Wind, Fire, and Pollution: Aerosol Nutrient Sources and Impacts on Ocean Biogeochemistry

A key Earth system science question is the role of atmospheric deposition in supplying vital nutrients to the phytoplankton that form the base of marine food webs. Industrial and vehicular pollution, wildfires, volcanoes, biogenic debris, and desert dust all carry nutrients within their plumes throughout the globe. In remote ocean ecosystems, aerosol deposition represents an essential new source of nutrients for primary production. The large spatiotemporal variability in aerosols from myriad sources combined with the differential responses of marine biota to changing fluxes makes it crucially important to understand where, when, and how much nutrients from the atmosphere enter marine ecosystems. This review brings together existing literature, experimental evidence of impacts, and new atmospheric nutrient observations that can be compared with atmospheric and ocean biogeochemistry modeling. We evaluate the contribution and spatiotemporal variability of nutrient-bearing aerosols from desert dust, wildfire, volcanic, and anthropogenic sources, including the organic component, deposition fluxes, and oceanic impacts. Expected final online publication date for the Annual Review of Marine Science, Volume 14 is January 2022. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The Bistatic Radar as an Effective Tool for Detecting and Monitoring the Presence of Phytoplankton on the Ocean Surface

A massive dust storm formed over the Sahara Desert in June 2020. The African dust cloud, which traveled over the tropical Atlantic’s main development region for hurricanes, resulted in the highest aerosol optical thickness (AOT) for the past two decades. Dust particles contained in dust clouds are at some point deposited on the ocean surface, impacting the ocean biogeochemistry through the supply of nutrients. Although there are remote sensing systems that can map the AOT, the locations of the aerosol particles deposited on the ocean surface remain unknown quantities with remote sensing measurements. In addition, the supplied nutrients are not static and are displaced by ocean currents. Nutrients trigger the phytoplankton (algae) blooms, which form a film on the ocean surface and affect the ocean surface tension. The change in ocean surface tension causes a local decrease of ocean surface roughness over the areas covered with phytoplankton. Bistatic radar data from the CYclone Global Navigation Satellite System (CYGNSS) mission can detect changes in the ocean surface roughness, expressed as an increase in reflectivity when the surface becomes smoother. Therefore, decreased ocean surface roughness correlated with a recent dust storm represents a key indicator of the presence of phytoplankton. In this paper, we present for the first time the capability of bistatic radar measurements to provide an effective tool to map information of areas covered with phytoplankton, establishing bistatic radar as the most reliable remote sensing tool for detecting phytoplankton blooms and monitoring their presence across the ocean surface. We present the analysis of low ocean roughness signatures in the bistatic radar measurements from the CYGNSS mission observed in the Gulf of Mexico after the Sahara’s dust storm circulation from Africa to the American continent from May to July 2020. CYGNSS data offer an unprecedented spatial and temporal coverage that allows for the analysis of those signatures at time scales of 1-day, robust to the presence of clouds and dust clouds. The described capability benefits the improvement of models, promoting a better constraint of the supply of dust into the ocean surface and a better understanding of the excess of nutrients that triggers the phytoplankton blooms. This new bistatic radar application enhances our understanding on the role of dust storms on ocean biogeochemistry and the global carbon cycle.

Bottom-Up Drivers for Global Fish Catch Assessed with Reconstructed Ocean Biogeochemistry from an Earth System Model

Identifying bottom-up (e.g., physical and biogeochemical) drivers for fish catch is essential for sustainable fishing and successful adaptation to climate change through reliable prediction of future fisheries. Previous studies have suggested the potential linkage of fish catch to bottom-up drivers such as ocean temperature or satellite-retrieved chlorophyll concentration across different global ecosystems. Robust estimation of bottom-up effects on global fisheries is, however, still challenging due to the lack of long-term observations of fisheries-relevant biotic variables on a global scale. Here, by using novel long-term biological and biogeochemical data reconstructed from a recently developed data assimilative Earth system model, we newly identified dominant drivers for fish catch in globally distributed coastal ecosystems. A machine learning analysis with the inclusion of reconstructed zooplankton production and dissolved oxygen concentration into the fish catch predictors provides an extended view of the links between environmental forcing and fish catch. Furthermore, the relative importance of each driver and their thresholds for high and low fish catch are analyzed, providing further insight into mechanistic principles of fish catch in individual coastal ecosystems. The results presented herein suggest the potential predictive use of their relationships and the need for continuous observational effort for global ocean biogeochemistry.

Riverine nitrogen supply to the global ocean and its limited impact on global marine primary production: a feedback study using an Earth System Model

A common notion is that negative feedbacks stabilize the marine nitrogen inventory. Recent modeling studies have shown, however, some potential for localized positive feedbacks leading to substantial nitrogen losses, in regions where nitrogen fixation and denitrification occur in proximity to each other. Here we include dissolved nitrogen from river discharge in a global 3-D ocean biogeochemistry model and study the effects on near-coastal and remote open ocean biogeochemistry. We find that at steady state the biogeochemical feedbacks in the marine nitrogen cycle, nitrogen input from biological N2 fixation, and nitrogen loss via denitrification, mostly compensate for the yearly addition of 22.8 to 45.6 Tg of riverine nitrogen and limit the impact on global marine productivity to < 2 %. Global experiments that regionally isolate river nutrient input show that sign and strength of the feedbacks depend on the location of the river discharge and the oxygen status of the receiving marine environment. Marine productivity generally increases in proximity to the nitrogen input, but we also find a decline in productivity in the Bay of Bengal and near the mouth of the Amazon River. While most of the changes are located in shelf and near coastal oceans, nitrogen supply from the rivers can impact the open ocean, due to feedbacks or knock-on effects.

Tuning ocean biogeochemistry in the Earth system — insights from the HAMburg Ocean Carbon Cycle model

&lt;div&gt;Ocean biogeochemistry as part of the Earth system impacts the uptake of atmospheric CO&lt;sub&gt;2&lt;/sub&gt; and storage of carbon in the ocean. In the ICON-O (Icosahedral non-hydrostatic general circulation model) ocean model, ocean biogeochemistry is represented by the HAMburg Ocean Carbon Cycle model (HAMOCC; Ilyina et al. 2013, Mauritsen et al. 2019, Maerz et al. 2020). Here, we present the results of an ongoing effort to tune HAMOCC (i.e. adapt parameters within the uncertainty range) to accommodate the ocean circulation simulated by ICON-O.&lt;/div&gt;&lt;div&gt;The tuning of biogeochemical models, including HAMOCC, has previously been an iterative, and a rather random process combining expert knowledge and a suite of parameter testings. A documented, systematic procedure, describing how to tune these models is lacking. Therefore, while tuning HAMOCC in ICON-O, we aim at filling this gap by structuring the process and documenting the steps taken to tune a biogeochemistry model in a global general ocean circulation model.&lt;/div&gt;&lt;div&gt;The ocean circulation has a large impact on the distribution of biogeochemical tracers, as biases in the circulation will, for example, impact the upwelling of nutrients or the CO&lt;sub&gt;2&lt;/sub&gt; exchange with the atmosphere. We investigate the impact of physical parameterization such as the Gent-McWilliam eddy parameterization and the vertical mixing scheme on the choice of HAMOCC tuning parameters. We then compare the spatial distribution of major state variables such as nutrients and alkalinity to observational data ( WOA; Garcia et al 2013, GLODAP; Key et al 2004) and evaluate the key tendencies such as CO&lt;sub&gt;2&lt;/sub&gt; surface fluxes and attenuation of particulate organic matter fluxes. Furthermore, we discuss the tuning steps, choices of the tuning parameters and their impact on the simulated biogeochemistry. The envisioned outcome of this work is a tuned ocean biogeochemistry component for the here used ICON-O model and a more generalized tuning procedure that can be applied to other models or HAMOCC in different model configurations (coupled runs, different resolution).&lt;/div&gt;&lt;div&gt;&amp;#160;&lt;/div&gt;&lt;p&gt;Garcia, H. E., et al. 2014: World Ocean Atlas 2013, NOAA Atlas NESDIS 76, Volume4: Dissolved Inorganic Nutrients (phosphate, nitrate, silicate), 25pp.&lt;/p&gt;&lt;p&gt;lyina, T., et al. 2013: Global ocean biogeochemistry model HAMOCC: Model architecture and performance as component of the MPI-Earth system model in different CMIP5 experimental realizations, J. Adv. Model. Earth Sy., 5, .&lt;/p&gt;&lt;p&gt;Key, R., et al. 2004: A global ocean carbon climatology: Results from Global Data Analysis Project, Global Biogeochem. Cycles, 18, 4, https://doi.org/10.1029/2004GB002247.&lt;/p&gt;&lt;p&gt;Maerz et al. 2020: Microstructure and composition of marine aggregates as co-determinants for vertical particulate organic carbon transfer in the global ocean, Biogeosciences, 17, 7, https://doi.org/10.5194/bg-17-1765-2020.&lt;/p&gt;&lt;p&gt;Mauritsen, T., et al. 2019: Developments in the MPI-M Earth System Model version 1.2 (MPI-ESM1.2) and Its Response to Increasing CO&lt;sub&gt;2&lt;/sub&gt;, J. Adv. Model. Earth Sy., 11, https://doi.org/10.1029/2018MS001400.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Modeling the physical drivers of the decadal variability of the Southern Ocean carbon uptake

&lt;p&gt;Observational estimates point to pronounced changes of the Southern Ocean carbon uptake in the past decades, but the mechanisms are still not fully understood. In this study we assess physical drivers of the Southern Ocean carbon uptake variability in a suite of global ocean biogeochemistry models with 0.5&amp;#186;, 0.25&amp;#186; and 0.1&amp;#186; horizontal resolution as well as in a 3-member ensemble performed with an Earth System Model (ESM) sharing the same ocean biogeochemistry model. The ocean models show a positive trend of the Southern Ocean CO&lt;sub&gt;2&lt;/sub&gt; uptake in the past decades, with a weakening of its rate of increase in the 1990s. The 0.1&amp;#186; model exhibits the strongest trend in the Southern Ocean carbon uptake.&amp;#160;&lt;span&gt;Different physical drivers of the carbon up&lt;/span&gt;take variability and of its trends (such as changes in stratification, ventilation, overturning circulation, and SST) are analyzed. A particular focus of this study is to assess the role of open-ocean polynyas in driving Southern Ocean carbon uptake. Open-ocean polynyas in the Southern Ocean have pronounced climate fingerprints, such as reduced sea-ice coverage, heat loss by the ocean and enhanced bottom water formation, but their role for the Southern Ocean carbon uptake has been as yet little studied. To this end we analyze conjunctly ESM simulations and an ocean-only sensitivity experiment where open-ocean polynyas are artificially created by perturbing the Antarctic freshwater runoff. We find that enhanced CO&lt;sub&gt;2&lt;/sub&gt; outgassing takes place during the polynya opening, because old carbon-rich waters come in contact with the atmosphere. The concomitant increased uptake of anthropogenic CO&lt;sub&gt;2&lt;/sub&gt; partially compensates the CO&lt;sub&gt;2&lt;/sub&gt; outgassing. When the polynya closes, the ocean CO&lt;sub&gt;2&lt;/sub&gt; uptake increases significantly, possibly fueled by abundant nutrients and higher alkalinity brought to the surface during the previous convective phase. Our results suggest that open-ocean polynyas could have a significant impact on the Southern Ocean CO&lt;sub&gt;2&lt;/sub&gt; uptake and could thus modulate its decadal variability.&lt;/p&gt;&lt;p&gt;&amp;#160;&lt;/p&gt;

Towards inferring the variability in oceanic CO2 fluxes at high latitudes using atmospheric O2 observations

&lt;p&gt;The oceanic CO&lt;sub&gt;2&lt;/sub&gt; sink displays year-to-year to decadal variabilities which are not fully reproduced by global ocean biogeochemistry models, especially in the high-latitude oceans. Oceanic CO&lt;sub&gt;2&lt;/sub&gt; is influenced by the same climate variability and the same ecosystem processes as oceanic oxygen (O&lt;sub&gt;2&lt;/sub&gt;), although in different proportions. Unlike for CO&lt;sub&gt;2&lt;/sub&gt;, oceanic O&lt;sub&gt;2&lt;/sub&gt; flux is not influenced directly by the rise in atmospheric CO&lt;sub&gt;2&lt;/sub&gt;, and therefore its variability reflects purely climatic and biogeochemical variability and trends. Therefore, natural climate variability and changes in oceanic processes controlling air-sea exchanges of CO&lt;sub&gt;2&lt;/sub&gt; can be studied by focusing on oxygen (O&lt;sub&gt;2&lt;/sub&gt;), where the signal is unencumbered by direct anthropogenic influence. A global time series of oceanic O&lt;sub&gt;2&lt;/sub&gt; flux was obtained by building a global O&lt;sub&gt;2&lt;/sub&gt; budget, with an approach similar to the one used for the global carbon budget. The global O&lt;sub&gt;2&lt;/sub&gt; budget is based on atmospheric O&lt;sub&gt;2&lt;/sub&gt; observations and fossil fuel statistics, and infers the partitioning of the land and ocean fluxes using constant C:O&lt;sub&gt;2&lt;/sub&gt; ratios for land processes. One key result of this analysis is that air-sea O&lt;sub&gt;2&lt;/sub&gt; exchange induced significant year-to-year variability in observed atmospheric O&lt;sub&gt;2&lt;/sub&gt;. Estimates of regional oceanic O&lt;sub&gt;2&lt;/sub&gt; fluxes were obtained from an atmospheric transport inversion analysis that inferred air-sea O&lt;sub&gt;2&lt;/sub&gt; exchange based on global atmospheric O&lt;sub&gt;2&lt;/sub&gt; observations and a global atmospheric transport model. For the Southern Ocean, a comparison was made between time series of winter oceanic O&lt;sub&gt;2&lt;/sub&gt; fluxes from this inversion method and winter mixed layer depths from Argo floats. Results from this comparison confirmed the previously suggested relationship between the winter ocean mixing and air-sea O&lt;sub&gt;2&lt;/sub&gt; exchange, which might be controlled by the climate variability induced by the Southern Annular Mode. Finally, these global and regional air-sea O&lt;sub&gt;2&lt;/sub&gt; fluxes were compared with outputs from six global ocean biogeochemistry models to examine their current skills in simulating O&lt;sub&gt;2&lt;/sub&gt; variability. Preliminary results suggested that all models underestimated the interannual variability in oceanic O&lt;sub&gt;2&lt;/sub&gt; fluxes, however they were able to simulate some of the observed multi-annual variability in O&lt;sub&gt;2&lt;/sub&gt; fluxes at high latitudes. We discuss the implications for the model&amp;#8217;s representation of the variability in CO&lt;sub&gt;2&lt;/sub&gt; fluxes.&lt;/p&gt;