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Transactions of the Karelian Research Centre of the Russian Academy of Sciences No. 3. 2019. P. 28-33 DOI: 10.17076/lim989
Труды Карельского научного центра РАН № 3.2019. С.28-33
УДК 551.482.213 + 528.8.04
ATMOSPHERiC COLUMNAR C02 ENHANCEMENT OVER E. HUXLEYI BLOOMS: CASE STUDiES iN THE NORTH ATLANTiC AND ARCTiC WATERS
E. A. Morozov, D. V. Kondrik, S. S. Chepikova, D. V. Pozdnyakov
Nansen International Environmental and Remote Sensing Centre, St. Petersburg, Russia
Blooms of a coccolithophore E. huxleyi are generally huge, occur annually and in the oceans of both Hemispheres. As a calcifying algal species, E. huxleyi is known to enhance the partial pressure of dissolved CO2 in the surface ocean, thus reducing its ability to absorb atmospheric CO2. Here we report on the results of our satellite study of CO2 enhancement in the atmospheric column over E. huxleyi blooms in the North, Greenland, Iceland and Barents seas. The study is based on OCO-2 and wind force and direction data, and E. huxleyi bloom masks developed by us earlier. Eight case studies are discussed herein relating to the time period 2015-2018. The results obtained are strongly indicative that, indeed, the phenomenon of E. huxleyi blooms noticeably affects the carbon fluxes between the atmosphere and the surface ocean: the quantified enhancement of CO2 content in the atmospheric column over the bloom area in five out of eight case studies proved to be in the range of 0.6-3.0 ppm. It is also shown that the magnitude of CO2 enhancement in the atmospheric column is significantly controlled by air advection in the boundary layer.
Keywords: satellite remote sensing; OCO-2 data; enhancement of atmospheric columnar CO2 content over E. huxleyi blooms in Subarctic and Arctic seas; Emiliania huxleyi; wind and atmospheric advection.
E. А. Морозов, Д. В. Кондрик, С. С. Чепикова, Д. В. Поздняков. УВЕЛИЧЕНИЕ КОНЦЕНТРАЦИИ С02 В АТМОСФЕРНОМ СТОЛБЕ НАД ОБЛАСТЬЮ ЦВЕТЕНИЯ E. HUXLEYI: КОНКРЕТНЫЕ СЛУЧАИ В ВОДАХ СЕВЕРНОЙ АТЛАНТИКИ И АРКТИКИ
В этой краткой статье сообщается о результатах спутниковых исследований по увеличению содержания СО2 в атмосферном столбе над цветениями E. huxleyi в Северном, Гренландском, Исландском и Баренцевом морях. Исследование базируется на данных космической станции OCO-2 по содержанию СО2 в атмосферном столбе, на спутниковых данных о силе и направлении приводного ветра, с использованием ранее нами разработанных масок цветений E. huxleyi. Всего было выявлено и исследовано восемь конкретных случаев за период с 2015 по 2018 гг., когда траектория пролета станции OCO-2 пролегала по области цветения и выходила за ее пределы, что позволяло оценивать влияние цветения E. huxleyi на содержание СО2 в атмосферном столбе. Полученные результаты однозначно свидетельствуют о влиянии исследуемого явления на потоки углерода на границе раздела «атмосфера-океан». В пяти случаях из восьми увеличение СО2 в атмосферном столбе над областью цветения E. huxleyi оказывалось в диапазоне 0,6-3,0 ppm. Выяснено, что
конкретная величина увеличения в некоторых случаях существенно зависит от адвекции воздушных масс в приграничном слое.
Ключевые слова: спутниковое дистанционное зондирование; данные ОСО-2; увеличение концентрации СО2 в атмосферном столбе над цветениями Е. huxleyi в морях субарктической Атлантики и Арктики; ЕтШапа huxleyi; ветер и адвекция воздушных масс.
introduction
Among marine biosystems, coccolithophores (class Prymnesiophycea) are the most productive calcite-producing organisms in the world's oceans [Thierstein, Young, 2013]. Dissolved carbon dioxide of atmospheric origin interacts with dissolved calcite with the formation of HCO- and Ca+2. Thus, any increase in the partial pressure of atmospheric CO2 leads to a shift between the marine suspended organic and inorganic carbon. This, in turn, is bound to affect the carbon cycle in the atmosphere - ocean surface balance.
In addition to the production of particulate cal-cite, coccolithophores are capable of increasing dissolved CO2 partial pressure within their blooming areas [Holligan et al., 1993; Kondrik et al., 2018].
Conjointly, these two mechanisms affect the carbon balance in surface ocean and tend to weaken marine carbon sinks, which has far-reaching consequences in terms of planetary climate change [IPCC, 2014].
Within the coccolithophore group, E. huxleyi is the most widespread species in the world's oceans [Westbroek et al., 1985; Moore et al., 2012]. It forms gigantic blooms with a surface of several thousand square kilometers [Kondrik et al., 2017], but sometimes exceeding one million square kilometers [Balch et al., 2014].
The aforementioned E. huxleyi bloom-driven enhancement of dissolved CO2 partial pressure can reduce, nullify or even reverse the flux of CO2 at the atmosphere-ocean interface. Indeed, Shutler et al. [2013] report on an average reduction in the monthly air-sea CO2 flux by about 55 % across the marine tracts encompassing extensive E. huxleyi blooms in the North Atlantic, whereas the maximum reduction over the time period 1998-2007 was registered at 155 %.
Here we present our results on several case studies in the North, Iceland, Greenland and Barents seas. The study was designed to quantify the atmospheric columnar CO2 over E. huxleyi blooms based on remote sensing data from the Orbiting Carbon Observatory OCO-2 that was put into orbit in 2014 to study CO2 concentration and spatio-temporal distribution in the Earth's atmosphere
[Crisp, 2015]. The areas targeted in the above seas were identified in advance making use of E. huxleyi bloom masks developed on the basis of ocean color data from the ocean-colour climate-change initiative OC CCI data archive [Sathyendranath, Krasermann, 2014].
Methodology
Previously, based on the developed bloom masking technology, i. e. the methodology of E. huxleyi bloom detection and contouring, the 1998-2018 time series of blooms of this alga were obtained for the Subarctic Atlantic and Arctic Seas [Kondrik et al., 2019; Selyuzhenok et al., 2019]. For the revealed locations of E. huxleyi blooms, the 2015-2018 OCO-2 data were subjected to sieve analysis in order to ascertain the cases of OCO-2 footprint trajectory crossing both the bloom area and adjoining bloom-free waters. The identified situations were further analyzed as case studies in order to investigate on a quantitative basis if there was any impact of E. huxleyi bloom areas on XCO2 registered by OCO-2. Thus, to assess the impact, XCO2 values registered along the OCO-2 footprint both over the bloom area and beyond it (either prior to reaching the bloom area or after leaving it) were compared. The resultant change in xCO2, i. e. AXCO2, was considered as a measure of the E. huxleyi bloom impact on the CO2 exchange at the atmosphere-sea water interface, and hence, of the change in the CO2 atmospheric columnar content.
All case studies also included the analysis of above water surface wind force and direction over the bloom area in order to clarify the issue of air mass advection across the satellite footprint trajectory.
Data sources
Wind data. 8-day averaged satellite data from Cross-Calibrated Multi-Platform (CCMP) data http://www.rmss.com/measurements/wind/ were employed for the time period prior to 2016 (http: www.rems.com/measurements/ccmp/). CCMP gridded surface vector winds are generated through concatenation of satellite, moored buoy,
Change of XCO2 (ppm) over E. huxleyi bloom areas in the Barents, Iceland, Greenland and North seas as recorded with the OCO-2 instrument within the time period 2015-2 018
Case number Sea Start of 8-day time interval XCO2 over bloom XCO2 beyond bloom axco2 Wind force (m/s) and direction
1 Barents 28.07.2015 393.6 393.0 0.6 4.6 E
2 South Iceland 12.07.2015 396.5 395.0 1.5 6.1 NNE
3 South Greenland 12.07.2015 397.0 394.0 3.0 3.0 WSW
4 South Iceland 24.05.2016 404.0 404.0 0 4.9 S
5 South Iceland 01.06.2016 404.0 402.0 2.0 3.3 ESE
6 North 17.05.2018 410.0 410.0 0 4.9 NE
7 North 18.06.2018 408.0 407.0 1.0 8.4 NW
8 North 26.06.2018 406.0 406.0 0 3.1 N
and simulated wind data. Thus conjoined, these mutually harmonized data qualify as Level-3 ocean vector wind analysis product. Through the involvement of improved and extended input data, the CCMP product was updated up to the CCMP V2.0 data set that is reachable at the Remote Sensing Systems (RSS) portal. This updated data set combines RSS-7 V. 7 radiometer wind speeds, QuikSCAT and ASCAT scatterometer wind vectors, wind speed actually measured from moored buoys, and ERA-Interim wind spatial distribution simulated with the Variational Analysis Method (VAM). The resultant product is four maps at a daily temporal and 0.25° spatial resolution.
In the case of the North Sea 2018, CCMP are unavailable, and in their stead ASCAT data, version 2.1 were exploited (http://www.remss.com/ missions/ascat/). To better harmonize scattero-metric and radiometric wind measurements, the ASCAT data were generated with the use of a new Geophysical Model Function, C-2015.
Thus, the wind vectors that are laid upon the maps illustrating our case studies represent 8-day wind force and direction averages specifically over the areas of E. huxleyi blooms.
Atmospheric C02 content. The column averaged dry air mole fraction, XCO2 is defined as the ratio of "the altitude-dependent CO2 number density integrated over the atmospheric column and the column abundance of dry air" [Crisp, 2015].
Having a 16-day ground-track repeat cycle, OCO-2 yields XCO2 values with single-sounding random errors in the range of 0.5-1 ppm at solar zenith angles up to 70°, and at the spatial resolution of 3 km2 in nadir, i. e. 1.25 km in width and ~2.4 km in length, which corresponds to a ~1.8 mrad instantaneous field of view and 3 Hz sampling. In 2018, the OCO-2 data processing algorithms were improved at NASA, and the current and retrospective products (L1B/L2 Version 8 and L2LiteFileVer-sion 9; October 10, 2018) were released (https://
docserver.gesdisc.eosdis.nasa.gov/public/proj-ect/OCO/OCO2_DUG.V9.pdf).
Only high quality data (i. e. unflagged data) were employed in our case studies. 8-day averaging of XCO2 data was implemented in this study.
E. huxleyi bloom masking. The 1998-2018 time series of E. huxley blooms in the Subarctic Atlantic and Arctic oceans was retrieved from Ocean Color Climate Change Initiative (OC CCI) data through the analysis of spectra of remote sensing reflectance, Rrs(A). The methodology is described in detail in [KSondrik et al., 2017, 2019]. Concisely, a typical Rrs spectrum from a E. huxleyi bloom exhibits a maximum at A = ~490 nm at the late stage of its development, when the surface water is predominantly populated by cocco-liths whereas E. huxleyi cells have already mostly died off. Accurate delineation of E. huxleyi blooms was based on fulfillment of the requirement that the spectral values of Rrs (sr-1) in the OC CCI standard spectral channels exceed the following statistically established thresholds: 0.001 at 412 nm, 0.008 at 443 nm, 0.01 at 490 nm, 0.008 at 510 nm, and ~ 0 at 670 nm. On this basis, masks of E. huxleyi blooms were plotted for the target Subarctic and Arctic seas to restore the chronicle of spatiotemporal variations of the bloom areas between 1998 and 2018.
Results and discussion
Here we present the results of eight satellite-based case studies from the Barents, Iceland, Greenland and North seas (Table, Fig., a-h). Note that red lines show the limits of the beyond-bloom areas used in this study for assessing AXCO2; black arrows indicate the force and direction of wind over the bloom area; black areas are E. huxleyi blooms.
As the Table shows, in 5 cases (The Barents, South Iceland, South Greenland, North seas) OCO-2 registered an increment of XCO2 over E. huxleyi bloom areas ranging between 0.6
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and 3.0 ppm. These numbers are fully consistent with the results we have obtained in the study of E. huxleyi -induced XCO2 in the Black Sea as registered in 2016-2017.
However, in three cases (the South Iceland, and North seas), no XCO2 enhancement was found. A combined OCO-2 and wind data analysis has shown that the explanation of the apparent absence of E. huxleyi blooming impact upon XCO2 might reside in the effect of above water air mass advection. Indeed, for cases 4, 6, and 8 the meteorological and E. huxleyi blooming conditions were specific. In case 4 the blooming area was essentially inhomogeneous/fractionized, and the wind direction was southern, i. e. bringing air masses from the parts of the sea free of any E. huxleyi bloom influence.
In case 8 there were very similar conditions in terms of wind-driven advection of above-water air from marine tracts void of E. huxleyi blooming. It is also worth mentioning that the blooming area was also significantly fractionized.
A special consideration should be given to case 6. At first glance, it appears that the E. huxleyi-driv-en AXCO2 signal should not be zero: the footprint trajectory traverses the bloom area, and the ad-vected air comes from a large portion of bloom. However, the number of OCO-2 pixels within the bloom area is rather small, rendering the AXCO2 retrievals insufficiently reliable.
Concluding remarks
Produced in the reaction of calcification inside the cell of E. huxleyi, CO2 becomes available for the reaction of photosynthesis with a result of a reduced uptake of dissolved CO2 from ambient water. Thus, surface marine waters within the bloom of E. huxleyi turn out to be less CO2 depleted. Moreover, the thus enhanced partial pressure of dissolved CO2 can either nullify the flux of atmospheric CO2 or even reverse it. This has been proven in shipborne surveys, and through spaceborne observations over the Black Sea: the enhancement of CO2 content in the atmospheric column proved to be within 1-2 ppm.
The eight case studies conducted with the employment of OCO-2 satellite data and presented in this concise report have shown that the impact of E. huxleyi blooming phenomenon on the atmospheric CO2 partial pressure over the North, Iceland, Greenland, and Barents seas proved to be of the same order of magnitude as over the Black Sea (0.6-3 ppm). It is also shown that the magnitude of CO2 enhancement in the atmospheric column is significantly controlled by the air advection in the boundary layer.
Arguably, this might be an indication of some inherent property of E. huxleyi, and the obtained results on the increment of CO2 in the atmospheric column over the blooms of this alga can be considered as representative of this phenomenon across the oceanic tracts, at least, in the Northern Hemisphere.
We express our gratitude for the financial support of this study provided by the Russian Science Foundation (RSF) under the project number 17-17-01117.
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Received February 05, 2019
СВЕДЕНИЯ ОБ АВТОРАХ:
Морозов Евгений Андреевич
старший научный сотрудник
Научный фонд «Международный центр по окружающей среде и дистанционному зондированию имени Нансена» 14-я линия В. О., 7, оф. 49, Санкт-Петербург, Россия, 199034
эл. почта: evgeny.morozov@niersc.spb.ru
Кондрик Дмитрий Вячеславович
научный сотрудник
Научный фонд «Международный центр по окружающей среде и дистанционному зондированию имени Нансена» 14-я линия В. О., 7, оф. 49, Санкт-Петербург, Россия, 199034
эл. почта: dmitry.kondrik@niersc.spb.ru
Чепикова Светлана Сергеевна
младший научный сотрудник
Научный фонд «Международный центр по окружающей среде и дистанционному зондированию имени Нансена» 14-я линия В. О., 7, оф. 49, Санкт-Петербург, Россия, 199034
эл. почта: sveta.chepikova@niersc.spb.ru
Поздняков Дмитрий Викторович
заместитель директора по науке, руководитель группы
водных экосистем, д. ф.-м. н., проф.
Научный фонд «Международный центр по окружающей
среде и дистанционному зондированию имени Нансена»
14-я линия В. О., 7, оф. 49, Санкт-Петербург, Россия,
199034
эл. почта: dmitry.pozdnyakov@niersc.spb.ru
CONTRIBUTORS:
Morozov, Evgenii
Scientific foundation "Nansen International Environmental
and Remote Sensing Centre"
14th Line, 7, Office 49, Vasilievsky Island, 199034
St. Petersburg, Russia
e-mail: evgeny.morozov@niersc.spb.ru
Kondrik, Dmitrii
Scientific foundation "Nansen International Environmental
and Remote Sensing Centre"
14th Line, 7, Office 49, Vasilievsky Island, 199034
St. Petersburg, Russia
e-mail: dmitry.kondrik@niersc.spb.ru
Chepikova, Svetlana
Scientific foundation "Nansen International Environmental
and Remote Sensing Centre"
14th Line, 7, Office 49, Vasilievsky Island, 199034
St. Petersburg, Russia
e-mail: sveta.chepikova@niersc.spb.ru
Pozdnyakov, Dmitry
Scientific foundation "Nansen International Environmental
and Remote Sensing Centre"
14th Line, 7, Office 49, Vasilievsky Island, 199034
St. Petersburg, Russia
e-mail: dmitry.pozdnyakov@niersc.spb.ru
