Spatial hydrochemical structure in surface waters of the Southern ocean between Africa and Antarctica

Some hydrochemical characteristics and, first of all, the main nutrients (phosphorus, nitrogen, silicon) can be used as markers for distinguishing different types of water masses and positions of the main fronts of the Southern Ocean. The seasonal and interannual variability of these characteristics also reflects the character of biological processes in the surface layer of the ocean, which is important for assessing biological productivity. The aim of this study was to analyze the main features of the spatial distribution of hydrochemical characteristics in the surface layer in the Atlantic and Indian Ocean sectors of the Southern Ocean between the Subantarctic Front and the shores of Antarctica and assess their seasonal (spring–autumn) and interannual variability for the observation period from 2008 to 2020. We describe the surface nutrient concentrations between Africa and Antarctica along the transects that cross the Subantarctic Front (SAF) in the north, the Polar Frontal Zone (PFS), Polar Front (PF) and Antarctic Zone water in the south. The findings revealed an increase in dissolved oxygen and nutrients towards the south. Nitrates changed values within the SAF from 15 μM to 24 μM, whereas values from 1.2 μM to 1.7 μM were observed for phosphates. Silicate increased considerably within the Polar Front, from 6.6 μM to 20.8 μM. An analysis was carried out of the seasonal and interannual variability of the hydrochemical conditions in the surface layer of the Southern Ocean. The interannual variability of the nutrients was determined by the spatial variability of the main fronts of the Antarctic Circumpolar Current (ACC) and the intensity of the largescale Weddell Gyre (WG). Since 2017, there has been an increase in the meridional transfer of waters: in the Antarctic Summer 2017–2018, there was a spreading of high-nutrient WG waters toward the north, and in the Summer 2019–2020, the low-nutrient waters anomaly was transferred far to the south (up to 60°S).
According to the data obtained, the seasonal dynamics of the nutrients in the surface layer of the Southern Ocean was rather weakly expressed. An exception is the high-latitude waters of the Cooperation and Davis Seas, where maximum seasonal variability of the hydrochemical characteristics was observed. The highest rate of nutrient consumption was observed in the coastal area of the Cooperation Sea near the fast ice edge from mid–December to early January and reached 3.2 μM per day for silicate, 1.8 μM per day for nitrates, and 0.12 μM per day for mineral phosphorus. The results of the long-term monitoring of the hydrochemical conditions in the Cooperation Sea made it possible to distinguish conditionally “warm” years with early vegetation (at the end of December) and intensive consumption of nutrients by phytoplankton, and “cold” years, when the formation of high-latitude “oases” in December–January was not observed.

1. Аrzhanova N.V., Burkal’tseva M.А. Providing of phytoplankton with nutrients in the Atlantic Ocean. Biologicheskie resursy Atlanticheskogo okeana. Biological resources of the Atlantic Ocean. Мoscow: Nauka, 1986: 111–133. [In Russian].

2. Gorshkov А.N., Gudoshnikov Yu.P., Smagin V.M. The main features of the spatial distribution of hydrochemical indicators in the Southern Ocean. Pelagicheskie ekosistemy Iuzhnogo okeana: Sbornik nauchnykh trudov. Pelagic systems of the Southern Ocean: Digest of scientific papers. Moscow: Nauka, 1993: 99–107. [In Russian].

3. Henley S.F., Cavan E.L., Fawcett S.E., Kerr R., Monteiro T., Sherrell R.M., Bowie A.R., Boyd P.W., Barnes D.K.A., Schloss I.R., Marshall T., Flynn R., Smith S. Changing biogeochemistry of the Southern Ocean and its ecosystem implications. 2020. Front. Mar. Sci. 2020, 7: 581. doi: 10.3389/fmars.2020.00581.

4. Аrzhanova N.V., Аrtamonova K.V. Hydrochemical structure of waters in the Antarctic krill fishery area. Trudy VNIRO. Proc. VNIRO. 2014, 152: 118–132. [In Russian].

5. Batrak K.V. Hydrochemical characteristics of various modifications of Antarctic waters. Okeanologiia. Oceanology. 2008, 48 (3): 371–378. [In Russian].

6. Maslennikov V.V. Klimaticheskie kolebaniya i morskaya ekosistema Аntarktiki. Climate fluctuations and the Antarctic marine ecosystem. Moscow: VNIRO, 2003: 295 p. [In Russian].

7. Campanelli A., Massolo S., Grilli F., Marini M., Paschini E., Rivaro P., Artegiani A., Jacobs S. Variability of nutrient and thermal structure in surface waters between New Zealand and Antarctica, October 2004–January2005. Polar Research. 2011, 30 (1): 7064. doi: 10.3402/polar.v30i0.7064.

8. Budillon G., Rintoul S.R. Fronts and upper ocean thermal variability south of New Zealand. Antarctic Science. 2003, 15: 141–152.

9. Orsi A.H., Whitworth Th. III, Nowlin W. D. Jr. On the meridional extent and fronts of the Antarctic Circumpolar Current. Deep-Sea Res. 1995, 42 (5): 641–673.

10. Struktura gidrofizicheskikh polej atlanticheskogo sektora yuzhnogo okeana i ikh vliyanie na planktonnye soobshhestva. The structure of hydrophysical fields in the Atlantic sector of the Southern ocean and their impact on plankton communities. Moscow: VNIRO, 1986: 63 p. [In Russian].

11. Maslennikov V.V., Popkov V.V. Position of the interaction zone of Antarctic waters of various modifications as an indicator of the northern boundary of the mass drift of Antarctic krill. Antarctica. Antarktika: Doklady Mezhduvedomstvennoi komissii po izucheniiu Antarktiki. Antarctica: Reports of the Interdepartmental Commission for the Study of Antarctica. Iss. 27. Moscow: Nauka, 1988: 134–142. [In Russian].

12. Fedulov P.P., Shnar V.N. Frontal zone and water structure of the Weddell Circle. Issledovaniia Ueddellovskogo krugovorota. Okeanograficheskie usloviia i osobennosti razvitiia planktonnykh soobshchestv: Sbornik nauchnykh trudov. Researches of the Weddell Circle. Oceanographic conditions and features of the plankton communities development. Digest of scientific papers. Moscow: VNIRO, 1990: 31–48. [In Russian].

13. Franck V.M., Brzezinski M.A., Coale K.H., Nelson D.M. Iron and silicic acid concentrations regulate Si uptake north and south of the polar frontal zone in the pacific sector of the Southern ocean. Deep Sea Res. Part II. Top. Stud. Oceanogr. 2000, 47: 3315–3338. doi: 10.1016/s0967–0645(00)00070–9.

14. Sapozhnikov V.V., Shiryaev V.A. The use of hydrochemical characteristics to clarify the dynamics of waters and assess the production and destruction processes in the areas of tropical upwellings. Promyslovo-okeanograficheskie issledovaniia produktivnykh zon morei i okeanov: Sbornik nauchnykh trudov. Commercial oceanographic studies of productive zones of the seas and oceans. Digest of scientific papers. Moscow: VNIRO, 1984: 151–165. [In Russian].

15. Hart T.J. Phytoplankton periodicity in Antarctic surface waters. Discovery Rep. 1942, 21: 261–356.

16. El-Sayed S.Z. History and evolution of primary productivity studies of the Southern Ocean. Polar Biol. 2005, 28: 423–438.

17. Martin J.H., Fitzwater S.E. Iron deficiency limits phytoplankton growth in the north-east Pacific Subarctic. Nature. 1988, 331: 341–343.

18. Sedwick P.N., Bowie A.R., Trull T.W. Dissolved iron in the Australian sector of the Southern Ocean (CLIVAR SR3 section): Meridional and seasonal trends. Deep Sea Res. Part I Oceanogr. Res. Papers. 2008, 55: 911–925. doi: 10.1016/j.dsr.2008.03.011.

19. Shafiee R.T., Snow J.T., Zhang Q., Rickaby R.E.M. Iron requirements and uptake strategies of the globally abundant marine ammoniaoxidising archaeon, Nitrosopumilus maritimus SCM1. ISME J. 2019, 13: 2295–2305. doi: 10.1038/s41396–019–0434–8.

20. Boyd P.W. The role of iron in the biogeochemistry of the Southern Ocean and equatorial Pacific: a comparison of in situ iron enrichments. Deep Sea Research II. 2002, 49: 1803–1821.

21. Chmyr V.D. Primary production in the waters of the Atlantic Antarctic in the pre-winter period of 1998. Biulleten’ Ukrainskogo Antarkticheskogo Tsentra. Bulletin of Ukr. Antarc. Center. 2002, 4: 141–142. [In Russian].

22. Millero F.J. Chemical Oceanography. 3rd ed. Taylor and Francis Group, 2006: 496 p.

23. Vedernikov V.I., Sapozhnikov V.V. Influence of various elements additives of mineral nutrition on the primary production of Antarctic phytoplankton. Trudy IOAN SSSR. Proc. IOAN USSR. 1978, 112: 58–68. [In Russian].

24. Volkovinsky V.V. Measurements of primary production in the Scotia Sea. Morskie biologicheskie resursy Antarktiki Antarctic marine biological resources. Moscow: Publ. House “Pishchevaya promyshlennost”, 1969: 160–168. [In Russian].

25. Oradovsky S.G. On the role of microelements in the formation of the primary productivity of sea waters. Osnovy biologicheskoi produktivnosti okeana. Ocean biological productivity fundamentals. Moscow: Nauka, 1971: 32–36. [In Russian].

26. Horne A.J., Fogg G.E., Eagle D.Y. Studies in situ of the primary production of an area of inshore Antarctic sea. J. Mar. Biol. Assoc. U. K. 1969, 49 (2): 393–405.

27. Makarov R.R. Some data on the abundance and age composition of mass copepods and euphausiid larvae in the coastal waters of Antarctica. Antarktika. Antarctica. 1995, 33: 88–106. [In Russian].

28. Batrak K.V., Maslennikov V.V., Antipov N.N., Klepikov A.V. Features of structure of the shelf and continental slope waters in the Sea of Cooperation according to the data of the expedition of R/V «Akademik Fedorov» in 2006. Arktika i Antarktika. Arctic and Antarctica. 2007, 5 (39): 112–124. [In Russian].

29. Rukovodstvo po khimicheskomu analizu morskikh i presnykh vod pri ehkologicheskom monitoringe rybokhozyajstvennykh vodoemov i perspektivnykh dlya promysla rajonov Mirovogo okeana. Guidelines for the chemical analysis of sea and fresh waters in the environmental monitoring of fishery reservoirs and promising fishing areas of the World Ocean. Moscow: Publ. House of VNIRO, 2003: 202 p. [In Russian].

30. Strickland J.D.H., Parsons T.R. A practical handbook of seawater analysis. Fisheries Research Board of Canada Bulletin. 1968, 167: 203 p.

31. Arzhanova N.V. The ratio of the content of silicon and phosphorus as an indicator of mixing waters zone in the area of the South Orkney Islands and the Antarctic Peninsula. Antarktika. Antarctica. Moscow: Nauka, 1982, 21: 95–100. [In Russian].

32. De Baar H J. W., de Jong, Bakker D.C. E., Löscher B. M., Veth C., Bathmann U., Smetacek V. Importance of iron for plankton blooms and carbon dioxide drawdown in the Southern Ocean. Nature. 1995, 373 (6513): 412–415. https://doi.org/10.1038/373412a0.

33. Antipov N.N., Klepikov A.V. Thermal structure of the upper layer of the ocean between Africa and Antarctica according to the oceanographic data of the AARI 2004–2010. Vklad Rossii v Mezhdunarodnyi poliarnyi god 2007/08. Ser. Okeanologiia i morskoi led. Contribution of Russia to the International Polar Year 2007/08. Ser. Oceanology and sea ice. Moscow; St. Petersburg: LLC “Paulsen”, 2011: 280–290. [In Russian].

34. Ionov V.V. Synoptic monitoring of surface thermal fronts of the Southern Ocean. Trudy VNIRO. Proc. VNIRO. 2015, 157: 143–150. [In Russian].

35. Artamonov Yu.V., Skripaleva E.A., Babij M.V., Galkovskaya L.K. Interannual variability of temperature fronts in the Southern Ocean. Sistemy kontrolia okruzhaiushchei sredy. Environmental control systems. 2009, 12: 280–282. [In Russian].