Phytoplankton variability in the Atlantic and Indian sectors of the Southern Ocean: a biogeochemical approach

Weir, Ian (2018-11-16)

Thesis (MSc)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: The Southern Ocean is identified as a key component in the global carbon cycle due to a unique combination of physical circulation and biological processes. In light of a predicted changing climate, understanding in-situ environmental and biological processes becomes fundamentally important for improving biogeochemical models. Phytoplankton variability in the Indian and Atlantic Southern Ocean are assessed both spatially and temporally, in terms of the unique physical and chemical environments encountered in the major oceanic zones of the Southern Ocean. The approach identified the Polar Front as an important biogeochemical boundary in both summer and winter waters, separating silicic acid replete, diatom-dominated southern waters from northern waters associated with lower silicic acid concentrations and greater flagellate contribution. Summer waters along a 0 °E meridian (Atlantic Southern Ocean) were characterized by high chlorophyll-a (up to 0.56 μg/L) concentrations and bloom conditions at certain stations, which, in some instances were correlated to an influx of trace metals. Studying a suite of trace metal distributions proved to be an important additional variable in understanding phytoplankton variability, as certain metals seemed to be preferentially utilized, possibly driving underutilization of other metals, although it is unclear whether these events were mutually exclusive. Furthermore, it allowed for the association of certain trace metals to specific phytoplankton groups e.g. Zn, Mn distributions were positively correlated with diatoms. Our results indicate a complex relationship between the phytoplankton community and trace metal distribution, as it was unclear whether trace metal distributions drive the community composition or the community composition drives trace metal distributions. . Winter waters along a 30 °E meridian (Indian Southern Ocean) were characterized by deep mixed layers, limited irradiance and cold surface waters with strong vertical mixing. Biomass indicators (biogenic silica, <0.77 μM; chlorophyll-a, <0.37 μg/L) point toward a winter water column that is more productive than previously thought and comparable to both summer (biogenic silica, <2.17 μM; chlorophyll-a, <0.57 μg/L) and spring (<0.36 μg/L) communities. Diatom contribution was shown to be more significant in the winter Antarctic Zone than summer waters, with heavily silicified Fragilariopis spp. being the greatest contributor to winter biomass, which may have implications for silicic acid cycling. Picoplankton biomass was also thought to be more important in summer waters than winter waters, which may have implications for micro- and macronutrient cycling, further demonstrating the importance of understanding seasonal progressions. This study highlighted the use of combining an array of environmental and physical variables in the interpretation of phytoplankton variability, further demonstrating the need for regional and seasonal differentiation in future studies.

AFRIKAANSE OPSOMMING: Die suidelike oseaan is geïdentifiseer as 'n belangrike komponent in die wêreldwye koolstof siklus, meestal as gevolg van 'n unieke kombinasie van fisiese sirkulasie en biologiese prosesse. In die lig van voorspelde klimaat verandering is ‘n begrip van in-situ omgewings- en biologiese prosesse fundamenteel in die verbetering van biogeochemiese modelle. Phytoplankton variasie in die Indiese en Atlantiese Suidelike Oseaan word geografies en oor tyd geassesseer deur verskeidenheid van biogeochemiese veranderlikes te kombineer met veelvuldige parameter lesings. Die benadering help met die karakterisering van beide somer en winter waters in terme van hulle geassosieerde oseaniese sones. Hierdie benadering het die Polêre Front geïdentifiseer as 'n belangrike biogeochemiese grens in beide somer en winter waters. Die suidelike oseaan verskil in terme van sy silika suur, diatoom-dominerende waters teenoor die noordelike waters met laer silika suur konsentrasies en groter flagellate dominasie. Somer waters langs die 0 °E meridiaan (Atlantiese Suidelike Oseaan) was gekenmerk deur hoë chlorofil-a konsentrasies en hoë alge groei toestande by sommige stasies, wat in sommige gevalle korreleer met die teenwoordigheid van spoor metale. Die veelvuldige parameter benadering wat 'n verskeidenheid spoor metale meet is uitgewys as ‘n fundamentele aspek, met sekere metale wat lyk of hul voorkeurig benut word, en heel moontlik die mindere gebruik van ander metale dryf. Die veelvuldige parameter benadering het verder toegelaat vir die assosiasie van sekere spoorelement metale met spesifieke phytoplankton groepe, bv. Zn, Mn verspreidings was gekorreleer met diatome. Ten spyte van die bogenoemde assosiasie was dit onduidelik of spoor metaal verspreidings die gemeenskap samestelling dryf of nie. In vergelyking, was die winter waters parallel met die 30 °E meridiaan (Indiese Suidelike Oseaan) gekenmerk deur diep gemengde lae, beperkte radiasie en koue oppervlak waters met sterk vertikale vermenging. Biomassa indikators verwys na 'n winter water kolom wat meer produktief is as voorheen bekend, en is vergelykbaar met beide somer en lente biomassas. In die Antarktiese Zone blyk die diatoom bydrae as meer beduidend in winter teenoor somer waters, met Fragilariopis spp. die mees bydraend tot winter biomassas. In somer waters blyk dit Picoplankton biomassa is meer belangrik as in die winter, met moontlike implikasies vir die biogeochemiese siklus. Die implikasie demonstreer verder die belangrikheid van insig en begrip in seisoenale veranderings. Met die gebruik van veelvuldige parameter veranderlikes in die interpretasie van phytoplankton variasie, het die studie die belangrikheid van geografiese en seisoenale verskille demonstreer.

Please refer to this item in SUNScholar by using the following persistent URL: http://hdl.handle.net/10019.1/104925
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