Browsing by Author "Weir, Ian"

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Links between the phytoplankton community composition and trace metal distribution in summer surface waters of the Atlantic Southern Ocean
    (Frontiers Media, 2019-06-06) Viljoen, Johannes J.; Weir, Ian; Fietz, Susanne; Cloete, Ryan; Loock, Jean; Philibert, Raissa; Roychoudhury, Alakendra N.
    This study assessed changes in the phytoplankton community related to macronutrient and bioactive trace metal distribution in surface waters of the Atlantic sector of the Southern Ocean, between Cape Town and Antarctica along the GEOTRACES GIPY_05 (mainly along prime meridian) transect in summer 2014–2015. Several general community structure features were reaffirmed, such as the restriction of cyanobacteria to the northern Subtropical Zone, while haptophytes, such as Phaeocystis, along with diatoms, dominate the community north of the Polar Front, and diatoms clearly dominate south of the Polar Front. These community structure changes were often linked with macro- and micro-nutrient composition changes. For example, the concentration of diatoms increased southwards with the availability of silica whereas the cyanobacterial contribution in the northern Subtropical Zone appeared to be linked with labile cobalt depletion. However, these links were not consistent along the entire transect, i.e., no individual nutrient, such as silica or iron, was linked to community composition changes across all water masses. Each station showed a rather unique combination of nutrient and community compositions. Our findings also indicated impacts on the phytoplankton community through trace metal distributions that could be related to a deep mixing event at ∼54∘S and to ice melt at ∼65 and 68∘S. The timing of sampling after such trace metal fluxes proved to be an important consideration, particularly where iron appeared to be preferentially depleted to near-limiting concentrations, possibly driving utilization of other metals. The study highlights the importance of considering a suite of trace metals when assessing controls of phytoplankton variability in the open ocean and emphasizes the need for higher resolution trace metal sampling and multi-element incubation studies to further study the complex relationships between phytoplankton and nutrients.
  • Thumbnail Image
    Item
    Phytoplankton variability in the Atlantic and Indian sectors of the Southern Ocean: a biogeochemical approach
    (Stellenbosch : Stellenbosch University, 2018-11-16) Weir, Ian; Fietz, Susanne; Stellenbosch University. Faculty of Science. Dept. of Earth Sciences.
    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.