Browsing by Subject "Phytoplankton"

Now showing 1 - 7 of 7
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    Antarctic sea ice phytoplankton growth rates and survival mechanisms
    (2025) Kumadiro, Lisa; Rampai, Tokoloho; Fawcett, Sarah; Fietz, Susanne
    Phytoplankton play an important role in the Southern Ocean food web being the primary producers of food, particularly in winter, and partaking in the uptake of CO2 from the atmosphere via photosynthesis. Despite being photosynthetic organisms, phytoplankton survive at the bottom of sea ice where there is very little irradiance for up to 6 months. Sea ice phytoplankton are understudied. This is mainly because in situ studies on sea ice are not only expensive but logistically difficult. Some researchers have elected to bring sea ice phytoplankton from the Southern Ocean to land-based facilities. This has seen some logistical difficulties as it meant either changing the habitat phytoplankton would have been for transportation, thus changing the species originally found in the Southern Ocean or transporting phytoplankton in ice cores and losing species due to brine drainage or osmotic stress from temperature changes in the core. The objectives of this study were to optimize a previously designed hybrid tank for the purpose of obtaining and preserving phytoplankton species from the Marginal Ice Zone of the Southern Ocean to land-based facilities. The study also included design of an environmental chamber to be used for housing phytoplankton obtained during experimentation. Responses to temperature and irradiance variation on phytoplankton from the Marginal Ice Zone of the Southern Ocean were then evaluated using the designed environmental chamber. The solid-liquid hybrid system known as the hybrid tank was successfully optimized by reducing the size of the tank, adding irradiation to the tank, and making improvements to the sampling protocol. The tank was used to obtain ice cores from the Southern Ocean to the University of Cape Town in winter 2022. Post the winter cruise one hybrid tank sample was melted, and microscopic analysis conducted on the sample. In comparison with transportation of phytoplankton in a solid core and in a liquid melt in the dark, the hybrid tank resulted in an increase in phytoplankton cell concentration. Furthermore, the optimized hybrid tank improved preservation of species transported when compared to the initial tank. A desktop environmental chamber made from Perspex and insulated with polystyrene was successfully designed. The environmental chamber offers temperature and irradiation control by making use of a cold plate attached to a chiller and an LED light. Experiments conducted on the diatom species revealed that all the sea ice species were shade adaptive being photo inhibited at irradiances beyond 42μmolm-2s-1 with the exception of Navicula spp, Cylindrotheca closterium and the unidentified pennates. The diatom species also preferred warmer environments i.e., 8°C to 5°C.
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    Combined fishing and climate forcing in the southern Benguela upwelling ecosystem: an end-to-end modelling approach reveals dampened effects
    (Public Library of Science, 2014) Travers-Trolet, Morgane; Shin, Yunne-Jai; Shannon, Lynne J; Moloney, Coleen L; Field, John G
    The effects of climate and fishing on marine ecosystems have usually been studied separately, but their interactions make ecosystem dynamics difficult to understand and predict. Of particular interest to management, the potential synergism or antagonism between fishing pressure and climate forcing is analysed in this paper, using an end-to-end ecosystem model of the southern Benguela ecosystem, built from coupling hydrodynamic, biogeochemical and multispecies fish models (ROMS-N 2 P 2 Z 2 D 2 -OSMOSE). Scenarios of different intensities of upwelling-favourable wind stress combined with scenarios of fishing top-predator fish were tested. Analyses of isolated drivers show that the bottom-up effect of the climate forcing propagates up the food chain whereas the top-down effect of fishing cascades down to zooplankton in unfavourable environmental conditions but dampens before it reaches phytoplankton. When considering both climate and fishing drivers together, it appears that top-down control dominates the link between top-predator fish and forage fish, whereas interactions between the lower trophic levels are dominated by bottom-up control. The forage fish functional group appears to be a central component of this ecosystem, being the meeting point of two opposite trophic controls. The set of combined scenarios shows that fishing pressure and upwelling-favourable wind stress have mostly dampened effects on fish populations, compared to predictions from the separate effects of the stressors. Dampened effects result in biomass accumulation at the top predator fish level but a depletion of biomass at the forage fish level. This should draw our attention to the evolution of this functional group, which appears as both structurally important in the trophic functioning of the ecosystem, and very sensitive to climate and fishing pressures. In particular, diagnoses considering fishing pressure only might be more optimistic than those that consider combined effects of fishing and environmental variability.
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    The effect of organic carbon and nitrogen additions on inorganic nitrogen uptake by phytoplankton and bacteria
    (1991) Van Wijk, Kim; Probyn, Trevor
    This study examines the effects of enrichment with organic carbon and nitrogen on inorganic nitrogen partitioning between phyto- and bacterio- plankton. Strongly preferential uptake of ammonium over nitrate was observed by both the phytoplanktonic and bacterial fractions, with RPINH4 values typically between 1 and 5. The bacterial fraction ( <0.8μm) was found to be responsible for as much as 48-75% of community uptake of ammonium; while the netplanktonic fraction was observed to take up approximately 50% of intact community uptake of nitrate. The addition of amino acids appeared to mediate bacterial competition for ammonium, indicating their preference for DON as a nitrogen source and allowing increased ammonium uptake by the nanoplanktonic fraction. The effect of glucose enrichment was complicated by the presence of protozoans, which appeared to be indirectly responsible for decreased ammonium uptake due to depletion of this substrate in the presence of added glucose. The nanoplankton appeared to be responsible for the least nitrogen uptake with respect to biomass, indicating that they may have been subject to competition pressure from both the bacterial and netplanktonic fractions.
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    Nitrogen uptake by phytoplankton in the Atlantic sector of the Southern Ocean during late austral summer
    (2011) Joubert, W R; Thomalla, S J; Waldron, H N; Lucas, M I; Boye, M; Le Moigne, F A C; Planchon, F; Speich, S
    As part of the Bonus-Good Hope (BGH) campaign, 15N-labelled nitrate, ammonium and urea uptake measurements were made along the BGH transect from Cape Town to ~ 60° S in late austral summer, 2008. Our results are categorised according to distinct hydrographic regions defined by oceanic fronts and open ocean zones. Nitrogen uptake (ρN) in the oligotrophic Subtropical Zone (STZ) was dominated by ρ urea, which contributed up to 70 % of ρN. High regenerated ρN in the STZ resulted in low f-ratios (f = 0.2). Size fractionated chlorophyll data showed that the greatest contribution (>50 %) of picophytoplankton (
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    Spatio-temporal variation in effects of upwelling on the fatty acid composition of benthic filter feeders in the Southern Benguela ecosystem: not all upwelling is equal
    (Public Library of Science, 2016) Puccinelli, Eleonora; McQuaid, Christopher David; Noyon, Margaux
    Variability in mesoscale nearshore oceanographic conditions plays an important role in the distribution of primary production and food availability for intertidal consumers. Advection of nutrient rich waters by upwelling usually allows the proliferation of diatoms, later replaced by dinoflagellates. We examined upwelling effects on the fatty acid (FA) signature of a benthic intertidal filter feeder to identify its response to pulsed variability in food availability. The study took place in two contrasting seasons and at two upwelling and two non-upwelling sites interspersed within the southern Benguela upwelling system of South Africa. We investigated the FA composition of the adductor muscles and gonads of the mussel Mytilus galloprovincialis to assess how FA are apportioned to the different tissues and whether this changes between upwelling and non-upwelling conditions. In situ temperature loggers used to identify upwelling conditions at the four sites indicated that such events occurred only at the upwelling centres and only in summer. Tissues differed strongly, with gonads presenting a higher proportion of essential FAs. This could reflect the faster turnover rate of gonad tissue or preferential retention of specific FA for reproductive purposes. FA composition did not vary as a direct function of upwelling, but there were strong dissimilarities among sites. Upwelling influenced mussel diets at one upwelling site while at the other, the expected signature of upwelling was displaced downstream of the core of upwelling. Condition Index (CI) and Gonad Index (GI) differed among sites and were not influenced by upwelling, with GI being comparable among sites. In addition, FA proportions were consistent among sites, indicating similar food quality and quantity over time and under upwelling and non-upwelling conditions. This suggests that the influence of upwelling on the west coast of South Africa is pervasive and diffuse, rather than discrete; while nearshore retention or advection of upwelled water is critical and site-specific so that the effects of upwelling differ even among sites categorised as upwelling centres.
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    A study of the phytoplankton of the South Western Indian Ocean
    (1964) Taylor, F J R; Schelpe, E A C L E; Day, J H O
    Although the phytoplankton of the waters off the west coast of South Africa (the Benguela Current region) has been the subject of several detailed studies in the past, data on that of the S.W. Indian Ocean has been almost entirely restricted to incidental references in the reports of expeditions which have passed through the area. Consequently, little has been known of the species composition and distribution of the phytoplankton, and nothing of its seasonal fluctuations. This study was designed to provide a broad picture of the phytoplankton of the area, the primary objective being a critical determination of the species composition. The material was collected by the S.A.S. Natal on four seasonal cruises in the area as a contribution to the International Indian Ocean Expeditions. A net-sampling technique was used to provide the maximum amount of material for quantitative analysis. The phytoplankton was found to be extremely rich in variety, 402 taxa being identified from the 98 samples collected. Of these 233 were diatom taxa, 157 dinoflagellate taxa, and the remainder being composed of members of the Chrysophyeeae (coccolithophorids), Cyanophyceae and Xanthophyceae. These are listed in the systematic section together with original references and other references used by the author for their identification. The local and general distributions or the taxa are described and many of the taxa are illustrated by line drawings or microphotographs. 5 new species are described, as well as 1 new variety, and it was found necessary to provide new names for several species. Full systematic details are given for all new or rare taxa.
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    Using a two-layered sphere model to investigate the impact of gas vacuoles on the inherent optical properties of M. aeruginosa
    (2013) Matthews, M W; Bernard, S
    A two-layered sphere model is used to investigate the impact of gas vacuoles on the inherent optical properties (IOPs) of the cyanophyte Microcystis aeruginosa. Enclosing a vacuole-like particle within a chromatoplasm shell layer significantly altered spectral scattering and increased backscattering. The two-layered sphere model reproduced features in the spectral attenuation and volume scattering function (VSF) that have previously been attributed to gas vacuoles. This suggests the model is good at least as a first approximation for investigating how gas vacuoles alter the IOPs. Measured R(subrs) was used to provide a range of values for the central value of the real refractive index, 1 + e, for the shell layer using measured IOPs and a radiative transfer model. Sufficient optical closure was obtained for 1 + e between 1.1 and 1.14, which had corresponding Chl a-specific phytoplankton backscattering, b(subbf)*, between 3.9 and 7.2 × 10-3 m2 mg-1 at 510 nm. The b(subbf)* values are in close agreement with the literature and in situ particulate backscattering measurements. R(subrs) simulated for a population of vacuolate cells was greatly enlarged relative to a homogeneous population. A sensitivity analysis of empirical algorithms for estimating Chl a in eutrophic/hypertrophic waters suggests these are robust under variable constituent concentrations and likely to be species-sensitive. The study confirms that gas vacuoles cause significant increase in backscattering and are responsible for the high Rrs values observed in buoyant cyanobacterial blooms. Gas vacuoles are therefore one of the most important bio-optical substructures influencing the IOPs in phytoplankton.