Browsing by Author "Probyn, Trevor"

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    The bio-optical detection of harmful algal blooms
    (2005) Bernard, Stewart; Probyn, Trevor; Shillington, Frank
    An analytical framework for the simulation and quantitative interpretation of ocean colour data is presented, providing an inverse reflectance algorithm designed for the detection of harmful algal blooms. The adopted framework focuses on establishing quantitative relationships between optically important algal intracellular properties and inherent optical properties (IOPs), such as the absorption and backscattering coefficients, and the resultant effects on remote-sensing reflectance. A principal aim of the study is to establish the determinant variables of the IOPs associated with natural algal assemblages, and provide a means of simulating these IOPs. Algal size is an important determinant of optical properties, and the study demonstrates algal IOP simulation, using equivalent particle size distributions that can be simply parameterised with regard to effective cell diameter. Statistical analyses of causal variability are also conducted on absorption data from a variety of natural algal assemblages, revealing the relative importance of cell size, intracellular Chi a concentration, and accessory pigment complement. An improved understanding of algal angular scattering is regarded as key to the analytical modelling of ocean colour, and the use of two-layered spherical models for the simulation of algal scattering properties is investigated. Preliminary validation of the combined use of the equivalent size and two-layered models indicates that they are capable of adequately simulating the remote-sensing reflectance properties of high biomass bloom waters.
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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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    The relative role of protozoans in the flux of phytoplankton nitrogen through pelagic food webs
    (1991) Matthews, Susan Gail; Probyn, Trevor
    Experiments were carried out during a cruise in the southern Benguela upwelling region in April 1989 to budget the nitrogen flux through the different microplanktonic compartments leading to copepods. Uptake of nitrate and ammonium by three different size classes of phytoplankton (net-, nano-, and picoplankton) was measured using ¹⁵N isotope techniques. Microzooplankton grazing on autotrophic picoplankton and nanoplankton was quantified by predator:prey dilution experiments. Between 7 and 52 copepods in species assemblages representative of the natural communities were incubated in 11 samples of ambient seawater to examine grazing rates on chlorotic and non-chlorotic microplankton. Copepod and microzooplankton excretion rates were also measured using ¹⁵N isotope techniques. Two experiments were performed at night and two during the day. Nitrogen uptake and regeneration studies revealed that phytoplankton of all size classes showed a consistent preference for ammonium, although nitrate was quantitatively more important for netplankton. Microzooplankton excretion fulfilled most of the phytoplankton ammonium demand, while copepod excretion was only detectable at night. Competition between microzooplankton and mesozooplankton for phytoplankton prey was minimal, in that the former appeared to graze mainly < 2 μm phytoplankton. Nevertheless, microzooplankton grazers had a significant impact on phytoplankton standing stocks. Microzooplankton grazing rates represented about 5 % of phytoplankton biomass under diatom bloom conditions and an average of 46% under post-bloom conditions. On the other hand, copepods removed 18% of phytoplankton biomass under bloom conditions and only 1 % under post-bloom conditions. Copepods appeared to demonstrate a preference for protozoan prey over phytoplankton, in that the percentage of carbon ingested as protozoans exceeded the percentage of carbon available as protozoans. Quantitatively, protozoans made up a highly variable component of the copepod diet. For example, at station 12, where the plankton assemblage was dominated by oligotrichous ciliates, 80 % of the ingested nitrogen ration consisted of protozoa. However, the total ingested ration at this station was only 0.6% of that at station 2, with a bloom assemblage, and it is unlikely that such a diet could support a large production potential. On average only 3 % of the nitrogen ingested by protozoa was subsequently transferred to copepods. Microbial pathways thus appear to have a minor role in the· transfer of nitrogen to higher trophic levels, their function being mainly the regeneration of nitrogen for primary producers.