Laboratory and field experiments examined several aspects of the interaction of freshwater phytoplankton species and plankton communities with nitrogen and phosphorus nutrient resources. The laboratory studies focused on the following three main areas: 1) effects of nutrient 'patchiness' on phytoplankton community structure; 2) kinetics of phosphate (PO₄⁻³ ) and ammonium (NH₄⁺) uptake of phytoplankton grown under non-steady-state but limiting rates of nutrient supply; 3) the effect of different N:P supply ratios on phytoplankton NH₄⁺ and PO₄⁻³ uptake kinetics and community structure. Nutrient 'patchiness' was simulated by altering the frequency of nutrient addition to cultures. Under conditions of infrequent addition (once per 18 days) dominance shifted to a larger species, and the average cell size of another species increased. Observations of PO₄⁻³ uptake kinetics were not consistent with most other studies where kinetics were determined under steady-state conditions. With respect to PO₄⁻³, the duration over which maximum uptake rates were sustained was species specific. There was a short lag before maximum uptake rates were realized, and whether maximum uptake rates occurred at the lowest or at intermediate dilution rates depended on the time scale over which the uptake measurements were made. NH₄⁺ uptake rates were found to be greatly enhanced during the first few minutes of uptake. When natural plankton assemblages were grown under N:P supply ratios of 5:1, 15:1 and 45:1 (by atoms), the treatments selected for different competitive dominants. An N:P ratio of 45:1 resulted in total dominance by Synechococcus sp.; cultures grown under 5:1 and 15:1 supply ratios were dominated by Synedra radians, Nitzschia holsatica and Scenedesmus sp. NH₄⁺ and PO₄⁻³ uptake kinetics were not the same in cultures grown under different supply ratios, and ratios of saturated PO₄⁻³ to NH₄⁺ uptake rates were a good indicator of the N:P supply ratio under which the cultures were grown. This relationship was used to derive an index termed the Relative Field investigations were conducted on an oligotrophic coastal lake. NH₄⁺ and PO₄⁻³ uptake rates of size fractionated plankton (< and > 3 um), at a range of substrate concentrations, revealed that a large portion of the total uptake (50-90 % and 65-85 % for NH₄⁺ and PO₄⁻³, respectively) was attributable to cells in the < 3 um fraction. In addition, saturating PO₄⁻³, uptake rates of the > 3 um cells were less sensitive to incubation time than smaller cells. The ratio of saturated PO₄⁻³ to NH₄⁺ uptake rates were consistent with nutrient bioassay experiments, and indicated that N:P supply ratios in the lake were in the range where both N and P could be limiting to phytoplankton growth. / Science, Faculty of / Botany, Department of / Graduate

Diel patterns of nitrogen (NO₃⁻, NH₄⁺, urea) uptake were investigated in natural assemblages of phytoplankton from neritic and oceanic environments off the coast of British Columbia. This is the first study to report nitrogen uptake rates and extensive measurements of ambient NH₄⁺ and urea concentrations in these waters. Calculated rates of N uptake, 15 based on ¹⁵N incorporation into particulate matter during time course experiments, were maximal during the day and minimal at night. Besides the obvious effects of irradiance, the amplitude of the periodicity in uptake rate was influenced by phytoplankton community composition, ambient nitrogen concentration, forms of nitrogen available, and depth of sampling. Uptake of nitrogen during the night and in artificial darkness were measurable proportions of daytime and light uptake rates, with the importance of dark uptake generally increasing with increasing N limitation. This is the first study of diel urea uptake by marine phytoplankton in the field. The ratios of dark to light urea uptake over a diel cycle were more similar to those of NO₃⁻than those of the other reduced N form, NH₄⁺. Rates of NO₆⁻ and urea uptake by phytoplankton in the shallow and deep chlorophyll layers of the Strait of Georgia were measured over a gradient of irradiances and results of these experiments could be fitted with a hyperbolic function similar to the Michaelis-Menten equation. Half-saturation constants (KLT) for light-dependent uptake of urea and NO₃⁻ ranged from 0 to 14% of the surface irradiance and dark uptake was a variable, but often substantial (> 50%) portion of the total (light + dark) uptake. The uptake response of nitrate-replete and -starved populations of the picoflagellate, Micromonas pusilla (Butch.) Manton et Parke, to urea, NH₄⁺ and NO₃⁻ perturbations was determined by both ¹⁵N accumulation and nutrient disappearance from the culture medium. Maximum specific uptake rates (Vmax) of NH₄⁺ were 0.13 h⁻¹, more than 2 times the Vmax of NO₃⁻ or urea (ca. 0.05 h⁻¹). The half-saturation constants (Ks) for urea, NH₄⁺ and NO₃⁻ were within ± 0.1 µg-at N•L⁻¹ of each other; the average value of 0.41 µg-at. N•L⁻¹ is within the range reported for small, oceanic diatoms. NO₃⁻ uptake was completely inhibited following NH₄⁺ addition (1-10 µg-at. N•L⁻¹), whereas urea addition resulted in only a 28% reduction in NO3- uptake. Starved cultures of M. pusilla exhibited variable uptake of NH₄⁺ and urea as a function of time, with an initial "surge" uptake response. This is the first laboratory study of N uptake by an eucaroyotic picoplankter and demonstrates that many of the transient uptake responses reported for diatoms, with which it competes in the field, are common to this picoplankter. Diel periodicity of nitrogen uptake and assimilation were measured in N-replete batch cultures of M. pusilla and also in N-limited cyclostat cultures (14L:10D) at three growth rates corresponding to ca. 75, 50 and 25% of it's maximal growth rate. Nitrate uptake was continuous and independent of the L:D cycle in the cyclostat cultures at the lowest dilution rate, but NO₃⁻ uptake rates exhibited pronounced periodicity in the batch and higher dilution rate cultures, a response similar to that seen in previous studies of cyclostat cultures of some diatoms. Diel patterns in cell division, mean cell volume, potential uptake rates and internal pools of NO₃⁻ were also observed and are discussed with respect to the nutritional status of the cells. The effect of irradiance on the uptake of NH₄⁺ and NO₃⁻ by M. pusilla was also described by Michaelis-Menten kinetics; with increasing N limitation the importance of light for nitrogen uptake decreased and dark uptake increased from 5-20% to 21-39% of NO₃⁻ and NH₄⁺ uptake rates, respectively, at saturating irradiance. / Science, Faculty of / Earth, Ocean and Atmospheric Sciences, Department of / Graduate

Bibliography: leaves 309-329. This thesis attempts to contribute to the understanding of how turbulence affects phytoplankton in freshwater systems, focussing on the temporal and spatial scales in phytoplankton dynamics ranging from photochemistry in the surface mixed layer to a small-scale shear and growth to intra-seasonal changes in community composition in a lake subject to high disturbances.

Bibliography: leaves 309-329. / xvii, 329 leaves : ill. (some col.) ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / This thesis attempts to contribute to the understanding of how turbulence affects phytoplankton in freshwater systems, focussing on the temporal and spatial scales in phytoplankton dynamics ranging from photochemistry in the surface mixed layer to a small-scale shear and growth to intra-seasonal changes in community composition in a lake subject to high disturbances. / Thesis (Ph.D.)--University of Adelaide, School of Earth and Environmental Sciences, 2003

Physical-biological interactions in the Southern Ocean were investigated using remote sensing data from several different satellite sensors. Satellite sea surface temperature data were used to study the dynamics of the Antarctic Polar Front (PF). Satellite ocean color data were used to estimate surface chlorophyll concentrations and their relation to various physical forcings within the Southern Ocean. A detailed study of phytoplankton blooms at the Antarctic Polar Front revealed that elevated chlorophyll concentrations (phytoplankton blooms) occur most often in areas where the PF interacts with large topographic features within the Southern Ocean. The physical dynamics of the PF are strongly influenced by the topography, and in turn strongly influence phytoplankton bloom dynamics. The analysis of satellite data from the modern Southern Ocean indicates that phytoplankton are limited by the availability of the micronutrient iron in most areas. This iron-limitation implies that the elevated iron inputs during glacial periods would have led to increased phytoplankton primary and export production and a stronger sink for atmospheric CO��� in the Southern Ocean. / Graduation date: 2000

Our view of phytoplankton has historically revolved around their inability to control their location in space. The term phytoplankton itself underscores this particular difference between phytoplankton and their sessile terrestrial counterparts. Yet there are other differences between land plants and the phytoplankton that are perhaps equally important, beyond this sessile-planktonic dichotomy, to their growth, survival, and productivity. For example, phytoplankton are microbes and thus are short-lived, with generational scales on the order of days or less. An intriguing question to ask is how today���s pelagic ecology would differ, had this temporal difference between plants and phytoplankton been initially emphasized, perhaps by naming these microbes phytoephemera instead? This dissertation addresses certain aspects of the ecology of phytoplankton that result from their having short generational scales. Because they are so short lived, phytoplankton need to adjust their photosynthetic physiology to cope with more rapid changes in irradiance than may matter to longer-lived plants. Photoacclimation on the hours-plus time scales has been studied extensively in the phytoplankton, because its temporal scales match those of vertical mixing processes in the ocean. Yet most phytoplankton exhibit faster photosynthetic responses as well, down to the time scales of seconds. These photosynthetic responses have received considerably less attention in phytoplankton ecology. This dissertation specifically examines these rapid, seconds-to-hour scale photosynthetic responses in phytoplankton. First, the physiological bases of rapid photosynthetic regulation were examined using a numerical model that shows how specific physiological changes in phytoplankton photosystems either constrain or enhance light harvesting. This model is stochastic, and thus replicates certain nonlinear aspects of light harvesting better than equation-based analytical models. Also in this dissertation, a laboratory study is described that examined rapid photosynthetic regulation in three model phytoplankton. Results suggest that rapid photosynthetic regulation is not only constrained to higher eukaryotic phytoplankton, but also occurs in the two dominant marine photosynthetic prokaryotes, Synechococcus and Prochlorococcus. Finally, rapid photosynthetic responses were examined in field assemblages at Station ALOHA in the North Pacific. This ocean region experiences considerable cloud cover, which may result in a strong degree of rapid photosynthetic responses, even in near-surface assemblages. / Graduation date: 2007

This thesis presents a quantitative model of interactions among phytoplankton, nutrients, bacteria and grazers in an enclosed marine ecosystem. The enclosed system was a 23 m deep, 9.6 m diameter column of surface water in Saanich Inlet, British Columbia. Dynamics of large- and small-celled diatoms and flagellates in response to observed irradiance and zooplankton numbers and observed or simulated nitrogen and silicon concentrations were modelled over a simulated 76-day period between July 12 and September 26. The model's predictions poorly matched the observed events in Controlled Experimental Ecosystem 2 (CEE2), but nevertheless provided some important insights into system behavior. Ciliate grazing probably prevented small-celled phytoplankton from increasing to large concentrations in CEE2. By virtue of their tremendous numbers, colourless flagellates were potentially the most important grazers on bacteria, much more important than larvaceans or metazoan larvae. Whereas small-celled phytoplankton were limited by grazers, large phytoplankton dynamics were not markedly affected by grazing. The average observed rate of 14C fixation in the surface 8 m was roughly consistent with an interpretation in which artificial additions of nitrogen contributed 62% of inferred net uptake of nitrogen by phytoplankton, mixing from subsurface water contributed 18%, bacterial remineralization 12%, and zooplankton excretion 9%. However, independent observations of rapid activity by microheterotrophs (presumably bacteria) suggested that 1*C fixation considerably underestimated net primary production. This yielded an alternative interpretation in which nutrient additions contributed 46% of inferred net uptake of nitrogen in the surface layer, mixing 13%, bacteria 35%, and zooplankton 7%. Dissolution of silica was responsible for the observed accumulation of silicic acid below 8 m depth in CEE2, but the importance of silica dissolution as a source of Si for diatom growth in the surface 8 m is uncertain. The model's major failing was its assumption of unchanging maximum growth rates of phytoplankton, and unchanging rates of exudation, sinking, and respiration. Physiological parameter values which accounted for the huge bloom of Stephanopyxis in CEE2 could not account for the ensuing collapse. Traditional modelling assumptions of slowly changing internal physiology, although adequate for marine systems dominated by physical factors such as seasonality or water movement, cannot capture the behavior of biologically dominated systems like the enclosed system considered here. / Science, Faculty of / Zoology, Department of / Graduate

Phytoplankton play a key role in the geochemical cycles of the Earth, are responsible for 50% of global carbon fixation, and through this, provide almost all of the energy for the entire marine trophic system. Understanding the dynamics of phytoplankton, and the species composition in relation to environmental factors is therefore of great importance. In this thesis a range of techniques to identify phytoplankton groups that use accessory pigment data obtained from high performance liquid chromatography are compared. While fixed indicator pigment:chl-a ratio approaches provide a quick and simple way of estimating phytoplankton distributions either on the basis of size-class or taxonomic group, the more sophisticated iterative approach of CHEMTAX divides the biomass into more categories and allows more flexibility to adapt to changes in indicator pigment:chl-a ratios caused by environmental variability. Combined with flow cytometric cell counts, depth-dependent trends in the intracellular concentration and composition of phytoplankton pigments can be identified. These data show an exponential decrease in the ratio of carbon-to-chlorophyll with depth, in response to decreasing light intensity. The relative as well as absolute concentrations of phytoplankton pigments are also seen to change with depth, particularly under stratified conditions, with the ratio of zeaxanthin to chlorophyll-a decreasing with increasing depth, and the ratio of chlorophyll-b to chlorophyll-a increasing. Cluster analysis is used to identify the main phytoplankton populations in the North West Atlantic, with communities of large, fucoxanthin-containing phytoplankton dominating in spring when mixing is strong, before being replaced by smaller cells upon the onset of stratification. The links between trends in phytoplankton photophysiology and abiotic conditions are also explored, with temperature being found to be the most important forcing factor. Size-class specific relationships between phytoplankton photosynthetic rates and temperature are identified, with the potential for use in remotely-sensed models of primary production.

Marine microorganisms are a key vector in global carbon cycling, supporting an annual flux of 5 – 12 gigatons of carbon to the ocean interior via the biological carbon pump. While methodological advances over the last half century have greatly advanced our understanding of the factors influencing variability in this flux, the contributions of individual components in the microbial food web remain poorly resolved. Utilizing a combination of laboratory, field and remote sensing studies, this dissertation addresses several different aspects of this challenge. In the second chapter, unsupervised learning methods are applied to a global bio-optical data set from biogeochemical Argo floats to identify six oceanic biomes characterized by distinct seasonal trends in vertical phytoplankton distributions. This study demonstrated the great potential for using data from autonomous profiling floats to generalize seasonal trends in vertical phytoplankton distributions across vast regions of the global ocean, while also providing new insight on the hydrological and biogeochemical drivers of this variability. The third chapter reports the development of a novel method for the direct measurement of chlorophyll a attributable to individual phytoplankton groups in natural samples via cell sorting by flow cytometry. Critically, this approach makes it possible to evaluate phytoplankton community structure in terms of a parameter measured by autonomous platforms, while simultaneously quantifying sources of variability not captured by existing methods. The fourth chapter investigates the environmental drivers of phytoplankton distributions within the Western Tropical South Pacific, providing a case study for the biogeographical provinces identified in chapter 2 while also investigating how biogeochemical gradients influence linkages between heterotrophic groups central to carbon cycling within the microbial food web. Chapter five reports series of experiments investigating cell physiology as a driver of predator-prey interactions between heterotrophic bacteria and algal phagomixotrophs—eukaryotic algae that supplement requirements for carbon and/or nutrients by ingesting smaller cells. By validating the predictions of a gene-based model of algal trophic modes, the results from these experiments point toward the potential widespread occurrence of phagomixotrophy amongst green algae, while highlighting potential sources of bias in field and laboratory studies of bacterivory. With global climate change expected to produce rapid changes in ocean circulation and biogeochemistry, the urgency of understanding the role of marine microbes in global biogeochemical cycling has never been greater. This dissertation represents an advance in this larger goal, providing an expanded framework for the broad distribution of microbial communities in addition to novel insight into the environmental and physiological drivers of microbial community structure from the global to cellular scale.

Future climate scenarios in the Baltic Sea project increasing sea surface temperature, as well as increasing precipitation and river runoff resulting in decreased salinity. These changes can severely impact the dynamics and function of brackish water communities, specifically phytoplankton. Phytoplankton are a significant source of organic matter to other trophic levels, and some species can be toxic. Their response to future climate conditions is of great relevance for the health of humans and aquatic ecosystems. The aim of this thesis was to assess the potential for climate-induced changes, such as decreasing salinity, to affect phytoplankton dynamics, physiology and chemical profiles in the Baltic Sea.      Phytoplankton successional patterns in the Baltic Proper consist of a spring bloom where diatoms and dinoflagellates co-occur and a summer bloom dominated by filamentous/colonial cyanobacteria. The consensus is that future warmer conditions will promote filamentous/colonial cyanobacteria blooms. This thesis shows that phytoplankton biomass in the spring bloom was lower in years with milder winters compared with cold winters. This suggests that in terms of annual carbon export to higher trophic levels, loss of biomass from the spring bloom is unlikely to be compensated by summer cyanobacteria. High frequency sampling of phytoplankton performed in this thesis revealed a strong relationship between the dynamics of pico- and filamentous cyanobacteria. Large genetic diversity was found in cyanobacterial populations with high niche differentiation among the same species. At community level, high temperature and low salinity were the main factors shaping the summer cyanobacterial composition. These conditions may promote the predominance of opportunistic filamentous cyanobacteria, e.g. Nodularia spumigena. This species produces various bioactive compounds, including non-ribosomal peptides such as the hepatotoxin nodularin. In this work, N. spumigena subpopulations evolved different physiological strategies, including chemical profiles, to cope with salinity stress. This high phenotypic plasticity ensures survival in future climate conditions. Under salinity stress, some subpopulations displayed shorter filaments as a trade-off. This indicates that the future freshening of the Baltic Sea may promote grazing on filamentous cyanobacteria and modify carbon flows in the ecosystem. In this thesis, Baltic N. spumigena chemotypes and genotypes grouped into two main clusters without influence of geographical origin. Thus, chemical profiling can be used to explore conspecific diversity in closely genetically related N. spumigena subpopulations.      Overall, this thesis has significantly expanded the knowledge on phytoplankton community and population responses to short- and long-term environmental changes, relevant to project the impacts of future climate conditions in the Baltic Sea.