Characterizing dissolved organic matter (DOM) composition remains a major unresolved problem in aquatic ecology. "Tryptophan-like" dissolved organic matter fluorescence (FDOM) was found to be a much better predictor of heterotrophic bacterial metabolism in 28 Quebec lakes than dissolved organic carbon (DOC), describing 52, 44, 51 and 55% of the variability in bacterial production (BP), bacterioplankton respiration (BR), total bacterial carbon consumption (TBCC), and total plankton community respiration (CR), respectively. In addition, the study provides indirect support for the view that FDOM represents a product of bacterial activity, rather than a bioavailable substrate. This is the first field study to show that fluorescence spectroscopy can be used to characterize an aspect of DOM composition that is related to bacterial metabolism, and provides results that encourage further exploration of the potential uses of DOM fluorescence spectroscopy as a predictive tool.

Alterations in the summer zooplankton community that have appeared since Johnson's (1972) study of a Michigan City (site M) transect in southeastern Lake Michigan were investigated. Vertical tows were made at 5, 10, 15, and 18 meters from June through August in 1987 and 1988 in order obtain data that could be compared with that of Johnson (1972).Subsamples analyzed were proportionately larger than those of Johnson (1972), in order to obtain a detailed profile of the species and numbers in the community, and to identify large, predatory zooplankton species that are believed to have been severely impacted by the explosive growth of the alewife population during the 1960's.The most common crustacean species collected were: Diacyclops thomasi, Leptodiaptomus minutus, Leptodiaptomus ashlandi, calanoid and cyclopoid nauplii and copepodids, Daphnia retrocurva, and Bosmina longirostris. The most common rotifer species collected were Keratella c. cochlearis, Keratella crassa, Kellicottia longispina, Conochilus sp., and Polyarthra sp.. Higher numbers of Epischura lacustris adults and copepodids, Leptodora kindti, Mesocyclops edax, Daphnia galeata mendotae, and the rotifers Asplanchna priodonta, Conochilus sp., Keratella crassa, and Ploesoma truncatum were recorded as compared to Johnson's 1972 data. The summer zooplankton community of this transect appears to have been represented in the summers of 1987 and 1988 by a greater number of large crustacean zooplankton species, as opposed to a 1970 community dominated by small microfiltrators and D. thomasi. / Department of Biology

Light is the primary source of energy in most of earth's ecosystems . In freshwater ecosystems the major interacting factors that determine the abundance and species composition of planktonic phototrophs, the primary utilizers of light, are nutrients, temperature and light. With increasing eutrophication and declining geographical latitude, nutrient availability becomes in excess of the organisms' requirements, water temperature is more favourable for growth, and community structure depends to a greater extent on light availability. This study focuses on the population dynamics of the bloom-forming cyanobacterium Microcystis aeruginosa Kutz. emend. Elenkin in subtropical Hartbeespoort Dam, South Africa. The objectives of the study were: to investigate the annual cycle, and the factors leading to the dominance and success of the cyanobacterium in this hypertrophic, warm monomictic lake, where light availability is the major factor limiting phytoplankton growth rates; to study the surface blooms and ultimately hyperscums that this species forms; and to assess the ecological significance of hyperscums. A 4. 5-years field study of phytoplankton abundance and species composition in relation to changes in the physical environment, was undertaken. The hypothesis was that M. aeruginosa dominated the phytoplankton population (> 80 % by volume) up to 10 months of every year because it maintained itself within shallow diurnal mixed layers and was thus ensured access to light. It was shown that wind speeds over Hartbeespoort Dam were strong enough to mix the epilimnion (7 - 18 m depth) through Langmuir circulations only 12 % of the time. At other times solar heating led to the formation of shallow ( < 2 m) diurnal mixed layers (Z[1]) that were usually shallower than the euphotic zone (Zeu; x = 3.5 m), while the seasonal mixed layer (zrn) was always deeper than Zeu. From the correspondence between vertical gradients of chlorophyll a concentrations and density gradients, when M. aeruginosa was dominant, it was implied that this species maintained the bulk of its population within Z[1]. Under the same mixing conditions non-buoyant species sank into dark layers. These data point out the importance of distinguishing between Zrn and Z[1], and show the profound effect that the daily pattern of Z[1], as opposed to the seasonal pattern of Zrn can have on phytoplankton species composition Adaptation to strong light intensities at the surface was implicated from low cellular chlorophyll a content (0.132 μg per 10[6] cells) and high I[k ](up to 1230 μE m⁻² S¯¹). Ensured access to light, the postmaximum summer populations persisted throughout autumn and winter, despite suboptimal winter temperatures, by sustaining low losses. Sedimentation caused a sharp decline of the population at the end of winter each year and a short ( 2-3 months) successional episode follCMed, rut by late spring M. aeruginosa. was again dominant. The mixing regime in Hartbeespoort Dam and the buoyancy mechanism of M. aeruginosa led to frequent formation of surface bloons and ultimately hyperscums. Hyperscums were defined as thick (decimeters), crusted, buoyant cyanobacterial mats, in which the organisms are so densely packed that free water is not evident. In Hartbeespoort Dam in winter M. aeruginosa formed hyperscums that measured up to 0.75 m in thickness, covered more than a hectare, contained up to 2 tonnes of chlorophyll a, and persisted for 2 - 3 monnths. Hyperscum formation was shown to depend upon the coincidence of the following preconditions: a large, pre-existing standing crop of positively buoyant cyanobacteria; turbulent mixing that is too weak to overcome the tendency of the cells to float, over prolonged periods (weeks); lake morphometry with wind-protected sites on lee shores; and high incident solar radiation. The infrequent occurrence of hyperscums can be attributed to the rare co-occurrence of these conditions. Colonies in the hyperscum were arranged in a steep vertical gradient, where colony compaction increased exponentially with decreasing distance form the surface. This structure was caused by evaporative dehydration at the surface, and by the buoyancy regulation mechanism of M. aeruginosa., which results with cells being unable to lose boyancy when deprived access to light from above. The mean chlorophyll a concentration and water content were 3.0 g 1¯¹ and 14 % at the surface crust, 1.0 g 1¯¹ and 77 % at a few mm depth, and 0.3 g 1¯¹ and 94 % at 10 cm depth, where M. aeruginosa cell concentration exceeded 109 ml¯¹. A consequence of the high cell and pigment concentrations was that light penetrated only 3 mm or less, below which anaerobic, highly reduced conditions developed. Nutrient concentrations in hyperscum interstitial water, collected by dialysis, increased dramatically with time (phosphate: 30-fold over 3 months; ammonia: 260-fold). Volatile fatty acids, intermediate metabolites in anaerobic decomposition processes, were present. Gas bubbles trapped within the hyperscum contained methane (28 %) , and CO[2] (19 %), the major end products of anaerobic decomposition, and no oxygen. The structure and function of M. aeruginosa in hyperscum was examined in relation to the vertical position of colonies and the duration of exposure to hyperscum condition. Colonies and cells collected from 10 em depth in the hyperscum were similar in their morphology (light and fluorescent microscopy) and ultrastructure (transmission and scanning electron microscopy) to those of colonies from surface blooms in the main basin of the lake. With declining depth over the uppermost 10 mm of the hyperscum cells appeared increasingly dehydrated, decomposed and' colonized by bacteria. studies employing microelectrode techniques demonstrated that photosynthetic activity of colonies at the surface of a newly accumulated hyperscum rapidly photoinhibited, substrate-limited, and then ceased within hours of exposure to light intensities > 625 μE m⁻² S¯¹. Photooxidative death followed. The dead cells dehydrated to form the dry crust, from underneath. and space was thus created for colonies rising Cells collected from 10 cm depth retained their photosynthetic capacity ([14]C-uptake experiments) throughout the hyperscum season, although a considerable decline in this capacity was noted over the last (third) month. Altogether the data indicated that spatial separation developed within the hyperscum, between a zone at the surface of lethal physical conditions, a zone beneath the surface of stressful and probably lethal chemical conditions, and a deeper zone of more moderate conditions, which nevertheless, deteriorated after 2 - 3 months. A conceptual model describing the fate of a colony entering a hyperscum was then proposed. According to this model, a colony that arrives below a hyperscum and is not carried away by currents, becomes over-buoyant in the dark and floats into the bottom of the hyperscum. With time it migrates towards, due to its own positive buoyancy, the buoyancy of colonies rising from underneath, and the collapse of cells at the top. It survives in the dark, anaerobic environment by maintaining low levels of basal metabolism while utilizing stored reserves. Depending on weather conditions, the colony mayor may not remain within the hyperscum long enough to reach the zone of decomposition near the surface, where it would die. With the aging of the hyperscum and the accumulation of trapped decomposition products, the zone of decomposition expands. Thus, a hyperscum is essentially a site of a continuous cycle of death and dehydration at the surface and upward migration of colonies from below to replace those that died, although not all colonies entering the hyperscum necessarily reach the lethal zone. The formation of hyperscums was shown to have no major influence on the annual cycle of M. aeruginosa in Hartbeespoort Dam. The seasonality of increase and decline of the planktonic population was similar from year to year, irrespective of whether or not hyperscums formed. The phenomenon of hyperscums demonnstrated that, as Reynolds and Walsby (1975) claimed, thick cyanobacterial water-blooms do form incidentally and have no vital function in the biology of the organism. water temperature did have a major effect on the annual cycle of this species in Hartbeespoort Dam. In temperate lakes the low water temperatures in autumn and winter (<10° C) cause M. aeruginosa to lose its ability to regain buoyancy in the dark, and consequently it sinks to bottom sediments. The higher ( > l2°C) minimum winter temperature in Hartbeespoort Dam leads to the maintenance of a relatively large residual planktonic population throughout the winter. Unlike the case in temperate lakes, the long-term survival of M. aeruginosa in warm-water lakes probably does not depend on winter benthic stocks for the provision of an inoculum for the following growth season. / Thesis (Ph.D.)-University of Natal, Pietermaritzburg, 1987.

I investigated the utility of nitrogen stable isotope signatures in zooplankton as water quality indicators in small lakes. The d15N of organisms such as fish, mussels, insects, and aquatic plants have been linked to land use in several studies, however, I believe pelagic zooplankton in lakes provide comparable information to these with the benefit of being commonly available and easier to collect. To determine the potential use of this tool in water quality assessments, I collected samples from a total of 61 lakes throughout British Columbia. To investigate the seasonal patterns and inter-annual consistency of calanoid copepods and Daphnia d15N, I analyzed data collected from eight coastal lakes over several years. Seasonal variability in zooplankton d15N was observed for several small, temperate lakes with peak values generally occurring in the winter and spring. Sampling schedules should therefore include this critical period of maximum d15N. Calanoid d15N was consistently higher and less variable than Daphnia d15N, and therefore selected as the preferred taxonomic group. A strong relationship between mean d15N and density of septic systems was demonstrated and spring calanoid d15N was consistent over time in the absence of changes in land use. Consistent seasonal patterns in zooplankton d15N on a year-to-year basis support the application of this parameter in water quality trend analysis. To determine the physical factors that influence spring calanoid copepod d15N, the role of several watershed and limnological characteristics in 22 British Columbia lakes was investigated. The density of residential lots, as a proxy for septic density, within the riparian zones of lakes and their tributary streams was the only consistent significant predictor of d15N. While this suggests residential land use and septic density influenced calanoid d15N in these lakes, there are likely additional factors that contribute to the final signature measured. These factors include: the contribution of different N sources and subsequent isotopic mixing, the physical characteristics of individual lakes and differences in lake-specific chemical or biological processing of N. Some weaknesses in the study were identified. Most of the lakes studied had residential development as the dominant land use and a better representation of other land use types, such as agriculture, would have been beneficial. The lakes studied were exposed to relatively low or high levels of septic densities and more lakes exposed to moderate levels of development may have explained more variation. Finally, the majority of lakes studied were located in coastal areas and greater regional representation of lakes may have illustrated regional differences in seasonal trends. In my research, I have demonstrated an application for calanoid d15N in water quality assessments and resource management. When used in conjunction with other information, such as land use and water chemistry, d15N provides insight to nutrient sources for a particular lake, tracks changes in water quality over time, and can help guide management decisions.

A dual stable isotope approach (delta13C and delta15N) was used to assess the importance of pelagic organic carbon in littoral secondary production and explore its predictability. Forty-seven sites were sampled in Lake Memphremagog (Quebec--Vermont) to characterize the isotopic position of the primary producers and filter-feeding freshwater mussels, as well as macrophyte biomass, chlorophyll-a concentration, and site exposure. The different sites spanned a wide range in the three environmental variables. For each site, littoral, terrestrial, and pelagic contributions to the diet of the mussels were calculated from mussel isotopic position, corrected for trophic enrichment. The mean contributions were: littoral---8%, terrestrial---27%, and pelagic---65%. However, the magnitude of the pelagic contribution was not related to macrophyte biomass, site exposure or chlorophyll-a concentration. The finding that the unionid mussels, a major littoral zone filter-feeder, obtain about two-thirds of their nutrition from pelagic zone particles washed into the littoral zone provides evidence for a close coupling in carbon flow of the littoral and pelagic zone. This study represents an important step towards a better understanding of carbon flow in aquatic food webs.

Thesis (D.Sc.)--University of Adelaide, Dept. of Geology and Geophysics, 2002. / "April 2002" Includes bibliographical references and list of the publications and papers submitted.

Thesis (Ph.D.)--Kent State University, 2006. / Title from PDF t.p. (viewed Mar. 5, 2009). Advisor: Robert T Heath. Keywords: bacterioplankton, phosphorus dynamics, bacterial bioenergetics, labile dissolved organic carbon, bacterial productivity, bacterial respiration, bacterial growth efficiency, Lake Erie, hypoxia. Includes bibliographical references (p. 357-397).

Thesis (Ph. D.)--State University of New York at Binghamton, Department of Biological Sciences, 2008. / Includes bibliographical references.

Thesis (M.Sc. Biology)--University of Waikato, 2009. / Title from PDF cover (viewed Apr. 22, 2010). Includes bibliographical references (p. 35-47, 104-120)

Thesis (Ph. D.)--University of Washington, 2002. / Vita. Includes bibliographical references (leaves 112-134).