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Preliminary investigations of Saanich Inlet, Vancouver Island, British Columbia, indicated that there was a stratification in the depth of occurrence, during daytime, of the euphausiid Euphausia pacifica and the amphipods Cyphocaris challengeri and Orchomenella pinguis. The presence of an oxycline and a gradient in the carbon dioxide concentration suggested that the stratification was associated with these environmental factors. A programme was conducted between July 1962 and June 1963 to determine whether the stratification was a persistent phenomenon and whether physical and chemical characteristics of the water were associated with the distribution of the organisms. Clarke-Bumpus plankton samplers were towed above, within, and below the oxycline on seven cruises. The survival of the species was measured above, within, and below the oxycline, for exposures of 1 to 12 hours for Euphausia pacifica and Cyphocaris challengeri and of 9 to 24 hours for Orchomenella pinguis, by lowering specimens in cylinders which were in communication with the environment. Laboratory experiments measured the oxygen consumption of the species when carbon dioxide produced by the organisms in experimental chambers accumulated and when it was absorbed. Stratification was a persistent phenomenon in which Euphausia pacifica occupied the upper position, Orchomenella pinguis the lower position and Cyphocaris challengeri an inter-mediate or upper position in the daytime aggregation of organisms. The position of the aggregation was not influenced by temperature or salinity, but was related to the concentration of oxygen, and carbon dioxide. The field experiments showed that Orchomenella pinguis could tolerate lower oxygen and higher carbon dioxide concentrations for much longer durations of exposure than could the other two species. The laboratory experiments indicated Orchomenella pinguis and Cyphocaris challengeri consumed less oxygen than Euphausia pacifica, but Cyphocaris challengeri was more susceptible to high concentrations of carbon dioxide than the other species. These facts offer partial explanations for the stratification of the species. / Science, Faculty of / Zoology, Department of / Graduate

Areas of hypoxia are found in coastal areas worldwide, and have become increasingly widespread. These areas vary in their duration and dissolved oxygen concentration from occasional diurnal hypoxia, as found in Corpus Christi Bay, Texas, seasonal hypoxia as in the northern Gulf of Mexico, to continuous hypoxia as found in oceanic oxygen minimum zones. The effects of exposure to low dissolved oxygen (DO) depend on the duration of exposure, the DO concentration and an organism’s tolerance to hypoxic conditions. Most studies have focused on lethal effects of hypoxia by comparing the abundance of benthic organisms and the species composition of benthic communities between hypoxic and normoxic areas. Sub-lethal effects of such as changes in reproduction may occur at less severe hypoxic conditions (by definition), but may still have effects at the population level. The goal of this study is to examine the sub-lethal reproductive effects of low DO on harpacticoid copepods. The life-history traits and reproductive biology of meiobenthic harpacticoid copepods make this group of organisms useful as test organisms to measure the sub-lethal effects of hypoxia on reproduction. It is hypothesized that changes in reproductive traits may be observed at DO concentrations higher than those that cause lethal effects because of the high energetic cost of female harpacticoid reproduction which may result in reduced fitness. Laboratory studies were conducted to examine the effects of low DO concentrations on survival, egg production, and the number and size of eggs. Harpacticoid population abundance, biomass and the abundance of ovigerous females were measured from field samples collected across sites that varied in their degree and duration of exposure to low DO. To contrast the effects of hypoxia in environments with different hypoxic exposures, field studies were carried out in the northwestern Gulf of Mexico in an area with near constant summer hypoxia, and in southeast corner of Corpus Christi Bay, Texas which experiences intermittent hypoxia often for less than one hour in the early morning hours during summer. At both field sites total copepod abundance, biomass and the abundance of ovigerous females were reduced at the most hypoxic site type compared the reference sites. In Corpus Christi Bay, total and ovigerous female abundance varied with exposure to hypoxia, but the relative declines in abundance and biomass were less dramatic than in the northern Gulf of Mexico. In the northern Gulf of Mexico, the percentage of ovigerous females within the total harpacticoids collected was reduced (p<0.001) to 3% at the Intermediate and 1% at the Hypoxic sites during the summers, but in the fall of 2007 at all transects had similar percentages (p = 0.81) of ovigerous females which ranged from 13 to 16%. Reference sites had a higher number of families with ovigerous females, and the relative abundances of those families were more evenly distributed at Reference sites compared to Hypoxic and Intermediate transects. Patterns of ovigerous female abundance with respect to DO concentration were similar across numerous harpacticoid families found at the study sites. Harpacticoid copepods in laboratory studies tolerated near anoxic DO conditions for longer than 120 hours. Average survival rates of the harpacticoids Schizopera knabeni and Nitokra affinis during periods of low DO with elevated ammonium concentrations were less than 50%, while survival rates for the other treatments were near 100%. The formation of an egg mass was reduced in the near anoxia treatment, but egg clutches did not form at all in the treatment that combined near anoxia and 10 micromolar ammonium. The DO concentration and presence of ammonium were important factors to the severity of population decline that were illustrated by population estimate calculations. Population estimates based on laboratory measurements of Nitokra affinis resulted in calculations of reduced survival and reproductive rates related to hypoxic exposure greatly decrease the potential population of the group, which may affect the copepod population’s ability to recover from hypoxic events. The effects of hypoxia on harpacticoid copepods are the result of the length of exposure, the concentration of low dissolved oxygen and exposure to other chemical fluxes that increase during periods of hypoxia. The measurement of reproductive indicators, such as the presence of an egg mass and the proportion of ovigerous females in a population, could be used to indicate the adverse environmental effects of low dissolved oxygen exposure. The studies from this dissertation are the first to document reproductive effects of low dissolved oxygen on harpacticoid populations in the field. Laboratory studies within this dissertation indicate the interaction between ammonium and hypoxia on survival and reproduction in harpacticoids. Future studies are needed to further determine the effects of hypoxia on the whole-life cycle of harpacticoid copepods. / text

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The areal hypolimnetic oxygen deficit model (AHOD) developed by Strom (1931) and Hutchinson (1938) was tested by examining the predictions which the model makes and by testing the assumptions which were made during the formulation of that model. The model was found to be incorrect. Rates of hypolimnetic oxygen depletion are strongly influenced by the morphometry and temperature of the hypolimnion. Lake morphometry influences at least two processes which affect oxygen concentrations. Significantly more oxygen is turbulently transferred into the hypolimnia of shallow lakes than deep lakes. However the maximum rates of vertical transport are always less than 15% of the observed rate of oxygen depletion. The morphometry of the hypolimnion exerts a much stronger influence upon the amount of respiration which occurs within the hypolimnetic water column. The amount of respiration measured in the water column increases as the thickness of the hypolimnetic water column increases. In oligotrophic lakes with shallow hypolimnia, less than 20% of the total amount of oxygen consumed in the hypolimnion is respired in the water column. In deep lakes more than 60% of the total amount of oxygen consumed is respired within the water column. Measured rates of water column respiration are strongly correlated with the temperature and amount of particulate organic matter present in the water column. / The rate of oxygen consumption within the hypolimnion is constant throughout the period of thermal stratification. Respiration does not depend upon the ambient concentration of oxygen present within the hypolimnion. Changes in oxygen concentration within any stratum of the hypolimnion of a lake can be predicted from a knowledge of the retention of phosphorus by the lake's sediments (Rp), the average temperature of the stratum during the period of thermal stratification (T), and the ratio of the volume of the stratum to the area of lake sediments horizontally contiguous to the stratum (V:SA). A simple statistical model developed from published estimates of rates of oxygen consumption (VOD mg/m('3)/day) predicts that / VOD = -6.6 + .0081T*Rp + 11.07T - 2.32T*Ln(V:SA). / The predictions of this equation agree very well with rates of oxygen depletion measured in 12 lakes which possess a diversity of physical and chemical characteristics. During the period of stratification, hypolimnetic oxygen concentrations can be estimated from the predicted rate of oxgyen depletion and an estimate of the initial oxygen concentration within the hypolimnion at the onset of stratification.

Species have specific tolerances to a variety of environmental variables including temperature, dissolved oxygen (DO) and turbidity. Changes in either of these variables can therefore be expected to affect predator-prey interactions in shallow water ecosystems. Temperature drives the metabolic rates of poikilotherms, including fish. Hypoxic conditions generally affect larger fishes to a greater degree than smaller fishes, though the presence of physostomous swim bladders in certain species can alter that relationship. Finally there are species of fish that rely on vision for food acquisition while other species rely on other senses such as chemical cues. Changes in turbidity levels could therefore affect foraging efficiency of visual foragers. This thesis examines the role that each of these environmental variables (temperature, DO and turbidity) can have on community composition and therefore predator prey interactions, with a specific focus on the role of temperature in structuring predator-prey interactions. Laboratory, field and theoretical studies suggest that as temperature increases, encounter rates between predators and prey will increase. Prey are more active, spend more time foraging, and increase their use of risky habitats in warmer environments in laboratory experiments. In the field, prey and predator activity and/or abundance is positively related to temperature. These laboratory and field studies suggest that temperature increases should result in increased predation rtes of prey. Finally, the results of a dynamic state dependent optimization model also suggest that periods of warming will result in a lowering of the probability of survival of the fathead minnow, Pimephales promelas, a prey species, over the-ice free season. A reduction in DO levels in aquatic ecosystems results in a reduction in the number of and/or activity of predators present. This should result in a reduction in predation risk to prey. However, when endothermic predators are factored in to this equation, this reduction in risk may not occur. The presence of avian predators of small forage fish are directly related to the level of DO in the water, regardless of the abundance of prey fish present. This relationship is likely a result of behavioural decisions of prey that occurs in hypoxic conditions. In periods of low DO, prey fishes may exploit areas of higher DO that are closer to the surface of the waters. While their piscine predators may not be able to tolerate the low DO levels regardless of the position of prey in the water column, avian predators appear to be able to cue in to this increase in availability of potential prey, reducing any benefits that might occur by occupying surface areas where DO levels might be slightly higher than lower in the water column. As compared to temperature and DO, turbidity does not appear to affect the potential risk of predation to forage fish. The catch per unit effort (CPUE) of foragers who rely on vision and those that rely on chemical cues to forages, were not related to turbidity levels. Turbidity levels were also not related to the abundance of avian predators. This suggests that in this generally turbid, shallow water ecosystem, changes in turbidity do not affect the overall species composition of the system. Predator-prey interactions in the system are also not likely to be affected by turbidity. In contrast to this, temperature and DO are likely to influence the interactions between predators and their prey in a shallow water ecosystem. Both increases in temperature and decreases in DO may result in increases in predation pressure on prey. While temperature increases will likely result in increased predation on prey by piscine predators, a reduction in DO, which often occurs as temperature increases, will likely result in increased predation on prey by avian predators, even as predation pressure by piscine predators decrease.

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