No description available.

No description available.

Long term data on the North Sea ecosystem are available for phytoplanktonic, zooplanktonic, berithic, fish, and seabird communities. Temporal changes in these have been examined by numerous researchers over the course of the 20th century, their main objective being to determine how the interannual dynamics of these communities are regulated. This study considers the long term ecosystem dynamics, and the mechanisms behind these dynamics, for the North Sea over the latter half of the 20th century, although it focuses upon the time series operated by the Dove Marine Laboratory, and the Continuous Plankton Recorder (CPR) survey in the central-west North Sea region. A review of the literature suggests that long term changes across the North Sea are ultimately driven by two processes. In the northern, western and central areas of the North Sea, the ecosystem is climatically driven, whilst in the southern and eastern regions, the signal of climate is masked by the large arithropogenic nutrient inputs into these regions. A comparison of the Dove and CPR zooplankton time series for the central-west North Sea area found that although their relative year to year fluctuations were similar, large differences were present in the absolute abundances recorded. Model derived catching efficiencies for the two sampling devices suggested that differences in absolute abundances were mainly due in some zooplankton taxa to a greater degree of active avoidance of the CPR sampling device. Further examination of these two series found that the long term zooplankton trends in the central-west North Sea were dissimilar to those observed for other North Sea regions. Inverse relationships between zooplankton abundance, and the position of the Gulf Stream North Wall, and with air temperatures were also observed. These dissimilar trends and inverse relationships were ultimately found to be due to the presence of an internal predation based mechanism. Cmatic influences were also found to indirectly influence the long term dynamics of the benthos in the central-west North Sea The primary factor influencing interannual variation in benthic abundance was phytoplankton productivity (Le. food), which in turn was related to climatic factors. However, at a second central-west North Sea benthic station, situated within a Nephmps norvegicus fishing ground, constant trawling disturbance of the benthos was the primary factor influencing both benthic abundance and species composition. Over the latter half of the 2O1 century, air temperatures and daily sunshine durations have increased in the central-west North Sea region, alongside changes in climatic proxy variables (e.g. NAO index). However, similar trends were not always observed in the biota. Ultimately, the long term dynamics of taxa, communities and ecosystems may be due to direct or indirect factors, yet interactions between a diversiy of internal and external factors, results in the complex behaviour of biological systems over time. This study shows that, a though the centralwest North Sea ecosystem is climatically driven, similar trends between climate and ecosystem components do not necessarily exist, and nor considenng the high comp exity of the ecosystem, should they be expected.

Marine phytoplankton produce the vast majority of primary production in the world’s oceans and sustain virtually all marine ecosystems. Despite this importance, it is currently unclear how global marine phytoplankton concentrations have been changing over the available oceanographic record, and what the causes and consequences of any such changes may be. In this thesis I use observational datasets, statistical modeling, theory, and experiments, to estimate how the global standing stock of marine phytoplankton (referenced by chlorophyll) has changed over the past century, and what the causes and consequences of any changes may be. I inter-calibrated shipboard measurements of upper ocean chlorophyll, transparency, and colour to generate a publicly-available global chlorophyll database spanning from 1890 to 2010. Generalized additive models and multi-model inference were used to estimate the magnitude and nature of changes over the available record, and to explore the effects of multiple oceanographic and climatic variables on these changes. Finally, I worked collaboratively to design and run a mesocosm experiment to test the mechanisms by which rising ocean temperatures influence phytoplankton and plankton community structure. I observed declining trends in upper ocean chlorophyll concentrations at local, regional, and global scales over the past century. Increasing trends were observed closer to coastlines, and were possibly related to increased land-based nutrient deposition there. I also observed inter-annual to multi-decadal fluctuations overlying the longterm trends, which were partly related to climate variability. Sea surface temperature was a consistently strong driver of observed chlorophyll trends. Strong negative effects of rising ocean temperatures on chlorophyll concentration were observed at mid, and low latitudes, and positive effects were observed at high latitudes. The overall effect of increasing temperature on chlorophyll was negative, yet the mesocosm experiment revealed that the primary mechanisms explaining this effect depend on the nature of the ecosystem. Under nutrient limitation, the physically-mediated effects (stratification) of increasing SST were dominant, while under nutrient saturation, the biologically-mediated effects (trophic) were dominant. This thesis provides new evidence that sustained declines in marine phytoplankton over the past century have occurred across multiple spatial scales and that rising ocean temperatures have contributed to this trend. The possible implications of this sustained decline are wide-ranging, with likely impacts on climate, geochemical cycling, fisheries, and ecosystem structure.

No description available.

Electron flow through Photosystem II (PSII) is essential to all life on Earth. The efficiency of this process in freshwater phytoplankton can be depressed by nutrient limitation, ultraviolet radiation (UVR), and excessive photosynthetically active radiation (PAR). The effects of nutrients and radiation on PSII function of natural communities were assessed using changes in the variable fluorescence of PSII (PSII VF), as determined by a pulse amplitude modulation (PAM) fluorometer. The net susceptibility to photoinhibition of PSII depends upon the sensitivity of the assemblage to photodamage and the efficiency of recovery. Damage and recovery rates were quantified by fitting changes in FV:FM during experimental spectral exposures to the model of Kok (1956). Phytoplankton from deep chlorophyll maxima were found to exhibit much higher photosynthetic impairment under UVR exposure than epilimnetic communities in two lakes, due to differences in both damage and recovery rates. In six temperate lakes of the Dorset-Haliburton region, the susceptibility to UVR-induced photoinhibition of PSII was found to be a function of the water transparency (dissolved organic carbon content) of the systems from which the plankton were isolated, with no obvious taxonomic pattern to the responses. Nutrient (nitrogen & phosphorus) supplementation of communities from the Dorset Lakes and from Lake Ontario did not have strong effects on PSII VF, and did not alter the response of Lake Ontario phytoplankton to spectral irradiance. Diurnal changes in FV:FM of Lake Ontario phytoplankton were modeled and average values for the upper half of the water column showed reasonable agreement with observed data; however it is suggested that the addition of a model to simulate vertical mixing could improve the depth-specific accuracy of the predictions. It is concluded that the light history (photoacclimation status) of phytoplankton is the major determinant of the sensitivity of PSII to UVR, and that the nutrient status and taxonomic composition of phytoplankton communities have a comparatively minor influence.

The role of crustacean zooplankton in changing the size structure of phytoplankton communities was examined in Lake Memphremagog, Quebec. The hypothesis that zooplankton have a selective effect on various algal size classes, within the net- and nannoplankton, was tested during different periods of the growing season in controlled field experiments. Depending on the pattern of loss rates imposed by zooplankton on different algal size fractions, it is shown that zooplankton can both accelerate and decelerate successional events in the phytoplankton. Manipulation of zooplankton biomass produced significant changes in algal phosphorus demand, estimated from observations on the short-term dynamics of orthophosphorus. It is shown that the presence of zooplankton reduces the phosphorus deficiency of the phytoplankton in early periods of the season. Empirical relationships are also presented, based on literature observations, to predict variation in feeding selectivity of cladocerans and variation in the biomass of zooplankton among lake ecosystems.

Graduation date: 1967

Thesis (M.S.)--Oregon State University, 1988. / Typescript (photocopy). Includes bibliographical references (leaves 140-142). Also available on the World Wide Web.

Thesis (Ph.D.)--Hong Kong University of Science and Technology, 2007. / Ph.D. in Marine Environmental Science. Includes bibliographical references. Also available in electronic version.