The top-down effects of consumers and bottom-up effects of resource availability are important in determining community structure and ecological processes. I experimentally examined the roles of consumers — both detritivores and predators — and habitat context in affecting nutrient cycling using the detritus-based insect community in bromeliad leaf wells. I also investigated the role of multiple resources in limiting plant productivity using meta analyses. The insect community in bromeliads only increased nitrogen release from leaf detritus in the presence of a predator trophic level. When only detritivores were present, the flow of stable isotope-labeled nitrogen from detritus to bromeliads was statistically indistinguishable from that in bromeliads lacking insects. I suggest that emergence of adult detritivores constitutes a loss of nitrogen from bromeliad ecosystems, and that predation reduces the rate of this nutrient loss. Hence, insects facilitate nutrient uptake by the plant, but only if both predators and detritivores are present. Moreover, predators can affect nutrient cycling by influencing the spatial scale of prey turnover. This mechanism results in a pattern opposite to that predicted by classic trophic cascade theory. Increasing habitat complexity can have implications for nutrient cycling by decreasing the foraging efficiency of both predators and their prey, and by affecting the vulnerability of predators to intraguild predation. Along a natural gradient in bromeliad size, I found that, depending on the relationship between community composition and habitat size, habitat complexity interacts with the changing biotic community to either complement or counteract the impact of predators on nutrient uptake by bromeliads. In contrast to the existing emphasis on single-resource limitation of primary productivity, meta-analyses of a database of 653 studies revealed widespread limitation by multiple resources, and frequent interaction between these resources in restricting plant growth. A framework for analyzing fertilization studies is outlined, with explicit consideration of the possible role of multiple resources. I also review a range of mechanisms responsible for the various forms of resource limitation that are observed in fertilization experiments. These studies emphasize that a wider range of predator and nutrient impacts should be considered, beyond the paradigm of single resource limitation or classic trophic cascades.

The top-down effects of consumers and bottom-up effects of resource availability are important in determining community structure and ecological processes. I experimentally examined the roles of consumers — both detritivores and predators — and habitat context in affecting nutrient cycling using the detritus-based insect community in bromeliad leaf wells. I also investigated the role of multiple resources in limiting plant productivity using meta analyses. The insect community in bromeliads only increased nitrogen release from leaf detritus in the presence of a predator trophic level. When only detritivores were present, the flow of stable isotope-labeled nitrogen from detritus to bromeliads was statistically indistinguishable from that in bromeliads lacking insects. I suggest that emergence of adult detritivores constitutes a loss of nitrogen from bromeliad ecosystems, and that predation reduces the rate of this nutrient loss. Hence, insects facilitate nutrient uptake by the plant, but only if both predators and detritivores are present. Moreover, predators can affect nutrient cycling by influencing the spatial scale of prey turnover. This mechanism results in a pattern opposite to that predicted by classic trophic cascade theory. Increasing habitat complexity can have implications for nutrient cycling by decreasing the foraging efficiency of both predators and their prey, and by affecting the vulnerability of predators to intraguild predation. Along a natural gradient in bromeliad size, I found that, depending on the relationship between community composition and habitat size, habitat complexity interacts with the changing biotic community to either complement or counteract the impact of predators on nutrient uptake by bromeliads. In contrast to the existing emphasis on single-resource limitation of primary productivity, meta-analyses of a database of 653 studies revealed widespread limitation by multiple resources, and frequent interaction between these resources in restricting plant growth. A framework for analyzing fertilization studies is outlined, with explicit consideration of the possible role of multiple resources. I also review a range of mechanisms responsible for the various forms of resource limitation that are observed in fertilization experiments. These studies emphasize that a wider range of predator and nutrient impacts should be considered, beyond the paradigm of single resource limitation or classic trophic cascades.

Oxygenic photosynthesis is responsible for virtually all of the biochemical production of organic matter in both marine and terrestrial ecosystems. Despite the large amount of research on phytoplankton, macroalgae have received less attention despite them being, on a per-area basis, one of the most productive ecosystems on earth. Furthermore, there has been a tendency of studies to measure primary production in single thalli, or monospecific stands. The lack of studies examining in situ production of whole assemblages using photorespirometry, as is common practice in soft-sediment systems, may be related to a lack of suitable apparatus. This research aimed to develop unique techniques and an apparatus for measuring primary production of intact macroalgal assemblages in laboratory and field conditions. Photorespirometry chambers were developed and tested on in situ macroalgal assemblages, giving information on the role of species identity, biodiversity, irradiance and community structure on overall primary production. Furthermore, the successful application of these methods was used to model annual primary production over local and regional scales, as well as the potential effects of human disturbance on production. In this study, photosynthesis-irradiance relationships (P-E curves) of intact intertidal algal assemblages showed no signs of saturation at high irradiance levels, as is typically seen in single species curves. Furthermore, diverse macroalgal assemblages showed a two-stage rise in production, with a significant enhancement of production at high irradiance. Evidence from this study suggests that the three-dimensional structure of natural assemblages, functional diversity and their interaction with a complex light environment is responsible for the unique P-E curves. The increased efficiency of light use in complex assemblages suggests an important role of species complmentarity in enhancing production with species diversity. This research also shows the potential consequences of disturbance on macroalgal assemblages, with the loss of several species causing a major decline in net production. The methods developed in this thesis have allowed simple modelling of annual rates of primary production and the parameters driving production of macroalgae over long time-scales. Respiration rates have a particularly large influence on production models and indicate that increasing temperature due to climate change could have significant consequences for net carbon fixation of macroalgae. This research gives valuable insight into the production of marine macroalgae and reinforces the notion that they are amongst the most productive systems on earth. These results revealed the importance of examining natural communities, as opposed to randomised assemblages and suggest a vital role of species diversity and community composition. Although there was no functional redundancy of the canopy forming species there did appear to be significant redundancy within the subcanopy assemblage. The identity of subcanopy species had little effect on production, but over longer temporal scales, as species come and go, they may help buffer the communities in terms of primary production. Furthermore, the relationship between biodiversity and ecosystem function (primary production), although driven by diversity, is moderated by resource levels. The complex relationship between irradiance, diversity and production shows the importance of resource levels in the enhancement of function with increasing biodiversity. Due to fundamental differences in terrestrial and marine systems, I was able to examine the effects of discrete levels of irradiance on production, which indicated an important role of complementary light use. This study represents advancements not only in the understanding of primary production in macroalgal assemblages, but also has implications for how diversity may enhance function in other autotrophic systems. The important role of enhanced efficiency of photon capture in multi-canopy layer communities may prove an essential process in ecosystems as diverse as macroalgal beds and tropical rain-forests.

In this thesis, I present a unique temporal study of phytoplankton, nutrient and environmental dynamics that focussed on the transitional period between sea-ice cover conditions and open waters in a coastal inlet of the Canadian Arctic during 2016. I also compared the 2016 experimental data with continuous observations made by the Ocean Networks Canada (ONC) underwater observatory. Surface seawater sampling was conducted in Cambridge Bay with high temporal resolution from June 16 to August 3, to measure phytoplankton carbon and nitrate utilisation, silica production, phytoplankton biomass, phytoplankton taxonomy and dissolved nutrients. Throughout the study period, nitrate concentrations averaged 0.67  0.08 µmol L-1, and chlorophyll a and primary production were low at 0.11  0.005 µg L-1 and 0.25  0.02 µmol C L-1 d-1, respectively. The presence of sea-ice reduced physical mixing, which resulted in low surface nitrate concentrations. Phytoplankton assemblages, production rates and biomass were dominated by small flagellated cells (<5 µm) until late July, yet increases in temperature and nitrate later in the season enabled larger Chaetoceros spp. diatoms to bloom. The Chaetoceros bloom coincided with a peak in silica production (0.429 µmol Si L-1 d-1), which was otherwise low, but variable. The time series was divided into three phases based on changes in environmental conditions, these phases were used to evaluate changes in biological dynamics. Phase I was characterised by sea-ice, low nitrate and increasing phytoplankton biomass and primary production. Phase II was a transitional period, with calm water conditions a drop in phytoplankton biomass, however, an increase in the mean nitrate concentration enabled more consistent carbon fixation. PIII had greater environmental variability driven by mixing events. The mixing of the water column in PIII enabled larger Chaetoceors spp. to become prevalent in the surface waters contributing increasingly to the biomass and carbon utilisation. Overall, the nutrient concentrations, levels of biomass and production rates in Cambridge Bay were more reflective of those from oligotrophic regions. When comparing experimental data with observations made by the ONC observatory, a strong relationship between carbon utilisation and apparent oxygen utilisation became evident. This finding suggests that long-term in situ observations can potentially be used to monitor biological rates in the Arctic. The temporal resolution of this field study adds a seasonal perspective to our understanding of Arctic ecosystems, complements studies with greater spatial and interannual coverage, and can contribute to future numerical modelling of Arctic change. Furthermore, this study provides a first-time comparison between experimentally-measured phytoplankton production and cabled observations in the Arctic. / Graduate / 2019-09-07

The top-down effects of consumers and bottom-up effects of resource availability are important in determining community structure and ecological processes. I experimentally examined the roles of consumers — both detritivores and predators — and habitat context in affecting nutrient cycling using the detritus-based insect community in bromeliad leaf wells. I also investigated the role of multiple resources in limiting plant productivity using meta analyses. The insect community in bromeliads only increased nitrogen release from leaf detritus in the presence of a predator trophic level. When only detritivores were present, the flow of stable isotope-labeled nitrogen from detritus to bromeliads was statistically indistinguishable from that in bromeliads lacking insects. I suggest that emergence of adult detritivores constitutes a loss of nitrogen from bromeliad ecosystems, and that predation reduces the rate of this nutrient loss. Hence, insects facilitate nutrient uptake by the plant, but only if both predators and detritivores are present. Moreover, predators can affect nutrient cycling by influencing the spatial scale of prey turnover. This mechanism results in a pattern opposite to that predicted by classic trophic cascade theory. Increasing habitat complexity can have implications for nutrient cycling by decreasing the foraging efficiency of both predators and their prey, and by affecting the vulnerability of predators to intraguild predation. Along a natural gradient in bromeliad size, I found that, depending on the relationship between community composition and habitat size, habitat complexity interacts with the changing biotic community to either complement or counteract the impact of predators on nutrient uptake by bromeliads. In contrast to the existing emphasis on single-resource limitation of primary productivity, meta-analyses of a database of 653 studies revealed widespread limitation by multiple resources, and frequent interaction between these resources in restricting plant growth. A framework for analyzing fertilization studies is outlined, with explicit consideration of the possible role of multiple resources. I also review a range of mechanisms responsible for the various forms of resource limitation that are observed in fertilization experiments. These studies emphasize that a wider range of predator and nutrient impacts should be considered, beyond the paradigm of single resource limitation or classic trophic cascades. / Science, Faculty of / Zoology, Department of / Graduate

Terrestrial ecosystems have continued to provide the critical service of slowing the atmospheric CO2 growth rate. Terrestrial net primary productivity (NPP) is thought to be a major contributing factor to this trend. Yet our ability to estimate NPP at the regional scale remains limited due to large uncertainties in the response of NPP to multiple interacting climate factors and uncertainties in the driver data sets needed to estimate NPP. In this study, we introduced an improved NPP algorithm that used local driver data sets and parameters in China. We found that bias decreased by 30% for gross primary production (GPP) and 17% for NPP compared with the widely used global GPP and NPP products, respectively. From 2000 to 2012, a pixel-level analysis of our improved NPP for the region of China showed an overall decreasing NPP trend of 4.65TgCa(-1). Reductions in NPP were largest for the southern forests of China (-5.38TgCa(-1)), whereas minor increases in NPP were found for North China (0.65TgCa(-1)). Surprisingly, reductions in NPP were largely due to decreases in solar radiation (82%), rather than the more commonly expected effects of drought (18%). This was because for southern China, the interannual variability of NPP was more sensitive to solar radiation (R-2 in 0.29-0.59) relative to precipitation (R-2<0.13). These findings update our previous knowledge of carbon uptake responses to climate change in terrestrial ecosystems of China and highlight the importance of shortwave radiation in driving vegetation productivity for the region, especially for tropical forests.

<p>Eutrophication, which is the most important degradation in water bodies, has been traced to the imposed loading of nutrients. Of interest is the fact that the process is often accompanied with undesirable effects, one of which is primarily the increased algae production at the surface and accumulation of biomass at the bottom and the secondary responses, which include a., change in species composition b. change in the biogeochemical cycle c. shift in the seasonal pattern and magnitude variability.</p><p>The biogeochemical cycle in response to hydrodynamic alterations may occur internally; however external loading often fosters the process over large spatial scales. In the quest of validating the above statement, we hypothesized that there is no difference in the mean concentration of nutrients in the Gulf of Bothnia from the overall mean concentration.</p><p>The analysis was done with a probability mapping method, in which all stations were grouped into a lattice. The cells are constructed using a grid system, i.e. x and y axis (longitude and latitude). Basically the method statistically tested for variables deviating from the over mean concentration. The variables analyzed are DIN, DSi, DIP and DIN: DSi.</p><p>Results of the analysis showed significant spatial variations in the nutrient distribution in the Gulf of Bothnia; such differences were observed in the coastal to the deep zones of the Gulf.</p>

No description available.

Eutrophication, which is the most important degradation in water bodies, has been traced to the imposed loading of nutrients. Of interest is the fact that the process is often accompanied with undesirable effects, one of which is primarily the increased algae production at the surface and accumulation of biomass at the bottom and the secondary responses, which include a., change in species composition b. change in the biogeochemical cycle c. shift in the seasonal pattern and magnitude variability. The biogeochemical cycle in response to hydrodynamic alterations may occur internally; however external loading often fosters the process over large spatial scales. In the quest of validating the above statement, we hypothesized that there is no difference in the mean concentration of nutrients in the Gulf of Bothnia from the overall mean concentration. The analysis was done with a probability mapping method, in which all stations were grouped into a lattice. The cells are constructed using a grid system, i.e. x and y axis (longitude and latitude). Basically the method statistically tested for variables deviating from the over mean concentration. The variables analyzed are DIN, DSi, DIP and DIN: DSi. Results of the analysis showed significant spatial variations in the nutrient distribution in the Gulf of Bothnia; such differences were observed in the coastal to the deep zones of the Gulf.

No description available.