The period of ice break-off in spring is a key event for many biogeochemical processes in lakes globallly. The biogeochemical processes occurring at ice break-off have the potential of influencing characteristics of lakes throughout spring and summer, including algal blooms and greenhouse gas emission. This makes it important to study lakes in the period of ice break-off. At ice break-off, soil bacteria from the catchment area usually enter the lake via spring floods and mix with the bacteria already occurring in the lake water. In this study, the effects of mixing soil- and lake microbial communities during ice break-off-like conditions were tested by performing an experiment under controlled conditions in the laboratory. In the experiment, light, microbial community composition and concentration of soil-derived organic matter were manipulated to simulate different conditions associated with ice break-off. The variables investigated were bacterial activity and functionality, measured as cell abundance and enzymatic activity, as well as primary production and concentration of dissolved organic matter. The results showed that a mix of soil and lake microbial communities had enzymatic activity patterns resembling lake communities, and then shifted to being more similar to soil communities. The experiment also showed that degradation of measured dissolved organic matter was not linked to biotic processes, and that the observed decrease was most likely due to photo degradation. Finally, the experiment showed that primary production, here measured as chlorophyll a, was only stimulated by the mixed community with light and added soil dissolved organic matter. The results found in this study are important as they show that microbial communities do alter their function and enzymatic activity based on composition. Furthermore, the result that primary production was only seen in the presence of light, soilderived organic matter and a mixed community of lake and soil bacteria may be seen as an indication that primary producers in lake ecosystems to some extent depend on the inflow of terrestrial microbes and organic matter. It also possible that the coalescence of microbial communities enables the communities to perform tasks they were unable to prior to coalescence (i.e. perform tasks that allows primary production to take place). These results give the basis for further, more detailed studies.

In urban areas, stormwater management ponds (SWPs) are built to mitigate polluted runoff. Although these ponds are increasing in numbers, their ecology is not well understood. Physical and chemical characteristics of 17 SWPs in the City of Ottawa were measured to determine the drivers of phytoplankton biomass (Chl. a) and primary production (PP). While total phosphorus was the best predictor of algal biomass in the ponds (as in lakes), the imperviousness of the catchment could also predict Chl. a. Planktonic PP in two ponds measured seasonally was more closely related to water residence time than to nutrient concentrations with rates approaching at times the theoretical maximum for aquatic systems. In one pond, whole ecosystem metabolism, estimated using diel changes in dissolved oxygen and δ18O-O2, suggested that these hypereutrophic systems were net sinks for carbon in the summer but likely sources to the atmosphere at other times of the year.

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In urban areas, stormwater management ponds (SWPs) are built to mitigate polluted runoff. Although these ponds are increasing in numbers, their ecology is not well understood. Physical and chemical characteristics of 17 SWPs in the City of Ottawa were measured to determine the drivers of phytoplankton biomass (Chl. a) and primary production (PP). While total phosphorus was the best predictor of algal biomass in the ponds (as in lakes), the imperviousness of the catchment could also predict Chl. a. Planktonic PP in two ponds measured seasonally was more closely related to water residence time than to nutrient concentrations with rates approaching at times the theoretical maximum for aquatic systems. In one pond, whole ecosystem metabolism, estimated using diel changes in dissolved oxygen and δ18O-O2, suggested that these hypereutrophic systems were net sinks for carbon in the summer but likely sources to the atmosphere at other times of the year.

[Truncated abstract] This work was the first large-scale biological oceanographic study to be undertaken in the coastal eastern Indian Ocean adjacent to Western Australia, and covered both northwest (Exmouth Peninsula to the Abrolhos Islands) and southwest (Cape Naturaliste to Cape Leeuwin) regions. The study area was dominated by the Leeuwin Current (LC), an anomalous eastern boundary current that transports tropical water poleward and prevents deep nutrients from reaching the surface by creating large-scale downwelling. Indeed, LC and offshore waters were consistently associated with low nitrate concentrations and low phytoplankton biomass and production (< 200 mg C m-2 d-1). However, the physical forcing of the LC was offset, during the summer months, by upwelling associated with wind-driven inshore countercurrents (Ningaloo and Capes Currents), which provided a mechanism to access high nutrient concentrations normally confined to the base of the LC. ... Limited seasonal investigations off the Capes region of southwestern Australia showed that the winter production scenario can be very different than summer conditions, with strong Leeuwin Current flow that meanders onto the continental shelf and entrains seasonally nutrient-enriched shelf waters. However, production in the LC was still low (≤450 mg C m-2 d-1) due to light limitation resulting from both increased light attenuation and reduced surface irradiance characteristic of the winter months. This investigation provides fundamental knowledge on physical-biological coupling off Western Australia, with implications for fisheries management in view of seasonal and inter-annual variability in the strength of both the Leeuwin Current and inshore countercurrents.

[Truncated abstract. Formulae and special characters in this field can only be approximated. See PDF version for accurate reproduction.] Solar salt producers use solar energy to evaporate seawater as it flows through a series of ponds. The Shark Bay Salt solar ponds, at Useless Inlet in Western Australia, vary in salinity from that of seawater to four times seawater, over the pond sequence. Water column photosynthesis and biomass decreased markedly with increasing salinity along the pond sequence, while benthic productivity increased as cyanobacterial mats developed. Correspondingly, net productivity shifted from autotrophy to heterotrophy in the water column and from heterotrophy to autotrophy in the benthos. Both shifts occurred at intermediate salinity in the pond sequence, where there was low production in both the water column and benthos. Within individual ponds, productivity, algal biomass and physico-chemical conditions were relatively constant over the year of study. Transitions between benthic and planktonic production along the pond sequence were driven mostly by direct responses to salinity stress, as well as the formation of a gypsum crust on the pond floors at higher salinity (>120 g kg-1). This transition is similar to that which occurs in saline lakes undergoing anthropogenic salinisation and identifies critical salinities for the restoration of these lakes.

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / The net exchange of CO2 between the atmosphere and the ecosystem (NEE), represent the balance of the ecosystem respiration (Reco) and gross primary production (GPP), which are directly related to temperature and photosynthetic active radiation (PAR) respectively. Studies related to the exchange of CO2 between ecosystems and atmosphere has been conducted in different ecosystems, with the objective of finding the contribution of different ecosystems to the atmospheric CO2. For this it is important to have a continuous data series. However, these series may fail causing gaps in time series data. Here, were studied gapfilling techniques( multiple linear regression , nonlinear regressions , Look-up Tables, Mean diurnal variation and Neural networks) and the technique of non-linear regressions excel the other, it is indicated to fill the gaps when available variables weather temperature and radiation, not available when these forcings , advise the use of Mean Diurnal variation . Three years of CO2 fluxes over irrigated rice were used to compare the techniques: two years at the Cachoeira do Sul site and one year at Paraiso do Sul site, both located at Rio Grande do Sul. As a result of gapfilling by nonlinear regression, NEE was estimated to be -92 ± 80gC / m² (2010/2011) and 429 ± 188gC / m² (2011/2012), for Cachoeira do Sul and -43 ± 72gC / m² (2003/2004), for Paraíso do Sul. / As trocas líquidas de CO2 entre a atmosfera e o ecossistema (NEE) representam o balanço entre a respiração do ecossistema (Reco) e a produção primaria bruta (GPP), que estão diretamente relacionados à temperatura e a radiação fotossinteticamente ativa (PAR), respectivamente. Estudos relacionados às trocas de CO2 entre ecossistemas e a atmosfera têm sido realizados em diferentes sítios experimentais com o objetivo de conhecer quanto cada ecossistema contribui para o CO2 atmosférico. Para tanto, é importante ter uma série de dados contínuos. Conquanto, estas séries podem apresentar falhas, gerando lacunas nas séries temporais dos dados, que devem ser preenchidas. Neste trabalho, foram estudas técnicas de preenchimento de dados (Regressões lineares múltiplas, Regressões não lineares, Look-up Tables, Mean diurnal variation), Redes neurais para estimativa do NEE anual sobre culturas de arroz irrigado no estado do Rio Grande do Sul. Foram utilizados três anos de dados de NEE coletados em dois anos para o sítio de Cachoeira do Sul e um ano para Paraíso do Sul. As diferentes técnicas foram avaliadas através da incerteza e do calculo do erro na estimativa do NEE para lacunas artificiais de cada técnica. A técnica de regressões não lineares mostrou-se mais adequada, sendo indicada para o preenchimento das falhas quando disponíveis as variáveis meteorológicas de temperatura e radiação. Quando não disponíveis, o uso da Mean diurnal variation é aconselhada. Como resultado do preenchimento de dados por meio da técnica de Regressão não linear, foram estimados os seguintes valores acumulados de carbono: -92 ± 80gC / m² (2010/2011) e 429 ± 188gC / m² (2011/2012), para Cachoeira do Sul e -43 ± 72gC / m² (2003/2004), para Paraíso do Sul.

In urban areas, stormwater management ponds (SWPs) are built to mitigate polluted runoff. Although these ponds are increasing in numbers, their ecology is not well understood. Physical and chemical characteristics of 17 SWPs in the City of Ottawa were measured to determine the drivers of phytoplankton biomass (Chl. a) and primary production (PP). While total phosphorus was the best predictor of algal biomass in the ponds (as in lakes), the imperviousness of the catchment could also predict Chl. a. Planktonic PP in two ponds measured seasonally was more closely related to water residence time than to nutrient concentrations with rates approaching at times the theoretical maximum for aquatic systems. In one pond, whole ecosystem metabolism, estimated using diel changes in dissolved oxygen and δ18O-O2, suggested that these hypereutrophic systems were net sinks for carbon in the summer but likely sources to the atmosphere at other times of the year.

The trophic state of lakes is commonly defined by the concentration of nutrients in the water column. High nutrient concentrations generate high phytoplankton production, and lakes with low nutrient concentrations are considered low-productive. This simplified view of lake productivity ignores the fact that benthic primary producers and heterotrophic bacteria can be important basal producers in lake ecosystems. In this thesis I have studied clear-water and brown-water lakes with respect to primary production, respiration and bacterial production based on allochthonous organic carbon. These processes were quantified in pelagic and benthic habitats on temporal and spatial scales. I also calculated the net ecosystem production of the lakes, defined as the difference between gross primary production (GPP) and respiration (R). The net ecosystem production indicates whether a lake is net heterotrophic (GPP &lt; R), net autotrophic (GPP &gt; R) or in metabolic balance (GPP = R). Net heterotrophic lakes are sources of carbon dioxide (CO2) to the atmosphere since respiration in these lakes, by definition, is subsidized by an external organic carbon source. External organic carbon is transported to lakes from the terrestrial environment via inlets, and can serve as a carbon source for bacteria but it also limits light availability for primary producers by absorbing light. On a seasonal scale, four of the clear-water lakes studied in this thesis were dominated by primary production in the soft-bottom benthic habitat and by respiration in the pelagic habitat. Concentrations of dissolved organic carbon (DOC) were low in the lakes, but still high enough to cause the lakes to be net heterotrophic. However, the lakes were not low-productive due to the high production in the benthic habitat. One of the clear-water lakes was studied also during the winter and much of the respiration under ice was supported by the benthic primary production from the previous summer. This is in contrast to brown-water lakes where winter respiration is suggested to be supported by allochthonous organic carbon. By studying lakes in a DOC gradient (i.e. from clear-water to brown-water lakes) I could draw two major conclusions. The lakes became less productive since benthic primary production decreased with increasing light extinction, and the lakes became larger sources of CO2 to the atmosphere since pelagic respiration was subsidized by allochthonous organic carbon. Thus, lake carbon metabolism can have an important role in the global carbon cycle due to their processing of terrestrial organic carbon and to their possible feedback effects on the climate system.

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