Global records of temperature show a warming trend both in the atmosphere and in the oceans. Current climate change scenarios indicate that global temperature will continue to increase in the future. The effects will however be very different in different geographic regions. In northern Europe precipitation is projected to increase along with temperature. Increased precipitation will lead to higher river discharge to the Baltic Sea, which will be accompanied by higher inflow of allochthonous organic matter (ADOM) from the terrestrial system. Both changes in temperature and ADOM may affect community composition, altering the ratio between heterotrophic and autotrophic organisms. Climate changes may thus have severe and complex effects in the Baltic Sea, which has low species diversity and is highly vulnerable to environmental change. The aim of my thesis was to acquire a conceptual understanding of aquatic food web responses to increased temperature and inputs of ADOM. These factors were chosen to reflect plausible climate change scenarios. I performed microcosm and mesocosm experiments as well as a theoretical modeling study. My studies had a holistic approach as they covered entire food webs, from bacteria and phytoplankton to planktivorous fish. The results indicate a strong positive effect of increased temperature and ADOM input on the bacterial community and the microbial food web. However, at the prevailing naturally low nutrient concentrations in the Baltic Sea, the effect of increased temperature may be hampered by nutrient deficiency. In general my results show that inputs of ADOM will cause an increase of the bacterial production. This in turn can negatively affect the production at higher trophic levels, due to establishment of an intermediate trophic level, consisting of protozoa. However, the described effects can be counteracted by a number of factors, as for example the relatively high temperature optimum of fish, which will lead to a more efficient exploitation of the system. Furthermore, the length of the food web was observed to be a strong regulating factor for food web responses and ecosystem functioning. Hence, the effect of environmental changes may differ quite drastically depending on the number of trophic levels and community composition of the system. The results of my thesis are of importance as they predict possible ecological consequences of climate change, and as they also demonstrate that variables cannot be examined separately. / <p>This thesis was supported by grants from the Swedish Research Council FORMAS to AA and SL (217-2006-674), the Centre for Environmental Research in Umeå (CMF) to UB, AA and SL, and by the Swedish strategic research program ECOCHANGE to Umeå University.</p>

Recent studies have characterized coastal estuarine systems as important components of the global carbon cycle. This study investigated carbon cycling through the microbial loop of Florida Bay by use of bacterial growth efficiency calculations. Bacterial production, bacterial respiration, and other environmental parameters were measured at three sites located along a historic phosphorus-limitation gradient in Florida Bay and compared to a relatively nutrient enriched site in Biscayne Bay. A new method for measuring bacterial respiration in oligotrophic waters involving tracing respiration of 13C-glucose was developed. The results of the study indicate that 13C tracer assays may provide a better means of measuring bacterial respiration in low nutrient environments than traditional dissolved oxygen consumption-based methods due to strong correlations between incubation length and δ13C values. Results also suggest that overall bacterial growth efficiency may be lower at the most nutrient limited sites.

Aquatic systems worldwide receive large amounts of organic carbon from terrestrial sources. This ‘allochthonous’ organic carbon (AlloOC) affects critical physical and chemical properties of freshwater ecosystems, with consequences for food web structures and exchange of greenhouse gases with the atmosphere. In the boreal region, loadings of AlloOC are particularly high due to leaching from huge organic deposits in boreal forest, mire and tundra soils. A main process of AlloOC turnover in aquatic systems is its use by heterotrophic bacteria. Applying a bioassay approach, I measured the respiration and growth (production) of bacteria in northern Sweden, in streams and lakes almost totally dominated by AlloOC. The objective was to elucidate how variations in AlloOC source, age, composition and concentration impact on its use by aquatic bacteria, and how AlloOC properties, in turn, are regulated by landscape composition and by hydrology. The bacterial respiration (30-309 µg C L-1 d-1) was roughly proportional to the concentration of AlloOC (7-47 mg C L-1), but not significantly related to AlloOC source or character. Bacterial production (4-94 µg C L-1 d-1), on the other hand, was coupled to the AlloOC character, rather than concentration. A strong coupling to AlloOC character was also found for bacterial growth efficiency (0.06-0.51), i.e. production per unit of assimilated carbon. Bacterial production and growth efficiency increased with rising concentrations of low molecular weight AlloOC (carboxylic acids, free amino acids and simple carbohydrates). While the total AlloOC concentrations generally were the highest in mire-dominated catchments, low molecular weight AlloOC concentrations were much higher in forested catchments, compared to mire-dominated. These patterns were reflected in a strong landscape control of aquatic bacterial metabolism. Moreover, high flow episodes increased the export of organic carbon from forests, in relation to the export from mires, stimulating the bacterial production and growth efficiency in streams with mixed (forest and mire) catchments. The potential of AlloOC to support efficient bacterial growth decreased on time-scales of weeks to months, as the AlloOC was aged in laboratory or lake in situ conditions. To conclude, landscape, hydrology and conditions which determine AlloOC age have large influence on bacterial metabolism in boreal aquatic systems. Considering the role of bacteria in heterotrophic food chains, these factors can have spin-off effects on the structure and function of boreal aquatic ecosystems.

Some of the major problems we face today are human induced changes to the nitrogen (N), phosphorus (P) and carbon (C) cycles. Predicted increases in rainfall and temperature due to climate change, may also increase dissolved organic matter (DOM) inflows to freshwater ecosystems in the boreal zone. N, P, C and light, are essential resources that most often limit phytoplankton (PPr) and bacterial production (BP) in the pelagic zone of lakes. PPr and BP not only constitute the total basal C resource for the pelagic aquatic food web, but also influence ecosystem function and biogeochemical cycles. In this thesis I studied how N, P, C and light affect the relative and absolute rates of PPr and BP, along a wide latitudinal and trophic gradient using published data, and in two in situ mesocosm experiments in a clear water oligotrophic lake. In the experiments I manipulated bottom-up drivers of production and top-down predation to examine how these factors interact to affect pelagic food web structure and function. The most important predictors of PPr globally (Paper I) were latitude, TN, and lake shape. Latitude alone explained the most variation in areal (50%) and volumetric (40%) PPr. In terms of nutrients PPr was primarily N-limited and BP was P-limited. Therefore bacteria and phytoplankton were not directly competing for nutrients. BP:PPr was mostly driven by PPr, therefore light, N, temperature and other factors affecting PPr controlled this ratio. PPr was positively correlated with temperature, but not BP, consequently, higher temperatures may reduce BP:PPr and hence the amount of energy mobilised through the microbial food web on a global scale. In papers II and III interaction effects were found between C-additions and top-down predation by young-of-the-year (YOY) perch. Selective predation by fish on copepods influenced the fate of labile C-addition, as rotifer biomass increased with C-addition, but only when fish were absent. Interaction effects between these top-down and bottom-up drivers were evident in middle of the food web, which is seldom examined in this type of study. Although the energy pathway from bacteria to higher consumers is generally longer than from phytoplankton to higher trophic levels, increased BP still stimulated the biomass of rotifers, calanoid copepods and YOY fish. However, this appeared to be mediated by intermediate bacterial grazers such as flagellates and ciliates. Light was an important driver of crustacean zooplankton biomass (paper IV), but the light:nutrient hypothesis was inadequate to predict the mechanisms behind the decrease in zooplankton biomass at low light. Instead, it appeared that reduced edibility of the phytoplankton community under low light conditions and reduced BP most strongly affected zooplankton biomass. Thus, the LNH may not apply in oligotrophic lakes where PPr is primarily N-limited, Daphnia is rare or absent and mixotrophic phytoplankton are abundant. N, P, C and light manipulations have very different effects on different parts of the pelagic food web. They influence the relative rates of PPr and BP, affect phytoplankton community composition, alter the biomass of higher trophic levels and change pathways of energy transfer through the pelagic food web. This thesis adds valuable information as to how major changes in these resources will affect food web structure and function under different environmental conditions and future climate scenarios. / Lake ecosystem response to environmental change

Inland waters process large amounts of organic carbon, contributing to CO2 and CH4 emissions, as well as storing organic carbon (OC) over geological timescales. Recently, it has been shown that the magnitude of these processes is of global significance. It is therefore important to understand what regulates OC cycling in inland waters and how is that affected by climate change. This thesis investigates the constraints on microbial processing of sediment OC, as a key factor of the carbon cycling in boreal lakes. Sediment bacterial metabolism was primarily controlled by temperature but also regulated by OC quality/origin. Temperature sensitivity of sediment OC mineralization was similar in contrasting lakes and over long-term. Allochthonous OC had a strong constraining effect on sediment bacterial metabolism and biomass, with increasingly allochthonous sediments supporting decreasing bacterial metabolism and biomass. The bacterial biomass followed the same pattern as bacterial activity and was largely regulated by similar factors. The rapid turnover of bacterial biomass as well as the positive correlation between sediment mineralization and bacterial biomass suggest a limited effect of bacterial grazing. Regardless of the OC source, the sediment microbial community was more similar within season than within lakes. A comparison of data from numerous soils as well as sediments on the temperature response of OC mineralization showed higher temperature sensitivity of the sediment mineralization. Furthermore, the low rates of areal OC mineralization in sediments compared to soils suggest that lakes sediments are hotspots of OC sequestration. Increased sediment mineralization due to increase in temperature in epilimnetic sediments can significantly reduce OC burial in boreal lakes. An increase of temperature, as predicted for Northern latitudes, under different climate warming scenarios by the end of the twenty-first century, resulted in 4–27% decrease in lake sediment OC burial for the entire boreal zone.

Both temperature and terrestrial dissolved organic carbon (TDOC) have strong impacts on aquatic food web dynamics and production. Temperature affects vital rates of all organisms and terrestrial carbon has been shown to alter the dynamics of phytoplankton and bacterial production and affect the trophic structure of planktonic food webs. As climate change predictions for the Baltic Sea suggests future increases in both terrestrial carbon run-off and increases in temperature, the aim of thesis was to adopt a system-ecological approach and study effects of these abiotic variables, not only on interactions within planktonic food webs, but also on the growth and consumption rates of one of the most common zooplanktivorous fish in the Baltic Sea, the three-spined stickleback Gasterosteus aculeatus. Results showed that three-spined sticklebacks display a high degree of resilience against increasing temperatures, as both growth rates as well as consumption rates on zooplankton were high at temperatures well over 20 °C. Furthermore, it was shown that the minimal resource densities required to sustain individual and population growth, actually decreased with increasing temperatures, implying that sticklebacks around their optimum temperature for growth at 21 °C will actually have an increased scope for growth. As stickleback population densities have increased over the last decade in the Baltic Sea and are now suggested to out-compete other coastal fish species for shared zooplankton resources, the results presented in this thesis suggest that increased water temperatures would only serve to increase sticklebacks competitive advantage. As the structuring role of this small zooplanktivore on pelagic communities might be considerable, further studies investigating competitive interactions as well as patterns of population abundances are definitely warranted. TDOC was overall shown to stimulate bacterial production and the microbial food web. Because of the longer trophic pathways required to transport carbon from bacterial production to higher trophic levels, the addition of TDOC always reduced food web transfer efficiency. However, it became apparent that the full effect of TDOC additions on pelagic food webs was complex and depended heavily not only on the existing trophic structure to which the carbon was introduced, but also on ambient temperature levels. When three-spined sticklebacks were part of food webs with significant TDOC inputs, the presence of fish, indirectly, through predator release of lower trophic levels, amplified the magnitude of the effects of carbon addition on bacterial production, turning the base of the system significantly more heterotrophic, which ultimately, impacted negatively on their own production. However, when a pelagic food web containing sticklebacks was simultaneously subjected to realistic increases in temperature and TDOC concentrations, food web efficiency and fish production increased compared to present day conditions. These results were explained by a temperature dependent increased production potential of zooplankton, sustained by an increased production of heterotropic microzooplankton via TDOC additions, which lead to higher fish production. Although the increased number of trophic linkages in heterotrophic food webs should have reduced energy transfer efficiency, these negative effects seem here to have been overridden by the positive increases in zooplankton production as a result of increased temperature. These results show that heterotrophic carbon transfer can be a viable pathway to top-consumers, but also indicates that in order to understand the full effects of climate change on trophic dynamics and fish production, abiotic variables cannot be studied in isolation.

Submitted by Ronildo Prado (ronisp@ufscar.br) on 2017-08-22T13:00:56Z No. of bitstreams: 1 DissRMFS.pdf: 2091397 bytes, checksum: 9416d9552b0e519672969fe9b02432ff (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-08-22T13:01:05Z (GMT) No. of bitstreams: 1 DissRMFS.pdf: 2091397 bytes, checksum: 9416d9552b0e519672969fe9b02432ff (MD5) / Approved for entry into archive by Ronildo Prado (ronisp@ufscar.br) on 2017-08-22T13:01:12Z (GMT) No. of bitstreams: 1 DissRMFS.pdf: 2091397 bytes, checksum: 9416d9552b0e519672969fe9b02432ff (MD5) / Made available in DSpace on 2017-08-22T13:01:18Z (GMT). No. of bitstreams: 1 DissRMFS.pdf: 2091397 bytes, checksum: 9416d9552b0e519672969fe9b02432ff (MD5) Previous issue date: 2017-03-10 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP) / Reservoirs located in tropical regions are main carbon (C) sources to the atmosphere, and bacterial metabolism is a key process that regulates those emissions. However, studies on the environmental drivers of bacterial metabolism in tropical reservoirs are scarce. By measuring metabolic rates and the limnological parameters in four cascading reservoirs that form a trophic state gradient, we determined the environmental drivers of bacterial metabolism in a tropical region, and compared them with those found in the literature (mainly from temperate regions). Our multiple regression models selected variables related to the trophic state as the main drivers of bacterial production (BP) and bacterial growth efficiency (BGE). On the other hand, bacterial respiration (BR), and consequently bacterial carbon demand (BCD), were weakly and negatively correlated to dissolved organic carbon (DOC), contrasting with the literature data. BR was always high, especially in less productive reservoirs where planktonic communities were limited by phosphorus. Nutrient limitation, high temperatures and high incident light intensity increased the environmental hostility, and cells must invest more energy in maintenance mechanisms, which directs the metabolism towards BR. This was observed in the reservoirs studied, especially in the more oligotrophic environments (Nova Avanhandava and Três Irmãos) where BR was higher and ECB lower. Our results indicate that the regulatory mechanisms of bacterial metabolism may vary according to latitude. / Reservatórios de regiões tropicais são fontes de carbono (C) para a atmosfera e o metabolismo bacteriano é um processo fundamental na regulação dessas emissões. No entanto, estudos que elucidem os fatores ambientais que determinam o metabolismo bacteriano em reservatórios tropicais são ainda escassos. Neste estudo foram medidas taxas metabólicas e parâmetros limnológicos em quatro reservatórios em cascata que formam um gradiente de estado trófico, com o intuito de determinar os reguladores do metabolismo bacteriano em uma região tropical e compará-los com dados obtidos a partir da literatura disponível (principalmente de regiões temperadas). Nossos modelos de regressão múltipla selecionaram variáveis relacionadas ao estado trófico como os principais reguladores da produção bacteriana (PB) e da eficiência de crescimento bacteriano (ECB). Foi encontrada uma relação fraca e negativa entre a respiração bacteriana (RB) e o carbono orgânico dissolvido (COD), diferente dos dados da literatura. As taxas de RB foram sempre elevadas, especialmente em reservatórios menos produtivos, nos quais as comunidades planctônicas estavam limitadas por fósforo. A escassez de nutrientes, as elevadas temperaturas e a alta intensidade de luz incidente aumentam o grau de hostilidade, e as células devem investir mais energia em mecanismos de reparação, o que direciona o metabolismo para a RB. Isso foi observado nos reservatórios estudados, especialmente, nos ambientes mais oligotróficos (Nova Avanhandava e Três Irmãos) nos quais a RB foi mais elevada e a ECB mais baixa. Nossos resultados indicam que os mecanismos reguladores do metabolismo bacteriano podem variar de acordo com a latitude. / FAPESP: 14/14139-3 / FAPESP: 11/50054-4

Submitted by Renata Lopes (renatasil82@gmail.com) on 2016-07-15T13:59:12Z No. of bitstreams: 1 saulomachadodesouzajacques.pdf: 663956 bytes, checksum: caccb1e96f0e94bf57e712aab0d16d45 (MD5) / Approved for entry into archive by Diamantino Mayra (mayra.diamantino@ufjf.edu.br) on 2016-07-19T15:55:35Z (GMT) No. of bitstreams: 1 saulomachadodesouzajacques.pdf: 663956 bytes, checksum: caccb1e96f0e94bf57e712aab0d16d45 (MD5) / Made available in DSpace on 2016-07-19T15:55:35Z (GMT). No. of bitstreams: 1 saulomachadodesouzajacques.pdf: 663956 bytes, checksum: caccb1e96f0e94bf57e712aab0d16d45 (MD5) Previous issue date: 2011-08-26 / Bactérias heterotróficas são importantes componentes do plâncton, participando ativamente do ciclo do carbono em ecossistemas aquáticos, como por exemplo, através da incorporação de carbono à biomassa pelo processo de produção bacteriana (PB). Alterações nos ecossistemas aquáticos influenciam a atividade bacteriana e, consequentemente, o ciclo do carbono nestes sistemas. Este estudo se propôs a avaliar a influência da temperatura, da concentração de carbono orgânico dissolvido (COD) e da disponibilidade de nutrientes sobre a PB em cinco lagoas costeiras tropicais, que representam um gradiente natural de concentração de carbono (12; 18,3; 46,7; 86 e 117,1 mg/L). Amostras de água de todas as lagoas foram incubadas em três temperaturas diferentes (25, 30 e 35º C), combinadas fatorialmente com a adição de nutrientes: carbono (C), nitrogênio (N), fósforo (P) e nitrogênio + fósforo (NP). Os resultados revelaram que a adição de nutrientes estimulou a PB, mas de maneira distinta em cada uma das lagoas. Em sistemas com maiores concentrações de COD, a adição de C estimulou a PB, enquanto que em lagoas com menores concentrações, a adição de N e P resultou maiores taxas de PB. No geral, o aumento de temperatura afetou negativamente a PB. Segundo os resultados observados, o efeito da temperatura e da adição de nutrientes sobre a PB é dependente de algumas características do estoque de carbono dos ecossistemas aquáticos, como sua qualidade para o consumo bacteriano. Desta forma, espera-se que a temperatura e a qualidade de C sejam mais importantes sobre as taxas de PB em ecossistemas com maiores concentrações de COD húmico, enquanto em ecossistemas com baixa concentração de COD estes dois fatores exercem fraca ou nenhuma influência sobre PB. Diante dos atuais eventos de eutrofização e alterações climáticas, espera-se observar alterações da atividade da comunidade vii bacteriana em ecossistemas aquáticos, com consequência para toda a teia alimentar. Os resultados deste trabalho apontam que fatores como a qualidade do substrato e a estrutura da comunidade bacteriana merecem especial atenção em estudos futuros para uma melhor compreensão da influência da temperatura e de nutrientes sobre a PB em ecossistemas aquáticos húmicos tropicais. / Heterotrophic bacteria are important components of plankton, actively participating in the carbon cycle in aquatic ecosystems and incorporating carbon biomass through the process of bacterial production (BP). Changes in ecosystems influence bacterial activity and thus the carbon cycle in these systems. This study aimed to evaluate the influence of temperature, dissolved organic carbon (DOC) concentration and nutrient availability on the BP in five tropical coastal lagoons, which represented a gradient of carbon concentration (12; 18,3; 46,7; 86 and 117,1 mg/L). Water samples from the lagoons were incubated at three different temperatures (25, 30 and 35º C) factorially combined with the addition of nutrients: carbon (C), nitrogen (N), phosphorus (P) and nitrogen + phosphorus (NP). The results revealed that the addition of nutrients stimulates the BP, but differently in each of the lagoons. In systems with higher DOC concentration, the addition of C stimulated the BP, while in lakes with lower DOC, the addition of N and P resulted in higher BP rates. In general, the temperature increase negatively affected BP. According to the results, the effect of temperature and nutrient addition on the BP is dependent of some characteristics of the carbon stock of aquatic ecosystems, as well on its quality for bacterial consumption. Thus, it is expected that the temperature and quality of C are more important on the rates of BP in the ecosystem with the highest humic carbon concentrations, while in ecosystems with low DOC, these two factors play weak or no influence on BP. Given the current events of eutrophication and climate change, a reduction in the activity of bacterial communities in aquatic ecosystems would be expected, with consequences for the entire food web. The present results point out that factors like substrate quality and ix the structure of the bacterial community deserve special attention in future studies for a better understanding of the influence of temperature and nutrient on BP in humic tropical aquatic ecosystems.

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.

No description available.