Evolution shapes ecosystems but the processes by which this occurs are not well understood. Adaptive change in resource expensive traits may underlie one such process, as evolution altering a species’ resource needs may effect how that species interacts with ecosystem resources. For this, Ecological Stoichiometry (ES) may be a tractable framework, as it simplifies organisms into elemental ratios and then applies mass-balance to predict changes in diet and waste interactions. ES detects variation in resource expensive traits as variation in elemental ratios, and predicts compensation via parallel changes in diet (e.g. high phosphorous individuals consume high phosphorus diets) and/or offsetting changes in waste (e.g. high phosphorous individuals release low phosphorus waste). To test the utility of this framework and improve our understanding of eco-evolutionary dynamics, I studied variation in phenotypic traits, genetics, elemental content and resource interactions within and across natural populations of highly regarded eco-evolutionary model species threespine stickleback (Gasterosteus aculeatus). First, I related heritable variation in phosphorus rich bony traits and genetics commonly under natural selection with variation in elemental content (N:P) to determine the magnitude and basis of intraspecific variation in N:P. Second, I investigated the ecosystem consequences of variation in elemental content by determining whether stickleback compensate through changes in diet choice and excretion rates. I found stickleback vary widely in elemental composition (3.0 – 9.4:1 N:P) which models explained well with four bone related traits: bone mineralization, body size, lateral plating and pelvis size (R2 > 0.52). Additional genetic models linked variation in Eda alleles (which underlie lateral plating) with a 12% shift in stickleback N:P. Stickleback compensated for this variation in N:P demand by altering diet choice rather than excretion rates, and by maximizing dietary inputs through changes in gut morphology. Within and across populations, high phosphorus stickleback consumed a larger proportion of high phosphorus prey and contained longer gastrointestinal tracts that more efficiency process diet resources. These results demonstrate that heritable variation in elemental composition is ecologically relevant with individual traits and genetics having large effects. As individuals compensated by altering resource acquisition rather than release, the direct ecological consequences of evolutionary change in these resource expensive traits is likely larger for food web structure and abundance than nutrient dynamics. / Graduate / 2018-12-19

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I investigated the drivers of nutrient cycling by heterotrophic microbes during leaf decomposition in streams. My research addressed two overarching questions: 1) how do exogenous and endogenous factors interact to drive microbial nitrogen (N) cycling during organic matter decomposition in stream ecosystems, and 2) what affect will the global increase in biologically active N have on these factors and resulting fluxes? I conducted studies in natural streams and laboratory mesocosms to address these questions and used general stoichiometric theory to conceptualize diverse microbial assemblages as a single functional unit within stream ecosystems. First, I described spatial and temporal patterns of N and phosphorus uptake and mineralization by leaf-associated microbial assemblages in five southern Appalachian streams which spanned a gradient of nitrate availability. I found wide variations in nutrient fluxes across time and space, perhaps due to macroinvertebrate-induced changes in microbial assemblage composition. Secondly, I explored the roles of endogenous and exogenous N in meeting microbial requirements. I isolated microbial biomass from leaves that had been labeled with N-15 and incubated in the same five Appalachian streams. The importance of exogenous N increased as decomposition progressed and was particularly important in streams with high N availability. Finally, I tested potential interactions between two exogenous drivers of microbial nutrient cycling: N availability and animal activity. I used mesocosms to test the effects of consumer nutrient recycling (CNR) and grazing by two shredders on microbial uptake under different N regimes. Animals only influenced microbial uptake under low N conditions. Shredder CNR generally stimulated uptake while grazing had a negative effect. My research provides a robust model describing N cycling by detritus-associated microbes over the course of decomposition. According to this model, microbes assimilate endogenous N during the initial stages of decomposition and immobilization of exogenous N becomes more important as decomposition progresses. The labeled substrate technique that I used to generate this model is an elegant way of testing the applicability of this model in other ecosystems. My results also suggest that anthropogenic activities that increase exogenous N availability have implications for N and C cycling in lotic systems. / Ph. D.

A demanda da população humana por alimentos, fibras e biocombustíveis impõe crescimento pronunciado na agricultura intensiva e, com ela, no consumo de fertilizantes. Este manejo de fertilizantes leva a alterações dramáticas nas quantidades e proporções de nutrientes em agroecossistemas que, por sua vez, podem influenciar comunidades biológicas por meio de alterações na biomassa, qualidade nutricional e composição de produtores. Uma tendência no cenário produtivo brasileiro recente é a expansão das culturas de biocombustíveis, notadamente da cana-de-açúcar (etanol) e da soja (biodiesel), sobre ambientes de cerrado. Estas culturas, que juntas cobrem nada menos que 30 milhões de hectares do território brasileiro, demandam padrões contrastantes de fertilização. A cana-de-açúcar exige altas proporções de nitrogênio relativo a fósforo, enquanto que a soja exige altas proporções de fósforo relativo a nitrogênio. Esta dissertação teve como objetivo testar os efeitos diretos e indiretos que as alterações nas quantidades e proporções de N e P que se seguem à conversão de ambientes nativos para o cultivo de cana-de-açúcar e soja têm sobre comunidades biológicas. Estes objetivos foram testados por meio de dois experimentos em mesocosmos aquáticos manipulando nutrientes, algas, girinos e ninfas de libélulas. O primeiro experimento seguiu um delineamento fatorial completo cruzando três concentrações de N (Nreferência, Nsoja, Ncana) e três concentrações de P (Preferência, Psoja, Pcana), e dois níveis tróficos (algas e girinos de Physalaemus cuvieri). Níveis de nutrientes manipulados simularam condições medidas em corpos d´água de cerrado, ou padrões de fertilização conhecidos para culturas de cana-de-açúcar e de soja. No segundo experimento pareamos essas concentrações (Nreferência Preferência, Nsoja, Psoja, Ncana Pcana) de forma a reduzir a dimensionalidade do experimento e assim permitir a inclusão de um terceiro nível trófico (algas, girinos de Hypsiboas faber e ninfas de libélulas). Incluímos ainda um tratamento no qual as ninfas de libélulas estiveram engaioladas para separar os efeitos da predação de girinos, dos efeitos da alteração nos seus atributos. As principais variáveis de resposta em ambos experimentos foram concentração de clorofila a como indicadora de biomassa de fitoplâncton, e mortalidade, massa final e estágio de desenvolvimento final de girinos como indicadores de rendimento de consumidores. Uma comparação na concentração de clorofila antes e depois da introdução de girinos procurou testar o efeito da adição de consumidores na biomassa de produtores. Conforme esperado, NT e PT medidos responderam expressivamente à manipulação de N e P. Nas semanas que antecederam a introdução de girinos, a biomassa algal cresceu em resposta ao aumento na concentração do nitrogênio, mas não do fósforo; esta tendência se manteve ao longo dos experimentos. Isso provavelmente ocorreu porque concentrações medidas de NT variaram da oligotrofia à hipertrofia; por sua vez, concentrações medidas de PT estiveram consistentemente em valores considerados hipertróficos. A introdução de girinos não exerceu efeitos evidentes sobre a concentração de clorofila a, possivelmente indicando que o alimento era abundante o suficiente para não ocorrer competição entre consumidores. O rendimento de consumidores foi significativamente influenciado pela manipulação de nutrientes. Tratamentos ricos em P levaram a uma redução no crescimento de Physalaemus cuvieri, e uma tendência à redução no seu desenvolvimento e sobrevivência. De forma similar, o cenário de fertilização de soja - portanto rico em P - levou a uma redução significativa na sobrevivência de H. faber. Por sua vez, o cenário de fertilização de cana-de-açúcar levou a um ganho de massa de H. faber 30% superior ao controle. Formulamos a hipótese que esta diminuição no crescimento e sobrevivência de girinos em cenários de fertilização ricos em P teria sido causada pela proliferação de cianobactérias, frequentemente impalatáveis ou mesmo tóxicas, e que tendem a ser favorecidas em ambientes de baixa proporção N:P. Ao contrário do que esperávamos, ninfas de libélulas não foram capazes de promover redução direta ou indireta no crescimento ou sobrevivência de girinos, ou influenciar a biomassa de fitoplâncton. Concluímos que as alterações nos níveis de nutrientes que acompanham a conversão de ambientes nativos para paisagens agrícolas tem efeitos importantes sobre cadeias alimentares aquáticas (controle \'bottom-up\'), e que estes efeitos são muito mais fortes do que aqueles mediados pela presença de predadores (controle \'top-down\'). No entanto, estes efeitos não são necessariamente intuitivos, uma vez que determinados padrões de fertilização influenciam negativamente o rendimento de elos intermediários da cadeia e, portanto, a transmissão da biomassa para níveis tróficos superiores. / The human demand for food, fibers and biofuels imposes a pronounced growth in intensive agriculture and, with it, in the consumption of fertilizers. Fertilizer management promotes dramatic changes in the quantities and proportions of nutrients in agroecosystems which, in turn, influence biological communities through changes in the biomass, nutritional quality, and composition of producers. A recent trend in the Brazilian productive scenario is the expansion of biofuel crops - notably sugarcane (ethanol) and soybean (biodiesel) - over the cerrados. These crops, covering no less than 30 million hectares in the country, demand contrasting fertilization patterns: sugarcane requires high proportions of nitrogen relative to phosphorus, whereas soybean requires high proportions of phosphorus relative to nitrogen. The objective of this thesis was to test the direct and indirect effects that changes in the quantities and proportions of N and P that follow conversion of native habitats to sugarcane and soybean fields have on biological communities. These objectives were achieved by means of two mesocosm experiments manipulating nutrients, algae, tadpoles and dragonfly naiads. The first experiment followed a full factorial design crossing three concentrations of N (Nreference, Nsoy, Ncane) and three concentrations of P (Preference, Psoy, Pcane), as well as two trophic levels (algae and Physalaemus cuvieri larvae). Nutrient levels simulated conditions measured in water bodies of the cerrado, or patterns of fertilization known for sugarcane or soybean crops. In the second experiment we paired these concentrations (Nreference Preference, Nsoy Psoy, Ncane Pcane) so as to reduce dimensionality and as such to permit inclusion of a third trophic level (algae, Hypsiboas faber larvae, dragonfly naiads). We included an additional treament with caged naiads to separate the effects of actual tadpole predation from those of changes in tadpole traits. The main response variables in both experiments were cholophyll a concentrations as na indicator of phytoplankton biomass, and tadpole mortality, final mass and final developmental stage as indicators of consumer performance. A comparison in the concentration of chlorophyll a before and after the introduction of tadpoles tested the effect of consumers on producer biomass. As expected, measured TN and TP responded strongly to N and P manipulations. In the weeks preceding the introduction of tadpoles, algal biomass increased in response to the concentration of N, but not to the concentration of P; this trend was maintained throughout the experiment. This probably happened because measured TN varied from oligotrophic to hypertrophic conditions; in contrast, measured TP consistenly corresponded to hypertrophic conditions. Tadpole introduction had no evident effects on chlorophyll concentrations, possibly indicating that food was abundant and competition among tadpoles, if present, was weak. Consumer performance was significantly influenced by nutrient manipulations. P-rich treatments led to a reduction in growth, and to a reduction trend in development and survivorship, of Physalaemus cuvieri. Similarly, the scenario of soybean fertilization - therefore P-rich - led to a significant reduction in Hypsiboas faber survivorship. The scenario of sugarcane fertilization led to a 30% increase in mass gain in Hypsiboas faber. We hypothesized that this reduction in tadpole growth and survivorship in P-rich environments was caused by a proliferation of unpalatable or even toxic cyanobacteria, which are favored in low N:P ratio environments. Contrary to our expectations, dragonfly naiads did not cause a direct or indirect reduction in tadpole growth or survivorship, nor influence phytoplankton biomass. We conclude that changes in nutrient levels that accompany the conversion of native habitats to agricultural fields can have strong effects on freshwater food chains (\'bottom-up\' control), and that these effects are much stronger than those mediated by predator presence (\'top-down\' control). However, bottom-up effects are not necessarily intuitive as certain fertilization patterns negatively influence the performance of intermediate consumers and, as such, the transmission of biomass to higher trophic levels.

No description available.

A demanda da população humana por alimentos, fibras e biocombustíveis impõe crescimento pronunciado na agricultura intensiva e, com ela, no consumo de fertilizantes. Este manejo de fertilizantes leva a alterações dramáticas nas quantidades e proporções de nutrientes em agroecossistemas que, por sua vez, podem influenciar comunidades biológicas por meio de alterações na biomassa, qualidade nutricional e composição de produtores. Uma tendência no cenário produtivo brasileiro recente é a expansão das culturas de biocombustíveis, notadamente da cana-de-açúcar (etanol) e da soja (biodiesel), sobre ambientes de cerrado. Estas culturas, que juntas cobrem nada menos que 30 milhões de hectares do território brasileiro, demandam padrões contrastantes de fertilização. A cana-de-açúcar exige altas proporções de nitrogênio relativo a fósforo, enquanto que a soja exige altas proporções de fósforo relativo a nitrogênio. Esta dissertação teve como objetivo testar os efeitos diretos e indiretos que as alterações nas quantidades e proporções de N e P que se seguem à conversão de ambientes nativos para o cultivo de cana-de-açúcar e soja têm sobre comunidades biológicas. Estes objetivos foram testados por meio de dois experimentos em mesocosmos aquáticos manipulando nutrientes, algas, girinos e ninfas de libélulas. O primeiro experimento seguiu um delineamento fatorial completo cruzando três concentrações de N (Nreferência, Nsoja, Ncana) e três concentrações de P (Preferência, Psoja, Pcana), e dois níveis tróficos (algas e girinos de Physalaemus cuvieri). Níveis de nutrientes manipulados simularam condições medidas em corpos d´água de cerrado, ou padrões de fertilização conhecidos para culturas de cana-de-açúcar e de soja. No segundo experimento pareamos essas concentrações (Nreferência Preferência, Nsoja, Psoja, Ncana Pcana) de forma a reduzir a dimensionalidade do experimento e assim permitir a inclusão de um terceiro nível trófico (algas, girinos de Hypsiboas faber e ninfas de libélulas). Incluímos ainda um tratamento no qual as ninfas de libélulas estiveram engaioladas para separar os efeitos da predação de girinos, dos efeitos da alteração nos seus atributos. As principais variáveis de resposta em ambos experimentos foram concentração de clorofila a como indicadora de biomassa de fitoplâncton, e mortalidade, massa final e estágio de desenvolvimento final de girinos como indicadores de rendimento de consumidores. Uma comparação na concentração de clorofila antes e depois da introdução de girinos procurou testar o efeito da adição de consumidores na biomassa de produtores. Conforme esperado, NT e PT medidos responderam expressivamente à manipulação de N e P. Nas semanas que antecederam a introdução de girinos, a biomassa algal cresceu em resposta ao aumento na concentração do nitrogênio, mas não do fósforo; esta tendência se manteve ao longo dos experimentos. Isso provavelmente ocorreu porque concentrações medidas de NT variaram da oligotrofia à hipertrofia; por sua vez, concentrações medidas de PT estiveram consistentemente em valores considerados hipertróficos. A introdução de girinos não exerceu efeitos evidentes sobre a concentração de clorofila a, possivelmente indicando que o alimento era abundante o suficiente para não ocorrer competição entre consumidores. O rendimento de consumidores foi significativamente influenciado pela manipulação de nutrientes. Tratamentos ricos em P levaram a uma redução no crescimento de Physalaemus cuvieri, e uma tendência à redução no seu desenvolvimento e sobrevivência. De forma similar, o cenário de fertilização de soja - portanto rico em P - levou a uma redução significativa na sobrevivência de H. faber. Por sua vez, o cenário de fertilização de cana-de-açúcar levou a um ganho de massa de H. faber 30% superior ao controle. Formulamos a hipótese que esta diminuição no crescimento e sobrevivência de girinos em cenários de fertilização ricos em P teria sido causada pela proliferação de cianobactérias, frequentemente impalatáveis ou mesmo tóxicas, e que tendem a ser favorecidas em ambientes de baixa proporção N:P. Ao contrário do que esperávamos, ninfas de libélulas não foram capazes de promover redução direta ou indireta no crescimento ou sobrevivência de girinos, ou influenciar a biomassa de fitoplâncton. Concluímos que as alterações nos níveis de nutrientes que acompanham a conversão de ambientes nativos para paisagens agrícolas tem efeitos importantes sobre cadeias alimentares aquáticas (controle \'bottom-up\'), e que estes efeitos são muito mais fortes do que aqueles mediados pela presença de predadores (controle \'top-down\'). No entanto, estes efeitos não são necessariamente intuitivos, uma vez que determinados padrões de fertilização influenciam negativamente o rendimento de elos intermediários da cadeia e, portanto, a transmissão da biomassa para níveis tróficos superiores. / The human demand for food, fibers and biofuels imposes a pronounced growth in intensive agriculture and, with it, in the consumption of fertilizers. Fertilizer management promotes dramatic changes in the quantities and proportions of nutrients in agroecosystems which, in turn, influence biological communities through changes in the biomass, nutritional quality, and composition of producers. A recent trend in the Brazilian productive scenario is the expansion of biofuel crops - notably sugarcane (ethanol) and soybean (biodiesel) - over the cerrados. These crops, covering no less than 30 million hectares in the country, demand contrasting fertilization patterns: sugarcane requires high proportions of nitrogen relative to phosphorus, whereas soybean requires high proportions of phosphorus relative to nitrogen. The objective of this thesis was to test the direct and indirect effects that changes in the quantities and proportions of N and P that follow conversion of native habitats to sugarcane and soybean fields have on biological communities. These objectives were achieved by means of two mesocosm experiments manipulating nutrients, algae, tadpoles and dragonfly naiads. The first experiment followed a full factorial design crossing three concentrations of N (Nreference, Nsoy, Ncane) and three concentrations of P (Preference, Psoy, Pcane), as well as two trophic levels (algae and Physalaemus cuvieri larvae). Nutrient levels simulated conditions measured in water bodies of the cerrado, or patterns of fertilization known for sugarcane or soybean crops. In the second experiment we paired these concentrations (Nreference Preference, Nsoy Psoy, Ncane Pcane) so as to reduce dimensionality and as such to permit inclusion of a third trophic level (algae, Hypsiboas faber larvae, dragonfly naiads). We included an additional treament with caged naiads to separate the effects of actual tadpole predation from those of changes in tadpole traits. The main response variables in both experiments were cholophyll a concentrations as na indicator of phytoplankton biomass, and tadpole mortality, final mass and final developmental stage as indicators of consumer performance. A comparison in the concentration of chlorophyll a before and after the introduction of tadpoles tested the effect of consumers on producer biomass. As expected, measured TN and TP responded strongly to N and P manipulations. In the weeks preceding the introduction of tadpoles, algal biomass increased in response to the concentration of N, but not to the concentration of P; this trend was maintained throughout the experiment. This probably happened because measured TN varied from oligotrophic to hypertrophic conditions; in contrast, measured TP consistenly corresponded to hypertrophic conditions. Tadpole introduction had no evident effects on chlorophyll concentrations, possibly indicating that food was abundant and competition among tadpoles, if present, was weak. Consumer performance was significantly influenced by nutrient manipulations. P-rich treatments led to a reduction in growth, and to a reduction trend in development and survivorship, of Physalaemus cuvieri. Similarly, the scenario of soybean fertilization - therefore P-rich - led to a significant reduction in Hypsiboas faber survivorship. The scenario of sugarcane fertilization led to a 30% increase in mass gain in Hypsiboas faber. We hypothesized that this reduction in tadpole growth and survivorship in P-rich environments was caused by a proliferation of unpalatable or even toxic cyanobacteria, which are favored in low N:P ratio environments. Contrary to our expectations, dragonfly naiads did not cause a direct or indirect reduction in tadpole growth or survivorship, nor influence phytoplankton biomass. We conclude that changes in nutrient levels that accompany the conversion of native habitats to agricultural fields can have strong effects on freshwater food chains (\'bottom-up\' control), and that these effects are much stronger than those mediated by predator presence (\'top-down\' control). However, bottom-up effects are not necessarily intuitive as certain fertilization patterns negatively influence the performance of intermediate consumers and, as such, the transmission of biomass to higher trophic levels.

For years considered pristine, glacial ecosystems are attracting more attention of biologists from various branches in last decades. Moreover, they are currently considered to be ecosystems forming the coldest biome on the Earth. The life on glaciers ranges from viruses and bacteria to highest consumers such as few groups of microinvertebrates. The most common are tardigrades (phylum Tardigrada) and rotifers (phylum Rotifera) which inhabit cryoconite holes on the glacial surface. Several studies highlight the importance of the role of these consumers. However, due to the dominance of prokaryotes on the glacial surface, these microinvertebrates are usually out of the major scope of most studies aiming at biological processes. The present study shows pioneering results on the isotopic composition of tardigrades and rotifers, which are the top consumers in cryoconite ecosystems, and is a foundation for the exploration of trophic pathways and interactions within cryoconite holes using elemental and stable isotopic analyses. It also presents information about the species composition of tardigrades and rotifers on different glaciers and in different parts of the ablation zone. We identified 5 species of tardigrades (Hypsibius sp., Hypsibius cf. dujardini, Pilatobius sp., Isohypsibius sp. and Cryoconicus...

Les cours d’eau de tête de bassin versant, malgré leurs tailles réduites, jouent des rôles importants pour le fonctionnement des milieux situés en aval, et rendent de nombreux services écologiques. Depuis l’émergence du River Continuum Concept, le fonctionnement de ces cours d’eau est considéré comme reposant principalement sur les apports allochtones de matière organique détritique par opposition aux écosystèmes aval de plus grande taille dans lesquels la production primaire autochtone est considérée comme prépondérante. Bien que les détritus soient, par définition, des ressources de moindre qualité pour leurs consommateurs que les végétaux vivants, le rôle des facteurs qui limitent la production biologique et les processus fonctionnels dans ces écosystèmes détritiques reste encore peu exploré. Par exemple la carence en éléments minéraux essentiels (notamment en azote et en phosphore) dans les détritus peut limiter le développement des organismes vivants dans les cours d’eau. De ce fait, la disponibilité en éléments minéraux dans la colonne d’eau, en stimulant la croissance, l’activité des décomposeurs et la qualité élémentaire des ressources par le processus d’immobilisation microbienne, pourrait contrôler pour partie les processus écologiques en jeu dans ces écosystèmes. En étudiant in situ la réponse des communautés microbiennes aux nutriments le long d’un gradient d’occupation des sols, nos travaux ont mis en évidence un effet positif des éléments dissous (N et P) sur la décomposition des litières et les activités enzymatiques des décomposeurs, et ce même pour les niveaux en nutriments les plus élevés du gradient, pour lesquels des effets inverses étaient attendus. Indépendamment, dans des cours d’eau anciennement soumis à de forts dépôts acidifiants, nous avons pu observer une récupération du processus de décomposition des litières. Néanmoins, cette restauration s’accompagnant d’un appauvrissement marqué en N, des perturbations du fonctionnement de ces cours d’eau pourrait se manifester à terme. Le rôle de la production primaire dans les cours d’eau de tête de bassin versant a ensuite été questionné. En effet, bien qu’étant mineur quantitativement, l’apport de carbone de forte qualité pourrait jouer un rôle non négligeable sur le fonctionnement des cours d’eau de tête de bassin versant. En particulier, l’importance et le rôle des apports en acides gras polyinsaturés (AGPI) via les biofilms phototrophes ont été questionnés. En se basant sur un suivi de terrain sur 15 cours d’eau des Vosges le long d’un gradient d’acidification, nos recherches ont tout d’abord pu mettre en évidence la présence de biofilms diatomiques riches en AGPI, malgré le caractère très forestier de ces cours d’eau. Dans un second temps, une expérience en conditions contrôlées a permis d’observer le rôle positif des apports, mêmes faibles, de ressources diatomiques sur la croissance et la survie du crustacé amphipode Gammarus fossarum. Sans cet apport, ces organismes se sont avérés incapables de maintenir leurs niveaux internes d’AGPI, suggérant qu’une consommation de producteurs primaires autochtones était indispensable à ces organismes détritivores. Nos résultats ont ainsi pu mettre en évidence les rôles fondamentaux que peuvent jouer la disponibilité en éléments dissous (N et P) et la qualité du carbone dans les ressources sur la production biologique et sur le processus de décomposition des litières dans les cours d’eau de tête de bassin versant. Ces résultats appellent désormais à investiguer plus profondément les interactions entre ces deux facteurs limitants, et à comprendre dans quels contextes l’un des facteurs pourrait suppléer l’autre. En particulier, il sera intéressant de rechercher comment diverses perturbations anthropiques (xénobiotiques, réchauffement climatique etc…) pourraient affecter l’intensité ou la nature de ces limitations / Despite their reduced sizes, headwater streams play important roles in the functioning of downstream ecosystems and provide numerous ecological services. Since the River Continuum Concept, the functioning of these streams has been considered as mainly relying on allochthonous supplies of detrital organic matter for their functioning, in contrast with downstream ecosystems of larger size where primary production is considered to be predominant. Despite that detritus represent, by definition, a resource of lower quality for its consumers than living plants, the nature of the factors limiting the biological production and functional processes in these detrital ecosystems remains sparsely explored. First, the lack of essential elements (in particular nitrogen and phosphorus) in detritus could be a major constraint for the development of living organisms in streams. As a result, the availability of mineral elements in the water column, by stimulating the growth and activity of decomposers, but also by increasing the elemental quality of resources through microbial immobilization, could control ecological processes in a higher extent than previously believed in these ecosystems. By studying in situ, the response of microorganisms to nutrients availability along a land-use gradient, our results have shown a positive effect of the dissolved elements (N and P) on the decomposition of leaf litters and the enzymatic activities of the decomposers, even for the highest nutrient levels of the gradient, for which adverse effects were expected. Independently, in streams formerly subjected to acidification, we observed a recovery of the process of leaf litter decomposition. Nevertheless, this recovery was accompanied by a marked N reduction in the ecosystem, huge changes in stream functioning might be expected in the medium to long term. The role of autochthonous primary production was then questioned. Indeed, while being quantitatively minor, this contribution of high quality carbon may play a significant role in the functioning of headwater streams. In particular, the importance and role of polyunsaturated fatty acid (PUFA) intakes via phototrophic biofilms was questioned. Based on a field survey of 15 rivers in the Vosges Mountains along an acidification gradient, our results first showed the presence of diatomic biofilms rich in PUFA, despite these streams are clearly in a forested context. In a second step, an experimentation carried out under controlled conditions permitted to observe the positive role of diatomic resources, even in low concentrations, on the growth and survival of the crustacean amphipod Gammarus fossarum. Without this input, these organisms were unable to maintain their internal levels of PUFAs, suggesting that the consumption of autochthonous primary producers might be essential for these detritivore organisms. All our results permit to highlight the fundamental roles that can be played by the availability of dissolved elements (N and P) and the carbon quality of the resources on the biological production and on leaf litter decomposition in headwater streams. These results stress the need for more profound investigation of the interactions between these two limiting factors, and for understanding in which contexts one factor could supplement the other. In particular, it will be interesting to investigate to what extent various anthropogenic disturbances (xenobiotics, global warming, etc...) could affect the intensity or the nature of these limitations