La prise de conscience des risques liés à la dégradation de la qualité des milieux lagunaires vis-à-vis de l’eutrophisation a abouti à la mise en œuvre d’actions de gestion visant à réduire les apports en nutriments. Certaines lagunes sont aujourd’hui en phase de restauration, avec une amélioration notable de la qualité de l’eau liée notamment à la réduction des biomasses phytoplanctoniques. En revanche, la réponse des macrophytes benthiques semble plus complexe et non linéaire. A travers une approche complémentaire basée sur l’analyse statistique des données d’observations acquises de 1998 à 2015 et sur des mesures in situ, cette thèse vise à identifier la dynamique temporelle et spatiale des macrophytes ainsi que leur rôle fonctionnel au cours du processus d'oligotrophisation. Au sein des lagunes méditerranéennes, la salinité et les teneurs en nutriments dans la colonne d'eau sont les principaux paramètres qui structurent la distribution des macrophytes benthiques. L’analyse statistique menée sur 21 lagunes polyhalines et euhalines a permis d'illustrer les premières étapes de restauration. La réduction des apports externes a conduit à un basculement rapide (3-4 ans) d’un système totalement dominé par le phytoplancton vers un système dominé par les macroalgues opportunistes et notamment les ulves. La réduction de la disponibilité en azote dans la colonne d'eau liée au développement des ulves favoriserait le développement des gracilaires (algue rouge) et des chaetomorphes (algue verte filamenteuse). Bien plus que la seule conséquence de la restauration, la présence d'espèces clés jouerait ainsi un rôle central dans la restauration en régulant les flux d'azote et de phosphore au sein du système et en créant les conditions favorables à l'apparition d'autres espèces plus pérennes. Bien que notre théorie décrive une tendance vers le retour des macrophytes pérennes pour les trajectoires de re-oligotrophisation, nous n’avons pas encore pu documenter ce retour dans les lagunes après une longue période d’absence. Ceci, montre l’importance de prendre en compte, entre autres, l’importance des sources internes d’éléments nutritifs et de la connectivité pour la recolonisation des angiospermes dans les lagunes. / The growing awareness of the negative impacts of the degradation of the water quality in coastal lagoons because of eutrophication has resulted in public action aiming at the reduction of the nutrient loadings into these lagoons. Hence, some of these coastal lagoons are currently undergoing ecological restoration and have shown a clear improvement of their water quality linked to decreasing phytoplankton biomass in the last 10 years. However, the response of the benthic macrophytes appears more complex, particularly by showing non-linear behaviour. By using complementary approaches including statistical analyses of long-term observations (1998-2015) and in situ measurements, this thesis aims to describe the dynamics of the macrophytes in space and time and to study their functional role during the oligotrophication process. Salinity, depth and nutrient concentrations in the water column represent the main factors that statistically explain the distribution of the benthic macrophyte communities in Mediterranean coastal lagoons. The first steps of the ecological restoration in eutrophied polyhaline and euhaline lagoons has been inferred from the statistical analysis of time series in 21 coastal lagoons. The reduction of the external nutrient loading results in a rapid regime shift (3-4 years) from a phytoplankton-dominated system to a macroalgae-dominated system, with particularly Ulva species. The decreasing inorganic nitrogen concentrations due to uptake by Ulva spp. in the water column appears to facilitate the development of Gracillaria spp. (red algae) and Chaetomorpha spp. (green filamentous algae). While the appearance of these macroalgal species is the result of the ecological restoration, these species also play a central role in this process by regulating the biogeochemical element fluxes. This way these species create more favourable conditions for perennial species. Although for re-oligotrophication trajectories in coastal lagoons our theory describes a tendency towards the return of perennial species, so far, we have not been able to document their return after a long period of absence. This shows, that among other factors, the internal nutrient loading and ecological connectivity are important factors to take into account for the recolonization of angiosperms in coastal lagoons.

Understanding and explaining spatio-temporal dynamics of ecological communities and their interaction with the environment is a central goal in ecology, but at the same time a very challenging task. Functional traits are a promising concept to achieve a better mechanistic understanding of variations in community structure. Traits are characteristics of organisms that directly or indirectly influence fitness and vary largely between different species. As traits functionally link communities with their bio-geochemical environment, they are well suited for explaining how community structure changes in response to changing environmental conditions, and how in turn communities shape their surrounding environment. Despite the potential of trait-based approaches to mechanistically explain the relationship between communities and their environment, they have rarely been applied in freshwater ecology. Therefore, this thesis explores spatio-temporal dynamics in functional traits of freshwater phytoplankton communities and their interaction with the environment, using the Rappbode Reservoir in Germany as a case study. Phytoplankton community data from a seasonally resolved dataset with 50 years of observation from the Rappbode Reservoir were used to translate taxonomic composition into ecologically meaningful functional trait values. The studied functional traits showed consistent, reoccurring seasonal developments that clearly mirrored environmental pressures over the year. From late autumn to spring nutrients and turbulence were high. Hence small celled, fast growing species that are able to rapidly incorporate existing nutrients and tolerate poor light conditions dominated (dominant traits: maximum growth rate, light affinity). In contrast, when turbulence and nutrients were low in summer, large cell size and more complex mechanisms to efficiently exploit mineral nutrients or acquire previously unexploited nutrient pools were key ecological strategies (dominant traits: phosphate affinity, mixotrophy, motility, nitrogen fixation). The similarity in observed trait patterns over several years indicates that despite the diversity and complexity of phytoplankton species dynamics, the seasonal succession is a highly ordered, predictable process, driven by trade-offs between different ecological strategies. During the 50 years of observation, nutrient concentrations in the Rappbode Reservoir decreased strongly (oligotrophication). A comparison between nutrient-rich and nutrient-deficient years revealed that the general intra-annual succession patterns of phytoplankton functional traits remained the same. However, the intra-annual succession patterns were more pronounced in nutrient-rich years. While functional community composition in summer changed little after oligotrophication, spring communities were largely affected by nutrient reductions and functional traits in spring became more similar to the ones in summer. Based on the quantitative analysis of functional traits, the thesis could generalize the existing patterns to provide a quantitative, functional template for seasonal succession patterns in lake ecosystems under nutrient-rich and nutrient-deficient conditions. Over the entire observation period, phytoplankton biomass did not respond to reduced phosphorus concentrations. This unexpected resistance of algal biomass against reductions in the limiting resource phosphorus was caused by changes in internal processes and functional traits of the phytoplankton community, allowing them to adapt to lower nutrient levels without a loss in total biomass. The main casual mechanism for the decoupling of inorganic nutrients from phytoplankton biomasses was an increase in phagotrophic mixotrophs and their ability to make bacterial phosphorus available for the entire phytoplankton community. Further, in nutrient-deficient years biomass losses in spring were compensated by high biomasses in summer. Higher summer biomasses compared to spring biomasses in nutrient-deficient years were probably also linked to a decrease in diatoms in spring. Diatoms are characterized by high sinking velocities. This trait in combination with phosphorus uptake by diatom cells leads to high downward nutrient exports from the pelagic zone. The decrease of diatoms in nutrient-poor years was related to less phosphorus losses over the season, leaving more nutrients for summer phytoplankton. The effect of species with high sinking velocities on seasonal phosphorus processing illustrates the importance of functional trait composition on biogeochemical cycling. A one-year monitoring campaign with a high temporal and depth resolution at Rappbode Reservoir allowed to investigate the impact of vertical phytoplankton trait distributions on the geochemical environment. During summer, a phytoplankton mass development occurred at the depth of the metalimnion, mainly represented by the cyanobacterium Planktothrix rubescens. The positive net growth at larger depth of this metalimnetic species is enabled through the possession of specific traits, e.g. buoyancy regulation and the ability for efficient light harvesting. The data indicated that the occurrence of metalimnetic species was connected to the formation of a metalimnetic oxygen minimum. The oxygen minimum occurred after the metalimnetic algae peak disappeared from the thermocline. Metalimnetic phytoplankton induced oxygen depletion probably through one of the following processes: Oxygen consumption by bacteria related to the degradation of dead organic material from metalimnetic algae or internal respiration of metalimnetic algae towards the end of the growing season leading to net oxygen losses due to the continuation of algal respiration. This constitutes a previously undiscovered mechanism for the development of metalimnetic oxygen minima. In conclusion, the thesis illustrates how trait-based approaches enhance ecological understanding of phytoplankton community dynamics. The trait-based approaches shed light on how phytoplankton communities respond to environmental gradients and how communities can affect their geochemical environment. The thesis also shows that functional traits can be used to reduce the complexity of communities through converting species information into ecologically meaningful functions and that they allow to link changes in community composition to corresponding features in the biogeochemical environment. The quantitative, trait-based approaches used in this study therefore improve our mechanistic understanding of community dynamics and are a step forward to higher predictability and generality in limnology.

Submitted by (edna.saturno@ufrpe.br) on 2017-02-08T11:32:54Z No. of bitstreams: 1 Antony Evangelista de Lima.pdf: 927439 bytes, checksum: 127455269dc34eb694da13c5b5c35cfd (MD5) / Made available in DSpace on 2017-02-08T11:32:54Z (GMT). No. of bitstreams: 1 Antony Evangelista de Lima.pdf: 927439 bytes, checksum: 127455269dc34eb694da13c5b5c35cfd (MD5) Previous issue date: 2012-06-29 / Conselho Nacional de Pesquisa e Desenvolvimento Científico e Tecnológico - CNPq / The present study evaluates the influence of the river flow on the trophic state of the reservoir cascade formed by the dams on the medium, submedium and low São Francisco River. Surface water sampling was conducted quarterly between December 2007 and September 2008, along the reservoirs of Sobradinho, Itaparica, Moxotó, PA-I,II,III, PA-IV and Xingó. The hydrological variables analyzed were inflow and outflow, and water transparency (DS), chlorophyl-a (CLO), total phosphorous (FT) and reactive soluble phosphorous (FSR) concentrations used for the estimation of the Carlson trophic state index modified by Toledo-Júnior (IETM), with posterior estimation of the average trophic state index. A Kruskal-Wallis test followed by the post-hoc Dunn´s test (p<0.05) was used to detect spatial (reservoirs) and temporal (months) differences between the variables analyzed. Principal component analysis (PCA) was used for each sampling month data in order to evaluate the variation of reservoir trophic state in response to changes in flow and nutrient concentration. Sobradinho differed from the other reservoirs for all variables and indexes analyzed, showing higher inflow in December 2007 and March 2008, and was regarded as eutrophic and mesotrophic throughout the study period. Itaparica, Moxotó, PA-I,II,III, PA-IV and Xingó reservoirs presented a relative uniformity, with higher inflow and outflow in December 2007 and September 2008, and a trophic state varying between mesotrophic and oligotrophic. A trophic state reduction along the reservoir cascade was detected, as evidenced by DS and CLO. However, the effect of reservoir water oligotrophication was attenuated by lateral contributions from the Moxotó and Pajeú tributary rivers, and effluents from the cities located along the system, particularly in the Paulo Afonso region, as evidenced by IETM(FT) and IETM (FSR). PCA exhibited a temporal change in the system trophic state, with a higher IETM similarity in March 2008. The PCA separated the Sobradinho and Xingó reservoirs, while Itaparica and the Paulo Afonso Complex reservoirs presented an intermediate trophic state, as demonstrated by the average trophic state index. / O presente trabalho teve como finalidade avaliar a influência da vazão sobre o estado trófico da água na cascata de reservatórios do médio, submédio e baixo rio São Francisco. Foram realizadas coletas trimestrais da água na superfície, entre dezembro/2007 e setembro/2008, nos reservatórios de Sobradinho, Itaparica, Moxotó, PA-I,II,III, PA-IV e Xingó. As variáveis utilizadas no estudo foram vazão afluente e defluente, além da transparência da água (DS) e as concentrações de clorofila-a (CLO), fósforo total (FT) e fósforo solúvel reativo (FSR), empregadas para o cálculo do índice de estado trófico de Carlson modificado por Toledo-Júnior (IETM), com posterior cálculo do índice de estado trófico médio. Foi realizada uma análise de variância de Kruskal-Wallis complementada pelo teste post-hoc de Dunn (p<0,05) para detectar diferenças espaciais (reservatórios) e temporais (meses para cada reservatório) entre as variáveis analisadas. Análises de componentes principais (ACP) foram realizadas individualmente para os meses de coleta, para avaliar a variação do estado trófico dos reservatórios em função da mudança das vazões e da concentração de nutrientes. O reservatório de Sobradinho mostrou-se diferentes dos demais para todas as variáveis e índices estudados, com maiores vazões afluentes nos meses de dezembro/2007 e março/2008, sendo classificado como eutrófico e mesotrófico ao longo do estudo. Os reservatórios de Itaparica, Moxotó, PA-I,II,III, PA-IV e Xingó mostraram uma relativa uniformidade de funcionamento, com maiores entradas e saídas de água em dezembro/2007 e setembro/2008, e estado trófico variando entre oligotrófico e mesotrófico. Houve uma variação no estado trófico ao longo do sistema, fato evidenciado por DS e CLO. O efeito da oligotrofização da água dos reservatórios ao longo da cascata, entretanto, foi atenuado pelo aporte lateral de nutrientes pelos rios tributários Motoxó e Pajeú e pelos efluentes das cidades no entorno do sistema, principalmente na região de Paulo Afonso, como evidenciado pelos IETM (FT) e IETM (FSR). As ACP exibiram a mudança no estado trófico do sistema ao longo do tempo, tendo a maior semelhança nos IETM entre os reservatórios sido registrada em março/2008. De modo geral, as ACP separaram os reservatórios de Sobradinho de Xingó, com Itaparica e o Complexo de Paulo Afonso apresentando um estado trófico intermediário entre os primeiros, conforme demonstrado pelo índice de estado trófico médio.