In the last two decades, particular interest has been given to the cycle of dimethylsulfide (DMS), a climate active gas, and its precursors the dimethylsulfoniopropionate (DMSP) and the dimethylsulfoxide (DMSO). DMS is involved in the Earth’s radiation budget while the DMS(P,O) are produced by a wide variety of micro- and macroalgae, corals, bacteria, or angiosperms in response to diverse environmental stresses. Several functions have been suggested for these sulfur compounds such as antioxidants, cryoprotectants, overflow mechanisms, osmolytes, zooplankton deterrents or signalling compounds. This research aims at improving the knowledge about the antioxidant role of DMS(P,O) within three major phytoplankton groups: diatom (i.e. Skeletonema costatum), Prymnesiophyceae (i.e. Phaeocystis globosa) and dinoflagellate (i.e. Heterocapsa triquetra). The experimental results demonstrate that cellular DMS(P,O) have the ability to lower cellular reactive oxygen species concentrations produced during high-light and chemically-induced oxidative stresses; thus supporting the antioxidant function. However, the initial DMS(P,O) concentrations of each species are not informative of their ability to tolerate a further oxidative stress, and their concentrations do not increase in high-light grown cells. The DMS(P,O) may then act as antioxidants without being part of the antioxidant response of the cell. We recommend analysing more constituents of the antioxidant system (i.e. enzymes, carotenoids, redox buffer) along with DMS(P,O) by-products and DMSP-lyase activity to better understand the cellular function of DMSP. Field measurements in the North Sea, including the Belgian Coastal Zone (BCZ) and the Northern North Sea (NNS), bring additional information on the DMS(P,O) variations. While abiotic parameters (nutrients, temperature, and incident light) influence the Chlorophyll-a (Chl-a) concentrations in the BCZ, this impact is not reflected in the DMS(P,O) concentrations. The latter depend on the succession of low- and high-DMSP producing species (i.e. diatoms and Phaeocystis, respectively). In the NNS in August, no distinct pattern can be drawn for the DMS(P,O) evolution regarding the phytoplankton diversity or abiotic parameters. Investigated by correlations between DMS(P,O), photoprotective pigments and incident light, the antioxidant function is not observed for this short-term period of sampling in a temperate sea. Based on Chl-a linear regressions, DMS(P,O) concentrations are successfully estimated with two distinct relationships for diatoms and Prymnesiophyceae in the BCZ. However, this estimation lacks accuracy in the NNS due to the mixed phytoplankton community observed. Further work will provide a better understanding about the antioxidant function – especially on the field – and its association with the phytoplankton diversity in temperate regions such as the North Sea. / Doctorat en Sciences agronomiques et ingénierie biologique / info:eu-repo/semantics/nonPublished

Climate change predictions include increasing precipitation and runoff events that expose phytoplankton communities to colored dissolved organic matter (cDOM) and nutrient pulses of varying intensity and frequency. The consequence of different nutrient/cDOM pulse regimes on phytoplankton communities and the role of legacy effects related to the characteristics of previous exposure regimes remain largely unresolved. To investigate this, we implemented add-on bottle experiments with water collected from a mesocosm experiment conducted in Lake Erken (Sweden) with four nutrient regime treatments with varying intensity and frequency. The nutrient additions of the mesocosm experiment lasted for three weeks and were followed by a two-week recovery period. At the end of both periods, water from three mesocosm treatments was filled into microcosms and either exposed to a second single nutrient pulse or left as control. The microcosms were incubated for 8 days in the lab and changes in phytoplankton biomass and composition were analyzed to determine the legacy effect of antecedent nutrient regimes on the community response to a second nutrient addition. The results showed that past nutrient regimes created legacy effects on the community, dampening further changes in response to a second nutrient addition. However, the effects were transient, since they were not detected in the second microcosm experiment. Further, the lack of legacy effects in the second microcosm experiment suggests that the communities from the mesocosm treatments in fact recovered from the nutrient regimes they were exposed to in the two-week period. Consequently, the results of the thesis suggest that legacy effects may be important in predicting phytoplankton community response to extreme nutrient and cDOM pulses but they might be temporary.

Made available in DSpace on 2016-06-02T19:29:50Z (GMT). No. of bitstreams: 1 TeseNMCN.pdf: 1178647 bytes, checksum: d18af7301e74a6724d6021f399155f9d (MD5) Previous issue date: 2003-06-30 / Financiadora de Estudos e Projetos / The phytoplankton communities of five lakes in the floodplain of Turiaçu River (MA) were analysed for taxonomic composition and evaluated regarding temporal dynamics. Changes on phytoplankton densities were analysed as well as the variations in physical and chemical water characteristics along a hydrological cycle, attempting to correlate them to the flood pulse and the consequent water level fluctuations. Samplings were carried out in lakes Cabeludo, Caboclo, Arrodiador, Bate Terra and Quebra Pote, five periods of representative phases: dry, water rising, flooding, water lowing and a second dry. Qualitative samples were taken by horizontal hauls with a 20 µm phytoplankton net, and quantitative samples were taken at the surface and at 20 cm above the bottom with a Van Dorn bottle. Samples were preserved with in 4% formalin and the phytoplankton was identified and enumerated in settling chambers under a Zeiss inverted microscope. Turbulence, rainfall and water level fluctuations were considered the main controlling factors of the successional dynamics. The lakes are shallow and nutrient rich, being eutrophic during the dry period and mesotrophic in the other hydrological phases. The phytoplankton communities of the five lakes are represented by 139 taxa belonging to 9 Classes: 22 Bacillariophyceae, 34 Chlorophyceae, 1 Cryptophyceae, 33 Cyanophyceae, 2 Dinophyceae, 20 Euglenophyceae, 1 Xanthophyceae and 25 Zignematophyceae. Most taxa are new records for Maranhão State. Cyanophyceae was the numerically dominant group in three periods: water rising (March), water lowing (August) and dry (December), with relative abundances varying from 23.13% in Lake Caboclo during the water raising period to 65.72% of the total, at the same lake, in the dry period. Bacillariophyceae were the most abundant organisms in the flooding period (May), with relative abundances varying from 38,14 % in Lake Bate-Terra to 49,70% in Lake Caboclo. The most abundant species of Cyanophyceae and Bacillariophyceae were Planktolyngbya circumcreta and Aulacoseira granulata, respectively. Total phytoplankton densities varied from 1725 ind.mL-1 in Lake Arrodiador, in the flood period, to 8272 ind.mL-1 in Lake Cabeludo, at the dry period. The highest densities occurred in dry periods, related to the greater nutrient availability and to the concentration factor resulting from the water level decrease. There was a tendency to higher species richness, lower dominance, higher equitativity and higher diversity of phytoplankton during water rising and water lowering. / As comunidades fitoplanctônicas de cinco lagos da planície de inundação do rio Turiaçu (MA) foram analisadas para a composição taxonômica e avaliadas de acordo com sua dinâmica temporal. As variações na densidade do fitoplâncton também foram avaliadas, assim como as mudanças nas características físicas e químicas da água dos lagos, ao longo de um ciclo hidrológico, buscando correlacioná-las com o pulso de inundação e as conseqüentes variações no nível de água dos lagos. As amostragens foram realizadas nos lagos Cabeludo, Caboclo, Arrodiador, Bate Terra e Quebra Pote, em cinco épocas representativas dos períodos: de seca, enchente, cheia, vazante e de um segundo período de seca, tendo sido realizadas coletas qualitativas, por meio de arrastos horizontais, com rede de 20 µm de abertura de malha e coletas quantitativas do fitoplâncton total, na superfície e 20 cm acima do fundo, com Garrafa de Van Dorn. As amostras foram preservadas em formalina 4% e quantificadas em câmaras de sedimentação, sob microscópio invertido Zeiss. A turbulência, a precipitação e as variações no nível d água foram consideradas os principais fatores controladores da dinâmica sucessional. Os lagos são rasos e suas águas ricas em nutrientes, sendo os lagos eutróficos nos períodos de seca e mesotróficos nos outros períodos hidrológicos. As comunidades fitoplanctônicas dos cinco lagos estão representadas por 139 táxons, distribuídos em 9 Classes: 22 Bacillariophyceae, 34 Chlorophyceae, 1 Cryptophyceae, 33 Cyanophyceae, 2 Dinophyceae, 20 Euglenophyceae, 1 Xanthophyceae e 25 Zignematophyceae. A maioria dos táxons são novas ocorrências para o estado do Maranhão. Cyanophyceae foi a classe numericamente mais abundante em três períodos: enchente (março), vazante (agosto) e seca (dezembro), com abundâncias relativas que variaram de 23,13% no lago Caboclo no período de enchente, a 65,72% do total, neste mesmo lago, no período de seca. Bacillariophyceae foi a classe mais abundante no período de cheia (maio), com abundância relativa variando de 38,14 % no lago Bate-Terra a 49,70% no lago Caboclo. As espécies mais abundantes das classes Cyanophyceae e Bacillariophyceae foram Planktolyngbya circumcreta e Aulacoseira granulata, respectivamente. As densidades totais do fitoplâncton variaram de 1.725 ind.mL-1 no lago Arrodiador, no período de cheia, a 8.272 ind.mL-1 no lago Cabeludo, no período de seca. As maiores densidades ocorreram nos períodos de seca, relacionadas à maior disponibilidade de nutrientes e ao fator de concentração, pela redução no nível de água. As comunidades tenderam a uma maior riqueza de espécies, menor dominância, maior uniformidade e maior diversidade nas fases de enchente e vazante.

In light of increasing harmful algal blooms and the need to protect human health and aquatic resources, proactive management approaches merit further study. For this purpose I conducted field samplings to characterize plankton community composition and laboratory experiments to test some approaches to new management schemes in the lower Rincon Delta. On site measurements and microscopic analysis showed that environmental parameters and plankton community composition varied considerably among sampling stations and sampling dates. A recent modeling study suggested that manipulation of freshwater inflow to estuaries might prevent phytoplankton blooms and enhance secondary productivity. To test this theory I conducted three semi-continuous design and flow-through incubation design experiments using natural plankton assemblages. I investigated the effect of two different pulsing regimes of inflow and nutrient loading on zooplankton densities, and phytoplankton biomass and diversity. Despite differences in zooplankton structure and phytoplankton community composition between the two experiment designs, the results confirmed that pulsed inflows might alter plankton dynamics. My findings showed that 3-day pulse treatments consistently supported greater zooplankton densities and higher phytoplankton species diversity when compared to 1-day pulse treatments. In addition, accumulation of phytoplankton biovolume remained low during 3-day pulse treatments. Differences in zooplankton performance between 3-day pulse and 1-day pulse inflow treatments were likely due to the ability of phytoplankton to uptake and store greater amounts of nutrients under conditions of 3-day pulse inflow. This resulted in food of higher quality for zooplankton, and might have supported greater zooplankton population growth rates. Additionally, in an attempt to understand the mechanisms leading to high biodiversity in aquatic ecosystems, I built a resource-storage model and studied the effects of resource-storage on competition of multiple phytoplankton species on multiple abiotic resources. I compared this model with a well-established multi-species competition model. My results showed that for certain species combinations a resource-storage-based model can generate dissimilar outcomes when compared to a model without resource-storage.

In light of increasing harmful algal blooms and the need to protect human health and aquatic resources, proactive management approaches merit further study. For this purpose I conducted field samplings to characterize plankton community composition and laboratory experiments to test some approaches to new management schemes in the lower Rincon Delta. On site measurements and microscopic analysis showed that environmental parameters and plankton community composition varied considerably among sampling stations and sampling dates. A recent modeling study suggested that manipulation of freshwater inflow to estuaries might prevent phytoplankton blooms and enhance secondary productivity. To test this theory I conducted three semi-continuous design and flow-through incubation design experiments using natural plankton assemblages. I investigated the effect of two different pulsing regimes of inflow and nutrient loading on zooplankton densities, and phytoplankton biomass and diversity. Despite differences in zooplankton structure and phytoplankton community composition between the two experiment designs, the results confirmed that pulsed inflows might alter plankton dynamics. My findings showed that 3-day pulse treatments consistently supported greater zooplankton densities and higher phytoplankton species diversity when compared to 1-day pulse treatments. In addition, accumulation of phytoplankton biovolume remained low during 3-day pulse treatments. Differences in zooplankton performance between 3-day pulse and 1-day pulse inflow treatments were likely due to the ability of phytoplankton to uptake and store greater amounts of nutrients under conditions of 3-day pulse inflow. This resulted in food of higher quality for zooplankton, and might have supported greater zooplankton population growth rates. Additionally, in an attempt to understand the mechanisms leading to high biodiversity in aquatic ecosystems, I built a resource-storage model and studied the effects of resource-storage on competition of multiple phytoplankton species on multiple abiotic resources. I compared this model with a well-established multi-species competition model. My results showed that for certain species combinations a resource-storage-based model can generate dissimilar outcomes when compared to a model without resource-storage.