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No. of bitstreams: 1 martins_sca_me_prud.pdf: 3974138 bytes, checksum: 73a1c2c28d4a0cbbde72b9e8a49211ce (MD5) Previous issue date: 2017-08-03 / Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq) / A matéria orgânica dissolvida (DOM) é a componente da água que pode ser usada como indicativo de sua qualidade, pois possui duas fontes: uma alóctone, relacionada com descargas de material terrestre, estando vinculada aos ácidos húmicos, e outra autóctone, associada às descargas fluviais ou produção própria do corpo hídrico estudado, estando relacionada aos ácidos fúlvicos. A matéria orgânica dissolvida colorida (CDOM) é a fração colorida da DOM, que pode ser usada como proxy para a observação desta última em águas interiores. O reservatório hidrelétrico de Funil (FHR) foi o corpo hídrico escolhido como área de estudo deste trabalho. Neste contexto, o objetivo geral desta pesquisa foi identificar e avaliar as mudanças no coeficiente de absorção da CDOM (aCDOM) na superfície da água ao longo do tempo (1995 – 2010), bem como entender a sua relação com mudanças no uso e cobertura da terra (LULC) na bacia de contribuição do FHR. Para alcançar tal objetivo foram realizados: (i) o mapeamento histórico de LULC (1995 – 2010, com 5 anos de intervalo) para detecção de mudanças; (ii) o estudo de um conjunto de modelos bio-ópticos baseados na literatura, bem como de um novo modelo empírico desenvolvido para estimar aCDOM via reflectância simulada (Rrs_simulated) para o sensor Thematic Mapper (TM); (iii) a distribuição espaço-temporal do aCDOM por meio da aplicação de um modelo bio-óptico em imagens TM/Landsat-5 de 1995 a 2010, e (iv) a análise das fontes possíveis de CDOM/DOM , assim como do comportamento/distribuição do aCDOM no FHR ao longo do tempo. Assim, o primeiro estudo desenvolvido nesta pesquisa foi o da parametrização do algoritmo maquinas de vetores de suporte (SVM) de acordo com as características da área de estudo para classificação supervisionada de LULC na bacia de contribuição do FHR. A detecção de mudança da classificação obtida para LULC demonstrou que a parametrização proposta para o SVM tornou o algoritmo capaz de diferenciar classes grandes e contínuas, classes estreitas e alongadas, além de áreas não contínuas e pequenas localizadas dentro de outra classe maior. A classificação obtida para o SVM apresentou boa avaliação estatística, com acurácia geral entre 86% e 96% para toda a série temporal, acurácia do produtor de 90%, acurácia do usuário maior do que 86% e índice Kappa entre 86% e 91%. Ainda, foi observado que o LULC desenvolvido na área de estudo se manteve relativamente estável ao longo da série histórica analisada. O segundo estudo realizado proporcionou o desenvolvimento de um modelo empírico em um comprimento de onda (485 nm) e uma razão de bandas (B4/B1) alternativos para estimativa de aCDOM via Rra_simulated para o TM/Landsat-5 (RMSE = 7%, Nash = 0.91). Este modelo também pôde identificar mesmo pequenas variações nos valores de reflectância via dados orbitais, assim como pode diferenciar variações sutis no aCDOM. Ainda, foram identificados dois padrões de comportamento da CDOM para o FHR: um associado ao LULC e à ocorrência de chuva/lixiviação, bem como outro relacionado à Clorofila-a (Chl-a) em situações de floração de algas. Os referidos estudos que compõe esta pesquisa foram padronizados como artigos científicos para a confecção deste documento. O primeiro estudo, sobre a parametrização do SVM, foi publicado na revista Modelling Earth Systems Environment – Springer (DOI 10.1007/s40808-016-0190-y). O segundo estudo, sobre a distribuição histórica do aCDOM está na etapa de revisão para futura submissão. / The dissolved organic matter (DOM) is a water compound related to water quality, since it has two sources: one allochthonous, related to terrestrial discharges that can be linked to humic acids, and another autochthonous, associated with river input and itself production, so related to or fulvic acids. The colored dissolved organic carbon (CDOM) is the colored fraction of DOM that could be used as a proxy for its occurrence in inland waters. The Funil hydroelectric reservoir (FHR) was chosen as the study site for this work. In this context, the general aim of this research was to identify and to evaluate the changes in CDOM absorption coefficient (aCDOM) at the water surface over time (1995 – 2010), and to understand its relationship with land cover land use (LULC) changes in FHR watershed. For match this goal, (i) a LULC historical mapping (1995 – 2010, with 5 years of interval) was made to change detection, (ii) a bio-optical model set and a new model were studied in order to estimate aCDOM from simulated reflectance (Rrs_simulated) for Thematic Mapper (TM) sensor, (iii) a aCDOM spatial and temporal distribution was obtained by applying a bio-optical model in TM/Landsat-5 imagery from 1995 to 2010, and (iv) the possible CDOM/DOM sources in FHR were found, as well aCDOM historical behavior/distribution over time was analysed. Thus, the first study was the support vector machine algorithm (SVM) parameterization according to study area characteristics to LULC supervised classification in FHR watershed. The obtained LULC change detection analysis demonstrates that the proposed SVM parameterization made the algorithm able to differentiate large and continuous classes, lengthy and thin areas, and non-continuous small areas located inside wide classes. The obtained classification had great statistics with overall accuracy among 86% and 98% over the time series, the producer accuracy of 90%, the user accuracy higher than 86%, and the Kappa statistics ranged from 86% to 91%. In addition, no significant changes in LULC were identified in the study site over all time series. The second study provides a bio-optical model at alternatives wavelength (485 nm) and a band ratio (B4/B1) for aCDOM estimation using simulated Rrs for TM/Landsat-5 (RMSE = 7%, Nash = 0.91). This model could identify even small variations in reflectance values from orbital data, as well as differentiate even slight alterations in aCDOM. Two significantly different aCDOM behaviors were also identified for FHR: one associated with LULC and rainfall/runoff occurrence, and other correlated to Chlorophyll-a high concentrations (Chl-a) in algal blooms situations. The referred studies that compose this research ware standardized as academic articles in this document. The first study, about SVM parameterization, was published yet in Modeling Earth Systems Environment – Springer (DOI 10.1007/s40808-016-0190-y). The second study, about aCDOM historical distribution is in the revision step to future submission.

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The purpose of this study was to examine the direct and indirect effects of ultraviolet-B (UV-B) radiation on freshwater protists. Laboratory experiments were conducted in order to determine the importance of photoenzymatic repair (PER) of UVBinduced DNA damage in the heterotrophic nanoflagellate Paraphysomonas sp. Investigations into the combined effects of UV-B and chromophoric dissolved organic matter (CDOM) were conducted in laboratory experiments in which protist cultures were exposed to UV-B radiation in the presence and absence of water amended to a higher CDOM concentration in order to determine the ability of CDOM to act as a UV-B filter and as a potential nutrient source. Field experiments examined the responses of natural communities of protists and bacteria to ultraviolet radiation (UVR) in the presence of high and low concentrations of CDOM. Ultraviolet-B radiation (UV-B; 280 – 320 nm) negatively affects many aquatic organisms, including heterotrophic flagellates, by directly damaging DNA. The quantity of UV-B reaching the surface of a lake varies with atmospheric chemistry, including stratospheric ozone, and the presence of large holes (Alldredge 1977) in this ozone shield during the last decade resulted in historically high UV-B levels. In aquatic systems, the nature of the damage to organisms depends on the intensity and duration of solar radiation, plus its attenuation in the water column. The amount of UV-B damage is highly dependent on the concentration of CDOM in the water column because CDOM strongly absorbs UV-B radiation. This protective role of CDOM is likely to be reduced in areas where warmer, drier climate decreases watershed runoff, which ultimately results in acidification and increased CDOM photodegradation. However, CDOM also may also act as an organic carbon source for bacteria and stimulate growth of the microbial food web, including bacterivores like heterotrophic flagellates. The effect of UV and CDOM interactions on the microbial food web is not well understood, but climate-related increase in CDOM in an oligotrophic lake could increase the heterotrophic microbial food web impact by reducing UV-B damage and increasing available resources. Since aquatic organisms, including protists, are differentially susceptible to UV-B radiation, climate change effects on CDOM and UV-B attenuation are likely to alter the ecology and community structure of aquatic systems. This thesis describes investigations into the direct and indirect effects of UV-B radiation on freshwater protozoa. The role of PER of direct UV-B induced DNA damage was examined in laboratory experiments that compared the survival and population growth of the heterotrophic nanoflagellate Paraphysomonas sp. at two environmentally relevant temperatures. The results from these experiments demonstrated the reliance of Paraphysomonas sp. on PER, with 100% mortality in the absence of the photorepair radiation that activates photoenzymatic repair enzymes. The ability of Paraphysomonas sp. to recover from exposure to UV-B radiation declined in flagellates adapted to 15°C relative to the same exposures at 20°C. Experiments examining the direct and indirect effects of UV-B radiation and CDOM on freshwater protists conducted in the laboratory and in an oligotrophic lake in the Pocono Mountains showed that potential DNA damage resulting from UV exposure is reduced and microbial growth may be enhanced with an increase in CDOM concentration. / Biology

There is large natural variation in light and nutrient conditions across lakes. In the boreal-subarctic region most lakes are small, shallow and nutrient poor. In such lakes there is often sufficient light to support primary production at the lake bottom. An expectation for the future is that colored dissolved organic matter (cDOM) of terrestrial origin will increase in these lakes. cDOM depresses the underwater light climate but is often associated with elevated pelagic nutrient concentrations. A dynamical model of a coupled benthic-pelagic food web was explored for how lake ecosystems might respond to altered light and nutrient regimes. The model predicts that mobile carnivores (fish) control grazers and release primary producers from grazing pressure. Primary producers are therefore limited by their resources and cross-habitat interactions are dominated by spatially asymmetric competition for light and nutrients. At high light and low nutrient supply benthic algae out-compete pelagic algae for nutrients diffusing from the sediment, whereas pelagic algae shade out benthic algae at lower light and/or higher nutrient supply. Biomass patterns of benthic and pelagic consumers follow the patterns of primary production. In contrast, habitat coupling through carnivore movement has only a weak impact on biomass patterns in the model food web. Model predictions were compared with data from boreal-subarctic lakes covering a broad range of cDOM concentrations. In agreement with model expectations the following relationships with increasing light attenuation were observed: benthic primary and secondary production decreased, pelagic primary production showed a unimodal trend, and pelagic nutrient concentrations as well as the proportion of fish feeding in the pelagic habitat increased. As a consequence, both primary and fish production were negatively related to pelagic nutrient concentrations across lakes. In a comparative study of boreal-subarctic lakes covering a broad range of cDOM concentrations, a similar negative relationship was found between pelagic total nutrient concentrations and the biomass of epilithic algae. This was surprising, because epilithon cannot access nutrients from the sediment. Patterns in epilithon biomass were largely driven by nitrogen fixing cyanobacteria, which in turn were positively related to light supply. The data suggest that nitrogen fixing autotrophs may have a competitive advantage over other epilithic primary producers in low-cDOM, low-nutrient, high-light environments, and that patterns in epilithic biomass, nutrient sequestration and elemental stoichiometry depend upon which functional group is dominant in the epilithic biofilm.

Coastal water quality is intricately linked to environmental factors, particularly meteorological and hydrological conditions. Weather conditions determine wind speed and direction, which in turn influence coastal currents, and these currents play a critical role in the transport and variability of water quality parameters. Understanding how these dynamic interactions impact water quality is essential for the effective management and preservation of coastal ecosystems. This dissertation investigates the effects of varying flow conditions on key water quality parameters in the Louisiana Continental Shelf, utilizing data from two research cruises. The study specifically focuses on how concentration variability and transport mechanisms of water quality indicators respond to different environmental conditions, including the passage of cold fronts and periods of non-frontal conditions. In this research, data were collected using a conductivity-temperature-depth (CTD) system and through the analysis of water samples to assess changes in nutrient concentrations, dissolved oxygen levels, chlorophyll-a, and Colored Dissolved Organic Matter (CDOM). These samples were collected under two distinct environmental conditions: following a cold front and during non-frontal conditions. Analytical methods were used to quantify the water quality parameters, while hydrodynamic modeling was applied to assess the transport and dispersion patterns of these indicators. The results show that cold fronts increase nutrient concentrations and decrease dissolved oxygen levels due to enhanced mixing and upwelling. The cold front activity also promotes southwestward transport of water quality parameters, driven by the region's hydrodynamic forces. Under stable weather conditions, nutrient and chlorophyll-a concentrations increase, but transport is limited, with reduced dispersion resulting from weaker hydrodynamic forces. CDOM component analysis revealed that cold front conditions lower CDOM concentrations and improve light penetration, while non-frontal conditions elevate CDOM concentrations, reducing light penetration and impacting aquatic ecosystem productivity. These findings enhance our understanding of how meteorological events, hydrological conditions, and water quality interact in coastal regions. The research demonstrates how weather conditions distinctly influence nutrient dynamics and CDOM behavior. These insights are crucial for developing effective water quality management strategies for the Louisiana Continental Shelf and similar coastal regions worldwide, supporting global efforts to mitigate challenges posed by climate change and human activity in coastal ecosystems.

2023 / The photobleaching of chromophoric dissolved organic matter (CDOM) is considered an important loss process for CDOM absorption in sunlit natural waters, where it can regulate the biota’s exposure to sunlight, surface solar heating, and dissolved organic matter (DOC) dynamics. Understanding the global significance of CDOM photobleaching is essential for assessing its impact on the light field of the surface ocean, related biogeochemical processes, and climate dynamics. Despite its importance, this sink remains poorly quantified, primarily due to the difficulty of: 1. determining photobleaching apparent quantum yields (AQY) that capture the dual spectral dependency of this process and are applicable to polychromatic sunlight; 2. constraining the variability of AQY across the land-ocean aquatic continuum under diverse environmental conditions. This dissertation seeks to quantify the global role of photobleaching as a sink of CDOM.Chapter 1 presents a simple method to determine CDOM photobleaching AQY matrix (AQY-M) for natural water samples that does not require any a-priori assumptions about the spectral dependency of photobleaching. It combines controlled irradiation experiments, a partial-least-square regression (PLSR) and an optimization procedure to produce AQY matrices that are spectrally coherent and optimized for modeling accurate photobleaching rates in natural waters. Water temperature and the solar-exposure history of CDOM had a major influence on the magnitude and spectral characteristics of the AQY-M. These factors should be considered when determining the AQY-M of samples, and provide constraints when modeling photobleaching rates in natural waters. Chapter 2 builds a model to constrain the natural variability of CDOM photobleaching AQY-M in natural waters. This model was developed using AQY-Ms determined for a set of water samples (n=27) collected from surface waters along the land-ocean aquatic continuum (range of salinity: 0-35.79 PSU). The analysis investigated the dependencies of the measured AQY-M on CDOM composition, water temperature, and exposure duration. The results demonstrated a strong relationship between the magnitude and spectral characteristics of the AQY-M and the CDOM composition and solar exposure duration. Terrigenous CDOM exhibited a higher overall magnitude of AQY-M compared to oceanic CDOM. Additionally, water temperature was found to influence the extent of photobleaching. The model used principle components (PCs) to re-present the spectral characteristics in the AQY-M, and used a series of linear regressions to describe PCs’ dependencies on these variables. The AQY-M prediction model perform well in the independent cross-validation with training samples (n=27) and in the validation with extra samples (n=12), showing promise for global applicability in estimating photobleaching AQY-M. Chapter 3 expands the investigation to a global scale, aiming to develop a comprehensive understanding of CDOM photobleaching significant, seasonal dynamics and sensitivities to climate change in the surface ocean. The chapter combines remote sensing and modeling approaches to create a global climatology of CDOM photobleaching rates. The chapter explores the seasonal variability (high in the summer and low in the winter) of photobleaching rates, calculates the annual global sink (~1.5% to 6.5%) of CDOM due to photobleaching, and examines the sensitivity (increase ~2% to 6% due to sea surface temperature increase in this century) of this process to climate change-induced variations in ocean physical properties. The annual CDOM photobleaching turnover rate in the mixed layer (~1 to 6 times) varied seasonally and, with the highest rates observed near the equator (~25 times at 350 nm) in the open ocean. Overall, this dissertation contributes to the field of CDOM photobleaching research by providing insights into the quantification of photobleaching rates, the natural variability of AQY-M in natural waters, and the global dynamics of CDOM photobleaching in the surface ocean. The methodologies and findings presented in these chapters enhance our understanding of CDOM photobleaching processes, their dependencies, and their implications for aquatic ecosystems and climate dynamics. / 2026-02-06T00:00:00Z

The Belgian Coastal Zone (BCZ) in the Southern Bight of the North Sea is a highly dynamic and optically complex area. This is due to high non-algal particles (NAP) and coloured dissolved organic matter (CDOM) content which in spring adds together with undesirable blooms of the haptophyte Phaeocystis globosa. There is a need for improving the algorithms for chlorophyll a (chl a) retrieval in these highly turbid waters and for developing algorithms for species detection in order to attempt to create an early warning bloom system. This information will contribute to the knowledge of the extent and magnitude of the P. globosa bloom in Belgian waters. In this study, pure cultures of the main taxa present in the BCZ, diatoms and P. globosa, were combined with field measurements of light absorption of total particles, phytoplankton and dissolved material, pigment determination and phytoplankton counts to address the main objectives. Sampling was performed during 8 cruises covering winter, spring, summer and late summer, and along nearshore-offshore gradients from 2003 to 2006. The area is characterised by a high spatio-temporal variability of IOPs due to the high dynamics of the area in terms of currents, salinity gradients and biological production. During spring the presence of P. globosa modulates the IOPs across all the area, the particle absorption is significantly higher than summer and there is no significant coast-offshore variability for phytoplankton and CDOM. The design of chl a retrieval algorithms assumes negligible absorption of NAP and CDOM in the near infrared (NIR) and the use of a fixed value of specific phytoplankton absorption. It is shown that neglecting the NAP and CDOM absorption in the NIR will have a significant overestimation impact in retrieval of chl a. On the other hand, the specific phytoplankton absorption was found to be highly variable (0.015 „b 0.011 m2 mg chl a-1). Both results will affect directly the retrieved chl a. The spatial variability of CDOM was significant varying between 0.20-1.31 m-1 in the marine area and between 1.81-4.29 m-1 in the Scheldt estuary. CDOM was found to be related to salinity with conservative mixing within the Scheldt estuary and during some seasons in the BCZ, however deviations from conservative mixing suggest other inputs to the CDOM pool. Analyses of the spectral slope of the CDOM absorption curve revealed two main CDOM pools in the area, an allochthonous one delivered by the Scheldt estuary and an autochthonous one associated with the phytoplankton spring bloom decomposition. Algorithms for CDOM retrieval will be affected if the variability in the relation between CDOM and salinity is not taken into account. The optical characterisation of diatoms and P. globosa from the BCZ in pure cultures revealed that the main differences in the phytoplankton absorption spectra were found at 467 and 500 nm corresponding to the absorption of the pigments chlorophyll c3 (chl c3) characteristic of P. globosa and fucoxanthin, respectively. Accordingly, both the absorption at 467 nm and the ratio 500/467 nm were successfully used to discriminate the two taxa in cultures and field samples. This latter indicator was not preserved in the reflectance signal due to degradation of the signal when passing from absorption to reflectance, and thus could not be used for algorithm development. The spectral feature at 467 nm was later used as the basis for the development of a flag-type algorithm to detect chl c3 using either absorption or water-leaving reflectance data. Also, the correlation between the algorithm¡¦s retrieved chl c3 and P. globosa cell number allowed the quantification of the bloom. The main findings of this thesis highlight the importance of the IOPs characterisation for the improvement and development of ocean colour retrieval algorithms in these highly complex waters.

Data from five research cruises performed between 1997 and 2001 were processed in order to investigate the potential for improving remote sensing algorithms in the Estuary and Gulf of St. Lawrence. Measured in situ parameters included concentration-dependent indicators of the three critical, optically-active constituents, chlorophyll, Coloured Dissolved Organic Matter (CDOM) and Suspended Particulate Matter (SPM). The radiometric dataset used to investigate different types of algorithms consisted of multi-band above-surface remote sensing reflectance (R[subscript rs]) estimates. These estimates were computed from downwelling surface irradiance and upwelling sub-surface radiance measurements acquired using a SeaWiFS Profiler Multichannel Radiometer (SPMR). The chlorophyll data varied from approximately 0.1 to 17.3 mg.m[superscript -3] in the study region which extended from stations near the Saguenay River to the outer extremes of the Gulf of St. Lawrence. The CDOM and SPM concentration indicators were lower in the Gulf compared to the Estuary. Moderate correlation between in situ measurements was found between chlorophyll and SPM, as well as between CDOM and SPM. Chlorophyll and CDOM were virtually uncorrelated. The standard SeaWiFS Case-I chlorophyll retrieval algorithm, OC4v4, was applied to SPMR data acquired over a significant number of sampling stations (N=169). Algorithm shortcomings were noted when the OC4v4 algorithm was applied directly to the study region. Specific shortcomings, the overestimation of low, and underestimation of high chlorophyll concentrations were consistent with previous findings in coastal regions and particularly with previous findings in the NW Atlantic and in high latitude regions. In addition, the algorithmic output was found to be fairly strongly correlated with CDOM and SPM. A perturbation approach, based on the analysis of residuals between OC4v4 estimates and in situ data, showed that the retrieved chlorophyll biases (overestimates) were dependent on SPM and CDOM (especially at low in situ chlorophyll concentrations). An analysis of the spectral parameters (band ratios and spectral slopes) with respect to in situ constituent concentrations showed that both band ratios and band slopes have a greater dependency on CDOM and/or SPM than on chlorophyll. This observation was supported by radiative transfer calculations which showed that the variability of the blue-to-green band ratios due to changes in CDOM and SPM concentrations could be greater than the variability due to changes in chlorophyll concentration. These findings showed that there was no adequate, single band-ratio algorithm for the remote sensing of chlorophyll in our study region. Systematic testing of a large combination of spectral parameters within the context of specific algorithmic formulations resulted in seven prescribed algorithms which provided slight to moderate improvement in the correlation coefficients and root mean square errors relative to in situ chlorophyll and significant decorrelation relative to CDOM and SPM parameters. In general, algorithms based on multiple spectral parameters were more accurate predictors of in situ chlorophyll. In addition to new algorithms, a set of previous algorithms developed by Jacques (2000) for a subregion of the Estuary were validated in the present study. This validation demonstrated a rather remarkable robustness of correlations between in situ and spectral parameters across time and for different types of instruments and measuring conditions. A relatively smaller number of matching SeaWiFS pixels (N=39) and in situ measurements were used to evaluate the performance of the SPMR-derived algorithms. The accuracy of all algorithms deteriorated when applied to satellite data (one possible reason being the shortcomings of the atmospheric correction algorithm, as underscored by the existence of negative values in the reflectance data). Nonetheless, the improvement of the two selected algorithmic formulations relative to the OC4v4 algorithm showed a certain robustness in the face of environmental influences such as atmospheric effects and sensor response variations. Model simulations showed significant shortcomings of the new algorithms in specific turbidity conditions. The selected algorithms were shown to achieve chlorophyll retrievals which were as good as or better than OC4v4 retrievals. Even though the APD<35% accuracy target of the SeaWiFS project could not be reached, new algorithms succeeded to decrease the APD of the remote estimations from 226% to 65% for SPMR data, and from 502% to 95% for SeaWiFS data. In general, our findings showed that the selected algorithmic formulations had the potential for improving chlorophyll retrieval in the St. Lawrence Estuary and Gulf."--Résumé abrégé par UMI.

La matière organique dissoute chromophorique (CDOM) est une fraction significative du pool global de matière organique dissoute (MOD) dans les océans. La CDOM absorbe une partie de la lumière dans le domaine du rayonnement ultraviolet (UV-R) et du visible. Une fraction de cette CDOM peut émettre une fluorescence lorsqu'elle est excitée par un UV-R. Cette fraction est alors appelée matière organique dissoute fluorescente (FDOM). La CDOM a donc d'une part, un effet positif, en protégeant les cellules contre les dommages causés par les UV-R, mais d'autre part, un effet négatif en réduisant la quantité de radiation disponible pour la photosynthèse. Les propriétés optiques de la CDOM, particulièrement sensibles aux processus physiques (abiotiques) et biologiques (biotiques), renseignent sur l'intensité des processus biogéochimiques en milieux aquatiques. Des suivis de la dynamique de la CDOM en zones côtière et hauturière en Méditerranée Nord occidentale ont permis de décrire différentes tendances temporelles claires, qui vont d’une faible à une forte saisonnalité et qui peuvent être découplées des variations du pool global de MOD caractérisé par les concentrations en carbone organique dissous (COD). Dans les zones tempérées, les événements météorologiques conduisent à des changements beaucoup plus brusques dans la frange littorale que dans l’océan, où les variations tendent à être plus progressives au cours de l'année. En outre, l'apport de nutriments et de polluants dans les zones côtières est fortement influencé par l'activité anthropogénique et ces entrées ne suivent pas nécessairement de tendances saisonnières nettes. Dans la présente étude qui effectue un premier bilan de la distribution et du devenir de la CDOM/FDOM aux stations d'observation à long terme du laboratoire Arago (stations côtière SOLA et hauturière MOLA) à partir respectivement d'un suivi hebdomadaire et mensuel, nous nous sommes attachés à extraire un signal cohérent et une variabilité claire des sources des différentes fractions de la MOD entre février 2013 et avril 2014 ceci, de manière à mieux comprendre les rôles respectifs des facteurs biotiques et abiotiques. Nos observations ont ensuite pu être replacées dans un contexte synoptique d'évolution climatologique des écosystèmes méditerranéens. / Chromophoric dissolved organic matter (CDOM) is a major fraction of dissolved organic matter (DOM). CDOM absorbs light over a broad range of ultraviolet (UV-R) and visible wavelengths. A small fraction of CDOM can emit fluorescence when excited by ultraviolet radiation; so called fluorescent dissolved organic matter (FDOM). CDOM plays a key role in regulating light penetration into the ocean, absorbing high-energy electromagnetic spectrum (visible and ultraviolet light) waves. On one hand, it protects aquatic organisms of potential photo-damage; in the other hand it induces a negative effect by reducing light for photosynthesis. The optical properties of the CDOM are sensitive to biological (biotic) and physical (abiotic) processes and for this reason the colored matter can provide valuable information about the biogeochemical processes in aquatic environments. CDOM monitoring in Mediterranean coastal areas has shown different temporal trends, which go from weak to strong seasonality. Interestedly, these temporal trends were uncoupled with those of the total dissolved organic carbon. In temperate areas, episodic meteorological events can induce much more abrupt changes in the littoral than in the open sea, where changes tend to be more gradual along the year. In addition, the input of nutrients and pollutants in coastal areas is strongly influenced by the anthropogenic activity on land, and those inputs do not necessarily follow seasonal trends. In the present study, weekly and monthly samplings were performed to investigate the temporal variability in SOLA and MOLA stations, respectively. The fluctuation of different fractions of dissolved organic matter (DOM) was evaluated from February 2013 to April 2014 and referred to long time-frame databases of SOLA and MOLA stations. Inorganic nutrients and chlorophyll shown the classical seasonal patterns, with a winter period characterized by an enrichment of surface waters favoring the spring bloom, followed by a calm period that allows the summer stratification and the depletion of nutrients in the photic zone. The stratification extended until autumn winds and low temperatures eroded the thermocline. In contrast, colored DOM fractions did not follow a clear temporal trend. Interestingly, dissolved organic carbon (DOC) exhibited the highest variability in summer, when the rest of parameters showed minimum variations. To explain this mismatch we proposed a sequence of abiotic and biotic phenomena driving the DOC dynamics. In the suggested conceptual frame, DOC dynamics depended strongly on episodic meteorological events (winds, rains, etc.) along the year, except in summer, where the biological factors were more relevant. In order to better understand the influence of biological factors, we examined the temporal trends of phytoplankton composition in relation to those of the different colored DOM fractions. We found that both phytoplankton and CDOM were strongly influenced by abiotic factors such as the intrusions of fresh waters, the vertical mixing due to convection and the light exposure. However we did not find a correlation between any of the CDOM fractions and any of phytoplankton groups. In addition, we use the dust deposition database of ADEPT project (ICM-CSIC, Barcelona) to investigate the potential role of atmospheric deposition in the CDOM temporal variability, and also performed two dust addition experiments with natural plankton communities collected in the Catalan coast.

No description available.