L’industrialisation de l’étang de Berre au cours du 20ème siècle s’est accompagnée d’importants rejets en éléments traces métalliques (ETM) qui ont été en partie accumulés dans les sédiments et sont aujourd’hui susceptibles d’être remobilisés vers la colonne d’eau ou d’être intégrés dans le réseau trophique et d’entrainer un risque écotoxicologique.La reconstitution de l’évolution temporelle et spatiale de la contamination des sédiments montre que les niveaux de contaminations actuels des sédiments de surface sont les plus bas depuis plusieurs décennies en lien avec l’efficacité des réglementations sur les rejets industriels mises en place dans les années 1970. Ces niveaux sont faibles à modérés en surface mais de très fortes contaminations existent quelques centimètres sous la surface des sédiments.Le rôle des oxy-hydroxydes de Fe ou de Mn et des sulfures dans le contrôle de la mobilité des ETM dans le sédiment et leurs flux à l’interface eau/sédiment a pu être démontré grâce à la modélisation du transport et des réactions des composés chimiques et des ETM dans les eaux interstitielles, de leurs profils de concentrations dans la fraction réactive de la phase particulaire et d’expérimentations en conditions contrôlées au laboratoire. L’oxygénation de la colonne d’eau constitue le principal paramètre influençant cette mobilité et ces flux, et l’influence d’une réoxygénation des fonds dans le Grand Etang est discutée.Enfin, la biodisponibilité des ETM et le stress (géno)toxicologique qu’ils peuvent constituer pour un organisme benthique cible, le polychète Nereis succinea, ont été évalués par l’estimation des fractions potentiellement biodisponibles dans les sédiments (extractions chimiques et Diffusive Gradient in Thin-films), par la mesure des concentrations bioaccumulées et par l’utilisation de biomarqueurs de défense (métallothionéines) et de dommages (tests de génotoxicité). Certains ETM qui sont fortement bioaccumulés représentent un risque potentiel et pourraient être impliqués dans la dégradation de la macrofaune benthique. / The industrialization of the Berre lagoon in the 20th century was accompanied by large releases trace metals, which were partially accumulated in sediments and are now likely to be remobilized to the water column or be integrated into the food chain and cause an ecotoxicological risk.The reconstruction of the temporal and spatial trends of sediment contamination shows that current levels of contamination of surface sediments have been the lowest for decades in agreement with the effectiveness of regulations on industrial releases set up in the years 1970. These levels are low to moderate in surface but very high contamination exist a few centimeters below the sediment surface.The role of Fe and Mn oxy-hydroxides and sulfides in controlling the mobility of ETM in the sediment and fluxes at the water/sediment interface has been demonstrated through the modeling of transport and reactions of chemical compounds and trace metals in the pore waters, their concentration profiles in the reactive fraction of the particulate phase and experiments under controlled laboratory conditions. The oxygenation of the water column is the main parameter influencing the mobility and fluxes and the influence of reoxygenation of bottom water column in the Grand Etang is discussed.Finally, the bioavailability of trace metals and adverse effects they may constitute for a target benthic organism, the polychaete Nereis succinea, were evaluated by estimating the potentially bioavailable fraction in sediments (chemical extractions and Diffusive Gradient in Thin-films), by measuring bioaccumulated concentrations and by the use of biomarkers (metallothioneins and genotoxicity assays). Some highly bioaccumulated trace metals pose a potential risk and might be involved in the degradation of the benthic macrofauna.

Les processus biogéochimiques benthiques liés à la dégradation de la matière organique sont étudiés depuis 30 ans. Beaucoup de travaux ont mesuré soit un grand nombre de paramètres sur quelques carottes, soit un nombre restreint de paramètres sur de nombreuses carottes. Dans le Golfe de Gascogne, 29 paramètres diagénétiques sont étudiés à de multiples stations depuis 1997 et dans le Bassin d’Arcachon depuis 2005. Ainsi la banque de données biogéochimiques benthiques constituée est actuellement la plus importante à notre connaissance. Les similarités et divergences entre l’environnement côtier de la lagune mésotidale d’Arcachon et la marge continentale du Golfe de Gascogne ont pu être discutées. Une étude de l’hétérogénéité spatiale à l’échelle de l’échantillonnage a permis de préciser la méthodologie d’échantillonnage dans le Bassin d’Arcachon et de discuter de la représentativité saisonnière des données de la banque de données du Golfe de Gascogne. Les carottes de la banque de données ne peuvent pas être réellement interprétées en terme de variabilité saisonnière, alors qu’une saisonnalité des apports a bien été mise en évidence par l’étude des particules en suspension dans la colonne d’eau. L’oxygène dissous est très sensible à la variabilité de ces apports et l’étude de sa distribution à l’aide de la banque de données du Golfe de Gascogne, ainsi que des teneurs en carbone organique particulaire (COP) montre que les flux de matière organique labile représentent la plus grande partie du carbone exporté au fond et que, pour une zone géographique peu étendue comme le Golfe de Gascogne, l’efficacité d’enfouissement du COP varie de 50% à 10%. La banque de données a également permis l’étude de la géochimie benthique du manganèse, ce qui a donné l’occasion de proposer une méthode de calcul du taux d’accumulation sédimentaire basée sur la géochimie de cet élément à l’état stationnaire. Enfin, des carottes longues réalisées dans les deux environnements ont permis de caractériser les processus diagénétiques anoxiques profonds dans un environnement à l’état stationnaire (le Golfe de Gascogne) et un autre à l’état transitoire (le Bassin d’Arcachon). / Benthic biogeochemical processes due to organic matter degradation have been studied for 30 years. Many works have investigated either multiple parameters on a few cores, or a couple of parameters on numerous cores. In the Bay of Biscay, 29 diagenetic parameters have been investigated at numerous stations since 1997, and in Arcachon Bay since 2005. Thereby, our benthic biogeochemical database is currently the most important to our knowledge. Similarities and differences between the coastal mesotidal lagoon of the Arcachon Bay and the continental margin of the Bay of Biscay have been discussed. Study of sampling-scale spatial heterogeneity has clarified the sampling methodology in the Arcachon Bay and allowed to consider the question of data seasonal signal in the Bay of Biscay database. The cores of the database cannot really be interpreted in terms of seasonal variability, while a seasonal signal has been demonstrated by the study of suspended particles in the water column. Dissolved oxygen is very sensitive to the variability of these inputs. Oxygen distributions and particulate organic carbon (POC) contents from Bay of Biscay database shows that labile organic matter flux represents most of the carbon exported to the bottom. The burial efficiency of POC varies from 50% to 10% for a narrow geographical area like the Bay of Biscay. The database has also allowed the study of benthic geochemistry of manganese, which has provided a method for the determination of mass accumulation rate based on the steady state. Finally, piston cores have been used to characterize the deep anoxic diagenetic processes in a steady state (Bay of Biscay) and a transition state (Arcachon Bay) environment.

Cette thèse participe à accroitre les connaissances sur les facteurs régissant la structures des communautés benthiques de la Grande Vasière (GV) du Golfe de Gascogne. Une attention particulière a été portée au rôle du chalutage de fond, dont l’intensité a été estimée à l’aide des données VMS. A l’échelle de la GV, nos résultats suggèrent que le chalutage de fond serait le principal moteur de structuration du mégabenthos (> 10 mm). Cependant, cette hypothèse n’a pu être formellement démontrée en raison de la co-variation entre l’activité de pêche et certaines caractéristiques environnementales. Par conséquent, les travaux ont été poursuivis sur une zone restreinte de la GV, choisie de manière à minimiser les variations dans l’habitat, tout en conservant un large gradient d’intensité de chalutageNous avons ainsi observé des modifications saisonnières et temporaires de la structure des communautés méga et macro-benthiques (>1 mm) en lien avec le chalutage. Ils proviendraient essentiellement d’une disponibilité alimentaire accrue pour les prédateurs-charognards de la mégafaune et de modifications des caractéristiques sédimentaires pour la macrofaune. En revanche, l’étude de la diversité fonctionnelle montre que ces changements n’ont que des répercussions mineures sur le fonctionnement de l’écosystème. Nos conclusions suggèrent que l’écosystème benthique de la GV a probablement été façonné par plusieurs décennies de pêche intensive et qu’il est actuellement adapté à des perturbations anthropiques chroniques / This PhD thesis aims at increasing the knowledge on factors influencing the benthic community structures of the « Grande Vasière » (GV) of the Bay of Biscay. Particular attention has been paid to the role of bottom trawling whose intensity has been estimated using VMS data. At the scale of the GV, our results suggest that bottom-trawling is the main driver of the megabenthic community structure (> 10mm). However, this hypothesis has not been formally demonstrated due to the co-variations between the trawling intensity and certain environmental characteristics. Hence, works continued on a restricted part of the GV that displayed minimal variations of environmental characteristics while exhibiting a wide gradient of trawling intensityResults suggest that bottom-trawling induces seasonnal and transient modifications on the mega and macrobenthic (> 1mm) communities. These changes could be due to an increase of food availability for the megabenthic predator-scavengers and to changes of sedimentary characteristics for macrofauna. However, investigation about functional diversity showed that these changes did not have major consequences on ecosystem functioning. We concluded that decades of bottom trawling may have shaped the benthic ecosystem of the GV and that it is currently adapted to frequent disruptions.

Cette thèse visait l’étude des micro-environnements géochimiques créés par la présence de l’herbier de Zostera noltei et de son réseau racinaire dans le Bassin d’Arcachon ainsi que ses effets 1) sur l’écologie des foraminifères benthiques, un groupe d’organismes présent en grand nombre dans les sédiments et dont l’influence sur les cycles biogéochimiques reste mal connue et 2) sur les espèces chimiques et les flux benthiques associés. Pour y répondre, une stratégie multidisciplinaire a été mise en place. L’étude écologique des foraminifères benthiques à l’échelle centimétrique, conduite dans des sédiments avec et sans végétation en février et juillet 2011, a montré la présence de trois espèces majeures aux métabolismes spécifiques. L’herbier de Zostera noltei semble influencer la densité et la profondeur de vie des espèces calcaires hétérotrophe (Ammonia tepida) et mixotrophe (Haynesina germanica), présentes près de la surface des sédiments. La troisième espèce (Eggerella scabra), au métabolisme anaérobie encore inconnu, est retrouvée sur au moins 7 cm de sédiment et semble préférer le carbone organique issu de la dégradation de l’herbier. En surface, la contribution des foraminifères à la reminéralisation aérobie a été évaluée à 7 % soit 5 fois plus que lors des études précédentes dans d’autres environnements marins. La présence de l’herbier influence également la géochimie du sédiment où des structures enrichies en fer dissous et appauvries en sulfure sont observables en 2D grâce aux gels DET-DGT développés ici. Le phosphate dissous semble dépendre de la demande des zostères et est observé à forte concentration uniquement sous forme de spots. Une tranche de sédiment, prélevée en vis-à-vis du gel DET-DGT visait la réalisation de cartes de la phase solide et de la densité des foraminifères. Bien que ce travail reste inachevé, des méthodes comme la micro-fluorescence et la micro-tomographie aux rayons X donnent des perspectives prometteuses. De nouvelles voies de recherches ont ainsi été ouvertes grâce à des méthodologies innovantes combinant physiologie, écologie et géochimie. / This thesis aimed to study the geochemical microenvironments created by Zostera noltei meadows and its root system in Arcachon Basin and to evaluate the effect of these microenvironments 1) on living benthic foraminifera ecology, a group of organisms present in large densities in sediment which impact on marine biogeochemical cycles remain poorly known and 2) on the chemical species and associated benthic fluxes. To fulfill these objectives, a multidisciplinary strategy was developed. The study of benthic foraminiferal ecology at centimeter scale, conducted in sediments with or without vegetation in February and July 2011, has shown the presence of three major species with specific metabolisms. Zostera noltei meadows seem to influence the densities and the living depth of heterotrophic (Ammonia tepida) and mixotrophic (Haynesina germanica) calcareous species, present in the sediment surface. The third species (Eggerella Scabra), which anaerobic metabolism is still to discover, is found throughout the sedimentary column and seems to prefer the organic matter coming from the degradation of the seagrass. In surface sediment, the contribution of these three species to aerobic remineralization was estimated at 7%, i.e. 5 times more than the maximum rates previously recorded in marine environments. The presence of the seagrass also influences geochemistry where enriched iron and depleted sulfide structures are observable in 2D thanks to DET-DGT gels developed here. Dissolved phosphorus depended on seagrass uptake and was highly concentrated only as scattered spots. A slice of sediment, taken face to face with the DET-DGT gel, was also conducted during this study aiming to map the solid phase and foraminiferal density. Despite this work is still in progress, methods such as X-ray microfluorescence and microtomography showed promising perspectives. New research pathways have been opened through technological developments and innovative approaches combining physiology, ecology, and geochemistry.

Submitted by Biblioteca de Pós-Graduação em Geoquímica BGQ (bgq@ndc.uff.br) on 2017-10-03T17:34:43Z No. of bitstreams: 1 Dissertação Bruno Allevato M. da SIlva.pdf: 2506067 bytes, checksum: a7fd88a86552d0a0f031f1eb05a0b288 (MD5) / Made available in DSpace on 2017-10-03T17:34:43Z (GMT). No. of bitstreams: 1 Dissertação Bruno Allevato M. da SIlva.pdf: 2506067 bytes, checksum: a7fd88a86552d0a0f031f1eb05a0b288 (MD5) / Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / Universidade Federal Fluminense. Instituto de Química. Programa de Pós-Graduação em Geoquímica, Niterói, RJ / A baía de Tamandaré, PE, Brasil, é um ambiente recifal que apresenta uma condição ambiental de baixa cobertura de corais. Entre as possíveis causas para esta situação, estão a alta pressão que o turismo exerce na cidade de Tamandaré e a entrada de águas continentais, com grande volume de sedimento e contaminada com resíduos agrícolas e domésticos. Estes fatores apresentam uma variação sazonal, de forma que o primeiro se intensifica no período seco (setembro a fevereiro) enquanto o segundo atua mais forte sobre a baía no perído chuvoso (março a agosto). Entretanto, mesmo com estas pressões antrópicas na baía de Tamandaré, suas águas são classificadas como oligotróficas. Objetivando de avaliar o sedimento recifal da baía de Tamandaré, foram estudados composição e tafonomia dos foraminíferos bentônicos presentes no sedimento de nove amostras do período seco e chuvoso. Os resultados apontaram para uma assembléia de foraminíferos sem características de ambientes recifais, com um Índice FORAM abaixo de 4 em todas as estação, com somente uma exceção no período seco, que classificaram a baía como tendo qualidade de água desfavorável para assentamento de corais. Sazonalmente, há uma diferença entre as assembleias dos períodos seco e chuvoso refletida no número total de gêneros encontrados nos dois períodos, sendo maior no período seco que no chuvoso. Entretanto, em análises estatísticas como SHE e CLUSTER, não foram detectadas variações sazonais, sobre tudo nas estações mais profundas. Entre as razões para a baixa variação das assembleias de foraminíferos nos dois períods, a pouca variação da salinidade e temperatura, influênciadas pelo baixo volume de chuva nos meses da coleta, podem ter sido o principal motivo. Foraminíferos com processos tafonomicos como quebrado ou alteração de cor, representaram 40% a 20% do total de testas analisadas, indicando a ocorrência de ressuspenção do sedimento. Três testas com alteração tafonomica da cor e um quarta sem mudança na cor foram analisadas em MEV/EDS e mostrara a ausência ou pouca distribuição de elementos como Fe e S diferente do que seria esperado e a presença de elementos comuns de argilo minerais. De forma geral, os resultados associaram a baía de Tamandaré como ambiente não usal para assentamento de corais, com assembleias de foraminíferos similares entre os períodos analisados, e tendo como principais fatores físico-químicos que influenciam na composição da assembleia a temperatura, salinidade e ressuspenção / Tamandaré bay, PE, Brazil, it’s a reef environment which shows a low coral coverage. Among the possible causes for this situation are the, high pressures which tourism exert in Tamandaré city and the flow of continental waters, with high volume of sediment and contaminated with agricultural and domestic waste. These factors shows seasonal variation, where the first intensify during the dry season (September to February) whereas the second it’s strongest in the bay during the wet season (March to August). However, even with these anthropic pressures in the Tamandaré bay, those waters are classified as oligotrophic. Aiming evaluated the reef sediment of Tamandaré bay, were studied composition and taphonomy of benthic foraminifera present in the sediment of nine samples of dry and wet season. The results point out a foraminiferal assemblage without characteristics of reef environment, where FORAM Index was below 4 in all stations, with just one exception in the dry season, classifying the bay as have water quality unfavorable for coral attach. Seasonality, there’s just one difference between the assemblage of dry and wet seasons reflected in the total number of generous found in the two seasons, where was bigger during the dry season when compared whit dry season. Otherwise, statistical analyses like SHE and CLUSTER, didn’t detected variations between seasons, manly in the deepest stations. Among the reason for the similar foraminifera assemblage between the two seasons, the few variation of salinity and temperature, influenced by the low rain during the month of collection, seems to be the main reason. Taphonomic processes in foraminifera like broke or color change, represented 40% to 20% of the total test analyzed, suggesting sediment ressuspection. Three tests with color change and one fourth without, were analyzed in MED/EDS and shows the absence or few distribution of elements like Fe and S different of what would be expected and the presence of ordinary elements in clay minerals. All in all, the results associated Tamandaré bay as a environment unusual for coral attach, with a similarity foraminifera assemblages between the season analyzed, and had been as main physic-chemistry factors influence in assemblage composition, the temperature, salinity and ressuspection

La compréhension des processus contrôlant le fonctionnement des mangroves est capitale au vu des services socio-économiques/écosystémiques rendus par ces écosystèmes et des menaces qu’ils subissent. En Guyane Française, les mangroves se développent rapidement en réponse aux contraintes sédimentaires récurrentes induites naturellement par les apports sédimentaires amazoniens. Cette thèse étudie la structure et la composition de trois classes de taille de communautés benthique (méso, macro, mégafaune), et les activités de bioturbation associées (remaniement sédimentaire biologique : RS, terriers) dans les jeunes stades de mangrove. 51 taxons de méso-macrofaune et 12 espèces de crabes (mégafaune) ont été identifiés. Bien que la biodiversité benthique soit spécifique à l'âge de la mangrove, des taxons tolèrent les modifications brusques du milieu, et cette persistance maintient des fonctions clés du remaniement sédimentaire tout au long du développement des mangroves. L’intensité du RS varie le long de ce gradient (21-146 g PS.m-2.cycle tidal-1). Si les crabes dominent le RS, cette thèse souligne la contribution effective de la méso-macrofaune aux transports particulaires. Les communautés benthiques montrent des capacités de bioturbation adaptées aux instabilités récurrentes du littoral. Les variations du RS au cours du temps pourraient indiquer l’état fonctionnel des mangroves. Cette étude conclut que l’hétérogénéité morpho-sédimentaire au sein des jeunes stades de mangroves influence la structuration spatiale des crabes, le remaniement sédimentaire qu’ils induisent, et la morphologie des terriers. Les microhabitats devraient être considérés lors de l'évaluation du rôle fonctionnel de la faune benthique des mangroves. / Understanding the processes that control mangroves functioning is essential regarding the socioeconomic/ecosystemic services these ecosystems provide and the increasing threats they suffer. In French Guiana, mangroves grow rapidly in response to natural sedimentary perturbations caused by sediment inputs from the Amazon River. This thesis characterizes the structure and the composition of benthic fauna communities (three size classes: meso-, macro- and megafauna) and the associated bioturbation activities (biological sediment reworking: SR, burrows) in mangrove early growth stages. 51 meso- macrofaunal taxa and 12 crab (megafauna) species were identified.Despite a mangrove age specific biodiversity, some taxa tolerate abrupt environmental changes, and such persistence maintains key sediment reworking functions along mangrove development.The SR intensity varies along this gradient from 21 to 146 g DW.m-2.tidal cycle-1. Burrower crabs dominate the SR but this thesis also outlines the non-negligible contribution of smaller-sized (meso- and macrofauna) organisms to particulate transport. Benthic communities’ bioturbation capacities are adapted to recurrent environmental instabilities. Changes of the biologicallyinduced SR may be used as a proxy of the functional status of mangrove ecosystems. This study concludes that morpho-sedimentary heterogeneity of early-growth mangrove stages influences crab spatial structuration, the crab-induced sediment reworking and burrow shapes. Thus, microhabitat specificities should be considered when evaluating the role of benthic fauna in mangroves ecosystem functioning.

A ciclagem de matéria orgânica (MO) no ambiente marinho é um processo-chave para o ciclo global de carbono. Os sedimentos costeiros são de suma importância para a ciclagem de carbono pois atuam como receptores de grandes quantidades de MO alóctone (i.e. terrestre) e autóctone (i.e. marinho). A miríade dos componentes orgânicos e suas diferentes características dificultam o entendimento das fontes de MO em ambientes costeiros. Este trabalho visou entender a origem e a composição da MO (através de biomarcadores lipídicos) e a dinâmica da comunidade microbiana (método ATP) em sedimentos superficiais de diferentes ecossistemas marinhos da costa sudeste do Brasil: (i) margem continental de Cabo Frio; (ii) sistema lagunar de Saquarema; (iii) áreas costeiras e a plataforma continental de Ubatuba; (iv) e a plataforma adjacente ao estuário de Santos. Os resultados apontaram uma origem predominantemente autóctone para a MO nestes sistemas, com contribuição terrestre reduzida e limitada à áreas próximas à costa. Processos oceanográficos e forçantes ambientais são cruciais para a composição da MO sedimentar e são discutidas para cada um dos ecossistemas estudados. / The cycling of the organic matter (OM) in the marine environment is a key process in the global carbon cycle. Coastal sediments are important to the global carbon cycle, since they receive large inputs from both marine and terrestrial OM. The myriad of organic compounds and their spectrum of reactivity complicate the understanding of OM sources in coastal environments. In this work, we aimed to access the origin and composition of the OM (through lipid biomarkers) and the microbial dynamics (ATP method) in surface sediments of diverse marine ecosystems from the SE Brazilian coast: (i) the continental margin off Cabo Frio; (ii) the lagoonal system of Saquarema; (iii) coastal and shelf areas from Ubatuba; and (iv) the continental shelf adjacent to Santos estuary. The results showed a dominance of autochthonous OM, with a minor fraction of the OM derived from terrestrial sources and restricted to areas close to the coast. Oceanographic processes and environmental forces are crucial to the composition of sedimentary OM and are discussed for each of those ecosystems.

Graduation date: 2006 / Structure-forming invertebrates belong to a polyphyletic group of primarily sessile and sedentary megafauna that can significantly enhance the complexity of physical habitats. A number of these organisms, including cold-water corals and sponges, are known to be slow growing and vulnerable to physical disturbance. In addition, as filter feeders, these invertebrates can indicate areas of consistently favorable conditions for feeding and growth. This study provides the first quantitative analysis of structure-forming invertebrate communities in many areas along the continental margin of Oregon and Washington. Geological surveys during 1992-95, using the occupied submersible, Delta, sampled an extensive area in this region, primarily on and around rock outcrops. The videos from these surveys were analyzed to inventory and catalog sessile structure-forming invertebrates and to document their associations with geological habitat types. Detailed data on geological substrate, invertebrate diversity, abundance, and density were compiled and analyzed. It was found that geological substrate and depth were reliable indicators of suitable habitat for most species included in the study. Gorgonian corals tended to concentrate in high densities in depths between 200-250m, at the southern edges of submerged rocky banks, and where hard rocky substrate was covered with a thick layer of sediment. Because of recent fishery regulation changes, this information can be used as baseline data for future studies on the effectiveness of closed areas on the recovery of structure-forming invertebrates from disturbance, particularly bottom trawling.

Aquatic sediments are, by surface, the largest habitat on Earth. A wide diversity of organisms inhabit these sediments and by their actions they have a large influence on and also mediate many ecosystem processes. Several of these processes, such as decomposition and remineralisation of organic matter are important on a global scale and are essential to sustain life on Earth. The main aim of this thesis was to use an experimental ecosystem ecology approach in order to study some of these ecosystem processes in marine sediments and how they are linked to biodiversity. Paper I and II found that an increased species richness of sediment deposit feeders increases the processing of organic matter from phytoplankton settled on the sea-floor, and that species-rich communities have a more efficient resource utilization of deposited organic matter. The results in paper IV and V also suggest that there is a link between microbial diversity in sediments and the degradation of organic contaminants. Paper V also shows that antibiotic pollution is a potential threat to natural microbial diversity and microbially mediated ecosystem services. The introduction of invasive species to ecosystems is another major threat to biodiversity and was studied in Paper II and III, by investigating the ecology of Marenzelleria arctia, a polychaete worm recently introduced in the Baltic Sea. Paper II suggests that M. arctia mainly utilize food resources not used by native deposit feeders, thus potentially increasing the benthic production in the Baltic Sea by increasing resource use efficiency. Paper III, however, show that M. arctia is protected from predation by the native benthic invertebrate predators, due to its ability to burrow deep in the sediment, suggesting that predation on M. arctia by higher trophic levels is restricted, thereby limiting trophic transfer. In conclusion, this thesis gives some examples of the importance of marine biodiversity for the generation of a few key ecosystem processes, such as organic matter processing and the degradation of harmful contaminants. / At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 2: Manuscript. Paper 3: Manuscript. Paper 4: In press.

The inhabitants of marine benthos must combat the ecological pressure caused by predation, competition and fouling through a series of mechanisms, one of which is chemical defense. This type of protection is particularly extended among sessile and/or sluggish organisms, such as sponges, soft corals or ascidians. The strategies to prevent predation are related to bad taste rather than to toxicity. Moreover, they must be considered along with nutritional quality, since the more nutritious the prey, higher quantities or more potent repellents are needed to gain protection. The production of defensive secondary metabolites is energetically expensive. For this reason, the Optimal Defense Theory (ODT) predicts that defenses must be allocated in the most valuable or more exposed structures or body-regions. In Antarctic communities, the main predators are asteroids, and defensive agents are hence expected to accumulate in superficial layers of potential prey. But dense populations of amphipods, which associate opportunistically with biosubtrata, obtaining both refuge and direct or indirect sources of nutrition, are also very influencing on these bottoms. It has been reported that chemical defenses are very common in Antarctic organisms, in accordance with our results. However, the research effort has not been the same for all the groups, and there are still many aspects to learn on the chemical ecology, like the identification of the implicated products, their mode of functioning or their localization and origin. This PhD has focused on three relevant groups of the Antarctic benthos, quite understudied: hexactinellid sponges, soft corals and colonial ascidians. Two influencing sympatric predators were selected, the sea star Odontaster validus, which is a known model predator, and for the first time, the amphipod Cheirimedon femoratus, used to perform feeding experiments for the detection of repellent chemical defenses. We designed a new protocol which provided numerous methodological profits, as well as a remarkable discriminatory potential for unpalatable activities. In a survey with 31 species of Antarctic organisms, a larger incidence of unpalatable activities was recorded towards C. femoratus than against the asteroid, especially in algae and sponges, in which amphipods may particularly influence defenses distribution for representing potential host-preys. Some organisms instead, seemed to exploit alternative defensive strategies. The ecological success of the three studied groups is probably related to the presence of chemical defenses. In hexactinellid sponges these are weak, yet compensated with a low energetic content, and derive from primary metabolites, such as steroid derivates. Some glucosphingolipids instead, could have a chemotaxonomical value as chemical markers in rossellid sponges. In soft corals, chemical protection is obtained from products originating from both, primary (wax esters) and secondary metabolism (sesquiterpenoids), which seem to cooperate in an additive way, and are likely exuded within the coral mucus in living specimens. The use of primary metabolites for defense represents an effective energy saving strategy. In colonial ascidians, defensive secondary metabolites of terpenoid and alkaloid nature with potent bioactivities predominate, and in some species these are accumulated in internal tissues. Presumably, this distribution is related to the production of chemically defended lavae. Some bioactive secondary metabolites isolated from various sources, like the meridianins, may suggest a broad evolutionary retention, or a symbiotic origin. Regarding bacterial antifouling, ascidians exhibited poor activity, while some soft corals did display inhibition. With this work we provide the identification, localization, and possible origin of several defensive agents in three relevant groups of Antarctic benthic invertebrates. / Los habitantes del bentos antártico combaten la depredación, la competencia y el recubrimiento desarrollando mecanismos como la defensa química. Ésta estrategia está particularmente extendida entre organismos sésiles y organismos de cuerpo blando. Las defensas repelentes contra depredadores han de considerarse junto con la calidad nutricional, pues las dietas muy energéticas enmascaran la repelencia. La producción de metabolitos secundarios defensivos es costosa. Por ello, la Teoría de Defensa Optimizada (ODT) prevé que han de localizarse en las regiones corporales más valiosas o expuestas. En las comunidades antárticas los principales depredadores son las estrellas de mar y se postula la concentración de defensas en áreas superficiales en las presas. Pero también influyen las poblaciones de anfípodos asociados a los biosustratos, obteniendo en ellos refugio y fuente de alimentación. Esta tesis se centra en las defensas químicas de tres grupos relevantes del bentos antártico relativamente poco estudiados: esponjas hexactinélidas, corales blandos y ascidias coloniales. Se seleccionaron dos depredadores simpátricos, la estrella Odontaster validus y, por primera vez, fue utilizado el anfípodo Cheirimedon femoratus. Diseñamos un nuevo protocolo con numerosas ventajas metodológicas además de un gran potencial discriminatorio, y observamos que en 31 especies hubo mayor repelencia hacia el anfípodo que hacia la estrella, sobretodo en algas y esponjas, que podrían representar potenciales huéspedes-presa. A partir de aquí estudiamos muestras de los tres grupos seleccionados. En hexactinélidas, las defensas químicas son más débiles y derivadas del metabolismo primario, pero compensadas con un bajo valor nutricional. Algunos glucoesfingolípidos, podrían tener valor quimiotaxonómico como marcadores de la familia Rossellidae. En corales blandos existen metabolitos de defensa primarios y secundarios operando sinérgicamente, y probablemente forman parte del mucus superficial. En ascidias coloniales, los metabolitos defensivos son secundarios y muy potentes; además, en algunas especies éstos tienden a acumularse en tejidos internos, presumiblemente para producir larvas protegidas químicamente. Las ascidias mostraron poca actividad antibacteriana, pero algunos corales exhibieron respuestas inhibitorias. Esta Tesis proporciona la estructura, distribución y posible origen de los metabolitos responsables de las actividades defensivas en tres grupos relevantes de invertebrados antárticos.