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Oil spill is a major source of pollution in Kuwait marine environment and oil dispersants are used as a method to combat oil spill but the adverse effects of either oil or dispersed oil is unknown to fish species local to Kuwait. Therefore, the toxicity of water-accommodated fraction (WAF) of Kuwait crude oil (KCO) and chemically enhanced water-accommodated fraction (CE-WAF) of KCO with three dispersants (Corexit® 9500, Corexit® 9527 and Slickgone® NS) were investigated against selected marine fish species local to Kuwait marine waters such as: sobaity-sea bream (Sparidentex hasta), hamoor-orange-spotted grouper (Ephinephelus coicoides), meidmullet (Liza Klunzingeri), and shea’am-yellow-fin sea bream (Acanthopagrus latus). Prior to exposure chemical characterization of KCO WAF and CE-WAFs was conducted for benzene, toluene, ethylbenzene and xylene (BTEX), polycyclic aromatic hydrocarbons (PAH), aliphatic and total petroleum hydrocarbons (TPH) compounds. Standardization experiments regarding oil loading and mixing duration revealed that 1 g KCO loading and 24 h mixing duration were the most appropriate experimental conditions to obtain a reproducible and stable WAF and CE-WAF solutions. In general, CE-WAF contained higher concentrations of TPH, PAHs and aliphatics compared to KCO WAF. Exposure to KCO WAF and CE-WAF had no adverse effects on hatching success of embryonated eggs of sea bream and orange-spotted grouper exposed but larvae hatched during exposure exhibited a toxic response. Considering larval sensitivity, pre-hatched larvae of four marine fish species were separately exposed to KCO WAF and their sensitivities from the most sensitive to the least sensitive were: sea bream>orangespotted grouper > yellow-fin sea bream > mullet pre-hatched larval stages. The sensitivities of pre-hatched larvae of sea bream and orange-spotted grouper to WAF and CE-WAF were of different degrees. For sea bream the LC50 values were around 0.120 g oil/L for both WAF and CE-WAF indicating that dispersant didn’t increase oil toxicity, whereas for orange-spotted grouper CE-WAF (LC50 0.010 g oil/L) was more toxic than WAF alone (LC50 0.93 g/L). The data obtained in this study showed that most resistant developmental stage of fish to the toxicity of WAF and CE-WAFs was the egg stage > ABSTRACT ©KARAM v larvae hatched during exposure > pre-hatched larvae. Exposure of pre-hatched larvae to KCO WAF induced developmental abnormalities in spinal curvature of larvae and the most prominent deformity types were lordosis, scoliosis and kyphosis compared to that of control larvae were no abnormalities were observed. Relating toxicity data obtained in the present experimental study to actual petroleum hydrocarbon concentrations in Kuwait marine area, it was observed that current contamination level with petroleum hydrocarbons is far less than the LC50 determined in this study suggesting that there isn’t any acute hazard to either fish egg hatching or larva survival.

The key problem faced by environmental scientists is to predictand recognize the damaging effects of chemical pollutants on natural biota. The aim of this thesis was to evaluate the potential for proteomics in ecotoxicology and environmental risk assessment (ERA), with the hypothesis that proteomic technologies (i.e. ProteinChip technology in combination with SELDI TOF MS) could be a useftil supplement to existing methods of environmental assessment, by providing a sensitive, non-invasive, rapid multi-endpoint assessment of effects of anthropogenic chemicals on organism in vivo. Three invertebrate species, Mytilus edulis, Hyas araneus and Strongylocentrotus droebachiensis was exposed to natural and anthropogenic chemicals in laboratory and field studies. Results revealed that proteomics was a sensitive endpoint, as all exposure regimes significantly affected protein expression. It was shown that plasma protein expression profiles contained information that was compound, dose, site, species and gender-specific. Regarding the latter; male and female organisms responded differently to all exposures both quantitatively (e.g. in terms of number of affected protein species) and qualitatively (e.g. in terms of tj^e of affected protein species). Furthermore, genders have shown opposite responses following the same exposure regime. Equally, species-specific responses were observed. Moreover, exposing organisms to graded levels of contamination under controlled laboratory conditions and in the field revealed that different subsets of proteomes were affected at different levels of exposure. This finding represents an opportunity for appljdng proteomics for both prognostic (e.g. early warning of potential adverse effects or assessment of recovery) and diagnostic purposes. Moreover, those protein features that were changed by all exposure concentrations showed complex dose-response relationships, including both linear and various types of biphasic response-curves. In summary, results from the present study indicate that proteomics have the potential to be a useftil tool in ERA. For example, identification of key molecules could elucidate mechanism of action related to mixture effects, gender and species-specific susceptibility to environmental pollutants as well as dose-response relationships at low doses. Furthermore, key proteins (i.e. putative biomarkers) could, be purified and coupled to e.g. a biosensor for automated monitoring.

This study aims to evaluate and advance our ability to predict the effects of disturbance on communities using statistical models. This is important because climate change is likely to alter the patterns of disturbance experienced by ecosystems. In the first chapter, I review the evidence for effects of climate change on the disturbance regimes experienced by marine ecosystems, particularly coral reefs. I also outline theories about how disturbance affects communities, and give background information on two field sites: Heron Island in the Great Barrier Reef, and Salthouse Dock in Liverpool. Chapter 2 is a comparison of four models for coral community dynamics at Heron Island; a discrete-time Markov model and a nonlinear continuous-time model, each with and without the effects of cyclones. The models are fitted using Bayesian methods. The addition of cyclones improved the fit of the model, with cyclones causing some coral mortality, although recovery was rapid. Recruitment appeared less important than the growth and mortality of existing corals. In Chapter 3, I use the best model from Chapter 2 to examine how different the dynamics at Heron Island for the years 1962-1992 might have been under a range of cyclone frequency scenarios. Under current or slightly increased cyclone frequency, coral cover could be either high or low, depending on the timing of cyclones. With increasing cyclone frequency, there was a transition from the outcome being contingent on chance events to being almost certain about the outcome. In Chapter 4, we take the two models with cyclone effects from Chapter 2, and run them forwards in time until the year 2008. We then compare the predictions with newly collected field data using Bayesian methods. The continuous-time model predicted significantly less coral than was observed in 2008. Conversely, the discrete-time model predicted significantly more coral than was observed. Overall, the predictions made by the continuous-time model are closer to the observations. Chapter 5 is a two-year experimental study on the effects of disturbance on marine sessile communities. A two-year study was carried out at the Salthouse Dock site to determine whether a disturbance regime would cause persistent changes in community composition. Recovery was rapid. A simple mathematical model suggests that recovery is almost inevitable in systems of this kind, especially for species with planktonic larval stages. Comparing the Heron Island and Salthouse Dock results highlights the importance of external recruitment in recovery from disturbance. I discuss the advantages and disadvantages of alternative modelling strategies, and argue that simple models based on extensive data may be useful.

This study encompasses twenty-seven SIDS and the Caribbean Sea. In order to detennine the pressure on the sea from anthropogenic activities both terrestrial and marine, four major components were investigated. These are (a) how land use activities on the islands are affecting the Sea (b) the effects of coastal and marine based activities on this marine area (c) the influences of natural events on the Caribbean Sea and the SIDS (d) how the region is responding to minimise the pressures on the sea via policies and programmes. In the first component, the DPSIR in combination with GIS was applied to three islands to demonstrate the causal links between economic activities and its effects on the Caribbean Sea. The activities on these islands have resulted in loss of reef covers, reduction and loss of commercial fish species and reduction in water quality. The second component was investigated by using spatial analysis to compose a vulnerability assessment of the Caribbean Sea. This was derived from mapping anthropogenic activities and habitats within the sea. The assessment demonstrated varying levels of vulnerability throughout the sea. This finding reinforces the need to manage the sea as a large marine ecosystem. The third component demonstrated that events such as hurricanes, tsunamis and effects of climate change are affecting the quality of the ecosystems in the Caribbean Sea and increasing the vulnerability of island communities. Data analysed for a 44 year period show that the highest number of successive hurricanes that made landfall in the Caribbean SIDS was in the 1990s. The fourth component was an analysis of the existing legal and institutional mechanisms that are being used in the region to respond to the issues in the marine environment. The analysis revealed that most of the current responses are within geo- political borders which have been less effective in dealing with the issues.

The use of antifouling booster biocides on boats and ships is of environmental concern as the leached products can affect non-target, highly susceptible phytoplanktonic organisms. Appropriate assessment of whether or not estuarine/marine phytoplankton are potentially at risk should include a combination of approaches, investigating toxic effects for both single species and natural phytoplankton communities and ideally include field verifications. This research addresses the application and comparison of methods to investigate toxic responses to four antifouling biocides Irgarol 1051®, diuron, Sea-Nine 211® and zinc pyrithione in laboratory-based experiments with single species and using natural phytoplankton assemblages. The applicability of photosynthetic parameters and pigment:chlorophyll-α ratios as biomarkers was initially employed in unialgal experiments. Variations in results for pigment:chlorophyll-a ratios following toxic exposures indicate that care must be taken when using CHEMTAX estimations and that further research into this topic is needed. Compositional changes in natural phytoplankton assemblages under toxicant exposure were evaluated using standard techniques (microscopy) and were compared to group-specific biomass estimation derived from CHEMTAX-High Performance Liquid Chromatography. This broad approach proved effective in detecting changes in the main phytoplankton groups, with prasinophytes and prymnesiophytes proving most susceptible and chlorophytes and dinoflagellates being comparatively resistant to the PSII inhibitor Irgarol 1051®. In addition, the impact of selected antifouling agents on photosynthesis and pigment chemotaxonomy was investigated for a natural phytoplankton community. Pigment signatures were determined by HPLC and growth was determined by Analytical Flow Cytometry (AFC). Primary production (estimated by 14C-uptake) was compared to photosynthetic efficiency (FV/FM) measured using Fast Repetition Rate Fluorescence (FRRF). Differences in species-specific sensitivity of the phytoplankton community were detected through pigment composition after 72 h exposures to zinc pyrithione (5 µg 1ˉ¹) and Sea-Nine 211® (10 µg lˉ¹) . The pigment zeaxanthin was proportionally increased indicating a relative increase in Cyanophyceae. This effect was corroborated by AFC. Both techniques (14C-uptake and FRRF) were in good agreement (r = 0.88) suggesting the impairment of primary production and FV/FM following exposure to the selected toxicants. Phytoplankton responses to toxicants may be influenced by fluctuations in a variety of environmental factors. The nutrient regime may alter the outcome of contamination by influencing the susceptibility of phytoplankton to a toxicant. A preliminary investigation of such effects on biocidal toxicity was conducted under controlled laboratory conditions to determine the extent of nutrient influence. Results indicated the influence of nutrient addition on the Irgarol 1051® toxic responses to cyanophytes. Besides experimental approaches, data were supplemented with a pilot survey in Plymouth coastal waters. Current concentrations of biocides together with environmental and biological data for this survey are presented and discussed as a diagnosis that provides an outline for future research.

Synthetic microplastics (≤5-mm fragments) are anthropogenic contaminants that are rapidly accumulating in coastal environments worldwide. The distribution, abundance, ecological impacts and fate of these pollutants are poorly understood. In this study, a novel reflectance micro-FT-IR spectroscopy method was developed to detect microplastics in sediments from 17 sites (UK), using polyethylene (PE) as the model polymer. Additionally, a 14-day laboratory microcosm experiment was used to characterise bacterial succession on low-density polyethylene (LDPE) fragments over time and across three sediment types (ranging from sand to silt) from the Humber Estuary, UK. In contrast with ATR-FT-IR measurements, micro-FT-IR measurements in reflectance mode were susceptible to refractive error when analysing irregularly shaped PE fragments. However, molecular mapping by reflectance micro-FT-IR spectroscopy successfully detected PE within polymer-spiked sediments and in a non-spiked sediment retentate. Moreover, reflectance micro-FT-IR spectra of PE were consistent across all 17 sampling sites. Bacteria were found to rapidly colonise LDPE fragments, with bacterial community structure and diversity differing significantly from those in bulk sediments, as demonstrated by scanning electron microscopy, T-RFLP analysis and 16S rRNA gene sequencing. The composition of LDPE-colonising assemblages within different sediment types increasingly converged over time, with 16S rRNA gene sequencing analysis identifying site-specific populations of the genera Arcobacter (Epsilonproteobacteria) and Colwellia (Gammaproteobacteria) as dominant members (up to 93% of sequences) of the plastic-associated communities after 14 days of exposure. Log-fold increases in the relative abundance of LDPE-associated bacteria occurred within 7 days of exposure with bacterial abundance differing significantly across sediment types, as shown by Q-PCR amplification of 16S rRNA genes. Attachment of bacterial cells and specifically of Colwellia spp. onto LDPE surfaces was demonstrated by CARD-FISH analysis. These results provide a foundation to both developing improved spectroscopy methods to detect microplastics, and characterising ecological interactions between microorganisms and microplastic debris within marine sediments.

An accurate assessment of population genetic subdivision is crucial in making informed decisions for the management and conservation of marine resources. However, obtaining a precise assessment is particularly challenging since population subdivision results from the interaction of many biological and environmental variables. Here, the patterns of population subdivision that can be attributable to gene flow in 8 marine fish species from 11 different locations in the Mediterranean Sea and the Eastern Atlantic Ocean are analysed. The species considered are Mullus barbatus, Mullus surmuletus, Tripterygion delaisi, Apogon imberbis, Symphodus tinca, Diplodus vulgaris, Serranus cabrilla and Oblada melanura. These species differ significantly in their dispersal capabilities as presumed by their diverse early-life stage characteristics, and were sampled over multiple habitats at two geographical scales. A total of 1640 individuals (mean 168/species) were surveyed at 63 species-specific and 10 cross-amplified microsatellite loci (mean 9.1/species). The overall results show that genetic structure patterns cannot be predicted confidently by the presumed dispersal capabilities of the species. Species such as S.tinca, M.barbatus and O.melanura, showed highly incongruent population structuring patterns to those expected. Furthermore, the results also suggest that species with extreme variations between their life history traits such as D.vulgaris and A.imberbis, can display similar patterns of genetic structuring in the presence of common oceanographic variables, whereas species with slight differences such as M.barbatus and M.surmuletus can display contrasting patterns of genetic partitioning. These results highlight the importance understanding the role of environmental features and life history traits in establishing ecologically meaningful connections among marine fish species. Furthermore, this study provides evidence of the usefulness of multi-species approaches in planning to incorporate population genetic information into conservation and management of marine fish resources.

Depuis plusieurs décennies la production mondiale de plastiques ne cesse d’augmenter, menant à une contamination des écosystèmes aquatiques à l’échelle de la planète qui a été récemment estimée à plus de cinq mille milliards de débris de plastiques flottants à la surface des océans. Les microplastiques (particules de plastique < 5 mm), introduits dans l’environnement aquatique directement en tant que microparticules (granulés plastiques industriels, cosmétiques, fibres textiles) ou lors de la fragmentation de plus gros débris plastiques, représentent une préoccupation scientifique et sociétale grandissante. Ces travaux de thèse se sont focalisés sur la contamination par les microplastiques de la rade de Brest (Bretagne, France), un système côtier macrotidal où l’activité anthropique est importante. Les objectifs de ces travaux sont (1) d’évaluer la contamination des matrices environnementales (eau de surface, sédiment subtidal et biote) par les microplastiques, et (2) d’identifier leur rôle potentiel en tant que vecteurs de contaminants chimiques et de bactéries dans la rade de Brest.Pour cela, des développements analytiques ont été nécessaires afin d’améliorer leur extraction des matrices environnementales ainsi que leur caractérisation morphologique et chimique via la microspectrométrie Raman. Les observations in situ ont montré que l’ensemble de l’écosystème de la rade de Brest est contaminé par les microplastiques, avec des concentrations de 0,24 ± 0,35, et 0,97 ± 2,08 (moyenne ± écart-type). L’eau de surface et le sédiment sont contaminés par le polyéthylène, le polypropylène et le polystyrène. Leur distribution à la surface de l’eau semble être liée à l’urbanisation très présente au nord de la rade, mais également à l’hydrodynamisme de cette région marine. Les premiers résultats concernant les bivalves marins ont montré un niveau relativement faible de contamination par les microplastiques (0,01 ± 0,04 et 0,08 ± 0,34 pour les moules et les coques, respectivement), cependant cela est probablement dû aux choix méthodologiques appliqués ici (exclusion des fibres). Les observations in situ ont montré que certains polluants organiques (HAP, PCB et pesticides) étaient détectés sur les microplastiques flottants à des valeurs (non détecté – 49763 ng.g-1, moyenne ± écart-type) similaires de celles retrouvées dans les sédiments et les bivalves locaux, ce qui suggère un risque faible dans le transfert des contaminants chimiques vers les organismes marins en cas d’ingestion. Enfin, les résultats concernant la colonisation bactérienne des microplastiques flottants ont montré des communautés distinctes de celles retrouvées dans l’eau de mer environnante, et l’identification du genre Vibrio en tant que biomarqueur discriminant de cette matrice. Dans l’ensemble, ces travaux fournissent une première évaluation approfondie de l’état de contamination de la rade de Brest par les microplastiques ainsi que de solides recommandations méthodologiques pour des travaux futurs. / World production of plastics has increased steadily for the past decades leading to a major contamination of the worldwide aquatic ecosystems recently estimated at more than five trillion plastic pieces floating the surface of the oceans. Microplastics (plastic particles < 5 mm) are introduced into aquatic environments directly as industrial raw material (plastic pellets, cosmetics, clothing) or indirectly via the fragmentation of larger plastics. This emerging contaminant represents an increasing ecological concern for science and society. The present study focused on the microplastic contamination of the Bay of Brest (Brittany, France), a macrotidal coastal ecosystem characterized by intense anthropogenic activity. The main objectives were: (1) to evaluate the contamination of environmental matrices (surface water, subtidal sediment and biota) by microplastics, and (2) to identify their potential role as vector of chemicals and bacteria in the bay of Brest.Methodological developments were first conducted to improve microplastic extraction from environmental matrices as well as their rapid morphological and chemical identification by Raman micro-spectrometry. The field investigations showed that the ecosystem of the bay of Brest is contaminated by microplastics with mean concentrations of 0.24 ± 0.35, and 0.97 ± 2.08 (mean ± standard deviation) in surface water and sediment, respectively. Microplastic contamination in surface water and sediment was dominated by polyethylene, polypropylene and polystyrene microparticles.Spatial microplastic distribution appeared to be related to proximity to urbanized areas and to hydrodynamic in the bay. Preliminarily results of microplastic contamination in marine bivalves demonstrated relatively low contamination (0.01 ± 0.04, and 0.08 ± 0.34 for mussels and cockles, respectively) by microplastics (mainly polyethylene and polypropylene fragments), however this could be partly related to the methodological limitation identified here (e.g. exclusion of fibers). Organic pollutant (PAH, PCB and pesticides) were detected on floating microplastics at levels (not detected – 49,763 ng g-1, mean ± SD) similar to those measured in sediment and bivalves suggesting low risks in transferring hazardous chemicals in local marine organisms upon microplastic ingestion. Finally, distinct bacterial community assemblages were demonstrated on microplastics as compared with surrounding surface water; the Vibrio genus was identified as a discriminant biomarker of the plastic matrix. Overall, this work provides a first and thorough assessment of the microplastic contamination in the bay of Brest and solid methodological recommendations for further work.

Les plages de sable sont des écosystèmes dynamiques couvrant 70% des côtes mondiales. Ces systèmes abritent un cortège spécifique unique et assurent des fonctions essentielles de nourricerie, de nurserie et d’épuration des eaux. Or à proximité des côtes, la population humaine est en développement constant ce qui accroît les multiples pressions d’origine anthropique sur les écosystèmes côtiers. L’apport en excès de nutriments constitue une menace majeure qui peut se traduire par le développement d’importantes biomasses de macroalgues opportunistes (eutrophisation). La fréquence et l’intensité de ces blooms, communément formés de chlorophycées à court cycle de vie (ulves) et appelés marées vertes (MV), s'amplifient sur les côtes françaises et dans le Monde menaçant le fonctionnement de systèmes prépondérants et uniques.La plupart des études visant à déterminer les effets des MV sur la structure et le fonctionnement d’écosystèmes sédimentaires ont été conduites dans des environnements abrités, micro- ou atidaux. Cette problématique est restée presqu’inexplorée dans des écosystèmes plus dynamiques (systèmes ouverts et macrotidaux) en raison des difficultés de mise en place d’échantillonnage et de détection des effets de stress d’origine anthropique, inhérentes à la variabilité de ces systèmes. Cette thèse a donc eu pour objectif principal de combler ce manque et produire des connaissances en étudiant les réponses in situ des communautés benthiques de plages de sable fin macrotidales en présence ou non de MV. Quatre études ont été menées à différentes échelles spatio-temporelles et en considérant différents compartiments biologiques pour répondre à cette question générale.Ce travail de thèse montre qu’à l’échelle de la région Bretagne (variabilité intégrée sur 2700km de côtes et 7 ans) les communautés benthiques d’écosystèmes dynamiques sont modifiées significativement et de manière conservative par la présence de MV. Ce travail démontre aussi que les marées vertes impactent différemment la faune benthique en fonction du type d’habitat (plages semi-exposées vs. exposées), de la profondeur (mediolittoral vs. infralittoral), et du compartiment biologique (macrofaune benthique vs. juvéniles de poissons plats). Ces comparaisons ont permis d’identifier la faune benthique de médiolittoral inférieur des plages exposées comme étant le système le plus affecté par les MV. L’étude des variations à fine échelle spatio-temporelle de ce dernier montre que les caractéristiques faunistiques (uni- et multi-variées) sont modifiées le long d’un gradient de couverture d’algues vertes. Par exemple, la β-diversité décroît significativement le long de ce gradient. Afin d’explorer les processus pouvant expliquer ces modifications, et déterminer si ces changements ont des répercussions sur le fonctionnement de l’écosystème « plage de sable », les effets de l’accumulation d’ulves sur le réseau trophique à différent(e)s niveaux/échelles ont été mesurés. Les résultats de cette étude montrent qu’une importante biomasse d’ulves induit un changement de la structure entière du réseau trophique et une modification importante du fonctionnement trophique des plages. Les expérimentations menées au cours de cette thèse montrent que les changements observés sont induits par des effets directs (consommation de débris d’ulves) et indirects (modifications d’autres sources de nourriture) de la présence des MV.Cette thèse propose un cadre de travail visant à mieux détecter les effets de stress anthropiques sur la structure et le fonctionnement d’écosystèmes dynamiques. Dans un contexte de changement global forçant les écosystèmes à faire face à de multiples stress, cette approche pourrait se révéler particulièrement utile pour démêler, comprendre et prédire les effets de perturbations induites par les activités humaines sur le fonctionnement des écosystèmes et constituer une aide à la gestion de ces environnements particuliers. / Highly dynamic systems, often considered as resilient systems, are characterised by abiotic and biotic processes under continuous and strong changes in space and time. Because of this variability, the detection of overlapping anthropogenic stress is challenging. Coastal areas harbour dynamic ecosystems in the for of open sandy beaches, which cover the vast majority of the world’s ice-free coastline. These ecosystems are currently threatened by increasing human-induced pressure, among which mass-development of opportunistic macroalgae (mainly composed of Chlorophyta, so called green tides), resulting from the eutrophication of coastal waters. The ecological impact of opportunistic macroalgal blooms (green tides, and blooms formed by other opportunistic taxa), has long been evaluated within sheltered and non-tidal ecosystems. Little is known, however, on how more dynamic ecosystems, such as open macrotidal sandy beaches, respond to such stress. This thesis assesses the effects of anthropogenic stress on the structure and the functioning of highly dynamic ecosystems using sandy beaches impacted by green tides as a study case. The thesis is based on four field studies, which analyse natural sandy sediment benthic community dynamics over several temporal (from month to multi-year) and spatial (from local to regional) scales. In this thesis, I report long-lasting responses of sandy beach benthic invertebrate communities to green tides, across thousands of kilometres and over seven years; and highlight more pronounced responses of zoobenthos living in exposed sandy beaches compared to semi-exposed sands. Within exposed sandy sediments, and across a vertical scale (from inshore to nearshore sandy habitats), I also demonstrate that the effects of the presence of algal mats on intertidal benthic invertebrate communities is more pronounced than that on subtidal benthic invertebrate assemblages, but also than on flatfish communities. Focussing on small-scale variations in the most affected faunal group (i.e. benthic invertebrates living at low shore), this thesis reveals a decrease in overall beta-diversity along a eutrophication-gradient manifested in the form of green tides, as well as the increasing importance of biological variables in explaining ecological variability of sandy beach macrobenthic assemblages along the same gradient. To illustrate the processes associated with the structural shifts observed where green tides occurred, I investigated the effects of high biomasses of opportunistic macroalgae (Ulva spp.) on the trophic structure and functioning of sandy beaches. This work reveals a progressive simplification of sandy beach food web structure and a modification of energy pathways over time, through direct and indirect effects of Ulva mats on several trophic levels. Through this thesis I demonstrate that highly dynamic systems respond differently (e.g. shift in δ13C, not in δ15N) and more subtly (e.g. no mass-mortality in benthos was found) to anthropogenic stress compared to what has been previously shown within more sheltered and non-tidal systems. Obtaining these results would not have been possible without the approach used through this work; I thus present a framework coupling field investigations with analytical approaches to describe shifts in highly variable ecosystems under human-induced stress.