The aim of this study was to evaluate the methane potential of 35 kg of blue mussels in a batch amanaerobic two-stage dry digestion system (pilot-scale), which consists of aleach bed reactor (LB) and an up-flow anaerobic sludge blanket reactor (UASB). We monitored the process daily by measuring temperature, pH, COD, VFA, NH4+ gas flow rate and gas content. The temperature was approximately 30 C in the LB and 36 C in the UASB reactor. The applied OLR was 1.5 g COD/l*d. After 37 days process run, we obtained a total methamne yield of 0.64 Nm3 respectively 0.29 Nm3/kg VS, of that 70% in the LB reactor and 30% in the UASB reactor.

I denna studie har förekomst av Polycykliska aromatiska kolväten (PAHer) i musslor undersökts. Syftet var att undersöka vilka PAHer som förekom, i vilka halter och ifall dessa nivåer överskred gränsvärdena för humankonsumtion. Vissa PAHer är skadliga för människan då de kan ge upphov till cancer. Vilda och odlade musslor jämfördes för att se om innehållet av PAHer skiljde sig åt. Musslor från sex olika provplatser undersöktes. Två prov handplockades vilda utanför Stenungssund, Sverige. Danska vildfångade musslor undersöktes, likaså vildfångade svenska musslor. Även odlade musslor från Irland och odlade musslor från Sverige undersöktes. Resultaten visar att fluoranten, fenantren, pyren, krysen, benzo(e)pyren och benzo(b)fluoranten var de ämnen som förekom i högst halter. Av de alkylerade PAHerna var 2-metylantracen-9,10-dion, 2-metylfenantren, 2-metylkrysen och 1-metylfluoranten vanligast. Halterna av oxy-PAHer var låga, förutom för antracen-9,10-dion, benzo(h)quinolin samt 2-metylantracen-9,10-dion. Utifrån de riktvärden som finns för humankonsumtion överskred inga musslor gränsvärdena. Inga skillnader kunde ses mellan odlade och vilda musslor. Sammanfattningsvis förekommer PAHer i musslor, men inte i sådan halt att musslorna inte är godkända som livsmedel. / In this study, the presence of PAHs in mussels were investigated. The aim was to examine the occurrence of PAHs, at which levels and if these levels exceeded the limits for human consumption. Some PAHs are toxic to humans as they can give rise to cancer. Wild and cultivated mussels were compared to see if the content of PAHs was different. Mussels from six different test sites were examined. Two wild samples were hand-picked outside Stenungsund, Sweden. Furthermore, wild clams from both Sweden and Denmark were sampled. Also cultured mussels from Ireland and cultured mussels from Sweden were investigated. The results show that fluoranthene, phenanthrene, pyrene, chrysene, benzo(e)pyrene and benzo(b)fluoranthene were the compounds that occurred at the highest levels. The most common alkylated PAHs were 2-methylanthracene-9,10-dione, 2-methylphenanthrene, 2-methylchrysene and 1-methylfluoranthene. The concentrations of oxy-PAHs were low except for anthracene-9,10-dione, benzo(h)quinoline and 2-methylanthracene-9,10-dione. Based on the guideline values available for human consumption, no mussels exceeded the limits. No differences could be seen between wild and cultivated mussels. To sum up, PAHs were found in mussels, but not at such concentration that the mussels would not be approved for human consumption.

Understanding factors that influence a species' distribution and abundance across their annual cycle is needed for range-wide conservation planning. Every year during spring migration, thousands of federally threatened (U.S.A.) and endangered (Canada) migratory Western Atlantic red knots (Calidris canutus rufa, 'red knot') use Virginia's barrier islands as stopover habitat to regain the fat required to continue flights to breeding grounds. Because the red knot completes one of the longest avian migrations in the world and relies on variable prey resources at its stopover grounds, the red knot exemplifies the challenges faced by long-distance migrant shorebirds. These challenges may be exacerbated by climate change, as long-distance migrants may be unable to adapt quickly to changing prey ranges and abundances, resulting in spatial and temporal mismatches between predators and prey. More specifically, as climate change causes ocean temperatures near Virginia's barrier islands to rise, organisms that live within the intertidal zone, like blue mussels (Mytilus edulis), are experiencing range shifts. Here, we 1) confirmed what prey red knots select in Virginia, 2) addressed the factors that affect red knot site selection, red knot flock size, and prey abundances across Virginia's barrier island intertidal shoreline during 2007 – 2018, and 3) predicted the origin of juvenile blue mussels, a key prey resource for red knots in Virginia. To determine which prey are most available to red knots in Virginia, we collected sand and peat substrate core samples from Virginia's ocean intertidal zone and counted the number of prey in each sample. We compared these prey availability data to prey DNA data obtained from fecal DNA metabarcoding analyses on red knot feces (n = 100) collected on peat and sand substrates between 2017 – 2019. Red knots consumed prey from Orders Veneroida (clams), Mytiloida (mussels), Diptera (flies), and Amphipoda/Calanoida (crustaceans). While crustaceans were the most abundant prey on both sand (70.80% of total prey counted) and peat (74.88%) substrates, red knots selected crustaceans less than expected given their availability. Red knots selected clams and mussels, supporting their status as bivalve specialists in Virginia. After determining which prey red knots consumed and selected in Virginia, we predicted the number of red knots using Virginia's barrier island stopover during their migratory stopover (May 14 – 27, 2007 – 2018) annually. We used confirmed prey, tide, distance to known roosts, and red knot winter counts from Tierra del Fuego to inform zero-inflated negative binomial mixed-effects regression models of red knot site selection and flock size in Virginia. We also used generalized linear mixed-effects regression models to determine how climatic and geomorphological factors affected prey abundances. Modeled red knot peak counts were highest in 2012 (11,644) and lowest in 2014 (2,792; x̄ = 7,055, SD = 2,841); the trend over time was variable but there was no evidence of a linear increase or decrease. Red knots selected foraging locations with more prey, though red knot flock size did not consistently relate to prey abundance. Tide, substrate, and water temperature affected prey availability. While different prey responded to these covariates in variable ways, prey generally were most abundant on peat banks at low tide. Given the importance of blue mussels in the red knot's diet and distribution in Virginia, if the blue mussel's range continues to contract northward, red knots could be faced with additional fat replenishment challenges. We analyzed the variation in blue mussels from 2010 – 2018 by collecting core samples on peat banks in Virginia and counting the number of blue mussels in the cores. To approximate the origin of Virginia's juvenile blue mussels and determine how continued ocean temperature warming may further affect the blue mussel's range contraction, we conducted oxygen stable isotope (δ¹⁸O_c) analyses on 74 blue mussel shell umbos (the first portion of the shell precipitated) and shell edges (the most recently precipitated shell) to compare and predict where different portions of the shell were formed. We compared blue mussel shell compositions to δ¹⁸O_c calculated in equilibrium with regional ocean water using recorded δ¹⁸O_w data and sea surface temperature data from ocean buoys between New Hampshire and Virginia. Blue mussel abundance/core sample declined over the duration of our study (Spearman's rank correlation coefficient: ρ(rho) = -0.31, p < 0.001), with the highest abundance in 2010 (x̄ blue mussels/core sample = 537.88, SE = 85.85) and lowest in 2016 (x̄ = 34.08 blue mussels/core sample, SE = 6.96). Blue mussel umbos (x̄ δ¹⁸O_c = -0.23‰, SE = 0.12) contained more positive δ18Oc than shell edges (x̄ δ¹⁸O_c = -0.53‰, SE = 0.20), suggesting that Virginia's blue mussels originated from ocean populations in more saline and/or colder water than that within Virginia's intertidal zone. Blue mussel umbo δ¹⁸O_c were not different than δ¹⁸O_c calculated in equilibrium with regional ocean water off the Virginia and Delaware coasts, suggesting that Virginia's blue mussels originated in ocean waters between Delaware and Virginia; however, they may have originated in waters as far north as New York in some years, potentially decreasing the risk of blue mussels being completely extirpated from Virginia in the near future. While red knots currently use spring migratory stopovers across the United States' Atlantic Coast, from Florida to New Jersey, the largest spring concentrations of knots are confined to the Delaware Bay and Virginia's barrier islands. Because these stopover grounds support large proportions of the red knot's migratory population, any changes in the factors that affect red knots at these stopover sites could have lasting implications for red knots. The blue mussel's range contraction and decline over time in Virginia, for example, is concerning from a conservation perspective. Red knots require easily accessible and abundant prey resources to efficiently replenish fat-stores needed for continued migration and breeding. Additionally, because red knots breed within a narrow period, any delays on stopover grounds could prevent red knots from breeding, even if they survive migration. Our research demonstrates that red knots use prey abundance as a determinant when selecting foraging locations, and that peat banks, while only sporadically available across the barrier islands at mid- to low-tides, contain higher prey abundances than sand. Thus, to continue maximizing the availability of prey in Virginia, measures should continue to be taken to allow natural island migration processes that encourage the presence of both sand and peat substrates. Beach nourishment and stabilization projects are often used on coastal beaches to prevent shoreline erosion; however, such actions prevent the formation of peat banks by blocking island migration processes. A reduction in peat banks could decrease the abundance of prey available to red knots, making weight gain during the critical stopover period more challenging for red knots. Additionally, beach nourishment through sand replenishment buries invertebrate prey, potentially causing mass prey mortality and reducing shorebirds' ability to access deeply buried prey. To prevent the loss of important peat banks on these islands, and to prevent disrupting predator-prey interactions, managers should continue their ongoing focus on allowing natural processes to occur on Virginia's barrier islands. / Doctor of Philosophy / Red knots (Calidris canutus rufa) are imperiled shorebirds that migrate thousands of kilometers each year from wintering grounds as far south as Argentina to breeding grounds in the Canadian Arctic. To migrate such long distances, red knots use stopover habitat, like Virginia's barrier islands, to regain the fat required to continue flights to breeding grounds. Climate change is causing the ranges of important red knot prey, such as blue mussels (Mytilus edulis), to shift. Red knots may be unable to quickly adapt to changes in prey abundances on stopover grounds, making it more challenging for them to gain the weight required for successful migration and breeding. Thus, understanding the factors that may affect red knots in Virginia are important for successful management of this shorebird. Here, we determined what prey red knots consume in Virginia, addressed the factors that affect red knots in Virginia, and predicted the origin of blue mussels, an important prey resource for red knots in Virginia. We collected sand and peat substrate samples from Virginia's ocean intertidal zone to determine what prey were available to red knots and collected red knot feces to determine what prey red knots consume. Red knots consumed clams, mussels, fly larvae, and crustaceans in Virginia and showed preference for clams and mussels over the more abundant crustaceans. Red knots selected foraging locations that had higher prey abundances than unused sites. The relationship between red knots and prey was affected by the tidal cycle, with the highest abundance of prey available and most birds foraging at low tide when peat banks were exposed. After confirming that red knots preferentially consume blue mussels, which is experiencing a northward range contraction due to increasing ocean temperatures, we analyzed how blue mussel numbers have changed over time in Virginia. Blue mussel numbers declined over the duration of our study. We also analyzed blue mussel shells to help determine where the blue mussels were spawned, as shells contain environmental clues that relate to the conditions within which the shell forms. The juvenile blue mussels red knots consumed on Virginia's peat banks likely originated in ocean waters between Delaware and Virginia, but potentially in waters as far north as New York, possibly using ocean currents to travel to Virginia's intertidal zone. Because migratory stopovers, like Virginia's barrier islands, support a large percentage of the total red knot migratory population, any changes in factors that affect red knots during their spring stopover, like prey availability, may negatively affect red knots. Because red knots need to quickly consume large quantities of prey to gain the weight needed for continued migration and breeding, managers should continue to prevent factors that negatively affect prey in Virginia. For example, we found that peat banks support high quantities of red knot prey; thus, continuing to ensure that peat banks are able to form on the islands is paramount. However, peat banks form through shoreline erosion and overwash, two natural processes which are often portrayed negatively. Beach nourishment and stabilization projects are commonly used in other coastal areas to prevent shoreline erosion and overwash along coastal beaches. If Virginia's barrier island beaches are nourished, natural processes like shoreline erosion may be less likely to occur. The absence of shoreline erosion on Virginia's barrier islands would preclude the formation of peat banks, likely decreasing prey availability for red knots. Beach nourishment also buries invertebrate prey, as sand is deposited along the beaches' intertidal zone. The disturbance caused by nourishment combined with the reduction in prey caused by it could negatively affect red knots in Virginia. Managers should continue to protect both peat and sand substrates by further allowing natural processes, like shoreline erosion, to occur on Virginia's barrier islands.

Environmental change is a major threat to marine ecosystems worldwide. Understanding the key biological processes and environmental factors mediating spatial and temporal species' responses to habitat alterations underpins our ability to forecast impacts on marine ecosystems under any range of scenarios. This is especially important for calcifying species, many of which have both a high climate sensitivity and disproportionately strong ecological impacts in shaping marine communities. Although geographic patterns of calcifiers' sensitivity to environmental changes are defined by interacting multiple abiotic and biotic stressors, local adaptation, and acclimation, knowledge on species' responses to disturbance is derived largely from short- and medium-term laboratory and field experiments. Therefore, little is known about the biological mechanisms and key drivers in natural environments that shape regional differences and long-term variations in species vulnerability to global changes. In this thesis, I examined natural variations in shell characteristics, both morphology and biomineralisation, under heterogeneous environmental conditions i) across large geographical scales, spanning a 30° latitudinal range (3,334 km), and ii) over historical times, using museum collections (archival specimens from 1904 to 2016 at a single location), in mussels of the genus Mytilus. The aim was to observe whether plasticity in calcareous shell morphology, production, and composition mediates spatial and temporal patterns of resistance to climate change in these critical foundation species. For the morphological analyses, the combined use of new statistical methods and multiple study systems at various geographical scales allowed the uncoupling of the contribution of development, genetic status, and environmental factors to shell morphology. I found salinity had the strongest effect on the latitudinal patterns of Mytilus shape. Temperature and food supply, however, were the main predictor of mussel shape heterogeneity. My results suggest the potential of shell shape plasticity in Mytilus as a powerful indicator of rapid environmental changes. I found decreasing shell calcification towards high latitudes. Salinity was the best predictor of regional differences in shell deposition, and its mineral and organic composition. In polar, low-salinity environments, the production of calcite and organic shell layers was increased, while aragonite deposition was enhanced under temperate, higher-salinity regimes. Interacting strong effects of decreasing salinity and increasing food availability on compositional shell plasticity predict the deposition of a thicker external organic layer (periostracum) at high latitudes under forecasted future conditions. This response potential of Mytilus shell suggests an enhanced protection of temperate mussels from predators and a strong capacity for increased resistance of polar and subpolar individuals to dissolving water conditions. Analyses of museum specimens indicated increasing shell calcification during the last century. Deposition of individual shell layers was more closely related to temporal changes in the variability of key environmental drivers than to alterations of mean habitat conditions. Calcitic layer and periostracum showed marked responses to alterations of biotic conditions, suggesting the potential of mussels to trade-off between the deposition of calcareous and organic layers as a compensatory response to strategy-specific predation pressure. These changes in biomineralisation indicated a marked resistance to environmental change over the last century in a species predicted to be vulnerable, and how locally heterogeneous environments and predation levels can have a stronger effect on Mytilus responses than global environmental trends. My work illustrates that biological mechanisms and local conditions, driving plastic responses to the spatial and temporal structure of multiple abiotic and biotic stressors, can define geographic and temporal patterns of unforeseen species resistance to global environmental change.

Eutrophication is a consequence of excess nutrients in the water which leads to increased algaegrowth, reduced water transparency and hypoxic bottoms. This is the biggest environmental problemfor the Baltic Sea which recently has resulted in stricter legislations and other initiatives to help theBaltic Sea to recover. However, the actions to reduce the nutrient input to the Baltic Sea have so farmainly been land-based. These actions seem to not be enough since the eutrophication continues tobe a problem for the Baltic Sea. Farming blue mussels has shown to have a mitigating effect on theeutrophication and could thus be a complementary action. Blue mussels are filter-feeding specieswhich means that they filter water for food and thus eat phytoplankton and accumulate nutrients atthe same time. When the blue mussels are removed from the sea, so is the nutrients accumulated inthe mussels, resulting in a mitigation of nutrients and thereby the eutrophication. Due to the brackishwater with the low salinity in the Baltic Sea, the blue mussels farmed there do not grow bigger thanaround 3 cm. This means that the mussels are not suitable for human food production and theharvested mussels need to be used for something else, even though the farming itself is anenvironmental action. Three possible mussel products from valorisation of the Baltic Sea blue musselshave been identified; producing mussel meal, biogas or compost.Region Östergötland is involved in a project, Baltic Blue Growth, with the main objective to study howto use mussel farming as an environmental measure and which of the three valorisation options is themost beneficial from an environmental perspective. This study is a part of their investigation to reachtheir goal and will study their mussel farm in St. Anna and the three valorisation options from anenvironmental perspective. The aim of this study is thus to investigate the net nutrient reduction froma mussel farm in the Baltic Sea in combination with the contribution to climate change. This is donefrom a life cycle perspective to include the valorisation of the mussels into the different productsmussel meal, biogas or compost. For this, an existing farm in the archipelago of St. Anna, Östergötland,Sweden is studied. The main results show that there is a nutrient reduction from the mussel farm andthis is not majorly affected regardless of which valorisation option that is chosen. However, the musselfarm does have an impact on climate change and the magnitude of the impact varies for the threevalorisation options. The results of the sensitivity analysis show that the result from the life cycle canbe improved with future improvements of the mussel farm and transportation. The nutrient reductioncan become larger and the impact on the climate change can be reduced. Outside the result from thelife cycle analysis it is discussed that there are other future improvement possibilities in the productionof the mussel products, which would impact the result. The mussel farm and the mussel products alsohave other positive impacts that is not included in the life cycle analysis but discussed in the study,such as increased water transparency, recycling of nutrients and reduction of over fishing. However,the mussel farm could also have negative impacts, such as emissions of microplastics and locallyincreased sedimentation which affect the hypoxia. Those are discussed in this study but the probabilityand possible impact of them are not fully investigated and need further research.

Coastal areas currently face a lot of anthropogenic pressures (Erlandsson et al, 2011; Wilcox et al, 2020) which might further increase through climate change (Maltby et al, 2022). This can lead to the disappearance of crucial, habitat engineering species like blue mussels (Tummon Flynn et al, 2020). The lack of those increases the risk of eutrophication and pollution, on the long run having negative consequences not only for biodiversity and ecological factors but also social and economic factors like income through fishing and risking coastal flood protection (Tummon Flynn et al, 2020; Schotanus et al, 2020; Cooley &amp; Doney, 2009). Blue mussels can form biogenic reefs, which are protected as important habitats under the EU Habitats Directive (Council of the European Communities, 1992) as well as the EU Water Framework Directive (European Parliament &amp; council, 2000). Nevertheless, actual implementation of protection and conservation of those reefs is lacking (Rees et al, 2018), which is why many of them are declining drastically (EEA, 2013). One reason for that lack of protection might be the diverse definitions applied to mussel reefs all over Europe (Stounberg, 2021), while another one could be that many of them are not mapped. In the present study, an attempt was made to map a potential blue mussel reef in the Roskilde Fjord in Denmark, which was first discovered by a previous study (Dahl et al, 2019:39) but not investigated to its extend as that study focused on other goals. A ROV was employed to collect video data of the seafloor of the respective fjord area over a period of three months. The recorded video and GPS data was then processed and analysed to produce a map in QGIS, showing if and where an area of, according to the Danish definition for blue mussel reefs, more than 2500m2 with more than 30% mussel cover and 3 year classes of mussels (Miljøstyrelsen, 2018), is present. Three areas in the chosen search area of this study were confirmed to fulfil those requirements, consistent with the previous findings of Dahl et al (2019). The actual blue mussel reef might be even bigger as other areas which have not been analysed in detail contain blue mussels as well. Therefore, protection measures based on best available scientific knowledge, interdisciplinary work and the knowledge of as many stakeholders possible, including social, economic and ecological aspects, should be developed for the blue mussel reef in Roskilde Fjord. One of them could be to make the reef part of the Natura 2000 protection area already existing in the same area.

The thesis deals with several aspects of monitoring of persistent organic contaminants (POPs) in biological matrices, for example choice of sample, sampling design, and statistical treatment of data both for temporal and spatial trends and for compliance towards a set target value. The efficiency has been evaluated through statistical power analyses. Contaminant data from more than 4 decades from the Swedish National Monitoring Programs for monitoring of contaminants in biota (marine, freshwater and human health), has been quantitatively evaluated both temporally and spatially and for compliance. The aim was also to evaluate the suitability of different matrices, i.e. herring (Clupea harengus), guillemot (Uria aalge) egg, cod (Gadus morhua), perch (Perca fluviatilis), eelpout (Zoarces viviparous), blue mussel (Mytilus edulis), pike (Esox lucius), Arctic char (Salvelinus alpinus) and human milk, for monitoring of POPs with the overall aim to improve the assessment within monitoring programs. The results show that variation can be reduced by using pooled samples including more specimens but fewer chemical analyses, which in turn generate a higher statistical power to a lower cost, at least in cases where the cost of collection and sampling is considerably lower than the cost of chemical analysis. However, there are also a number of advantages using individual samples, such as information of sample variance and maximum value, which allows the choice of an appropriate central measure and direct adjustment of confounding factors. Generally, the levels of polychlorinated biphenyls (PCBs), dichlorodiphenyltrichloroethanes (DDTs), hexachlorocyclohexanes (HCHs) and hexachlorobenzene (HCB) have decreased both in marine and freshwater biota but concentrations are still higher in the Baltic compared to e.g. the North Sea. The levels of dioxinlike-PCBs and polychlorinated dibenzo-p-dioxins/polychlorinated dibenzofurans (PCDD/Fs) have decreased in human milk over time, but not to the same extent in fish and guillemot egg from the Baltic and the freshwater environment. This may be explained by the dietary advice developed by the Swedish Food Administration with the goal that girls, reproductive aged, and pregnant women should eat less food containing high levels of PCDD/Fs. Thus the levels in milk could continue to decrease at the same rate although the temporal trend in the environment has slowed down or leveled out. The most essential regarding the choice of species and matrices for contaminant monitoring, is that the species and organ fit the purpose of the monitoring.

Dans le contexte actuel de changement global et particulièrement de réchauffement climatique et de pollution chimique chronique, se pose la question de la vulnérabilité des écosystèmes côtiers et notamment des populations d’invertébrés filtreurs jouant un rôle structurant dans la plupart de ces écosystèmes. Dans ces travaux de thèse, une approche couplée d’écophysiologie et de génomique fonctionnelle a été mise à profit afin de mieux comprendre les processus moléculaire, cellulaire et physiologique de réponse à une contamination chimique chronique modérée et d’évaluer la capacité à faire face à une augmentation de température chez des populations naturelles de moules bleues (Mytilus spp.) de la Rade de Brest. Les résultats obtenus révèlent qu’une contamination chronique modérée induit des réponses adaptatives au niveau subcellulaire chez les moules exposées, prévenant ainsi l’apparition de souffrances physiologiques et permettant aux populations de se maintenir dans un environnement variable. Ces réponses impliquent en particulier, une activation des mécanismes de défense cellulaire (métabolisme énergétique et défenses antioxydantes) et d’élimination des xénobiotiques. Par ailleurs, l’exposition à une augmentation de température en conditions expérimentales ne met pas en évidence de sensibilité particulière au stress thermique chez les moules provenant d’un site exposé à une contamination chronique modérée. En revanche, les résultats indiquent que ces dernières pourraient avoir une plus grande capacité à compenser grâce à des réponses adaptatives, les effets délétères générés par une augmentation de température. Cependant, le fort coût énergétique généré par la contamination chimique ainsi que les effets importants du stress thermique mis en évidence dans ces travaux indiquent que la combinaison de ces deux sources de stress pourrait provoquer des dysfonctionnements métaboliques et représenter à l’avenir, une menace pour les populations naturelles de moules bleues. Ces travaux de thèse ont également contribué au développement de nouvelles méthodologies permettant respectivement, de quantifier des contaminants environnementaux dans des tissus biologiques marins, d’étudier des réponses génomiques précoces de stress et de mesurer des paramètres physico-chimiques in situ. Ces méthodologies pourront contribuer à améliorer les performances du diagnostic de l’état de santé des populations naturelles de moules et au delà, d’espèces-sentinelles de mollusques bivalves dans un contexte de contamination chimique mais également de changements climatiques. / In the current context of chronic chemical pollution and on-going climate change, coastal ecosystems, and in particular keystone filter-feeding bivalve populations inhabiting them, appear vulnerable. In this thesis, an approach coupling ecophysiology and functional genomics was used to study the molecular, cellular and physiological responses of wild blue mussel populations of the Bay of Brest to a moderate chronic chemical contamination, and to assess the ability of these populations to face a heat stress. Results indicate a moderate chronic chemical contamination induces adaptive responses in wild mussel populations from the Brest harbour area, which prevents severe physiological disturbances and sustain long-term population survival. These responses include an activation of cellular stress response [energetic metabolism and antioxidant defences) and xenobiotic elimination mechanisms. Furthermore, experimental heat stress exposure does not highlight a higher sensitivity to a temperature increase in mussels sampled in a moderately contaminated area but suggests that these mussels could have a better ability to offset the adverse effects of heat stress thanks to adaptive responses. However, the high energetic cost of chemical contamination and the strong effects of heat stress presented in this work, suggest combined chemical and heat stress could be a future threat for wild blue mussel populations. This work also contributes to the development of new methodologies to, respectively, quantify environmental contaminants in marine biota, study early warning genomic stress responses and to survey physicochemical parameters in situ. These methodologies contribute to improving the health diagnostics of natural mussel populations and thus, appear as useful tools to assess health of bivalve sentinel species populations in biomonitoring studies, in a context of chemical contamination and climate change.