En milieu intertidal rocheux, les communautés dominées par des canopées d'algues brunes (Phaeophyceae) sont particulièrement répandues. Malgré leur accessibilité, leur fonctionnement reste encore largement méconnu. En particulier, l'influence de la saisonnalité et du gradient d'émersion sur le métabolisme, le réseau trophique ou encore le processus de succession a été peu étudiée. Ce travail de thèse a donc eu pour objectif d'analyser le fonctionnement de deux communautés de Fucus établies à des niveaux marégraphiques différents, en portant une attention particulière aux conditions environnementales. Sur les côtes bretonnes (France), les communautés de Fucus vesiculosus Linnaeus et de Fucus serratus Linnaeus sont respectivement caractéristiques des étages médiolittoral moyen et inférieur. Des mesures de métabolisme (production primaire et respiration) réalisées in situ à différentes périodes de l'année ont mis en évidence une forte influence de la saisonnalité et le rôle primordial de la canopée dans les flux de carbone. Le métabolisme s'est avéré être plus élevé à l'émersion qu'à l'immersion, où l'intensité lumineuse constitue régulièrement un facteur limitant pour la production primaire. Par une approche de modélisation basée sur ces mesures et sur des données environnementales acquises à haute fréquence, un bilan annuel de production primaire a été calculé pour chaque communauté. Ces bilans confirment que ces systèmes sont fortement productifs et permettent de préciser leur régulation à l'échelle des cycles de marées. Le réseau trophique de ces communautés, analysé via les isotopes stables du carbone et de l'azote, apparait basé sur l'utilisation de nombreuses sources. / Along intertidal rocky shores, communities dominated by canopy-forming brown algae (Phaeophyceae) are particularly widespread. Their functioning is however largely understudied, especially concerning the influence of seasonality and shore height on primary production and respiration, on food webs or on successional process. The aim of this study was to investigate the functioning of two intertidal Fucus communities which are established at different tidal levels. During this study, a special consideration was given to the effects of environmental conditions. On the coasts of Brittany (France), communities dominated by Fucus vesiculosus Linnaeus and Fucus serratus Linnaeus are characteristic of mid-intertidal and low mid-intertidal levels, respectively. In situ measurements of metabolism (primary production and respiration) carried out at different periods of a year highlighted a strong seasonal influence on carbon fluxes and emphasized the importance of canopies in the metabolism of whole communities. Metabolism was higher during emersion than during immersion periods, for which light intensity regularly limits the primary production. Based on these measurements and on high-frequency measurements of environmental parameters, an annual primary production was calculated by modelling for both communities. These annual estimations confirmed that these communities are among the most productive systems of coastal region and specify their regulation at the tidal cycles scale. Food webs, which were analysed through carbon and nitrogen stable isotopes, are based on the use of almost all available sources of organic matter by consumers, and revealed a strong conservation of food webs across seasons. Thecolonisation survey of bare substrates shows that Fucus species improve the settlement success of numerous species and increase rapidly primary production of whole assemblages. Overall, despite some differences in metabolism and successional sequences according to the shore height at whichthey are located, these communities exhibited similar functioning due to the canopy which damns the environmental conditions.

In the future, the cultivation of algae for food may become increasingly important in Sweden. But this area is still new in Europe and most of the harvest today comes from wild stocks. In Sweden, cultivation is only available on the west coast, but research is underway to see if it is possible to start cultivating macroalgae in the Baltic Sea as well. The Baltic Sea has completely different biological conditions than the west coast, and one of the challenges is the low salinity, which limits the species that can live in the Baltic Sea. This is something that the Marine Center in Simrishamn is also researching in the project Tångkusten, of which this study is a part of. The purpose of this study is to use a literature study to compile available knowledge about which algae that has the potential to be cultivated in the future and what the life cycles of these algae look like. An inventory of the macroalgae found locally in Hanöbukten was also carried out. Based on this information, possible cultivation methods and the advantages of each species are discussed, but also the challenges that may arise.  The results showed that the most abundant species were filamentous brown algae, bladder wrack and saw wrack . The species deemed most cultivable are gut weed and sea lettuce. Clawed fork weed, bladder wrack and saw wrack are also considered to have some cultivation potential. Among the biggest challenges are the growth of epiphytic algae and finding a suitable site for cultivation. / Projektet Tångkusten på Marint centrum i Simrishamn