Description: In Algen-Herbivore Studien wurde bereits gezeigt, dass durch Fraßfeinde wie z. B. Littorina littorea oder Idotea baltica (Rohde und Wahl, 2008; Weinberger et al., 2011) eine chemische Abwehrreaktionen in der Braunalge Fucus gegen Grazer ausgelöst werden kann. Vergleichbare Studien zu einer durch Makrofouler induzierten chemischen Abwehr gibt es jedoch bisher kaum. In der vorliegenden Arbeit sollte daher getestet werden, ob in Fucus vesiculosus eine chemische Antifoulingreaktion durch die Gegenwart von Amphibalanus improvisus induziert werden kann. Um diese Fragestellung zu bearbeiten wurde ein Experiment entworfen, in welchem F. vesiculosus in An- und in Abwesenheit von A. improvisus unter Laborbedingungen zwei Wochen gehältert wurde. Im zweiten Teil des Experiments wurden die Oberflächenextrakte dieser Fucus-Thalli auf ihre Antifoulingwirkung mit Hilfe eines Feld-Bioassay getestet. Im Bioassay sollte die Antifoulingwirkung der Extrakte in Abhängigkeit von der Expositionsdauer im Wasser untersucht werden. Für die Ausbringung der Oberflächenextrakte im Wasser wurden 2 Methoden entwickelt und getestet, die sogenannte „Donut“- und die „Sandwich-Methode“. Entgegen den Erwartungen hatten die extrahierten Metabolite der Fucus-Thalli, die in Gegenwart von A. improvisus gehältert worden waren, keine Antifouling- sondern bei kurzen Laufzeiten eine siedlungsfördernde Wirkung. Dieser Effekt verschwand allerdings mit zunehmender Expositionsdauer.

Description: In times of climate change, sea water temperatures are predicted to increase. However, the strength of warming varies over different latitudes, causing diverse reactions in marine organisms. This study provides experimental information on how sea anemones are affected by global warming. For this purpose, Aiptasia diaphana were kept for 43 days at five different temperature regimes in order to determine the upper thermal tolerance limit and changes in weight and size close to this maximum. The sea anemones survived temperatures of up to 33°C for 6 weeks showing declines in weight and size with increasing temperatures. Survival analysis revealed an abrupt end of the thermal tolerance within only 2° C of warming. This study indicates that - in addition to survival - weight and size are both suitable characteristics to investigate warming effects in A. diaphana. This thesis was part of the GAME project 2015, an experimental study at a global scale, investigating whether latitudinal differences influence the upper thermal tolerance of sea anemones. The maximum temperature the anemones survived differed between sites substantiating the hypothesis that sea anemones from lower latitudes live closer to their upper thermal tolerance limit than those from higher latitudes. However, the discrepancy between the temperature increase predicted by climate models and the maximum deviation tolerated by the sea anemones varied considerably between locations. In order to make reliable predictions about future distributions of species, the necessity of combining global trends and local characteristics has to be emphasized.

Description: This manuscript reports the first sightings and collection of the swimming crab Cronius ruber (Lamarck, 1818) on the coast of Madeira Island, Portugal. After the recent record in the Canary Islands, this represents a further step northward on this species’ expansion in distribution in the eastern Atlantic. The crab was first spotted during underwater visual census surveys done by scuba diving in July 2018 and was repeatedly observed during the following months, in different locations on the south coast of Madeira. Analysis of temperature data from several geographic locations where C. ruber is present was performed to assess how thermal regimes and ongoing changes may influence this recent distribution shift. Current temperature trends in Madeira suggest that the arrival and establishment of C. ruber to Madeira might have been facilitated this thermophilic species, adding evidence for the ongoing tropicalization of this area. Finally, the current spread of C. ruber in both Canaries and Madeira island systems highlights the need for a long-term monitoring program targeting this and other non-indigenous species (NIS).

Description: Mass mortality events involving marine taxa are increasing worldwide. The long-spined sea urchin Diadema africanum is considered a keystone herbivore species in the northeastern Atlantic due to its control over the abundance and distribution of algae. After a first registered mass mortality in 2009, another event off the coasts of Madeira archipelago affected this ecologically important species in summer 2018. This study documented the 2018 D. africanum mass mortality event, and the progress of its populations on the southern coast of Madeira island. A citizen science survey was designed targeting marine stakeholders to understand the extent and intensity of the event around the archipelago. Underwater surveys on population density prior, during and after the mass mortality, permitted an evaluation of the severity and magnitude of the event as well as urchin population recovery. A preliminary assessment of causative agents of the mortality was performed. The event was reported in the principal islands of the archipelago reducing the populations up to 90%. However, a fast recovery was registered during the following months, suggesting that the reproductive success was not compromised. Microbiological analyses in symptomatic and asymptomatic individuals, during and after the event, was not conclusive. Nevertheless, the bacteria Aeromonas salmonicida, or the gram-negative bacteria, or the interaction of different types of bacteria may be responsible for the disease outbreak. Further studies are needed to assess the role of pathogens in sea urchin mass mortalities and the compound effects that sea urchins have in local habitats and ecological functioning of coastal marine ecosystems.

Description: Globally, climate change can affect biodiversity and the environment in many ways. One of the most pressing challenges for the marine environment is the change in seawater temperature. Such changes can be observed in long-term averages as well as sporadic, distinct extreme events like marine heatwaves and cold spells. Both processes profoundly impact species' physiology and distribution in the marine realm. Furthermore, due to their isolation, oceanic islands represent peculiar habitats that are expected to be more vulnerable to disturbances like species introductions or changes in climatic conditions. In this context, Macaronesia constitutes a group of highly isolated archipelagos in the Northeast Atlantic formed by the Azores, Madeira, Canary Islands, and Cape Verde island systems. This thesis focused on analyzing the different aspects of changes in sea surface temperatures at Madeira Island and their impact on local species. Furthermore, it represents the first comprehensive regional study of marine heatwaves and cold spells performed on an oceanic island and a pioneering effort for the Macaronesia region. Ongoing changes in sea surface temperatures can be separated into two different elements: changes in long-term average temperatures and distinct extreme events. To this end, both aspects of ocean warming were analyzed for the Madeira Archipelago. Satellite data analysis showed an increase of 0.8°C in the average temperatures over the last four decades at Madeira. The comparison of different time periods showed that current warming trends are accelerating. The analysis of temperature extreme events showed that temporal trends in heatwave and cold spell frequencies were both driven by the underlying warming trends. After removing the long-term trend, both marine heatwaves and cold spells at Madeira occurred at a rate of an average of about two events per year, reaching maximum intensities of ~3.5°C and lasting on average ~11 days. This thesis is structured in two sections: The first section includes two chapters that outline the impact long-term changes in seawater temperatures can have on species distributions by presenting two examples of ongoing tropicalization processes at Madeira Island. The second section includes two chapters testing the effect of changes in temperature on local species experimentally. In the context of tropicalization, the second chapter describes the first records and capture of the pantropical swimming crab Cronius ruber at Madeira Island. Cronius ruber was first recorded during monitoring surveys in the summer of 2018 and afterward repeatedly sighted in three different locations along the island's south coast in the following months. Secondly, the spread of the circumtropical spotfin burrfish Chilomycterus reticulatus at the Madeira Archipelago is investigated in Chapter 3. After being previously considered a vagrant species at Madeira Island, the occurrence of C. reticulatus around the archipelago has been examined using data from historical records, scientific monitoring campaigns, and a citizen science survey. The spotfin burrfish was recorded on all islands in the archipelago, and its first sightings have increased over time. Moreover, following a risk assessment screening (AS-ISK), C. reticulatus showed only a medium risk of becoming invasive. The arrival of C. ruber and the spread of C. reticulatus around Madeira Island already marks the second step of the species' gradual northward expansions after similar processes had been previously recorded at the Canary Islands in a similar fashion. The analysis of thermal environments across both species' distributional ranges strengthens the hypothesis that ongoing sea surface temperature warming promotes these recent poleward shifts and consecutive spread in the Madeira Archipelago. The second part of this thesis tested the sensitivity of two selected invertebrates towards climate change-related changes in sea surface temperatures experimentally. The experiments covered both aspects of current ocean warming: increases in long-term average temperatures and distinct extreme events. Comparing the performance of polyps and ephyrae of the scyphozoan Aurelia solida under different temperatures and salinities in Chapter 4 showed changing optima at different life stages. At fully marine conditions, polyps survived all tested temperatures from 12 to 28°C, while asexual reproduction peaked at intermediate temperatures of 20°C. Ephyrae performed better at slightly lower temperatures and showed the highest survival rates at 15°C. As a result, although long-term increases in average temperatures could initially promote the asexual life cycle of A. solida at the polyp stage, warmer winter temperatures might eventually result in lower ephyrae production and survival and ultimately inhibit the link to the pelagic life stage altogether. Finally, Chapter 5 investigates the effects of an experimental simulation of marine heatwaves on the performance of two invertebrates. Realistic scenarios were created by analyzing regional heatwave and cold spells and consecutively using these local heatwave metrics to simulate a realistic heat event under current and future conditions. Polyps of A. solida and adult rockpool shrimps, Palaemon elegans, were subjected to these heatwave simulations. While both species generally performed well under all the different scenarios, A. solida polyps showed significant increases in asexual reproduction during the recovery phase after the future heatwave simulation. Hence, future conditions at Madeira could potentially boost the species' abundance at this particular life stage. Overall, the present thesis represents an overview of changes in local seawater temperatures on a subtropical oceanic island in both long-term averages and distinct extreme events and outlines how these changes shape the local marine ecosystem. The processes described at Madeira Island follow a stepping stone pattern in Macaronesia and could potentially be applied to other oceanic island systems.

Description: Jellyfish outbreaks are conspicuous natural events in marine ecosystems that have a substantial impact on the structure and dynamics of marine ecosystems and different economic sectors of human activities. Understanding the life cycle strategies of jellyfish species is therefore critical to mitigate the impacts these organisms may have. In this context, the present study investigated the effect of different temperature and salinity regimes on the rearing success of the jellyfish Aurelia solida in microcosm experiments on two different life stages: polyps and ephyrae. Polyps showed high survival rates across the different conditions (except at 28◦C/20 psu) and reproduced asexually in all combinations, with the highest budding activity at 20◦C and 30 psu. Strobilation occurred mainly at 16◦C and 35 psu. Although ephyra survival was highest at low salinities (20 psu) and lower temperatures (10 and 15◦C), the highest growth rates were reached at intermediate temperatures (20◦C). The comparison to other Aurelia species underlines the differences between even closely related species. Given the high tolerance capacity that A. solida presented in the experiments, the species has the potential to cope well under current climate change scenarios and possibly adapt successfully to other regions and ecosystems.

Description: Ctenophores are fragile gelatinous organisms whose diversity and distribution are relatively unknown. For the first time, the occurrence of four planktonic species, namely Ocyropsis crystallina, Eurhamphaea vexilligera, Cestum veneris, and Beroe sp., was reported from Madeira Archipelago waters (NE subtropical Atlantic). This report represents the northernmost records in the Eastern Atlantic Ocean for O. crystallina and E. vexilligera.