Description: The Arctic is the region on Earth expected to experience the highest rate of warming caused by climate change. Ocean warming is directly and indirectly decreasing oxygen concentration in the ocean, therewith confronting marine biota with a change of two crucial abiotic factors. Polar cod Boreogadus saida is an Arctic key stone species due to its central position in the food web. In order to contribute to a better understanding of its upper thermal limits and the synergistic effects of warming and decreasing oxygen availability on its metabolic and swimming capacity, Polar cod were acclimated to a temperature hypothesised to belong to its upper thermal limit (10°C) over 10 months. Using static and swim tunnel respirometry 10°C were found to clearly belong to the pejus temperature range of Polar cod although aerobic scope and swimming capacity were maintained at this temperature. No metabolic compensation was observed for standard metabolic rate that increased by a factor of five. A significant PO2 effect on maximum metabolic rate and aerobic scope was observed when measuring metabolic and swimming capacity at decreasing ambient oxygen levels. Polar cod displayed oxy regulation over the whole PO2 range tolerated. Critical velocity stayed stable until 40% ambient O2 saturation whereas gait transition velocity decreased non-significantly at 50% O2. Temperature had a strong negative effect on hypoxia tolerance by increasing Pcmax and Pcrit to 12.53 and 5.22 kPa O2, respectively. We observed that water masses of 10°C can be tolerated in short-term by Polar cod but do not allow for population survival. Hypoxia tolerance was found to be strongly decreased at the long-term incubation temperature but still remained high in inter-species comparison and with respect to 10°C as pejus temperature. Future research should address hypoxia tolerance of Polar cod during acute warming to understand the physiological impacts during marine heatwaves.

Description: Understanding cetacean distribution is essential for conservation planning and decision-making, particularly in regions subject to rapid environmental changes. Nevertheless, information on their spatiotemporal distribution is commonly limited, especially from remote areas. Species distribution models (SDMs) are powerful tools, relating species occurrences to environmental variables to predict the species' potential distribution. This study aims at using presence-only SDMs (MaxEnt) to identify suitable habitats for fin whales (Balaenoptera physalus) on their Nordic and Barents Seas feeding grounds. We used spatial-block cross-validation to tune MaxEnt parameters and evaluate model performance using spatially independent testing data. We considered spatial sampling bias correction using four methods. Important environmental variables were distance to shore and sea ice edge, variability of sea surface temperature and sea surface salinity, and depth. Suitable fin whale habitats were predicted along the west coast of Svalbard, between Svalbard and the eastern Norwegian Sea, coastal areas off Iceland and southern East Greenland, and along the Knipovich Ridge to Jan Mayen. Results support that presence-only SDMs are effective tools to predict cetacean habitat suitability, particularly in remote areas like the Arctic Ocean. SDMs constitute a cost-effective method for targeting future surveys and identifying top priority sites for conservation measures.

Description: The climate in the summer months is essential for ecosystems and society. However, climate change is causing lasting changes in the characteristics of the summer climate. In order to better understand the summer climate, to capture changes in a statistically meaningful way and to develop climate sce- narios for the future, long-term climate observations and reliable climate models are needed. These three points are addressed in this thesis with the help of three main research questions. The first question examines the prevailing large-scale climate patterns which are elaborated using the climate signature of the oxygen isotope ratio in tree ring cellulose (δ18Ocel) over the past 400 years. An empirical orthogonal function analysis reveals two different modes of variability. The first mode is related to multi-seasonal anomaly patterns associated with the El Niño-Southern Oscillation. The second mode of δ18Ocel variability, which captures a north-south dipole, is associated with a regional summer atmospheric circulation pattern that has a distinct centre over the North Sea. To further exploit the climate sensitivity of δ18Ocel tree-ring records, the first grid-based reconstruc- tion of the European summer vapour pressure deficit (VPD) for the last four centuries is presented. This reconstruction is used to answer the second question of what trends in VPD have occurred in Europe over the last 400 years. The simultaneous increase in temperature and decrease in precipita- tion starts from mid-17th century in Central Europe and the Mediterranean region and relates to a positive VPD trend. This trend towards higher VPD continues throughout the observation period. In addition to studying the past summer climate with the help of a tree ring network, climate models provide valuable information on future scenarios which are highly relevant for society and ecosys- tems. Therefore, this thesis addresses the question of whether simulations with different climate models from a climate model comparison project are suitable for making reliable statements about future drought conditions and what influence the amount of greenhouse gases has on drought oc- currence. Based on a comparison between simulated and observed drought conditions for the period 1971-2000, reasonable agreement can be found between climate model simulations and the observa- tions. However, climate models cannot reproduce drought trends in observations for recent decades for large parts of the Northern Hemisphere. Furthermore, it is shown that drought occurrence is projected to increase significantly in arid regions under three different future scenarios, with the se- verity of droughts depending on greenhouse gas emissions. For regions currently less affected by prolonged droughts, such as the European continent, the climate models show that the probability of drought occurrence increases significantly under the warmest future scenario. Thus, this thesis presents new perspectives on past, present and future European summer climate using a δ18Ocel tree ring network, climate observations and climate model simulations.

Description: The concept of “blue growth,” which aims to promote the growth of ocean economies while holistically managing marine socioecological systems, is emerging within na-tional and international marine policy. The concept is often promoted as being novel; however, we show that historical analogies exist that can provide insights for contem-porary planning and implementation of blue growth. Using a case-study approach based on expert knowledge, we identified 20 historical fisheries or aquaculture ex-amples from 13 countries, spanning the last 40–800 years, that we contend embody blue growth concepts. This is the first time, to our knowledge, that blue growth has been investigated across such broad spatial and temporal scales. The past societies managed to balance exploitation with equitable access, ecological integrity and/or economic growth for varying periods of time. Four main trajectories existed that led to the success or failure of blue growth. Success was linked to equitable rather than open access, innovation and management that was responsive, holistic and based on scientific knowledge and monitoring. The inability to achieve or maintain blue growth resulted from failures to address limits to industry growth and/or anticipate the im-pacts of adverse extrinsic events and drivers (e.g. changes in international markets, war), the prioritization of short-term gains over long-term sustainability, and loss of supporting systems. Fourteen cross-cutting lessons and 10 recommendations were derived that can improve understanding and implementation of blue growth. Despite the contemporary literature broadly supporting our findings, these recommenda-tions are not adequately addressed by agendas seeking to realize blue growth.