Stress ecology in times of global change – single and combined effects of ocean acidification, temperature and food availability on different life stages of the barnacle Amphibalanus improvisus = Stressökologie in Zeiten des Klimawandels – Einzel- und interaktive Effekte von Ozeanversauerung, Temperatur und Nahrungsverfügbarkeit auf verschiedene Lebensstadien der Seepocke Amphibalanus improvisus

Increasing atmospheric CO2 affects seawater pH and chemistry. This process, commonly known as ocean acidification (OA), has led to a decrease in oceanic seawater pH by 0.1 since the industrial revolution. Oceanic models show that mean pH may fall from the current 8.1 units to 7.8 and 7.5 by 2100 and 2300, corresponding to levels of about 1000 and 2000 µatm pCO2, respectively. Coastal habitats have been described to differ substantially from open ocean conditions. Not only absolute mean values and annual or daily fluctuations but even future predictions differs considerably from open ocean norms. These characteristics evolved over many years and likely formed species or populations, which are more robust to future OA than species or populations from more stable oceanic environments. Calcifying species as well as early life-history stages of marine organisms are considered to be mainly affected by OA. The acorn barnacle Amphibalanus improvisus is a dominant marine calcifier within the western Baltic Sea and has a complex life cycle with various stages such as feeding nauplii and non-feeding cyprids as well as settled calcifying juveniles. Thus, this species is an ideal organism to address a wide range of hypotheses. In this thesis, I investigated the sensitivity of the A. improvisus towards OA stress in combination with additional environmental parameters such as temperature and food availability. The organisms for this study came from the Kiel Fjord, Germany and the Tjarno Archipelago, Sweden, which, to a certain extent, allows interpretations on the population level. As one of the first cases, this study evaluates the entire life cycle of an invertebrate towards OA in combination with additional stressors. Nauplius larvae of A. improvisus were affected neither by moderate ( 1500 µatm) OA under summer temperature conditions of 20 °C in the Kiel population or under slightly elevated temperatures (25 °C) in the Tjarno population. However, in cooler waters (12 °C) severe OA drastically slowed down the larval development of Kiel individuals. Warming generally increased the survival as well as the rate of development in barnacle nauplii but cypris larvae also suffered increased mortality with increasing temperatures. Cyprid size (Tjarno) and settlement (Kiel) were unaffected by temperature and OA but survival was enhanced under severe OA in the Kiel population. While survival, growth, condition index, reproduction, shell strength as well as development of the F1 generation of juvenile barnacles from Kiel were rather unaffected by OA over 20 weeks, moulting frequency increased with increasing acidification of the seawater. Net-calcification was reduced under increasing acidification with negative impacts on the shell maintenance of adults. This was true for barnacles cohorts collected in summer but when a cohort of juvenile barnacles was collected in autumn and investigated under OA scenarios over the winter, severe OA negatively impacted juvenile growth after 10 weeks. Seawater warming by 4 °C temporary effected the growth of juvenile barnacles. It also decreased the condition index and increased the breaking resistance, i.e. led to an increased investment in shell production compared to body growth. Juvenile barnacles from Tjarno showed a generally higher sensitivity to OA with reduced growth and survival under moderate and severe OA when combined with food limitation and under severe OA when food availability was increased. Food availability in general, seems to be the major factor driving the performance of juvenile barnacles (increasing growth, condition index, reproduction and shell strength), in both the Kiel and the Tjarno populations. In the present study, barnacles have been shown to withstand predicted near-future OA well, with if at all, only sub-lethal effects on the different life-history stages. Larval stages of both the Kiel and the Tjarno populations were tolerant to even the highest OA levels applied (3250 µatm pCO2, pH 7.5). Nevertheless, the sensitivity of juveniles to OA differs remarkably between barnacle populations. Juveniles of A. improvisus from Tjarno responded more sensitively to OA than juveniles from Kiel. Juveniles of A. improvisus from Tjarno responded more sensitively to OA than juveniles from Kiel. These population-specific differences might be explained by the high natural variability in pH/pCO2 over the year in the Kiel Fjord and the possibility of barnacles to adapt to these fluctuations over many generations. Assuming that this adaptation is potentially also valid for other barnacle populations, A. improvisus from Tjarno will likely adapt to anthropogenic OA in the future. In conclusion, barnacles might be pre-selected to withstand strong small-scale as well as large-scale fluctuations in their natural environments. Nevertheless, habitat characteristics and thus population specific differences seem to play a role in determining the resilience of A. improvisus to OA. Even seasonality likely controls the resilience of this species to OA. Since OA is expected to increase more drastically in shallow coastal habitats in the future, the absolute OA tolerance limits of this species are still to be evaluated in more detail. Additionally, the synergistic effects of OA and factors such as warming or desalination need to be considered when future predictions are made. Community structures depend on the responses of various organisms and there will be “losers” and “winners” in the fate of future OA. Thus, although single species such as A. improvisus will be not lethally impacted in a future acidified ocean, community structures are likely to change due to the higher sensitivity of other organisms.