Cold-water corals in a changing ocean: Effects on their physiological performance

Abstract

Cold-water corals (CWCs) are abundant worldwide, with their distribution mainly determined by physico-chemical factors and food availability. Despite predominantly occurring in deep waters, CWC habitats are also affected by the impacts of global climate change, such as ocean acidification and warming as well as reduced primary productivity and oxygen concentration. Several previous studies have already investigated the effects of environmental changes on different CWCs species, but mainly focused on laboratory experiments under controlled conditions of a single parameter and the study of adult corals. To date, there is limited information on the effects of environmental changes on different life stages of CWCs, the combined effect of multiple factors, the impact of variable rather than constant conditions and the importance of food availability for the resilience of CWCs to environmental changes.

Therefore, this thesis aimed to better understand the physiological response of CWCs to changes in their environment. In both aquarium and in situ experiments, I examined how CWCs cope with changes in single and several combined factors under stable and variable environmental conditions, taking into account the response of different life stages and the influence of differences in food availability.

For this purpose, I have investigated the short- and long-term physiological response of three life stages of the CWC Caryophyllia huinayensis to different conditions of aragonite saturation (Ωarag), temperature and feeding in an aquarium experiment, all as individual factors as well as their interactions (manuscript 1). Aragonite undersaturation did not affect the corals, but elevated temperature and reduced feeding showed a clear negative effect. Juvenile and adult corals responded differently to temperature changes and reduced food supply as calcification rates of early juveniles were most affected, while adult corals showed highest mortality rates. The clear feeding effect and the delayed response of corals after more than three months under constant treatment conditions underline the need for long-term experiments and the inclusion of food supply as an important factor.

However, CWCs are not exposed to stable environmental conditions in their natural habitat, as a year-long in situ experiment in Comau Fjord in northern Patagonia (Chile) showed (manuscript 2). We took advantage of the ubiquitous occurrence of the CWC Desmophyllum dianthus in this fjord and transplanted corals between spatially close habitats with contrasting physico-chemical conditions to investigate its ability to acclimatise to changing environmental conditions. This reciprocal transplantation experiment revealed the fast acclimatisation potential of most investigated traits of D. dianthus to a new environment, as calcification and respiration rates were clearly determined by the environment, demonstrating a high phenotypic plasticity. Unexpectedly, corals at greater depth (300 m) had higher calcification rates than corals in shallow waters (20 m), despite aragonite undersaturation in deep waters. I was able to show that natural environmental variability is inversely correlated with CWC calcification and therefore, stable environmental conditions in deep waters of the fjord are beneficial for the corals.

Food availability and energy reserves are assumed to be important parameters for the ability of CWCs to cope with environmental changes, but little is known so far about their in situ food sources. Therefore, we investigated the in situ biochemical composition, including fatty acid composition and lipid classes, and trophic ecology of D. dianthus in more detail on a spatial (manuscripts 3 and 4) and seasonal scale (manuscript 4) in Comau Fjord to determine which biotic and abiotic factors influence it the most and how energy reserves can be related to coral calcification. Energy reserves strongly correlated with calcification as deep corals had a higher amount of total energy reserves and storage lipids, indicating that deep corals receive more food. Examination of fatty acid trophic markers (FATM) in conjunction with differences in lipid classes revealed differences of the zooplankton community the corals are feeding on in the two water depths. In addition, the energy reserves of novel deep corals (transplanted from shallow into deep waters) also increased rapidly, underscoring the findings of manuscript 2 on the rapid acclimatisation potential of D. dianthus.

Overall, this thesis improves our understanding how environmental changes affect CWC physiology and biochemical composition by considering different CWC life stages, the interaction of multiple environmental factors, natural environmental variability and potential differences in food availability. This thesis highlights for the first time the relevance of differential effects across CWC life stages when studying the impact of environmental changes. I show that early life stages may represent an important bottleneck for the resilience of CWC populations. Since I was able to show that CWC calcification is affected by environmental variability, future studies should consider both natural fluctuations in environmental parameters and the interactive effects of multiple environmental parameters to gain a more realistic understanding of the in situ response of CWCs in a future changing ocean. The composition of fatty acids and lipid classes, as well as the inclusion of reduced food supply as another parameter in the multiple driver experiment, provided a better understanding of the importance of food availability and composition for the resilience of CWCs to current and future environmental changes. In addition, the in situ biochemical composition of CWCs gave new insights into their natural diet and the relationship between energy reserves and calcification.