How marine organisms cope with changing climate

Abstract

As anthropogenic CO2 levels continue to rise, the oceans are becoming warmer and more acidic. Organisms need to adjust to such environmental changes and display a variety of mechanisms to maintain their fitness in novel conditions. These adjustments can operate at various levels of biological organisation: from cellular levels to organismal physiology and behaviour. Such adaptive responses of species will determine their persistence under future ocean warming and acidification conditions. If organisms are capable of maintaining fitness after long-term exposure to a stressor this can be indicative of acclimation potential. However, their sensitivity to stressors is linked to life stage. Early life phases are considered to be the most vulnerable to fluctuations in the environment. If detrimental effects occur during an organism’s early life this could modify its capability to handle stress at later life stages. The physiological and behavioural adjustments that are triggered in response to changing conditions can lead to modifications in the phenotypic distributions of traits within a population. Analysing the variation of phenotypical traits offers an insight into the capacity of populations to persist by acclimating to their environment. In this thesis I evaluated the sensitivity of marine organisms to ocean warming and acidification and their various coping mechanisms. I reveal that ocean acidification and warming can alter the behaviour of fish species by increasing their anxiety (chapter 2), boldness (chapter 3 and 5), or feeding rates (chapter 2). Modifications in feeding behaviour were linked to physiological and to changing environmental conditions, creating a feedback mechanism between their cellular and behavioural responses that helped organisms maintain their fitness (chapter 3). However, altered behaviours in a population are not always accompanied by physiological changes, as in chapter 5 I also found changes in risk taking behaviours that did not alter the body condition of temperate or tropical fishes. The direction of responses (negative, positive or neutral) exhibited by a species in response to changing conditions will depend on their specific physiological requirements that determine their sensitivity to stressors. Using a meta-analysis in chapter 4 I showed that when facing climatic stressors, the growth and survival of diverse marine species vary according to their species-specific physiological requirements. For example, negative responses in growth were observed in calcifying organisms and positive responses were found for primary producers. Life stage was key in determining survival, as eggs and larvae showed to be more vulnerable to stressors than older juvenile and adult stages. The sensitivity of early life stages was also found in laboratory experiments performed in this thesis (chapter 2). A mouthbrooder species was used to test early stage sensitivity, and I showed that the parental environment of the mouthbrooder fish did not provide protection to embryos from acidified conditions. Enriched CO2 conditions exerted negative effects on the behaviour of their juvenile stage by increasing their anxiety. The distinct species-specific responses in physiology and behaviour have the potential to modify the distribution of phenotypical traits. I revealed that ocean acidification and warming can alter the phenotype distribution of risk taking behaviours (chapter 5). The redistribution of phenotypical traits has the potential to re-shape populations interactions as more dominant species are selected for under future conditions. Additionally, under naturally acidified and warming conditions I found that some species experience a loss of risk-taking phenotypes, as their phenotypic variability was reduced compared to the control conditions. This behavioural homogenisation puts populations of animals at risk to increasing global environmental change. The coping strategies that species use by adjusting their physiology and behaviour can enable them to maintain their fitness under climate change. If species maintain fitness during their entire life span and in future generations, then species will have a greater chance to persist under climatic disturbances. Understanding species sensitivity and their potential to acclimate to environmental change will help improve how we anticipate the future of adaptive capacity of organisms to warming and acidifying oceans.