Climate change impacts on ecosystem functioning and services provided by temperate benthic ecosystem

The world is going through severe climatic changes and this transition period is characterized by peculiar phenomena. Despite the mean global temperature increase, episodes of higher frequency and longer heatwaves will occur more often. Heatwaves have already disrupted resilience of macrophyte species, resulting in massive mortality of seagrasses and macroalgae. Macrophytes are responsible for considerable amounts of carbon storage and sequestration in marine coastal ecosystems. Many macrophytes are also considered foundation species since they harbor high species diversity, which results in large capacity to structure a community. Thus, heatwaves are able to cause shifts in biodiversity and ecosystem functioning. The goal of this thesis was to improve the understanding of the mechanistic effect of climate change on temperate coastal marine ecosystems based on macrophytes meadows. The first chapter showed that grazing intensity (that also depends on temperature) contributed to control the amount of carbon stored on the macroalgae and modified the trend to a non-linear pattern. The lowest carbon storage capacity was found between 20 and 22 °C, following the consumption intensity of the grazers. The second chapter, through the analysis of attributes and the application of ecological network analysis, I demonstrated that three sequential heatwaves jeopardized the capacity of the ecosystem to store carbon, since photosynthesis declined and the size of the ecosystem decreased. Moreover, three heatwaves caused a simplification on the pathways of carbon circulation, which makes the ecosystem more vulnerable to further disturbances. In the third chapter, I analyzed with a qualitative network model (loop analysis) the impact of sequential heatwaves on the capacity of the ecosystem to deliver services. The provision of water purification and climate regulation services was impaired, while the capacity of habitat provision did not change after the exposure to heatwaves. Therefore, the conclusions of this thesis are that: (1) both changes in temperature regimes (i.e. average constant and heatwaves) have profound effects on single species physiological performance and modify trophic interactions, thus altering energy circulation in food webs; (2) three consecutive heatwaves during spring/summer represented a threat for the health of the benthic ecosystem studied due to the reduction of productivity and the lowered diversity of energy flows, which increased vulnerability of the system; (3) besides having affected the functioning of the ecosystem, the heatwaves also harmed the capacity of the ecosystem to provide water purification and climate regulation services. This thesis showed that more knowledge on ecosystem functioning and services can be generated by combining experimental and modeling approaches. Furthermore, focusing on whole ecosystems rather than on isolated responses of single organisms expands our comprehension on the effects of changes in temperature regimes, which may assist further climate change mitigation measures.


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