"THE ROLE OF C-CYCLE INTO THE ECOSYSTEM FUNCTIONING OF MARINE SYSTEMS AND THE INTERACTION WITH ANTHROPOGENIC DRIVERS"

The functioning of marine ecosystems is closely tied to the carbon cycle, a fundamental process that regulates the flow of energy and matter in the oceans and supports the biological communities within them. Oceans absorb approximately 30% of human-produced atmospheric CO₂, making them crucial reservoirs in mitigating global warming. This process is largely driven by the biological pump, a mechanism that transfers carbon from the ocean’s surface to the deep sea through primary production and the sedimentation of organic matter. However, human-caused drivers of change, like climate change and fishing, are threatening these balances by both affecting the carbon cycle and by compromising the resilience of marine species whose functions are strictly related to it. The objective of this PhD thesis is to analyse the impact of two key human-caused drivers of change, climate change and fishing (specifically trawling), on the carbon cycle in marine ecosystems, with a focus on planktonic and benthic systems. These pressures can induce fundamental changes in the oceans' capacity to sequester carbon, making it crucial to understand how these processes affect ecosystem health and the services they provide, such as climate regulation. In the context of climate change, marine plankton plays a pivotal role in the carbon cycle, transferring atmospheric carbon to the deep ocean through photosynthesis and the sedimentation of organic particles. The first two chapters of this thesis explore the effects of climate change on planktonic systems, a key group for marine ecological balance. The first chapter provides a comprehensive review of the existing scientific literature, showing how ocean acidification and warming impact plankton diversity and their role in the carbon cycle. Key findings include how these changes may reduce primary productivity, altering the efficiency of the biological pump and, in turn, global carbon sequestration. The second chapter presents an experiment at a naturally acidified hydrothermal site in Panarea Island (Mediterranean Sea), examining the metabolic response of plankton under different acidified conditions. Although the community's structural composition remained stable, metabolic activity from acidified samples was significantly reduced, highlighting the risk of functional degradation masked by structural resilience. The study also analysed the Arrhenius Break Point to understand the plankton’s response to thermal and acidification stress, offering insights into potential long-term shifts under continued climate pressure. The second part of the thesis focuses on benthic ecosystems and the impact of trawling on macro epifaunal and infaunal communities across the Mediterranean continental shelves along gradient of trawling disturbance. Trawling has been selected as representing a fishing practice that significantly affects biogeochemical cycles, including the carbon cycle. The benthic communities, comprising organisms living “on” and “in” the seafloor, plays an essential role in ecosystem functioning and the carbon cycle, through processes like bioturbation, enhancing nutrient recycling and the remineralization of organic matter, thus contributing to carbon sequestration and primary productivity. In the third chapter, the infaunal biodiversity was examined in relation to depth and sediment grain size at two gulfs (Gulf of Castellammare del Golfo “CG” and Gulf of Catania “CT”), revealing how these natural gradients influence community composition. Although the study did not directly assess the role of infauna in carbon dynamics, the findings provide valuable knowledge baseline into how benthic diversity supports broader ecosystem functions, including carbon flux regulation, specifically under untrawled conditions like the ones revealed inside the untrawled areas of CG. The fourth chapter focuses on secondary production, a critical measure of ecosystem productivity and energy transfer through food webs. Secondary production, reflecting the rate at which benthic organisms convert energy into biomass, directly influences the carbon cycle by driving sedimentation and organic matter recycling. The analysis of epibenthic species' secondary production in the Gulf of Castellammare “CG” highlighted significant variability in production rates, suggesting that species like the suspension-feeder ophiuroid Astrospartus mediterraneus and deposit-feeder pagurid Pagurus bernhardus play vital roles in the carbon dynamics of marine sediments. This chapter underscores the importance of considering benthic species’ functional roles in carbon sequestration processes. A further key component of this research involved measuring oxygen consumption as a proxy for ecosystem functioning. Oxygen consumption provides insights into the metabolic activity of marine communities, allowing for the assessment of biogeochemical processes such as organic matter decomposition and carbon cycling. In the study, oxygen consumption was measured showing how different species contribute to the benthic system’s metabolic activity. The experiments also analysed the content of carbon biopolymers in sediments before and after acclimation, shedding light on benthic species' role in carbon recycling. Finally, the fifth chapter investigates the impact of trawling on bycatch, non-target epibenthic species accidentally caught by trawling, through an experiment simulating the stress experienced by captured and released species. The experimental setup was based on Local Ecological Knowledge (LEK), ensuring it reflected real-world conditions. The study assessed the effects of fishing-induced stress by measuring oxygen consumption immediately after the capture on board and evaluating species' recovery after 24 hours. This dual approach provides valuable insights into the long-term impacts of fishing on bycatch resilience, something that will affect both species survival and then species functions impairment with direct link on the carbon cycle. This research provides novel, sometimes preliminary, insights to the understanding of the effects of climate change and fishing on the carbon cycle in marine communities and ecosystems. By combining laboratory experiments, manipulations, natural and human driven gradients, biodiversity and functioning indices and measures, the thesis confirms the complexity of interactions between biogeochemical processes and anthropogenic pressures. Some of the findings offer interesting insights to be used to inform sustainable management of oceans, particularly in developing conservation strategies that safeguard both biodiversity and critical ecosystem functions like carbon sequestration.