| Summary: | Highly oligotrophic marine waters cover about a third of the Earth’s surface and have diverse communities that are affected by multiple stressors, such as ocean acidification, warming, marine litter, pollution, overfishing. The low nutrient Mediterranean Sea has high biodiversity and endemism but is heavily impacted by these human pressures and a recent major influx of invasive alien species that compete, predate and infect indigenous species. Aquaculture effluents and eutrophication alter water quality and add pressure on coastal ecosystems that are already subject to habitat loss and degradation due to coastal developments. This PhD dissertation reviews human impacts on highly oligotrophic marine ecosystems (Chapter 2). It then presents case studies that cover multiple impacts around Cyprus. A major impact to the Mediterranean Sea is immigration of Indo-Pacific species, which has accelerated in recent years due to climate change and the widening of the Suez Canal. Chapter 3 compares populations of the seagrass Halophila stipulacea both in their alien and native environments. It also presents the detection of the lionfish Pterois miles providing the first evidence that a lionfish invasion in the Mediterranean is underway. Shallow rocky reefs covered by canopy-forming brown seaweed and meadows of Posidonia oceanica are declining rapidly throughout the Mediterranean. I studied these important ecosystems across the most industrialised coastline of Cyprus. Chapter 4 characterises macroalgal communities along an impact gradient and shows that macroalgae are robust indicators to assess the ecological status of coastal waters. Communities dominated by canopy-forming Cystoseira at pristine sites, shifted to turf and filamentous opportunistic seaweed on industrialised coastlines. Chapter 5 characterises sediment chemical variables, delivers the first habitat cartography, describes structural descriptors of P. oceanica and assesses the ecological condition across Vasiliko Bay. Despite mounting pressures, ancient P. oceanica meadows in Vasiliko Bay have among the highest shoot densities across the whole Mediterranean. However, lower shoot densities and more epiphytes were noted near fish farm sites. In Chapter 6 monitoring of P. oceanica descriptors from fixed plots established at the lower limits of the meadows near fish farms shows that a management decision to relocate farms into deeper waters had prevented further declines in seagrasses. Chapter 7 presents Integrated Multi-Trophic Aquaculture (IMTA) trials carried out off southern Cyprus. The concept of IMTA is to cultivate organisms (e.g. seaweed, filter feeders, detritivores) around finfish cages, converting waste effluents into product and mitigating environmental impacts. Trials lasted three years and identified opportunities and challenges that impede IMTA adoption by fish farms in warm oligotrophic conditions. Taken as a whole, this thesis demonstrates that ecology of the eastern Mediterranean Sea is changing rapidly due to multiple anthropogenic stressors, but strategic management can help halt, and in some cases reverse, the damage done.
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