Tracing Localised Nutrient Pollution Events to Posidonia oceanica Epiphyte Community Assemblages, in the Eastern Aegean Sea

Abstract The occurrence of localised eutrophication events in small coastal communities of the Mediterranean threatens seagrass ecosystem functionality, necessitating the reliable identification of the onset and magnitude of these processes. This study examined the suitability of epiphyte assemblages on Posidonia oceanica leaves as an indicator of anthropogenic nutrient enrichment, for the first time in the eastern Aegean Sea. Seagrass biometric and meadow parameters were studied at control and potentially disturbed sites (n=3), in addition to epiphyte community composition and structure at a depth of 5 m during winter 2022. Sites were classified using a modified version of the Land Use Simplified Index. Potentially disturbed meadows exhibited decreased leaf length and shoot density, and increased epiphyte cover and biomass. Epiphyte community composition shifted significantly, with clear differences observed in red coralline algae, filamentous brown algae, bryozoans, and annelids. Results highlighted that reduced water quality, which is discontinuous and therefore difficult to tackle with a single direct measurement in the water column, from point and diffuse sources including a sewage outlet, agricultural activity and tourism, strongly affected seagrass bed condition. Here, substantial epiphyte cover was observed, causing direct (shading) and indirect (intensified grazing; physical leaf instability) effects. The study revealed that epiphyte communities, in combination with the index, serve as accurate and cost-effective indicators of coastal water pollution. Regular monitoring could aid in detecting higher nutrient loading at an early stage and potentially prevent long-term meadow decline.