Effects of nutrients on respiratory rates of algal turf communities of the North Adriatic Sea

Marine environments are increasingly facing urbanisation-related problems, such as eutrophication, which led algal turf to colonise artificial structures of coastal areas. These communities are generally considered to be net-autotrophic, and therefore used in greening infrastructures to provide ecosystem services. However, recent evidence question the overall efficacy of algal turf in sequestering carbon: specifically, under high nutrient pressure the turf-associated bacterial community increases in its respiratory demand while the covering debris limits the light available to photosynthesis, resulting in a net O2 consumption and, thus, CO2 production. Therefore, it is paramount to fully understand if there is actually a shift from a net production to a net consumption of O2 in algal turf under eutrophic conditions, in order to verify if they could still act as a carbon sink in urbanised coastal environments. Through a month-lasting field experiment in the Venice Lagoon (Italy), we compared respiratory rates of algal turf communities grown under fertilisation regime and arthropod exclusion treatment to rates of control communities. Surprisingly, results revealed a net consumption of O2 in both fertilised and control turf. Further investigations suggested that due to the high environmental/experimental temperatures (28°C - 30°C), turf algae were already stressed, meaning that also warm temperatures could be a major respiration driver. This outcome might be helpful to understand algal turf role in global warming scenarios, such as heat waves.