Effects of eutrophication and heatwave events on the oxygen dynamics of algal turf

Algal turfs are sparse to thick mats of filamentous macroalgae that colonize intertidal and subtidal hard substrata. Turfs are replacing native macroalgal forests in many coastal ecosystems globally, particularly in urbanized marine systems, with significant impacts on ecosystem functions and services. The recruitment of algal turfs has been related to both eutrophication and warming, but no studies have explored the combined effects of these two stressors on established turfs, and the potential role of the associated microbiome in mediating the effects of these stressors remains elusive. This study investigates: first, the oxygen productivity of algal turfs growing in oligotrophic or eutrophic conditions at different temperatures (20°, 22°, 24°, 28° and 32° C) and, second, how an heatwave event can further impact the system. Turfs were sampled from urban artificial structures in Venice Lagoon and grown in lab conditions exposed to two treatments: “fertilized”, which mimicked eutrophic conditions, and “control”, which mimicked baseline oligotrophic conditions. The oxygen dynamics of both treatments were recorded via an in-situ respirometry in dark and light conditions. In the second experiment the turf was grown at 28° C and in the same nutrient conditions and then exposed to a simulated marine heatwave event. The results of the first experiment showed that under baseline temperature conditions the turf is autotrophic regardless of nutrient load until 28° C, where the fertilized turf switches to a heterotrophic regime. In the second experiment, after the heatwave exposure the turf showed net oxygen consumption regardless of nutrient treatment, which can lead to negative impacts on the productivity of the system. To contextualize the relevance of the work, sea temperature data were analyzed to discover the number and intensity of heatwaves affecting the Venice Lagoon. The analysis confirmed the global trend of the increase in frequency and severity of Marine Heatwave Events in the Venice lagoon.