Effects of an Experimental Heatwave on Phytoplankton Morpho-Functional Traits from Two Natural Vulnerable Mediterranean Ecosystems

Phytoplankton are photosynthetic microorganisms suspended in water, playing a fundamental role in biogeochemical cycles and aquatic food webs. The cell is the basic biological unit of phytoplankton, and its morphological features, particularly cell geometric shape, volume, and surface to volume ratio (S/V) strongly influence physiological processes and adaptive capacity. These morpho-functional traits determine the ability to acquire light and nutrients, sinking rate, and resistance to grazing. Global warming is increasingly changing aquatic ecosystem structure and functioning. Climate models predict a marked rise in the frequency, duration, and intensity of heatwaves extended periods of abnormally high temperatures. This phenomenon is becoming more frequent globally, with particular intensity in the Mediterranean region. In this proposal, we focus on morpho-functional traits of organisms incubated in a laboratory experiment simulating a summer heatwave, conducted in July 2024. The experiment involved two natural plankton communities, collected from an artificial lake (Bidighinzu Lake) and a coastal lagoon (Cabras Lagoon) in Sardinia (Western Mediterranean), including multiple trophic levels (from picoplankton to mesoplankton). Each community was incubated for 15 days and exposed at two cross treatments in triplicates: increased temperature (+ 5 °C) compared to the control at environmental temperature and the presence/absence of mesozooplankton. With the aim to understand which phytoplankton morpho-functional trait persist and dominate during the heatwave, in this study we grouped phytoplankton of each community according to several morphological traits: cell geometric shape, cell volume, cell area, cell S/V, colony, presence of flagella. This trait-based morphological approach can provide a valuable framework to know the effects of heatwaves on phytoplankton communities and therefore on the functioning of Mediterranean vulnerable aquatic ecosystems. To date, similar studies have not yet been conducted.