Coastal lagoons as transitional aquatic ecosystems are hotspots of biodiversity and highly productive, thus providing numerous ecosystem services. They are subjected to increasing anthropogenic pressures and the effects of climate change, notably in the Mediterranean region. Specifically, extreme events, such as heatwaves are predicted to be more intense and frequent in the Mediterranean, and to date their effects on planktonic food webs in transitional ecosystems have been poorly studied.

In July 2024 we simulated a summer heatwave in laboratory to investigate its experimental effects on an entire plankton community collected from Cabras Lagoon, the largest shallow costal lagoon in Sardinia (Western Mediterranean). With the aim to distinguish direct warming effects from those indirect mediated by grazing, we exposed the natural community for 15 days to +5°C increased temperature compared to a control (environmental temperature) in presence and in absence of planktonic apical predators, mainly represented by rotifers and the ctenophore Mnemiopsis leidyi in Cabras Lagoon.

Preliminary, we observed significant taxonomical shifts throughout the experiment:

• increased affirmation of rotifer resting stages under heatwave;
• among ciliates, Oligotrichia were replaced by Euplotia and Scuticoliatia under heatwave in absence of apical predators, and only by Euplotia under heatwave in presence of apical predators;
• among phytoplankton, Cyanophyceae and Bacillariophyceae increased under heatwave in absence of apical predators, while Chlorophyceae and picocyanobacteria decreased under heatwave in presence of apical predators.

First observations also highlighted a strict negative correlation among abundances of ciliates Oligotrichia and heterotrophic nanoflagellates following the experimental manipulation, suggesting predator-prey dynamics.

This study provides new insights on how heatwaves affect the size structure of a planktonic trophic web in Mediterranean coastal lagoons where fishing is one of the main economic activities.