Effects of an experimental summer heatwave on a small-sized plankton community from a Mediterranean coastal lagoon

Coastal lagoons are vulnerable ecosystems subjected to increasing anthropogenic pressures and the effects of climate change, notably in the Mediterranean region. Extreme events, such as heatwaves are predicted to become more intense and frequent, and to date their effects on planktonic food webs in transitional ecosystems have been poorly studied. We simulated a 14-days lasting summer heatwave in laboratory to investigate its experimental effects on the whole natural plankton community collected from Cabras Lagoon (Western Mediterranean). We exposed the natural community to +5 °C increase in temperature compared to a control (environmental temperature) in presence and in absence of planktonic apical predators. A long planktonic trophic web was detected, composed of several trophic levels dominated by very small-sized taxa: Mnemiopsis leidyi – rotifers – ciliates – Chlorophyceae as phytoplankton – heterotrophic nanoflagellates – picocyanobacteria and bacteria. This revealed low energy available for planktivorous fish in the lagoon which has a great economic value in relation to fishery's activities. The simulated summer heatwave strongly affected the heterotrophic community, triggering defense mechanisms in rotifers, altering ciliate abundance (with short-term increases and long-term declines), and reshaping their taxonomic composition. Warming also indirectly influenced heterotrophic nanoflagellates through intensified ciliate grazing. In contrast, bacterial and autotrophic components remained largely stable, sustaining heterotrophic changes without significant decline. This study provides new insights on how heatwaves can affect the structure of a small-sized planktonic trophic web in coastal lagoons, contributing to a broader understanding of heatwave impacts on coastal lagoon functioning and productivity under future warmer scenarios.