Marine heatwaves effects on quantity, composition, and turnover of sedimentary organic matter in a Mediterranean coastal lagoon: A benthocosm study

Marine heatwaves (MHWs), caused by climate change, are becoming increasingly frequent and more intense, impacting benthic marine ecosystems. Nonetheless, knowledge about MHWs effects on the sediment biogeochemistry of coastal lagoons are almost unknown. We show that MHWs of different intensity (+4, +8 °C) simulated in benthocosm altered quantity, biochemical composition and nutritional quality of sedimentary organic matter obtained from a Mediterranean coastal lagoon. Sedimentary contents of carbohydrates, chlorophyll-a, and total phytopigments and biopolymeric C decreased significantly (by ca. −25, −37, −34, −15 %, respectively) already at +4 °C; at +8 °C those changes were accentuated for almost all variables (by −34, −48, −50, −14 %, respectively), including lipids (by −16 %), ultimately causing a drop in the nutritional quality of food for the benthos. This was plausibly the result of interacting (possibly synergistic) factors including the decay of labile molecules exposed to warmer conditions, an increased consumption due to the stimulation of heterotrophic activity by warmer temperatures, and an increased degradation (up to 31 % at +4 °C) due to the stimulation of extracellular enzymes. We can anticipate that more frequent, intense and long-lasting MHWs will influence at large sedimentary biogeochemistry in coastal lagoons and their ability to store blue C. However, since our experiments did not control for other factors possibly interacting at the microscale (e.g. microbial communities’ responses to warming and hypoxia), we foster the need of further studies to disentangle the effects of MHWs from those of other climatic stressors. • Simulated heatwaves of different intensity altered lagoon sediment biogeochemistry. • Heatwaves decreased the nutritional quality of sedimentary food for the benthos. • Heatwaves had differential effects on different molecules. • Heatwaves accelerated organic matter degradation. • Global warming will alter lagoons' ability to store blue carbon.