Land–sea connectivity after megafire: ash-derived nutrient pulses drive short-term responses in Mediterranean coastal ecosystems

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Abstract
Wildfires are becoming more frequent and severe under a warming and drying climate, with Mediterranean regions emerging as global hotspots of megafire activity whose impacts extend from upland catchments to coastal waters. Although the terrestrial effects of fire are well established, the mechanisms by which wildfire by products influence marine ecosystems remain poorly resolved. Here, we investigate whether and how a Mediterranean megafire (12,000 ha) altered post fire fluxes of nutrients, sediments properties, and combustion derived compounds, causing changes in coastal biogeochemistry and ecosystem responses across land–sea and depth gradients. Ash-derived runoff generated a transient pulse of macronutrients and organic matter (TP, TDN, DOC and Si), producing short lived increases in turbidity and chlorophyll a in coastal waters. Stable isotopes and C/N ratios in primary producers indicated enhanced nitrogen uptake from riverine inputs, with intertidal macroalgal forests—particularly Ericaria spp.—showing marked short-term recovery. In contrast, responses were minimal in seagrass Posidonia oceanica and geniculate coralline algae, and absent in both subtidal macroalgal communities and deep coralligenous habitats. Our results show that megafires can temporarily reconfigure land-sea nutrient fluxes generating short-lived increases in primary productivity in oligotrophic Mediterranean coasts. As wildfire activity intensifies under climate change, understanding land–sea connectivity will be essential for predicting cascading impacts on coastal ecosystem functioning.

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