Land-sea connectivity after megafire: Post-fire runoff-associated nutrient pulses and short-term responses in...

Wildfires are becoming more frequent and severe, 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 materials mobilised from burned catchments influence marine ecosystems remain poorly resolved. We investigate whether and how a Mediterranean megafire (~12,000 ha) was followed by changes in nutrient concentrations and coastal water column conditions and whether these changes were associated with short-term ecosystem responses across land-sea and depth gradients. Post-fire runoff was associated with transient riverine pulses of total phosphorus (TP), total dissolved nitrogen (TDN), and potassium (K), whereas coastal waters showed variable, element-specific responses, mainly involving TP and dissolved silicon (Si). Satellite-derived Chl-a showed local positive departures from pre-fire baseline conditions during the runoff window, but no statistically supported disturbance-specific response. Underwater light records provided descriptive evidence of reduced light availability at the disturbed site during the earliest available post-runoff monitoring period. Stable isotopes and carbon-to‑nitrogen (C/N) ratios in primary producers were consistent with site-specific changes in nitrogen availability, while intertidal macroalgal forests, particularly Ericaria spp., showed marked short-term recovery. Responses were minimal in Posidonia oceanica meadows and geniculate coralline algae, and absent in subtidal macroalgal communities and deep coralligenous habitats. Overall, post-fire runoff was associated with temporary changes in land-sea nutrient connectivity in oligotrophic Mediterranean coastal systems. Biological responses emerged most clearly in shallow intertidal macroalgal habitats, whereas responses were weaker or undetectable across subtidal and deeper habitats.