Zooming in on the summer 2024 mucilage outbreak in the northern Adriatic: Shifts in microbial abundances and organic matter dynamics

TY - JOUR

T1 - Zooming in on the summer 2024 mucilage outbreak in the northern Adriatic

T2 - Shifts in microbial abundances and organic matter dynamics

AU - Budiša, Andrea

AU - Simonović, Niki

AU - Paliaga, Paolo

AU - Najdek, Mirjana

AU - Ciglenečki, Irena

N1 - Publisher Copyright: Copyright © 2026. Published by Elsevier Ltd.

PY - 2026/10

Y1 - 2026/10

N2 - In summer 2024, three mucilage events occurred along the western Istrian coast of the northern Adriatic—natural phenomena characterized by amorphous aggregates of organic matter that have recurred for three centuries, particularly during the 1990s and early 2000s. These events developed under exceptionally warm, low-salinity, and stratified conditions driven by the advection of nutrient-rich Po River waters. Each event was initiated by phytoplankton blooms releasing large quantities of surface-active dissolved (DOM) and particulate organic matter (POM), fostering the formation of polysaccharide-rich matrices that supported microbial enrichment. Within the mucilage, heterotrophic prokaryotes, cyanobacteria, and heterotrophic nanoflagellates reached abundances up to an order of magnitude above long-term regional means, exceeding previous records from gelatinous plankton blooms and eutrophic outbreaks. The elevated microbial abundances within these aggregates, together with lower pH and oxygen saturation and shifts in organic matter composition, suggest enhanced microbial processing of organic matter and are consistent with aggregate compaction and sinking during mucilage ageing. Differences among the June, July, and August events reflected variations in hydrographic forcing, bloom intensity, and pycnocline depth, which modulated aggregate structure, microbial abundance, and surface residence time. The August outbreak—the most extensive event—exhibited pronounced diurnal oscillations in microbial abundances, with afternoon peaks at the surface and pycnocline coinciding with wind-driven mucus accumulation. Overall, mucilage aggregates appear to function as transient biogeochemical reactors and microbial hotspots that may accelerate organic matter cycling in the northern Adriatic, with important implications for predicting their recurrence and ecological impacts under future climate-driven warming and altered riverine inputs.

AB - In summer 2024, three mucilage events occurred along the western Istrian coast of the northern Adriatic—natural phenomena characterized by amorphous aggregates of organic matter that have recurred for three centuries, particularly during the 1990s and early 2000s. These events developed under exceptionally warm, low-salinity, and stratified conditions driven by the advection of nutrient-rich Po River waters. Each event was initiated by phytoplankton blooms releasing large quantities of surface-active dissolved (DOM) and particulate organic matter (POM), fostering the formation of polysaccharide-rich matrices that supported microbial enrichment. Within the mucilage, heterotrophic prokaryotes, cyanobacteria, and heterotrophic nanoflagellates reached abundances up to an order of magnitude above long-term regional means, exceeding previous records from gelatinous plankton blooms and eutrophic outbreaks. The elevated microbial abundances within these aggregates, together with lower pH and oxygen saturation and shifts in organic matter composition, suggest enhanced microbial processing of organic matter and are consistent with aggregate compaction and sinking during mucilage ageing. Differences among the June, July, and August events reflected variations in hydrographic forcing, bloom intensity, and pycnocline depth, which modulated aggregate structure, microbial abundance, and surface residence time. The August outbreak—the most extensive event—exhibited pronounced diurnal oscillations in microbial abundances, with afternoon peaks at the surface and pycnocline coinciding with wind-driven mucus accumulation. Overall, mucilage aggregates appear to function as transient biogeochemical reactors and microbial hotspots that may accelerate organic matter cycling in the northern Adriatic, with important implications for predicting their recurrence and ecological impacts under future climate-driven warming and altered riverine inputs.

KW - Microbial abundance

KW - Mucilage

KW - Northern Adriatic

KW - Organic matter

KW - Phytoplankton bloom

KW - Thermohaline stratification

U2 - 10.1016/j.marpolbul.2026.120005

DO - 10.1016/j.marpolbul.2026.120005

M3 - Article

AN - SCOPUS:105041602304

SN - 0025-326X

VL - 231

JO - Marine Pollution Bulletin

JF - Marine Pollution Bulletin

M1 - 120005

ER -