Sediment-mediated eutrophication in coastal lagoons: A Systematic review of benthic nutrient fluxes and management implications

Shallow coastal lagoons are among the ecosystems most vulnerable to eutrophication; their sediments, which act as nutrient reservoirs under oxic conditions, become diffuse sources of nitrogen and phosphorus in hypoxic conditions, prolonging the eutrophic state despite reduced external inputs. This systematic review, conducted according to PRISMA-ScR, searched Web of Science, Scopus and Google Scholar; 82 publications were selected after double screening, and quantitative data from 26 lagoon/estuary ecosystems across four continents were analyzed. The meta-synthesis of 327 benthic flux measurements shows a median of 1.9 mmol·m -2 ·d -1 for phosphorus in lagoons (approximately ×2 of shallow lakes) and 3.8 mmol·m -2 ·d -1 in tropical environments, revealing a high thermal and redox sensitivity of P-Fe locks. For nitrogen, the median net flux reaches 11 mmol·m -2 ·d -1 ; 85% of positive fluxes come from NH 4 + efflux (DNRA dominant in eutrophic contexts), while 34% of cases indicate a sink via N 2 production (denitrification/anammox). Analysis of the controlling factors identifies background oxygen as the main lever, followed by organic load, NO 3 - availability, bioturbation, salinity and temperature, which together explain 82% of the variance in PO 4 3- and NH 4 + fluxes. On this basis, effective management of eutrophic lagoons rests on two inseparable pillars: (i) sustainable reduction of external inputs (a prerequisite) and (ii) treatment of internal stocks (targeted dredging, reactive capping, oxygenation/mixing) in a diagnostic-dependent and adaptive framework, complemented by nature-based solutions to stabilize gains. This summary highlights the operational importance of sediments in eutrophication trajectories and provides orders of magnitude and levers to guide the restoration of coastal lagoons.