Multi-year renewal of green tides: 18 years of algal mat monitoring (2003–2020) on French coastline (Brittany region)

To better understand the green tide phenomenon impacting French coastline and give guidance to stakeholders to elaborate effective mitigation plan, an extensive survey has been deployed on Brittany hot spots. Based on 18 years monitoring database, the objectives of this work were to investigate the inter-and-intra annual evolutions of Ulva beaching, and to assess the parameters driving green tide annual renewal. The yearly cumulated area of Ulva mat on the Brittany coast averaged 2,42 ± 0,84 ha, of which 55 ± 12 % % was reported within Saint Brieuc-Binic Bay, with a maximal beaching generally observed in July. The renewal of green tide at spring time seems to be correlated with the Ulva bloom from the previous autumn period, particularly in bays with low exposure to swell. The residual stock, defined as Ulva fragments maintained in healthy conditions during the winter season, appears as highly dependent on nitrogen summer flows. •The yearly cumulated area of Ulva mat on the Brittany coast averaged 2,4 ± 0,8 ha.•More than half of Ulva beaching occurred at Saint Brieuc-Binic Bay.•Green tide renewal is tighly linked to the residual algal stock of the last year.•Large nitrate fluxes at the end of summer could promote the renewal of green tides.•The evolution of green tides will be driven by the future mitigation strategies.