{"id":1014,"date":"2026-05-12T11:43:34","date_gmt":"2026-05-12T09:43:34","guid":{"rendered":"http:\/\/greenseas.fr\/?p=1014"},"modified":"2026-05-12T11:44:27","modified_gmt":"2026-05-12T09:44:27","slug":"restitution-greenseas-1","status":"publish","type":"post","link":"http:\/\/greenseas.fr\/index.php\/2026\/05\/12\/restitution-greenseas-1\/","title":{"rendered":"Restitution GreenSeas #1 : Histoires d’apparitions : l’\u00e9mergence des mar\u00e9es vertes en Bretagne (1960s-1980s)"},"content":{"rendered":"\n

Jeudi 4 juin 2026 (10h-12h)<\/strong><\/p>\n\n\n\n

Intervenant\u00b7es : <\/em><\/p>\n\n\n\n

Justine Louis (CEVA), Alix Levain (CNRS, EMR POSSEA-AMURE), Sylvain Ballu (CEVA), Herv\u00e9 Squividant (Institut Agro, UMR SAS)<\/p>\n\n\n\n

R\u00e9sum\u00e9 : <\/em><\/p>\n\n\n\n

Les premi\u00e8res occurrences de mar\u00e9es vertes sont un sujet d\u2019int\u00e9r\u00eat pour une grande diversit\u00e9 d\u2019acteurs sociaux, dont certains ont pu collecter des informations destin\u00e9es \u00e0 conforter des hypoth\u00e8ses fortes sur leur caract\u00e8re tr\u00e8s ancien et leur naturalit\u00e9 ou, au contraire, la brutalit\u00e9 de leur surgissement, en lien avec les changements majeurs ayant affect\u00e9 les infrastructures paysag\u00e8res et les pratiques agricoles dans la p\u00e9riode qui le pr\u00e9c\u00e8de imm\u00e9diatement.
S\u2019ajoutent, sur la longue dur\u00e9e, des \u00e9volutions des enjeux associ\u00e9s \u00e0 cette datation : en particulier, d\u00e8s lors que l\u2019implication des acteurs publics dans la lutte contre les mar\u00e9es vertes est abord\u00e9e avec un point de vue critique et a fortiori par le prisme de l\u2019exposition des populations \u00e0 un risque sanitaire, la mise en \u00e9vidence de l\u2019installation du ph\u00e9nom\u00e8ne sur le temps long, ainsi que la recherche de sources montrant l\u2019\u00e9cart entre la d\u00e9tection du ph\u00e9nom\u00e8ne, la formalisation des alertes, et une forme de prise en charge publique, maintiennent la question de l\u2019\u201dorigine\u201d des mar\u00e9es vertes \u00e0 l\u2019agenda.<\/p>\n\n\n\n

La datation pr\u00e9cise des premi\u00e8res occurrences d\u2019efflorescences massives d\u2019algues, et en particulier des ulves, sur les sites rep\u00e9r\u00e9s et suivis par le CEVA depuis sa cr\u00e9ation en 1982, est ainsi \u00e0 la fois un enjeu scientifique et politique. C’est aussi un d\u00e9fi m\u00e9thodologique, qui implique de croiser diff\u00e9rents types d’expertise, pour d\u00e9velopper une approche critique de sources aussi diverses que des t\u00e9moignages, des photographies a\u00e9riennes, des articles de presse ou des \u00e9tudes scientifiques.<\/p>\n\n\n\n

Dans ce contexte et dans le cadre du projet GreenSeas, le chantier engag\u00e9 par le CEVA, AMURE et l’Institut Agro autour de la datation vise d\u2019une part \u00e0 contribuer \u00e0 l\u2019analyse r\u00e9trospective int\u00e9gr\u00e9e de la trajectoire des socio-\u00e9cosyst\u00e8mes vuln\u00e9rables \u00e0 l\u2019eutrophisation c\u00f4ti\u00e8re, et d\u2019autre part \u00e0 consolider et publiciser le travail de datation de l\u2019 \u201corigine\u201d des mar\u00e9es vertes. La participation des trois \u00e9quipes de recherche au projet constitue une opportunit\u00e9 in\u00e9dite pour croiser les sources h\u00e9t\u00e9rog\u00e8nes collect\u00e9es par le pass\u00e9 et construire ainsi des interpr\u00e9tations plus robustes des traces de blooms<\/em> anciens, mais aussi pour explorer de nouvelles sources, produites par d\u2019autres institutions et\/ou t\u00e9moins du ph\u00e9nom\u00e8ne.<\/p>\n\n\n\n

A l’occasion de cette premi\u00e8re restitution, le groupe de recherche interdisciplinaire \u00ab\u00a0Datation\u00a0\u00bb (Justine Louis, Alix Levain, Sylvain Ballu, Herv\u00e9 Squividant) pr\u00e9sentera les m\u00e9thodes de collecte et d’analyse des sources qu’il a mobilis\u00e9es, les premiers r\u00e9sultats et la fa\u00e7on dont ils seront rendus accessibles \u00e0 un public \u00e9largi. Rendez-vous le 4 juin, rejoignez-nous nombreuses et nombreux!<\/p>\n\n\n\n

Abstract<\/em> : <\/p>\n\n\n\n

The earliest occurrences of green tides are a subject of interest to a wide range of stakeholders, some of whom have gathered information intended to support strong hypotheses regarding their ancient origins and natural character or, conversely, the suddenness of their emergence, linked to major changes that affected the landscape and agricultural practices in the period immediately preceding them.
In the long term, there are also evolving issues associated with this dating: in particular, when the involvement of public actors in the fight against green tides is approached from a critical perspective and, a fortiori, through the lens of the population\u2019s exposure to a health risk, the demonstration of the phenomenon\u2019s establishment over the long term, as well as the search for sources showing the gap between the detection of the phenomenon, the formalization of alerts, and some form of public response, keep the question of the \u201corigin\u201d of green tides on the agenda.<\/p>\n\n\n\n

The precise dating of the first occurrences of massive algal blooms, and in particular of Ulva, at sites identified and monitored by the CEVA since its creation in 1982, is thus both a scientific and political challenge. It is also a methodological one, requiring the integration of different types of expertise to develop a critical approach to sources as diverse as eyewitness accounts, aerial photographs, news articles, or scientific studies.<\/p>\n\n\n\n

In this context and as part of the GreenSeas project, the initiative launched by CEVA, AMURE, and the Institut Agro on dating aims, on the one hand, to contribute to the integrated retrospective analysis of the trajectory of socio-ecosystems vulnerable to coastal eutrophication, and, on the other hand, to consolidate and publicize the work on dating the \u201corigin\u201d of green tides. The participation of the three research teams in the project presents a unique opportunity to cross-reference the diverse sources collected in the past and thus construct more robust interpretations of traces of past blooms, as well as to explore new sources produced by other institutions and\/or witnesses to the phenomenon.<\/p>\n\n\n\n

On the occasion of this first presentation, the interdisciplinary research group \u201cDating\u201d (Justine Louis, Alix Levain, Sylvain Ballu, Herv\u00e9 Squividant) will present the methods used to collect and analyze the sources they have mobilized, the initial results, and how these will be made accessible to a wider audience.<\/p>\n\n\n\n

Lien de connexion : <\/p>\n\n\n\n

Lien de connexion : https:\/\/webinaire.numerique.gouv.fr\/meeting\/signin\/invite\/79946\/creator\/40565\/hash\/e930bdbd49f91aad74793ea724345aaa6ce8d77f<\/a><\/p>\n\n\n\n

<\/p>\n\n\n\n

<\/p>\n","protected":false},"excerpt":{"rendered":"

Jeudi 4 juin 2026 (10h-12h) Intervenant\u00b7es : Justine Louis (CEVA), Alix Levain (CNRS, EMR POSSEA-AMURE), Sylvain Ballu (CEVA), Herv\u00e9 Squividant (Institut Agro, UMR SAS) R\u00e9sum\u00e9 : Les premi\u00e8res occurrences de mar\u00e9es vertes sont un sujet d\u2019int\u00e9r\u00eat pour une grande diversit\u00e9 d\u2019acteurs sociaux, dont certains ont pu collecter des informations destin\u00e9es \u00e0 conforter des hypoth\u00e8ses fortes […]<\/p>\n","protected":false},"author":27,"featured_media":1017,"comment_status":"closed","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"_acf_changed":false,"footnotes":""},"categories":[36],"tags":[],"class_list":["post-1014","post","type-post","status-publish","format-standard","has-post-thumbnail","hentry","category-restitutions-greenseas"],"acf":[],"_links":{"self":[{"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/posts\/1014","targetHints":{"allow":["GET"]}}],"collection":[{"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/users\/27"}],"replies":[{"embeddable":true,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/comments?post=1014"}],"version-history":[{"count":3,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/posts\/1014\/revisions"}],"predecessor-version":[{"id":1018,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/posts\/1014\/revisions\/1018"}],"wp:featuredmedia":[{"embeddable":true,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/media\/1017"}],"wp:attachment":[{"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/media?parent=1014"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/categories?post=1014"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/greenseas.fr\/index.php\/wp-json\/wp\/v2\/tags?post=1014"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}