Bioturbation at the water-sediment interface of the Thau Lagoon: impact of shellfish farming

Sabine Schmidt; Jean-Louis Gonzalez; Pascal Lecroart; Jacek Tronczy?ski; Isabelle Billy; Jean-Marie Jouanneau
April 2007
Aquatic Living Resources;Apr2007, Vol. 20 Issue 2, p163
Academic Journal
Quantifying of sediment reworking processes provides new insights into benthic ecosystem functioning of the Thau lagoon, an important European shellfish farming area. In order to evaluate bioturbation rates of surface sediments, profiles of 7Be (half-life: 53 days) and 234Th (half-life: 24.1 days) were measured in cores collected since December 2001 to December 2006. Several sites were selected to sample the diversity of the Thau lagoon: C4, T10, and T11 in the middle of the lagoon, C5, T7 and T8 nearby oyster farming, T2 and T4 in the western edge, T12 in the eastern part closed to industry. 234Th in excess (234Thxs; i.e. supplied to sediment by settling particles) and 7Be both show seasonal variations in activities and in penetration within sediment. Taking into account the moderate sedimentation rates of the Thau lagoon, sites (0.1?0.4?cm per year), the penetration of both short-lived radionuclides to variable depths, from 1 up to 8 cm, indicates efficient biological mixing of upper sediments. Bioturbation rates (Db) to the distribution of excess 234Th (234Pb-Db) and of 7Be (7Be-Db) range between 1 and 35?cm2?y-1, depending on site and season. Surface sediment mixing of the Thau lagoon is primarily controlled by the quality of particle input, i.e. the food supply to the benthic fauna, mainly governed by the hydrological and seasonal conditions. But locally shellfish production is a key parameter that influences bioturbation through biodeposition. Quantifier les processus de remaniement s?dimentaire procure une vision nouvelle du fonctionnement des ?cosyst?mes benthiques de l''?tang de Thau, lieu d''une importante production ostr?icole. Afin d''?valuer les taux de bioturbation des s?diments de surface, des profils de deux traceurs radioactifs, le b?ryllium 7Be (T1/2 = 53 jours) et le thorium 234Th (T1/2= 24.1 jours) ont ?t? mesur?s ? partir de carottes s?dimentaires pr?lev?es entre d?cembre 2001 et d?cembre 2006. La s?lection de plusieurs sites a permis de couvrir la diversit? de l''?tang de Thau: C4, T10, et T11 dans le bassin central, C5, T7 et T8 au niveau des secteurs ostr?icoles, T2 et T4 ? l''ouest du bassin, et T12 ? l''est pr?s de la zone industrielle. Le 234Th en exc?s (234Thxs; i.e. apport? au s?diment par la s?dimentation des particules) et le 7Be pr?sentent des variations saisonni?res de leurs activit?s et de leurs p?n?trations dans le s?diment. Compte tenu des faibles taux de s?dimentation report?s pour l''?tang de Thau (0.1?0.4?cm par an), la p?n?tration ? des profondeurs variables (de 1 ? 8?cm) des deux radio?l?ments ? courte dur?e de vie r?v?le un m?lange efficace des s?diments superficiels en lien avec l''activit? biologique. Les taux de bioturbation (Db) calcul?s au moyen des profils de 234Th en exc?s (234Th-Db) et de 7Be (7Be-Db) sont compris entre <1 et 35?cm2?an-1, selon les sites et la saison consid?r?s. Le m?lange des s?diments d''interface de l''?tang de Thau appara?t principalement contr?l? par la qualit? du flux de particules, apport de nourriture pour la faune benthique, en lien avec l''hydrologie et les saisons. Localement, l''activit? ostr?icole est le facteur pr?pond?rant qui influence la bioturbation via les biod?p?ts.


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