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.