Abundance, Population Structure and Production of Scrobicularia plana and Abra tenuis (Bivalvia: Scrobicularidae) in a Mediterranean Brackish Lagoon, Lake Ichkeul, Tunisia

Abundance, growth and production of the deposit‐feeding bivalves were studied in the Ichkeul wetland, northern Tunisia, from July 1993 – April 1994. Scrobicularia plana (Da Costa , 1778) occurred at annual mean densities (biomasses) of 299 ± 65 to 400 ± 100 individuals/m 2 (22.54 ± 3.00 to 34.27 ± 3.96 g ash‐free dry mass (AFDM)/m 2 ) depending on the study area. The annual mean density of Abra tenuis (Montagu , 1803) amounted to 640 ± 74 individuals/m 2 during the whole study period, in contrast the biomass rose from 2.87 g AFDM/m 2 in July to 10.29 g AFDM/m 2 in April. Both species were largely dominated by age class I. Although not very successful, recruitment presented a two‐period pattern: the main period at the beginning of spring, and a secondary one in late summer/autumn. S. plana rarely exceeded 40 mm and lived for only 2 years, while most individuals of A. tenuis lived for only 15–18 months growing to a length of 12 mm. The annual bivalve deposit‐feeder production for the whole lagoon system (90 km 2 ) was 8.24 g AFDM/m 2 (5.26 g C/m 2 , 0.65 g N/m 2 ). The annual P/ $ \bar B $ ratio was about 0.4 and therefore in the same order of magnitude as estimates from other brackish coastal waters. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)