Contribution à l’étude écologique de l’herbier à Posidonia oceanica (L.) Delile (1813) de la frange côtière de Mostaganem : Etat de santé et relation entre plante et échinoderme

Abstract

This study interests the state of health of Posidonia oceanica meadows of Mostaganem coasts. Shoot density was measured and phenological parameters were followed from February to July 2011 at Stidia and Hadjadj areas at - 3 m mean depths. In order to study the feeding behavior of the aspidochirotid Holothurians inhabiting the Posidonia oceanica meadows and to have an idea on relationship between the Posidonia and the holothurians component, organic matter throughout digestive tract, faecal matter and into the preferred biota of Holothuria (H.) tubulosa, Holothuria (L.) poli, Holothuria (P.) forskali and Holothuria (P.) sanctori were analyzed and granulometrics analysis of the sediment particles ingested by Holothuria (L.) poli and into the biota was realized, in order to access the selectivity degree of this species by calculating the electivity index. Posidonia meadow of Stidia is installed on rocky substratum, whereas Hadjadj one is installed on soft substratum. At Hadjadj area average densities of (505.6 ± 316.9), (193.6 ± 59.13) and (379.2 ± 157.50) have been respectively obtained for HS1, HS2, HS3 stations and in Stidia area the average densities (181.4 ± 75.40), (350.8 ± 188.57) where obtained for stations SS1 and SSM respectively. This means that the Posidonia oceanica meadows of Stidia and Hadjadj areas have abnormal densities. At both localities, the studied phenological parameters shows an increase from the end of the winter - early spring (February and March) to late spring - early summer (May and June). Seagrasses beds of both localities show a slight sign of disturbance. However, the seagrass bed is in better health in Hadjadj area. Aspidochirotid holothurians species show organic matter selectivity. Holothuria (P.) sanctori is the most selective species, followed by Holothuria (P.) forskali, Holothuria (L.) poli and Holothuria (H.) tubulosa. Faeces are richer in organic matter than the surrounding sediment. Holothuria (L.) poli prefer the fine and very fine sediment fractions, which are richest in organic matter. The results support "optimal foraging" theory and show that these holothurians species recognize the richer organic coated particles and that Holothuria (L.) poli can distinguish between sediment particle with or without organic matter.