COMPARISON OF DEMOGRAPHIC PARAMETERS RELA TING TO NATURAL-POPULATIONSOF THE POLYCHAETE NEREIS-VIRENS (SARS) AND NEPHTYS-CECA (FABRICIUS)

Variations in demographic parameters between intertidal populations of Nereis virens and Nephtys caeca were studied. These polychaetes are m ajor component of the Macoma balthica community in the Lower St. Lawre nce estuary (Quebec, Canada). We compared spatial distribution between differently oriented shores in a small geographic area. Our results r evealed an interpopulation variability that appears to be dependent on hydrodynamical processes, edaphic parameters and biological interacti ons. In north and northeast oriented flats, the higher densities of yo ung individuals of Nereis virens are found in the higher tidal levels where we observed larval recruitment and the highest organic matter co ntents. On the other hand, the lower densities of adult worms are foun d in the lowermost part of the intertidal zone, in which we observed t he lowest organic matter contents and sandy sediment. As for the south west facing tidal flats, the highest densities of immature individuals were observed in gravelly sediment of the lowermost tidal level. Also , mature individuals of Nereis are found on the uppermost part. Larval recruitment occurs in the central area of these bays. For the other s pecies, Nephtys caeca, intertidal populations were dominated by juveni le individuals. Nevertheless, we remarked some bigger worms on the low er tidal level. The densities of Nephtys are higher in the sandy sedim ent and lower in the rich organic sediment. Interpopulation variabilit y of Nereis appears to be dependent on many hydrodynamical effects (la rval spatial patterns, hypsometric segregation of sediment organic mat ter, etc.) and the thickness of the colonizable sediment layer. Our re sults suggest that adult-juvenile interactions play an important role in population regulation of this polychaete. Interpopulation variabili ty of Nephtys caeca seems to be mainly caused by the sediment texture, most probably dependent on hydrodynamical processes. We noted that la rval settlement of Nephtys occurs in the most depth zones.