RESPONSE OF SUBLITTORAL HARD SUBSTRATE INVERTEBRATES TO ESTUARINE SEDIMENTATION IN THE OUTER HARBOR OF BILBAO (N SPAIN)

Four sampling stations with relatively similar hard-bottom substrates were investigated to determine if there were differences between the s ublittoral invertebrates near the Nervion River mouth (N. Spain) and t hose closer to the open sea, and if faunal differences could be attrib uted to the impact of estuarine sedimentation. Four 100 m long transec ts were surveyed using photographic and quadrat techniques, yielding d ata on percentage cover of all invertebrates from 136 visual inventori es. Data were analysed to determine whether any community response to sedimentation was evident. Ordination (multidimensional scaling) and c lassification (group-average clustering) based on Bray-Curtis dissimil arity measures separated the stations at three levels based on the dif ferent faunal assemblages. The analysis suggests the presence of a gra dient along the estuary discharge, mainly affecting the east side of t he outer part of Bilbao harbour, since the estuarine plume flows to th e east. The influence can be expressed in terms of retrogression of th e variety of species and disappearance of the algal fraction, which is progressively replaced by an animal assemblage of tolerant and opport unistic suspension feeders. Several 'indicator' species were identifie d on the basis of strong density differences between sites; matrix cor relations presented here were found to produce complementary and consi stent results. The role of estuarine sedimentation disturbance as the main factor controlling subtidal assemblages in outer Bilbao harbour i s discussed.