Soft bottom mollusc assemblages and pollution in a harbour with two opposing entrances

Conventional harbours, provided with only one entrance and devoid of channels, are enclosed areas with low water renewal, high sedimentation rate and high concentrations of pollutants in sediments; the soft bottom communities are characterised by low species richness and low values of the diversity and evenness indexes. The harbour of Ceuta, North Africa, presents an unusual structure since it is provided with two opposing entrances and a channel, which increase the water renewal across the middle of the harbour. These unusual characteristics turn the harbour of Ceuta into an adequate locality to test its environmental implications on macrofaunal assemblages. In the present study, the spatial distribution of mollusc community associated with soft bottoms was studied in relation to the influence of environmental factors on the harbour of Ceuta and nearby areas, North Africa. Twenty-one stations (15 inside and six outside the harbour) were sampled and 26 variables were measured in the sediment of each station: depth, %sand, organic matter, lipids, P, N, Al, As, B, Ba, Ca, Cd, Co, Cr, Cu, Fe, K, Li, Mg, Mn, Na, Ni, Pb, S, Sr, Zn. The special configuration of Ceuta harbour created a great variability in sediment characteristics and environmental measures depending on the stations and, due to this heterogeneity, the mollusc species richness in the sediments inside the harbour (45 species) was higher than in conventional harbours. The multivariate approach based on MDS analysis was much more sensitive than univariate techniques to detect differences between internal and external stations of the harbour of Ceuta. The percentage of sand was the main factor to affect the distribution of the mollusc assemblages according to the BIO-ENV procedure and the CCA. SIMPER showed that the bivalves Parvicardium exiguum, Ervilia castanea, Spisula subtruncata and Digitaria digitata were the species that most contributed to the dissimilarity between internal and external stations; P. exiguum, S. subtruncata and D. digitata were more abundant in the internal stations whereas E. castanea was more abundant in the external stations. The bivalves P. exiguum, Abra alba and Corbula gibba and the opistobranch Retusa obtusa dominated in the most enclosed stations inside the harbour where sediments contained very high values of organic matter, lipids, and heavy metals. The data of the present study might assist in the building of new harbours in the future.