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The harbour at Ceuta is one of the most important harbours in the Strait of Gibraltar. The sediments are moderately polluted with organic matter and heavy metals but the harbour has two opposing entrances and a connecting channel which increases water renewal and dissolved oxygen across the harbour. For these special conditions, the value of the soft bottom polychaete community as a bioindicator, and possible advantages of the presence of two harbour entrances on biotic assemblages, were studied. Twenty-one stations were selected, and 27 variables were measured in the sediment of each station. The polychaete species richness and Shannon diversity values were similar inside and outside the harbour. Nevertheless, the Pielou evenness index was significantly higher in the external stations due to high densities of some species of polychaetes such as Pseudomalacoceros tridentata and Capitella capitata inside the harbour. The multivariate approach based on polychaete species composition was much more sensitive than univariate analyses at discriminating between internal and external stations. The pollution gradient and granulometric parameters were the main factors affecting polychaete distribution. Polychaete species richness and diversity in sediments inside Ceuta harbour were higher than in conventional harbours due to the positive effects of increased water renewal. These results should be taken into consideration in design, construction and remodelling of future harbours.

Polychaete assemblages and sediment pollution in a harbour with two opposing entrances