| Summary: | Several accounts have been produced on the seaweeds of the west coast including the Saldanha and Langebaan System, but none has looked at how human activities may have affected the seaweed diversity of Saldanha Bay. The major human activities in the Bay include shipping and mariculture. The major human impacts resulting from these activities are species introductions which often lead to a change in species composition in an area. Six sites were sampled in various visits in autumn and winter 2012. Two of the sites were natural wave exposed rocky shore sites while the rest were sheltered and man-made. Detailed collections of seaweeds were made. The seaweeds were identified to species level and preserved either as herbarium specimen or in formalin. Specimens that it was not possible to identify using morphological features were preserved in silica gel for DNA analysis. Three samples of the red algal genus Gruteloupia, collected from aquaculture systems, which could not be identified using morphological features, were sent to Ghent University where the rbcL gene region was sequenced. Collected species were compared with species lists from previous studies in the same area. The results from this study showed that most of the species collected were west coast species, with a few species with south coast distributions. The numbers of species in the Bay since the study of Day showed an increase from 23 by Day to 58 by Simons' 1960 collection. His 1976 collection reduced to 47. The most species so far recorded are by Schils' 1997 collection with 72. From this study, 69 species were recorded of which 23 are potentially new records. Of these records, two were south coast species Gelidium reptans and Laurencia peninsularis. They were recorded for the first time in the system and they are range extensions. Furthermore, the results provide molecular evidence that two species of Grateloupia have been introduced into Saldanha Bay. Bayesian analysis showed that one of the species was Grateloupia turuturu, a Japanese species which has spread to a number of world regions which culture the Pacific oyster. The other was distinct from other Grateloupias previously recorded in South Africa, and did not match any species in Genbank. The potential vector of introduction for Grateloupia turuturu, importation of the Pacific oyster Crassostreo gigas, is discussed.
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