Effects of ship traffic on archipelago waters off the Långnäs harbour in Åland, SW Finland

The relatively shallow archipelago waters off the Långnäs harbour area, eastern Åland (SW Finland) were studied before and after an expansion of the ferry traffic on July 1, 1999. The large ships (150-200 m long) caused increased vertical mixing, turbidity and stirring of the sediments, especially in the turning area outside the harbour. The vertical distribution of silicate as well as disturbed temperature conditions indicated artificial upwelling. Ship traffic also contributes to the eutrophication of the archipelago waters via increased recycling of nutrients due to elevated bottom temperature and increased mixing depth in the fairway, As the bloom-forming cyanobacterium Nodularia spumigena could be favoured in the fairway or adjacent areas, the occurrence of its toxin nodularin-R was monitored. However, only low toxin concentrations were recorded in the whole study area in 1999.