Effects of ship traffic on archipelago waters off the Langnas harbour in Aland, SW Finland

The relatively shallow archipelago waters off the Langnas harbour area, eas tern Aland (SW Finland) were studied before and after an expansion of the f erry traffic on July 1, 1999. The large ships (150-200 m long) caused incre ased vertical mixing, turbidity and stirring of the sediments, especially i n the turning area outside the harbour. The vertical distribution of silica te as well as disturbed temperature conditions indicated artificial upwelli ng. Ship traffic also contributes to the eutrophication of the archipelago waters via increased recycling of nutrients due to elevated bottom temperat ure and increased mixing depth in the fairway. As the bloom-forming cyanoba cterium Nodularia spumigena could be favoured in the fairway or adjacent ar eas, the occurrence of its toxin nodularin-R was monitored. However, only l ow toxin concentrations were recorded in the whole study area in 1999.