A new ecological scenario was established in the Oslofjord at beginning of this century, when alien macroalgal species like the invasive Japanese fucoid Sargassum muticum and the red algae Dasya baillouviana were observed for the first time. In this study I surveyed a total of 33 sites in the inner Oslofjord through 2005/2006, most sites previously studied in detail by Larsen (1995), before the introduction of S. muticum. The aim of the study was to monitor the abundance of Sargassum muticum and Dasya baillouviana to better understand their possible ecological impact on recipient biota and habitat. In addition, I studied the reproduction and life cycle of S. muticum in the fjord based on field observation in October, March and May. From a small boat I monitored the macrobenthic algae by using an underwater videocamera.
Sargassum muticum was found at 18 of 33 sites. The alga was attached to a wide variety of hard substrate like solid bedrock, small stones, gravel and blue mussels. It was found from 1 – 4 m depth. In the late summer/autumn it became overgrown by epiphytes and detached.
Dasya baillouviana was previously known from a single find in 1994 . In 2005 it was found at 9 of 33 stations, mainly in the eastern part of the examined area. D. baillouviana is a summer annual species growing in the upper sublittoral zone. It was recorded on a wide range of hard substrates from solid bedrock to gravel.
The introduced species are apparently rapidly increasing in abundance in inner Oslofjord, and this trend will possibly continue, in view of available substrate and habitats. This is especially the case for S. muticum, which may settle in docks and marinas where the native fucoids are not successful.
From a biodiversity point of view this development may have both positive and negative consequences and continued studies are required.