Long-term impact of the invading Sargassum muticum to the indigenous macroalgal community in Limfjorden, Denmark

Kristina Clement Bødker Diernæs

Student thesis: Master thesis


Sargassum muticum (Yendo) Fensholt originates from Asia but is today probably one of the most common invasive macroalgae in the world. In Danish water, S. muticum was first observed in Limfjorden in 1984 and this was followed by a colonisation to most parts of Limfjorden. S. muticum is a monoecious and highly fecund macroalgae that has a pseudo-perennial life-cycle. When arrived in a new area, the success of invasion of S. muticum seems to be depending on the high accumulation of biomass, strong reproductive properties and on interspecific competition with indigenous macroalgae for light.
Based on data from 10 transects in Limfjorden from 1990 to 2012, it was found that S. muticum increased in cover from 1990 to 1998, followed by a decrease in 1999 and a stabilisation towards 2012. Species diversity and species richness showed changes that could be related to the cover of S. muticum. In concordance, multivariate community analysis revealed significant changes in the macroalgal community structure closely related to the increased cover of S. muticum. The dominance of S. muticum was also detected by an increased importance to similarity in community structure, which happened to the detriment of several indigenous species. When cover of S. muticum stabilised the importance to similarity in the macroalgal community structure persisted. Some of the possible impacted species seemed however to recover. Temporal changes in cover of functional form groups and the selected species; showed that cover of many of the indigenous species belonging to the thick leathery and coarsely branched macroalgae decreased while S. muticum cover increased. However, only F. serratus and C. fragile were inversely correlated to S. muticum, indicating that these two species seemed to be impacted by S. muticum in competition for light. It is further suggested, that the decreased cover of the other indigenous macroalgae could be related to the tendency to higher water temperatures. Nevertheless, cover of many of the possible impacted species (and functional form groups) seemed to stabilise and it is suggested that equilibrium already was reached between S. muticum and the indigenous macroalgal community 15 years after the introduction. Even though S. muticum seemed to stabilise in Limfjorden, changes in community structure still occurred. In the light of global climate changes and the expected increase in invasion rates, it is predictable that the macroalgal community structure in Limfjorden would change significantly in this century.

EducationsEnvironmental Biology, (Bachelor/Graduate Programme) Graduate
Publication date24 Jun 2018
Number of pages53
SupervisorsMorten Foldager Pedersen