The invasive macroalgae Gracilaria vermiculophylla - Effects of Salinity, Nitrogen availability, Irradiance and Grazing on the growth rate

The invasive macroalgae Gracilaria vermiculophylla has appeared increasingly in otherwise nonindigenous areas including seven geographically separated fjords in Denmark. These fjords now contain large quantities of G. vermiculophylla. In our study several eco-physiological traits of G. vermiculophylla were investigated in order to examine if it has competitive advantages over local macroalgal species, therefore posing a threat to the environment. Also, to assess the possible future distribution of G. vermiculophylla in Danish estuaries based on these traits. The results showed that G. vermiculophylla has a competitive advantage over native algal species. The grazing preference experiments showed that the two local grazers Idotea baltica and Littorina littorea preferred the local macroalgae Ulva lactuca and Fucus vesiculosus over G. vermiculophylla. G. vermiculophylla showed high tolerance to a broad range of salinities (from 10 to 45ppt) and can grow and probably prosper in most of the Danish estuaries, but does not seem to have a competitive advantage based on salinity alone. A light experiment where G. vermiculophylla was grown at various irradiances (0 to 163 μmol photons ms) showed that G. vermiculophylla is able to utilize low amounts of light (40 μmol photons ms), while maintaining maximum growth rate, which is an advantage in the eutrophic Danish estuaries, where the irradiance is limited. Nitrogen experiments showed that G. vermiculophylla is able to utilize small concentrations of nitrogen (approximately 0,5μM) and still maintain maximum growth. It is very probable that G. vermiculophylla has sufficiently competitive abilities to keep pace with the native macroalgae as it is already well established in some Danish estuaries. It may very well be able to out-compete some native species with consequences to the ecosystems.