Growth and biomass development of the introduced red alga Gracilaria vermiculophylla is unaffected by nutrient limitation and grazing

The invasive red alga Gracilaria vermiculophylla was first recorded in Denmark and Sweden in 2003 and has since been reported from several sites in Denmark, Sweden and northern Germany. The abundance of G. vermiculophylla is typically high at more eutrophic sites, whereas it remains relatively low at more oligotrophic sites. We recorded seasonal variations in growth and biomass development at 2 sites with contrasting nutrient status (eutrophic versus oligotrophic) to investigate whether nutrient limitation of growth and loss of biomass due to grazing from invertebrate herbivores could explain observed variations in biomass. The biomass of G. vermiculophylla at the eutrophic site (Holckenhavn Fjord) was ca. 300-fold larger than at the oligotrophic site (Fyns Hoved). Growth rates ranged from almost 0 in early spring to ca. 0.08 d(-1) in mid-summer and did not vary among sites. The seasonal pattern of growth was correlated to insolation and water temperature, suggesting that nutrient availability played a minor role in controlling growth. Experimental nutrient enrichment confirmed these findings; nutrient enrichment enhanced the level of tissue nutrients, but only had a marginal effect on growth. Grazing losses were insignificant throughout the entire study period in both systems. Hence, site-specific variations in biomass could not be explained by differences in grazing pressure. Given the lack of support for nutrient limitation or herbivory to explain the observed variations of G. vermiculophylla biomass, we suggest that physical exposure caused by wind-driven waves may be the factor that controls biomass of G. vermiculophylla in these shallow estuaries.