Nutrient (N and P) dynamics of the invasive macroalga Gracilaria vermiculophylla: nutrient uptake kinetics and nutrient release through decomposition

The invasive alga Gracilaria vermiculophylla was introduced to Europe two decades ago and has since become dominant in many shallow estuaries. Gracilaria vermiculophylla is a relatively fast-growing alga that thrives well at low nutrient availability in summer, suggesting that it uses nutrients efficiently, which might give it a competitive advantage over many native species. We studied therefore the nutrient dynamics of G. vermiculophylla and compared it to those of a range of native macroalgal species. Nutrient uptake rates (NH+4, NO−3 and PO3−4) were compared to growth-related requirements and we found that G. vermiculophylla needs relatively high nutrient concentrations to sustain fast and non-limited growth. This compares to the nutrient dynamics of many fast-growing algae, and we found thus no indication that G. vermiculophylla should have any particular advantage relative to other, sympatric species. The nutrient storage capacity of G. vermiculophylla was, in contrast, relatively large and comparable to that of more slow-growing algae, which, when combined with the low nutrient uptake experienced in summer, could explain how G. vermiculophylla can sustain non-nutrient limited growth through most of the growth season. The biomass of G. vermiculophylla can be massive and estimates showed that gross nutrient uptake could exceed the amount of nutrients received from land. The turnover of Gracilaria biomass is, however, fast and nutrients bound in the resulting detritus are quickly mineralized during decomposition, which is especially important during late summer when water temperatures are high. Invasion and subsequent dominance by G. vermiculophylla may thus affect local nutrient cycling significantly.