Nitrogen availability limits phosphorus uptake in an intertidal macroalga.

Nutrients such as nitrogen (N) and phosphorus (P) limit primary productivity, and recent anthropogenic activities are changing the availability of these nutrients, leading to alterations in the type and magnitude of nutrient limitation. Recent work highlights the potential for N and P to interact to limit primary production in terrestrial and freshwater systems. However, mechanisms underlying co-limitation are not well described. Documentation of ambient nutrient levels and tissue nutrients of the intertidal macroalga Fucus vesiculosus for 2 years in the southern Gulf of Maine, USA, indicates that N availability may be impacting the ability of F. vesiculosus to access P, despite relatively high ambient P concentrations. To experimentally validate these observations, F. vesiculosus individuals were enriched with N or P for 6 weeks, followed by an uptake experiment to examine how the interactions between these nutrients affected macroalgal N and P uptake efficiency. Results illustrate that exposure of seaweed to different nutrient regimes influenced nutrient uptake efficiency. Notably, seaweeds enriched with N displayed the highest P uptake efficiency at low, biologically relevant, P concentrations. Our results confirm that N availability may be mediating the ability of primary producers to access P. These interactions between limiting nutrients have implications for not only the growth and functioning of primary producers who rely directly on these nutrients but also the entire communities that they support.