Nitrate fertilisation does not enhance CO₂ responses in two tropical seagrass species

Seagrasses are often considered "winners" of ocean acidification (OA); however, seagrass productivity responses to OA could be limited by nitrogen availability, since nitrogen-derived metabolites are required for carbon assimilation. We tested nitrogen uptake and assimilation, photosynthesis, growth, and carbon allocation responses of the tropical seagrasses Halodule uninervis and Thalassia hemprichii to OA scenarios (428, 734 and 1213 μatm pCO₂) under two nutrients levels (0.3 and 1.9 μM NO₃⁻). Net primary production (measured as oxygen production) and growth in H. uninervis increased with pCO₂ enrichment, but were not affected by nitrate enrichment. However, nitrate enrichment reduced whole plant respiration in H. uninervis. Net primary production and growth did not show significant changes with pCO₂ or nitrate by the end of the experiment (24 d) in T. hemprichii. However, nitrate incorporation in T. hemprichii was higher with nitrate enrichment. There was no evidence that nitrogen demand increased with pCO₂ enrichment in either species. Contrary to our initial hypothesis, nutrient increases to levels approximating present day flood plumes only had small effects on metabolism. This study highlights that the paradigm of increased productivity of seagrasses under ocean acidification may not be valid for all species under all environmental conditions