title: Nitrate fertilisation does not enhance CO2 responses in two tropical seagrass species.
abstract: 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.
authors.affiliation: College of Marine and Environmental Science, James Cook University, Townsville Queensland 4811, Australia.|Australian Institute of Marine Science, Townville MC Queensland 4810, Australia.|Experimental Marine Ecology Laboratory, Department of Biological Sciences, National University of Singapore, 14 Science Drive 4, Blk S3, #02-05, 117543, Singapore.
authors.forename: Y X
authors.initials: YX
authors.lastname: Ow
authors.affiliation: Australian Institute of Marine Science, Townville MC Queensland 4810, Australia.
authors.forename: N
authors.initials: N
authors.lastname: Vogel
authors.affiliation: College of Marine and Environmental Science, James Cook University, Townsville Queensland 4811, Australia.|Centre for Tropical Water &Aquatic Ecosystem Research (TropWATER), James Cook University, Cairns, Queensland 4870, Australia.
authors.forename: C J
authors.initials: CJ
authors.lastname: Collier
authors.affiliation: College of Marine and Environmental Science, James Cook University, Townsville Queensland 4811, Australia.
authors.forename: J A M
authors.initials: JA
authors.lastname: Holtum
authors.affiliation: Australian Institute of Marine Science, Townville MC Queensland 4810, Australia.
authors.forename: F
authors.initials: F
authors.lastname: Flores
authors.affiliation: Australian Institute of Marine Science, Townville MC Queensland 4810, Australia.
authors.forename: S
authors.initials: S
authors.lastname: Uthicke
mesh_terms: D000143:Acids / Q000737:chemistry / Q000378:metabolism; D035961:Alismatales / Q000145:classification / Q000254:growth & development / Q000378:metabolism*; D018533:Biomass; D002244:Carbon / Q000378:metabolism; D002245:Carbon Dioxide / Q000378:metabolism*; D005308:Fertilizers; D031321:Hydrocharitaceae / Q000254:growth & development / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D009566:Nitrates / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism; D009792:Oceans and Seas; D010101:Oxygen Consumption; D010788:Photosynthesis; D018515:Plant Leaves / Q000254:growth & development / Q000378:metabolism; D027343:Rhizome / Q000254:growth & development / Q000378:metabolism; D012623:Seawater / Q000737:chemistry; D013045:Species Specificity; D013997:Time Factors; D014329:Tropical Climate
chemical_list: D000143:Acids; D005308:Fertilizers; D009566:Nitrates; D002245:Carbon Dioxide; D002244:Carbon; D009584:Nitrogen