Future increases in oceanic carbon dioxide con-centrations (CO2(aq)) may provide a benefit tosubmerged plants by alleviating photosyntheticcarbon limitation. However, other environmentalfactors (for example, nutrient availability) may al-ter how seagrasses respond to CO2(aq)by regulatingthe supply of additional resources required tosupport growth. Thus, questions remain in regardto how other factors influence CO2(aq)effects onsubmerged vegetation. This study factoriallymanipulated CO2(aq)and nutrient availability,in situ, within a subtropical seagrass bed for350 days, and examined treatment effects on leafproductivity, shoot density, above- and below-ground biomass, nutrient content, carbohydratestorage, and sediment organic carbon (Corg). Clear,open-top chambers were used to replicate CO2(aq)forecasts for the year 2100, whereas nutrientavailability was manipulated via sediment amend-ments of nitrogen (N) and phosphorus (P) fertilizer.We provide modest evidence of a CO2effect, whichincreased seagrass aboveground biomass. CO2(aq)enrichment had no effect on nutrient content,carbohydrate storage, or sediment Corgcontent.Nutrient addition increased leaf productivity andleaf N content, however did not alter above- orbelowground biomass, shoot density, carbohydratestorage, or Corgcontent. Treatment interactionswere not significant, and thus NP availability didnot influence seagrass responses to elevatedCO2(aq). This study demonstrates that long-termcarbon enrichment may alter the structure ofshallow seagrass meadows, even in relativelynutrient-poor, oligotrophic systems.

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• Does Nutrient Availability Regulate Seagrass Response to Elevated CO2?