This project investigated uptake of nitrate by the aquatic angiosperm Ruppia megacarpa Mason. Nitrate uptake by leaf and root/rhizome tissues of R. megacarpa was studied using two-compartment chambers separating aboveground from belowground biomass.

Uptake of nitrate was studied for each tissue separately by adding nitrate to the medium and measuring depletion over time. The results indicate that both leaf and root/rhizome tissues were able to acquire nitrate. Uptake was modelled (Michaelis-Menten) as a function of external concentration. The model estimated the values of nitrate uptake parameters K_s as 1.3µM-N and V_max as 0.68µmol g^-1 DW hr^-1 for leaves. K_s and V_max values for root/rhizome nitrate uptake were 18µM-N and 0.78µmol g^-1 DW hr^-1, respectively. Leaves were more effective in uptake at low external nitrate concentrations compared to roots/rhizomes, which showed net efflux of nitrate at concentrations below 10.2µM-N.

Interaction between tissues was determined by adding ¹⁵N-labelled nitrate to leaf and root compartments separately or combined then measuring the depletion of nitrate from the medium and ¹⁵N incorporation into tissues, aboveground (leaf and epiphyte) and belowground (root and rhizome). The results indicate that there is no interaction between above-and belowground tissues of R. megacarpa and that they acquire nitrate independently of each other. Uptake of nitrate by leaves of R. megacarpa was not affected by the availability of this nutrient to the belowground tissues. Neither was the uptake by root/rhizome tissues influenced by the presence of nitrate to the leaves. There was also no evidence of translocation of the nutrient between tissues. Uptake of ¹⁵N by epiphytes accounted for 50% of total N taken up by the plant.

Combined with environmental data, the results suggest that R. megacarpa can take advantage of occasional high concentrations of water column nitrate, whilst maintaining a continuous uptake of sediment nitrate. These nitrate uptake patterns of R. megacarpa may play an important role in the cycling of nitrogen in Wilson Inlet, with high nitrate concentrations in the sediment porewater and relatively low nitrate concentrations in the water column for long periods of the year.