if the concentration of NO3- in the water is uniformly 5 microMolar, how many moles of N are there per square meter?

There is 1 mole N in 1 mol NO3^-.

5 micromolar = 5 x 10^-6 mols N/L
5 x 10^-6 mols N/L x (1 L/0.001 m^3) = ?? mols N/m^3.
I don't know how you go from cubic meters to square meters.