Living components in a river ecosystem would include all plants, all animals, all microorganisms. Every fish species, every arthropod species, every reptile and amphibian species, all the birds, etc.
I think genetic variation, but i'm not completely sure
Other biological molecules that contain the element of N would be nucleic acids such as DNA and or RNA. There are found in the nucleotides that constitute or make up these nucleic acids.
A nucleotide consists of all of the following except a nitrogenous base. The are of course the main building blocks of genetic information in many life forms.
