What effect would there be on the marine environment if there was a significant decrease in the number of blue-green alga in an area?

Blue-green algae or cyanobacteria reduce nitrogen and carbon dioxide under cellular respiration. If there is a significant decrease in the cyanobacteria, there would be a decrease in productivity because the algae "fix" nitrogen needed for protein production. There would be a reduced nitrogen and protein in the marine environment because the algae produce nitrogen needed for protein synthesis.

Gay-lussac's law i think.