Complex sentence: When alligator weed invades waterways it can reduce water flow and quality by preventing light penetration and oxygenation of the water, it can reduce water bird and fish activity and cause the death of fish an native plants.
Split into simpler sentences: When alligator weed invades waterways it can reduce water flow and quality by preventing light penetration and oxygenation of the water. It can also reduce water bird and fish activity and cause the death of fish and native plants.