Cultural eutrophication (excessive plant growth resultingfrom nutrient enrichment by human activity) is theprimary problem facing most surface waters today. It isone of the most visible examples of human changes to thebiosphere
These sentences are important because they describe what cultural eutrophication is and declares that it is a major problem. The author wants to make it clear how eutrophication is a problem and starts the introduction being clear so there is not misunderstanding.