Concentration perspective
While giving a nod to the gross factor, keep the urine problem in perspective. If an acre of water, 10 feet deep, had the state maximum phosphorus level (8 parts/billion), it would take 40,000 average doses of pee to bring it up to 10 parts/billion, where algae blooms start to happen. There are other phosphorus sources of course, but headless weekender contributions are insignificant compared to the impact of the boats they arrive in. Prop wash and boat wake add measurable phosphorus to the water column by stirring up bottom sediment, where centuries of nutrient deposits reside.
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