There is always a rate of nutrients input that is too much for an ecosystem to handle, maybe the estuary cant handle this load. My point was that nutrients can enter freshwater basins and never leave. N will leave but P is there forever. This is why nutrient runoff is a big deal. Lakes can be forever changed into fishless slime pools. Nutrient runoff into the sea has been going on forever but in the case of freshwater basins there is a normal nutrient regime that has been going on since before man and if you change it the lake will change.