Nutrients--nitrogen, phosphate, potassium--occur naturally in the environment. They're all around us all the time as part of the life cycle.
There are many sources of nutrients that can lead to problems in water. Municipal waste water that's not treated, and even when it is treated, still contains nutrients. Livestock run-off from manure, from large-scale livestock operations, also cause... Phosphorus, for example, occurs in large quantities in the soil. When municipalities and provincial governments take marshes and drain them, create drainage channels to prevent flooding, there is a huge amount of erosion that occurs and the phosphorus that naturally occurs in the soil all goes into lakes and rivers.
Some fertilizer can be lost during application, but there is tremendous economic pressure on farmers when they're applying fertilizer, which they have to buy at large cost to put on their farms, to ensure that that fertilizer stays where it's put. Everyone involved in doing things in the environment, particularly in agriculture, has to be aware that these things need to be managed with best management practices.
Are the products inherently toxic? As the environmental group was saying, referring to the dictionary definition, are they poisons? No.