Which of the following is most a source of nitrogen that can lead to eutrophication of aquatic ecosystems? explain how you got your answer choice
a. runoff of fertilizer from yards b. treated sewage
c. runoff of manure from farms d. all of these

All of these are sources of nitrogen leading to eutrophication of aquatic ecosystems.

Explanation:

Synthetic inorganic fertilizers and are huge sources of the macronutrient nitrogen.

Organic manure also contains high amounts of nitrogen since they are made from remains of plants and animals.  

When fertilizers or manure are used on a larger scale in yards and farms, some amount left unused by the plants remain on the soil .

Water treatment of sewage water leads to release of nitrogen into soil due to treatment processes.

This soil nitrogen is biodegraded to nitrates by the soil bacteria in the soil through biological nitrogen removal processes like denitrification and ammonification.

These nitrates dissolve in water and get leached or carried away by water and settle in aquatic ecosystems like ponds, river, streams, ocean etc.

Accumulation of excess amounts of this nutrient rich fertilizers increases the nutrient level of the aquatic ecosystem causing eutrophication which there by promotes production of aquatic biomass like algae leading to algal bloom.  

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