How Fertilizer Can Suffocate a Bay: Nutrient Pollution Explained

The nutrients that make a garden grow can kill the animals in a bay. When too much nitrogen and phosphorus reach the water, they work the way fertilizer works on land: they feed growth, in this case the growth of algae, far beyond what the water can support. That is nutrient pollution, and the damage it does comes not from the nutrients themselves but from what happens after the algae bloom.

The Chesapeake Bay, before and after heavy rain washed suspended matter into its waterways. Credit: NOAA.

How the nutrients get there

Much of it runs off the land. In towns and cities, rain carries lawn and garden fertilizer into the waterways, along with waste from pets and wildlife. The Chesapeake Bay, part of the largest watershed in the Northeast, shows how quickly this adds up. A visualization of the area after exceptionally heavy rainfall in 2011 compares the silt, mud and debris suspended in its waterways before and after the storms. All of that runoff ended up in the bay.

Nature supplies nutrients too. Rocks and soil in a watershed release them as they weather, and ocean currents mix them into coastal water. But scientists pay most attention to the nutrients people add, because along the coast those far outweigh the natural ones. Wastewater treatment plants, runoff from urban streets during storms and farming all contribute, and as more people move to the coast, more nutrients follow them into the water.

Why an algae bloom ends in low oxygen

Scientists call the process eutrophication. A heavy bloom of algae clouds the water and blocks the light that underwater plants such as seagrasses need to grow. When the algae and the seagrass die, they decay, and decay uses oxygen. The oxygen dissolved in the water falls, sometimes low enough to kill fish, crabs, oysters and other aquatic animals.

So the harm runs in a chain: more nutrients, more algae, less light, more decay, less oxygen. Each link makes the next one worse, which is why a single cause — too much fertilizer in the wrong place — can end in dead fish and shellfish.

Sources

Based on "What is nutrient pollution?" from NOAA's National Ocean Service, a work of the United States government in the public domain.