Harmful Algal Blooms & Runoff

How River Runoff Creates Algal Blooms on Our Coasts

Nitrogen and phosphorus act like high-octane fuel for algae. When heavy rains wash them from farms, lawns and streets into rivers, coastal waters get a massive, unnatural “feeding.” In the Chesapeake Bay, farms and animal operations supply about 40% of the nitrogen and 45% of the phosphorus, and stormwater from developed land adds roughly another fifth.

Source: Chesapeake Bay Program

"I've spent 40 years diving these reefs. I've seen them when they were vibrant cities, and I've seen them turn into ghost towns. We aren't just saving 'pretty fish' — we're saving the living infrastructure that keeps our oceans and our coasts alive."

— Ken Smith, Oceans4Us

Some algal blooms follow natural seasonal cycles, but in shallow coastal waters, nutrient surges from runoff combined with warm water are a major driver of the worst blooms. A severe harmful algal bloom can kill fish by the thousands, close shellfish beds, and release toxins that can even become airborne along beaches.

1 — The “Fuel” for the Bloom

Excess nitrogen and phosphorus from farms and streets flood coastal waters during heavy rainfall, triggering explosive algal growth.

2 — How a Bloom Becomes a Dead Zone

Thick algae blocks sunlight from the seagrass below. When the algae die, bacteria break them down and use up the oxygen. Below about 2 milligrams of oxygen per liter, water is “hypoxic”: fish flee or suffocate, and crabs, oysters and clams can’t escape.

3 — The Coastline Trap

River water doesn’t simply spread out into the open sea. Earth’s rotation tends to turn river plumes and hold them against the coast, concentrating nutrients right in the shallows where seagrass beds, fish nurseries and reefs are most vulnerable.

That’s why Oceans4Us starts upstream. Cleaner creeks and rivers mean less fuel for the next bloom. See the River to Reef connection →