How Estuaries Shape Our Oceans — and Why That Matters

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June 8, 2025

We can't talk oceans without estuaries. When we think of the ocean, we often picture vast, open waters. But the vibrance and wildlife of our open oceans begins where rivers meet the sea—in estuaries.

This World Oceans Day, dive into the science of estuaries—the most productive ecosystems in the world, the heartbeat of our oceans, and the very ecosystem we're working to restore right here in New York City.

What is an Estuary? Estuaries are dynamic ecosystems where freshwater from rivers blends with saltwater from the ocean. Habitats like salt marshes, mangroves, seagrass beds, mudflats, river deltas, and lagoons characterize estuaries around the globe.

Unlike the vast open ocean, estuaries are shallow, nutrient-rich zones that trap and concentrate resources. While the open ocean has abundant space but fewer nutrients, estuaries offer concentrated nutrients, abundant sunlight, and sheltered conditions that support rapid growth and high biodiversity.

These vibrant, productivity hotspots are essential for maintaining the overall health and diversity of our oceans. Here's how they do it.

Estuaries feed our oceans oxygen… Their shallow, nutrient-rich waters allow sunlight to nurture plant life underwater, like salt marsh grasses and seagrasses which fuel and allow our oceans to breathe.

and plant materials — When these plants die and decompose, they become essential food sources at the base of ocean food chains. Nearly 50% of the energy feeding coastal fisheries originates from decomposing estuarine plants.

and us — Moving all the way up the food chain, estuaries support over 70% of commercial fish species that fill our markets and grocery stores with seafood.

They nurse the ocean's next generation — These protected waters offer safe nurseries and abundant food, providing ideal conditions for juvenile fish and invertebrates before they head out into the open sea. Over 90% of all marine species depend on estuaries during critical points in their life cycles.

They protect the ocean's travelers — Estuaries also provide crucial stopovers for millions of migratory birds and marine mammals like humpback whales. Nearly 75% of migrating fish species use estuaries when moving between freshwater rivers and salty oceanic water.

They keep our oceans biodiverse — The gradients of salt content, plant life, water depth, and dynamic weather create an abundance of habitat. Without estuaries, roughly 80% of all the life in and around our oceans would vanish completely.

They help our oceans store carbon — Salt marshes, mangroves, and seagrasses trap "blue carbon" in their sediment. Though seagrass meadows cover just 0.1% of the seafloor, they store an impressive 10–18% of the ocean's carbon.

They protect oceans from pollution — Estuaries are the ocean's natural buffers, trapping, redistributing and storing downstream pollutants, sediments, and runoff. Wetlands remove up to 90% of sediments and nitrogen.

and reduce erosion — Estuarine habitats reduce storm surge and wave energy. They can reduce wave height by as much as 95% over the first 100 meters of marsh.

They fostered our largest cities — Including New York City, 22 of the world's 30 largest cities are located on or around estuaries.

And they are disappearing. Over the past two centuries, we've overharvested, landfilled, and/or paved over nearly two-thirds of the world's estuary habitat. Before industrialization, global salt marshes covered approximately 100,000 square miles—roughly the size of Bangladesh. Today, less than 35,000 square miles remain.

Our home, New York City, highlights this stark reality. Once boasting one of the world's most productive estuaries, today only about 2% of New York Harbor's original estuarine habitat remains. At the Billion Oyster Project, we're actively working to reverse this trend—restoring the NY/NJ Harbor Estuary's oyster reef habitat–our ecosystems original engineers–and involve New Yorkers in the process.

This World Oceans Day, we want every city dweller to know you have a direct impact on your oceans. By supporting our work at Billion Oyster Project, or volunteering with your city's waterfront restoration initiatives, your small actions ripple outward to the open ocean.

Want to learn more about cities restoring ocean health?

Don't miss Pete Malinowski, our Executive Director, on the TEDxBoston stage for his recent talk, "Can oysters turn New York City waterways back into thriving habitats?"

Sources: American Geophysical Union, Encyclopedia of Puget Sound, NOAA Fisheries, Peconic Estuary Partnership, USDA Forest Service, Frontiers in Marine Science.

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