What Is an Estuary and Why Is It Important

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The Importance of Estuarine Ecosystems

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By Emily Caffrey

Where a river meets the sea. Yes, these two unlikely waterways eventually find each other! Their coastal rendezvous point hosts a flourishing ecosystem of brackish water—a fresh water and salt water mixture. There’s much more to it than their salt mixture, so really, what is an estuary? 

We’ll give you a walk-through, or should we say a swim-through, of estuarine types, benefits, features, and creatures. 

Types of Estuaries

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While estuaries can also be called bays, lagoons, swamps, inlets, salt marshes, and much more, there are four specific types of estuaries. Each type is different from the last. They differ from one another by their geology—by their formation. 

  1. Coastal plain estuary: an estuary created by a sea level rise. The risen sea fills in an existing river valley. These are also referred to as drowned river valleys.
  2. Tectonic estuary: an estuary created by shifting of earth’s tectonic plates. In other words, earthquakes! When two tectonic plates separate or when one folds underneath the other, the resulting depression may fill with water to create a new estuary.
  3. Bar-built estuary: an estuary created when a lagoon or bay is protected by a barrier-island, such as a sandbar. The barrier islands form parallel to the coastline, which bars the estuary from ocean water. 
  4. Fjord estuary: an estuary created by glaciers. After glaciers retreated, the narrow openings flooded with seawater creating steep-walled estuaries.

Estuary vs. Delta

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The main difference? Sediment—sand, clay, and silt. While estuaries and deltas both involve rivers meeting larger bodies of water, a delta differs by continuously depositing sediment. Deltas form when fast-moving rivers meet slow-moving bodies of water. As soon as they converge, sediment carried by the fast-moving river drops. 

Why? The slow-moving water body isn’t strong enough to carry it further. These sediment deposits build on top of each other and eventually rise above sea level! Because deltas become so shallow, they make for surrounding fertile land and agricultural opportunities. On the other hand, an estuary doesn’t support fertile land but is more suitable for fishing activities.

Another key difference is the tidal activity—the rise and fall of water levels. An estuary is more likely to form with high tides when the ocean rises to meet the mouth of a river. A delta is more likely to form with low tides. 

Tidal activity also affects the salinity—the salt content in the water. At high tide, sea water moves into the estuary making the salinity rise, while low tide drains sea water out decreasing salinity. These ever-changing tides draw in a variety of animal species suited for both high and low salinities.

Animals and Estuarine Ecosystems

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Estuaries are crawling with critters like snails, fish, rays, migratory birds, otters, oysters, sea turtles, and even sharks and crocodiles! Plant life includes seagrasses, marsh grasses, and mangroves. Each animal and plant has their own unique adaptations and food requirements that dynamic estuarine waterways abundantly provide.

Every creature plays an important role, also called a niche, in this ecosystem that creates an intricate food web keeping cycles and populations in check. These food webs provide animals with more than one food source to survive. This, in combination with the fluctuating salinity levels, make estuarine habitats booming with biodiversity.

Let’s look at some examples! 

Phytoplankton, a microscopic plant, is responsible for much of the oxygen production in estuaries. This is the foundation of all life in these ecosystems! In addition to oxygen, phytoplankton is a food source for slightly larger zooplankton.

Zooplankton is then fed on by many filter feeding animals like crabs, small fish, shrimp, and other shellfish. In this category, oysters are a keystone species—an organism that an ecosystem depends on. Their niche is filtration. Just one oyster filters up to 50 gallons of water a day! This eliminates harmful pollutants and promotes cleaner, clearer water. 

Larger predators including fish, sharks, crocodiles, and birds feed on the animals in the prior paragraph, while animals like sea turtles feed on grasses and jellyfish. 

An imbalance of any population within the food web can cause an ecosystem crash. It is vital that all ecosystem processes remain in equilibrium for both wildlife and humans to enjoy the benefits of estuaries.

Why are Estuaries Important?

Not only are estuaries important to their environments, but they provide resources for humans, too. 

For nature:

Estuarine habitats make for one of the most biologically productive ecosystems. Productivity refers to the rate at which energy is added to the bodies of organisms in the form of biomass. Biomass is simply the mass or weight of an organism. The higher the productivity in an ecosystem, the healthier its plants and animals. 

Calm waters of estuaries provide safe havens for small fish, shellfish, shore animals, and migratory species of bird. Estuaries are often referred to as nurseries because they provide habitat for these small fish and shellfish to develop. Many species of fish also use estuarine waterways to reproduce. The nutrient-rich water provides an endless feast for small fish and shellfish. 

For humans:

There is economic value of estuaries due to their recreational tourism, commercial fisheries, and educational output in scientific studies. These water bodies also serve as ports for transportation and shipping. The communities near and far rely on estuarine waterways for food sources, jobs, and consumer products deliveries.

An essential function of estuaries that humans benefit from is their protective properties. They provide coastal buffers from storm surge and flooding. Plants in estuarine habitats also hold onto sediments to decrease erosion and loss of shoreline. These plants and shellfish such as oysters, clams, and muscles help filter pollutants from the water. This filtration creates a healthy water source for both animals and humans alike. 

Threats to Estuaries

Unfortunately, the wildlife and the benefits from estuaries are threatened by a variety of factors including:

  • Overharvesting and overfishing
  • Growing populations in coastal watersheds
  • Dredging
  • Industrial and nutrient pollution 
  • Oil and gas drilling
  • Garbage pollution

Too much of one thing becomes a problem, but you can help by joining Ocean Blue Project.

It is Ocean Blue Project’s goal to remove one million pounds of ocean plastic by 2025! We need you to make that happen by caring for all waterways. 

You can make simple changes to help these habitats by joining forces with your community to clean up your local environment. Organizing a river or ocean clean-up can remove waste before it harms estuaries and their ecosystems. Without removal of these plastics and pollution, a build-up of garbage will lead to massive losses in aquatic ecosystems and their ability to continuously provide their needed benefits.

You can also reduce your plastic output by giving single-use plastics the cold shoulder. Switch your plastic water bottle for an eco-friendly reusable water bottle or turn to reusable containers instead of throwaway containers and bags. 

Spread the Word!

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Author Bio: Emily Caffrey is a copywriter living in Baltimore. She loves scuba diving, kayaking, and creating ocean-inspired artwork.

Estuaries are where rivers and oceans collide. Their ecosystems are important resources for animals, humans, and water quality.