Basin Blues
What is an Estuary?
Functions of the Estuary
The Lake Pontchartrain Estuary
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Lessons on the Lake

The Magic of the Estuary

"If there is magic on this planet, it is contained in water."

Loren Eiseley
Immense Journey


  1. Define an estuary.
  2. Describe characteristics of the estuary.
  3. List the functions of the estuary.
  4. Visit an estuary, making a permanent record with a photo or nature journal.
  5. Participate in creative problem solving, brainstorming problems and solutions, and developing criteria to solve a given problem.

Multiple Intelligences Learning Activities:

Describe and tell stories about estuary photos.
Develop a script for a news program.

Photograph estuary areas and organize them into a photo journal or
visual presentation.

Listen to sounds of the estuary and substitute nature sounds for
words of familiar songs, performing original musical pieces.

Write personal feelings about the estuary photos.

Investigate a scenario; identify problems; select a specific problem;
brainstorm solutions; select possible solutions to the problem
using acceptable criteria.

illustration of a blue crab

The Basin Blues

Shhhh! Buzz, buzz, shhhh! Slap, slap, crunch, crack, yipes! Hum, hum, sputter, splash, splat! Buzz, buzz, hummmmm!

There's a place where critters abound, where water is deep and rich, where animals munch on grassy green stalks, and then take the time for a dip....

There's a place, where grasshoppers go, where nutria travel and trail, where birds of all kinds of feathers will flock together, and fish look fishy-eyed at the world!

There's a place where a crab is a king, and a snail like a jewel will sway, There's a place where fisher people will go and will stay all night and all day!

There's a place where the sun and energy it brings shines from daylight to dusk, then a silvery white moon will sparkle the night and the sounds, whoooo, whaaaaa, kurplunk, swish and glop, will make all of us stop!

Basin Blues
What is an Estuary?
Functions of the Estuary
The Lake Pontchartrain Estuary

What is an Estuary?

An estuary is...
a semi-enclosed body of water that has a free connection with the open sea and within which the sea water is measurably diluted with freshwater that is derived from land drainage.

illustration of estuary characteristics
Click the illustration for a larger view.


  • An estuary is a place where seawater is measurably diluted by freshwater from land drainage.

  • The mixture of fresh and salt water provides a variety of habitats for animals

  • and plants in the area.

  • Salinity is a measurable quantity:

    • Freshwater is described as having 0 - 0.5 ppt (parts per thousand) of salt dissolved in the water.

    • Seawater is 20 - 35 parts per thousand. Imagine if you took 35 grams or parts of salt (table salt will do!) and dissolve it in 1,000 parts of water...you have just made sea water!


  • There are areas of the estuary which are characteristically freshwater areas.Other areas of the estuary, however, will have varying degrees of salinity because of the introduction of saline water from the sea.

  • Freshwater in the estuary comes from rivers, creeks, bayous, and streams which drain toward the estuary.

  • Freshwater amounts will vary with variations in rainfall.

  • During periods of heavy rainfall the estuary becomes less salty, since

  • more freshwater is added.

  • Animals living in the estuary must tolerate wide ranges of salinity and, therefore, are called euryhaline biota. If you travel from the top of the estuary toward the sea, salinity, as well as types of animals and plants will change.

Tidal Influence

  • The salinity of an estuary may change on a daily basis due to tides and winds.

  • Tides are the daily or twice daily movement of water in and out of an estuary or coastal area. There are high and low tides. High tides are determined by the high water mark on the shoreface, the sand on the beach. High tides bring high salinity water and add nutrients to the estuary, flushing away waste products, impurities, or even pollution. As the high tide gradually falls to its lowest point, it becomes the low tide, and the cycle starts over again.

  • Winds are movements of air which blow from offshore, moving seawater into the estuary. Winds also blow from the land toward the sea, moving water out of the estuary and drying areas which are normally wet.


  • When an ecosystem or a habitat has many different plants and animals it exhibits biodiversity. Biodiversity contributes to the stability of ecosystems. If there are many producers, consumers and decomposers, there is less chance that loss of one species will impact the entire ecosystem.

  • Estuarine animals and plants must tolerate changing salinities and other conditions such as fluctuating temperature, water level, currents, and levels of oxygen. These changing conditions are stressful to many animals and plants. Few species can tolerate these conditions; therefore, the ecosystem is less diverse! For example, while we may have many species of fish in some estuarine areas, scientists tell us that a coral reef contains four times as many species as are found in Lake Pontchartrain, a typical estuary.

  • What the estuary lacks in biodiversity it makes up for in abundance of the species that inhabit the area. To a mosquito this means that there is a lot of grass from which it can suck plant juices; to a frog, it means there are a lot of mosquitoes to catch at night; to a blue heron, it means there are a lot of frogs to catch!

Muddy Substrates:

  • Sediments such as sands, silts, and clays are found in estuaries and are derived from river systems.

  • The specialized environment of muddy substrates is home for burrowing worms, clams, microscopic bacteria, fungi, and other specially adapted animals.

  • If you plan on living in the estuary prepare to get your feet muddy!
illustration of 7 frogs
Basin Blues
What is an Estuary?
Functions of the Estuary
The Lake Pontchartrain Estuary

illustration of the functions of the estuary


  • The estuary is home to many species of plants and animals. As a home it provides protection, food, and space in which animals and plants find similar members of their own species. P>Animals in the estuary may be well suited to:

    • Living on tall grass, like the periwinkle snail that lives on Spartina alterniflora;

    • Burrowing in the muddy substrates, such as, Rangia cuneata clams and marine worms;

    • Hiding under rotting vegetation as crawfish do;

    • Clinging among the stalks of Spartina alterniflora like grass shrimp.

Nursery Area

  • Many species, such as the shrimp and menhaden (pogie) use estuarine areas to nurture and protect their young.

  • Larval shrimp and juvenile menhaden move into the estuary where

  • they grow to maturity because:

    • There is an abundance of food;

    • The estuary is shallow, protecting juveniles from predators.

    • Submerged aquatic vegetation, such as Vallisneria americana (Turtle grass), provide beds of vegetation for young animals to find protection.
illustration of the life cycle of the shrimp

Fishery and Recreation

  • Fisheries such as shrimp, crab, mullet and oyster depend on high productivity of the estuarine areas.

  • Many people, such as commercial fishers, use estuaries for their livelihood.

  • An example is the crab fisher.

    • Crab fishers put out crab traps in shallow bays and inlets.

    • These baited traps "catch" blue crabs in a baited wire cage from which they cannot escape.

    • Crab fishers harvest only crabs of a certain size, this assures that there are enough adults left to reproduce the next year.

    • The crab fishery is economically important in the Lake Pontchartrain Estuary because people who fish depend on the crab harvest for their livelihood.

The estuary with its high productivity is a place where boating, sailing, recreational fishing, shrimping, and crabbing activities take place. In this diverse habitat many people enjoy the relaxing atmosphere and derive pleasure from these leisure activities.

What a wonderful area to enjoy!

Basin Blues
What is an Estuary?
Functions of the Estuary
The Lake Pontchartrain Estuary

The Lake Pontchartrain Estuary

Lake Pontchartrain is a 630 square mile estuary which receives freshwater from six major sources:
  1. Tangipahoa River
  2. Tchefuncte River
  3. Tickfaw River
  4. Amite River
  5. Bogue Falaya River
  6. Lacombe Bayou


The Lake Pontchartrain estuary receives saltwater from the Gulf of Mexico, entering Lake Pontchartrain through the Rigolets and Chef Menteur Passes. A smaller lake, Lake Maurepas, is connected to Lake Pontchartrain at its western edge by Pass Manchac; its contribution of freshwater serves to dilute lake salinity.

Salinity in the estuary ranges from 0.5-15 ppt. The highest salinities are found near the Rigolets and Chef Menteur Passes as high salinity water is pushed from the passes into Lake Pontchartrain.

An additional source of high saline water is the Industrial Canal on the south shore of Lake Pontchartrain. This water is channeled through the Mississippi River Gulf Outlet and is largely responsible for changing freshwater habitat areas in the lower Mississippi River Delta to saltwater areas.


The freshwater from six major sources measurably dilutes high saline waters; for example, salinities near the Tangipahoa River are 0 ppt.

Under certain flooding conditions in the Mississippi River, the Bonnet Carre Spillway is opened and provides additional freshwater to the estuary.

The quality of water entering the estuary from the rivers, bayou and spillway is of concern to those who use the estuary. This water quality may be contaminated by:

  • Urban runoff: oil from city streets or untreated sewage;

  • Agricultural wastes: manure from dairy feed lots;

  • Chemical Pollution: toxins released into the river;

  • Sediment: from poor soil conservation practices;

  • Excess nutrients: fecal material and fertilizer from farms along the Mississippi;

  • Litter or garbage: waste from careless boaters or picnicers.

Tidal Influence:

Tides in Lake Pontchartrain are produced by winds.

These tides are minimal, and the tidal range is three to ten inches under most conditions.

The winds also drive a counterclockwise current in Lake Pontchartrain, causing water to circulate in that pattern. This means that what gets into Lake Pontchartrain usually stays there!

Estuarine Wetlands:

Historically, the land surrounding the Lake Pontchartrain estuary contained acres d acres of wetlands, (marshes, swamps and bottomland hardwood forests). This huge brackish-water (0.5-15 ppt salinity) estuary and its surrounding wetlands functioned as diverse habitat for freshwater and marine species.

Historically, the wetland areas acted as filters for water entering the lake from land drainage; the water in Lake Pontchartrain remained clean and useable.

Today many of the wetland areas have disappeared:

  • Affecting water quality in Lake Pontchartrain and

  • Producing loss of habitat for many species which surround the lake.HA


Although there are many habitats in the Pontchartrain Basin, we will focus on the surrounding wetlands and the lake bottom.


The healthy vegetated wetlands surrounding the Lake provide protection and food for many estuarine animals such as:

  • Mammals: raccoons, opossums, squirrels, deer;

  • Reptiles and Amphibians: yellow-bellied water snakes, red-eared slider turtles;

  • Birds: American Egrets, red-winged blackbirds, Little Blue Herons, American Bald Eagles;

  • Invertebrates: blue crabs, crawfish, shrimp, oysters;

  • Fish: juvenile menhaden (pogie), least killifish, speckled trout, red snapper.

Lake bottom or benthic substrate of Lake Pontchartrain:

The silty bottom of this large lake is habitat for many invertebrate animals; among them is the Rangia cuneata clam:

  • Rangia was a favorite food of local Native American inhabitants.

  • When purged of silt and fried in a batter, these clams are quite delicious!

  • The shells of the dead clams were used to make roadbeds throughout the country. Economically the shells were valuable, and an industry which dredged up living and dead clam shells developed.

  • Since shell dredging which increased lake turbidity was halted ten years ago, the turbidity in the lake is clearing. Al ong the edges of the lake, where sandy bottom sediment is found, favorable conditions are supporting the growth of submerged aquatic vegetation. The vegetation is Vallisneria americana or turtle grass. It is important because:

    • Submerged aquatic vegetation, or grass-like plants which grow under water, are returning with the decrease in turbidity.

    • Submerged aquatics are home for many species, like blue crabs and juvenile fish species.

    • Submerged aquatics provide oxygen and are a source of food for some species in Lake Pontchartrain.

    • Submerged aquatics are part of a healthy ecosystem and indicate that Lake Pontchartrain is beginning its recovery from environmental problems.
Basin Blues
What is an Estuary?
Functions of the Estuary
The Lake Pontchartrain Estuary


View Chapter:

©1998 Lake Pontchartrain Basin Foundation

Lessons on the Lake is published by the
Lake Pontchartrain Basin Foundation
Metairie, LA

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Basin Blues
What is an Estuary?
Functions of the Estuary
The Lake Pontchartrain Estuary