Shoreline Ecosystems

Shoreline (littoral) ecosystems are those that follow the coastlines, where land and water meet. Think of them as the transition from land to sea. They’re generally wet or at least damp most of the time, and salty, but water and salt content vary based on how far inland you go and on the terrain and climate of the area. In some areas, you may find an entire ecosystem living in and around a body of water not much larger than a puddle, whereas other shoreline ecosystems are beneath the ocean’s surface most, if not all, the time.

In this article, we lead you on a meandering journey down the shoreline to visit a variety of ecosystems and meet the characteristic and fascinating residents of each.

Digging life in the sand

What a great feeling it is to spread your beach blanket out on the sand and relax the afternoon away in peace and solitude. Well, not to ruin it for you, but you’re not alone. Even though you may not see any people milling about, you’re actually lying on a very crowded beach teaming with life, and we’re not just talking about those seagulls trying to steal your lunch. Above and below the surface of the sand are a variety of lifeforms, most of which are too tiny for you to see.

But don’t let their tinyness fool you; among these lifeforms are some really tough cookies. After all, beach living is a hard-knock life. Imagine being small and living on a beach. You go from being underwater, wet and salty one moment, to being exposed, dry, and hot the next. If you’re small enough, you have to deal with the tight squeeze between sand particles, and if you’re large enough, those same particles are constantly scraping against you and getting into your eyes and nose and, well, everywhere else. To top it all off, unless you’re lucky enough to live in a bay or other protected area, you’re tossed around all day by unrelenting waves.

Living here are tiny microbes too small to see, along with algae, plankton, sand dollars, crabs, and snails. Big animals like the sandy shoreline, too, including fish, rays, and sharks, which scour the shallow waters for food; turtles (for nesting); birds (for feeding); and pinnipeds (for napping).

One notable inhabitant that may have you grabbing your blanket and heading back to the motel is the innkeeper worm (shown). They generally hang out below ground (thank god for small favors) but occasionally wash up on beaches by the thousands when they get spooked out of their burrows.

Living life on the rocks

To live on a rocky seashore, you have three options: find a tide pool to hang out in, wedge yourself in a nook or cranny, or hang on for dear life. (A tide pool is a puddle in a rocky depression that fills with sea water at high tide, remains relatively full at low tide, and is refilled at high tide with fresh sea water and nutrients.)

The tide pool option is the cushiest. It’s sort of like having your own apartment with a bunch of diverse and colorful roommates — mussels, snails, starfish, barnacles, anemones, urchins, limpets, crustaceans, seaweed, and even small fish and octopus. Nooks and crannies are a little more exposed to waves and wind, so they tend to favor animals that can either hold on or retreat deeper into holes when the going gets rough, such as crabs, sea snails, chitons (pronounced “kite-ins”), and sometimes starfish. The harshest real estate on the rocky shore is right where the water meets the land. All these residents can do is hold on for dear life as they are frequently battered along these rocky high-energy coastlines. (High-energy refers to frequent large waves crashing against the shore.)

Two of the toughest hangers-on are barnacles and chitons. Barnacles are sessile (immobile) crustaceans that commonly cement themselves to rocks, piers, bridges, and boat hulls. Imagine stationary crabs in cone-shaped shells they grow themselves (see the following figure). They generally grow in colonies that look like rough concrete, but if you look closer, you can see tiny animals reaching out to grab any food that happens to float past.

Chitons are mollusks that look like turtles without heads, legs, or tails (shown). They can lie flat to wedge themselves between rocks or curl up like armadillos. You can tell a chiton when you see one by the eight overlapping armored plates on its back.

Rocky shores are also home to larger animals that spend a significant amount of time out of the water, including penguins, a wide variety of sea birds, and pinnipeds including seals, sea lions, and elephant seals.

Mixing it up in the estuaries

Estuaries are partially enclosed areas along the coastline where fresh water from rivers and streams flow into the ocean. Here, fresh water and saltwater mix, causing brackish water (not too salty, not too fresh, but somewhere in the middle). The salinity of the water changes throughout the day and the seasons depending on the tide, air temperature, rainfall, and other conditions.

If that’s not enough to mix you up, depending on the estuary, it can include a variety of habitats such as coral reefs, oyster reefs, rocky shores, mudflats, salt marshes, and mangroves. For example, Chesapeake Bay has three different habitats such as oyster reefs: home to oysters (duh!), mud crabs, and small fish; seagrass, where sea horses, blue crabs, and fish hang out; and open water, where you can find sea turtles, rays, and more fish.

Estuaries are also home to the largest reptile in the world — the estuarine crocodile, which can grow up to 21 feet long. Crikey! That’s one huge reptile with a nasty temper, to boot, but don’t worry — they live only in the tropical estuaries of Australia and Southeast Asia.

Estuaries are in constant decline due to coastal development, dredging, overfishing, and pollution.

Muddling through the mudflats

A mudflat (aka tidal flat) is just what it sounds like—a flat, barren expanse covered in mud that usually forms where river meets sea. The river carries particles of mud downstream and dumps them in the ocean. When the tide comes in, it slows the flow of the river allowing the particles to settle. Typically, wave action in these areas is minimal, so the fine particles aren’t washed out to sea.

Mudflats are covered with water at high tide and uncovered at low tide, at which time they can become some of the muckiest, most foul-smelling places on the planet at least to humans. The stench comes from the swamp gases, methane (CH₄) and hydrogen sulfide (H₂S), produced by bacteria that live in the mud and help break down organic matter to make it more digestible. These bacteria, along with algae living on the surface and detritus (decomposing plants, animals, and other organic matter), provide a rich source of food to anchor the food web. Protozoa and nematode worms feed on the bacteria; crustaceans, mollusks, and polychaete (pronounced “poly-keet”) worms feed on the bacteria, detritus, and the nematodes; and larger animals in turn feed on them. Mudflats are also vitally important stopover areas for migrating shore birds who rely on mudflat residents such as crabs, fish, and mollusks to fuel their long journeys.

Looking at a mudflat, you might conclude that it’s a smelly wasteland, and you wouldn’t be the first. People have destroyed many of these valuable ecosystems by filling them in and building on top of them in the name of coastal development. But that’s a big mistake because, in addition to serving as a huge source of food to coastal ecosystems, mudflats also help prevent coastal erosion. They’re also wonderful places to hike and watch birds. However, if you decide to hike a mudflat (an activity cleverly called “mudflat hiking”), be sure you go with a guide who knows the tides well. Muck, especially sandy muck, is like quicksand. If you don’t know what you’re doing, you may find yourself up to your chest in muck with the high tide rushing in.

Settling down in salt marshes

Salt marshes are like mud flats but with vegetation. They usually form after enough mud and peat (decomposing plant matter) has accumulated to raise the ground level to a point at which it’s high enough to support the growth of saltwater grasses and other plants that have adapted to living in shallow saltwater. Salt marshes are flooded and drained daily or at least occasionally by the overflow of ocean water, which ensures a steady supply of salt.

Like mudflats, salt marshes are home to snails, mussels, worms, and crabs, but they also attract fish and shrimp looking for food or a place to lay their eggs amidst the relative safety of the plants that offer plenty of nooks and crannies to hide in. Ducks, geese, wading birds and many migratory bird species also come to feed on the abundant vegetation, seafood, and insects. Mammals are attracted to salt marshes, too, with mice, rats, and raccoons coming to feed on the bounty; even the occasional coyote has been known to frequent salt marshes for the fine dining experience.

Clearly, salt marshes are very important to an enormous range of species . . . including humans. They provide essential food, refuge, and/or nursery habitat for many fisheries species that are vital to many coastal economies, including shrimp, blue crab, and many fish. In addition, they filter runoff water and help fight erosion.

Salt marshes can be found all over the world. In the United States, half of all the salt marshes are located along the Gulf Coast of Texas, Louisiana, Alabama, Mississippi, and Florida, but thanks to oil and gas development they are disappearing fast.

Meandering through the mangroves

Mangroves are a group of tropical and subtropical trees and shrubs that live in the intertidal zone. They range in size from small shrubs to huge 40-foot-tall trees and are adapted to living in saltwater. Some species get rid of excess salt thru their leaves while others have special pores that keep much of the salt from entering in the first place.

So, mangrove trees are cool, but what have they done for us lately? Well . . . a lot, including the following:

• Provide a habitat for algae, insects, mollusks, crustaceans, fish, sharks, birds, dolphins, manatees, turtles, lizards, crocodiles, alligators, snakes, frogs, small mammals, a few marine mammals, and more insects than you can probably imagine.
• Serve as a nursery where many fish and other sea creatures lay eggs, give birth, and protect and feed their young. Mangrove forests are directly or indirectly responsible for a considerable percentage of the global fish catch.
• Protect the coastlines and coastal communities from storm surges, hurricanes, typhoons, waves, and floods. (Every 330 feet of mangrove forest can reduce wave height by up to 66 percent.)
• Filter the water, improving clarity and quality.
• Store carbon, which mitigates global warming. One acre of mangrove alone can sequester 1,450 pounds of carbon a year, which is the amount of carbon produced by driving your car across the United States three times. That’s also more carbon than can be stored by an acre of any inland forest. (See Chapter 21 for details.)
• Contribute to the world economy. Worldwide, mangroves contribute about $1.6 billion each year to local economies.

Mangroves not only protect land but also create it. Mud collects around the network of roots leading to the development of shallow mudflats.

Any discussion around the mangrove ecosystem is more like a tale of two ecosystems—the one below water and the one above it—as explained next.

Living among the roots

Some mangroves have roots that reach from above the surface of the ocean down into the substrate below. This enables them to survive the ups and downs of the tides, but it also creates a tangle of roots that makes an ideal habitat for small ocean creatures, including the following:

• Invertebrates (animals without backbones): Snails, barnacles, bryozoans, tunicates, mollusks, sponges, polychaete worms, isopods, amphipods, shrimps, crabs, and jellyfish all live in or near the network of roots.
• Fish: A wide variety of fish live inside or adjacent to the root system, many of them, such as sharks, in the juvenile phases of life where they’re safe from bigger fish that can’t get in between the roots. Fish types vary according to water temperature, salinity, turbidity (cloudiness), and other characteristics of the water. In addition, salinity can change with seasons, drawing more or fewer freshwater species that can tolerate some salinity.
• Marine mammals: Farther out from shore, you can find some marine mammals, including bottlenose dolphins, manatees, and dugongs. Dolphins are typically in search of fish, whereas manatees dine on the vegetation.

Keeping your head above water

Above the waterline, mangroves support an even greater diversity of creatures, most of which are terrestrial or amphibious, including the following:

• Reptiles: Crocodiles, alligators, various snakes and lizards, and land turtles and (farther offshore) sea turtles. Also, some freshwater turtles nearer to the headwaters may frequent the mangroves.
• Amphibians: Amphibians aren’t generally drawn to mangroves because of the salinity of the water, but you may find some tree frogs and toads.
• Birds: Many species of birds live in the mangroves or feed there, including egrets, heron, spoonbills, ducks, grebes, loons, cormorants, falcons, eagles, owls, vultures, and more.
• Mammals: Mammals that frequent the mangrove forests include panthers, raccoons, skunks, mink, river otters, bobcats, possum, rabbits, rats, and even deer.