Imagine what it would be like if you lived in a town that gets flooded every day. Sometimes you’re underwater, sometimes you’re not. Sometimes it’s awfully hot and dry, sometimes you’re miserably cold and wet. To survive, you would need to figure out where best to stay at certain parts of the day, when and what routes to take to find food, where to keep your food so it doesn’t get wet or washed away, how to keep cool if the sun is scorching hot, how to stay dry when the waters come, et cetera, et cetera. Basically, if you lived in a place like that, you would have to become really good at adjusting to constant change.
Well, some creatures on earth do live in places where conditions are always changing. There are “borderlands” — where the land meets the sea, or where freshwater meets saltwater, for example — where the resident organisms have become really good at finding ways to thrive in an environment that’s constantly in flux.
How do they do it?
That’s what we will explore in this lesson on the fascinating world of estuaries and intertidal zones.
An estuary is a place where freshwater from a river mixes with saltwater from the sea. (This combination of fresh and saltwater is called brackish water.)
As you might imagine, estuaries are affected by things coming from either direction: from the sea or from the river.
From the sea, there are the tides, the waves, and the salt. There are daily high tides and low tides so the water level is always changing. Waves can grow huge, especially when there are storms. And because this water brought in by the tides and the waves is salty, the level of salinity (how salty the water is) in the estuary also keeps changing. For this reason, the fish and other organisms that live in estuaries need to have the ability to tolerate different levels of saltiness in the water.
Mangroves are the perfect example of plants that have become expert in living in estuaries. To survive in the brackish water, some types of mangroves have filters in their roots that remove the salt from the water before absorbing it. Other types of mangroves have special structures in their leaves that actively spit out excess salt.
From the river, there is freshwater flowing from upstream, the sediments it picks up, and everything else that falls into the river and is carried along to the sea. When there are heavy rains, a great amount of freshwater flows into the estuary, making it less salty than usual.
Because estuaries link the river and the sea, they are important stops in the journey of fish that migrate from the river to the sea or from the sea to the river. The estuaries are where these fish are able to adjust little by little to a level of salinity that is different from what they are used to. For example, eels that live in the river but return to the sea in order to breed can avoid getting shocked by the sudden change in salt levels from freshwater to saltwater by spending some time in the brackish water of estuaries.
Estuaries are nicknamed “the nurseries of the sea” because a lot of fish lay their eggs there where it’s relatively safe from big predators and because there are a lot of nutrients brought in by both the river and the sea.
However, when there is too much nutrient — such as when water runs off from farms containing a huge amount of fertilizer, goes into the river, and is carried down to the estuary — it can cause the growth of too much algae. This harmful algal bloom (HAB) can eat up all the oxygen in the water as they decay and can even release poisonous toxins.
Soil eroded by rains can also get deposited into the estuary, making the water turbid (or muddy), drifting into the gills of fish, and covering the places where the fish lay their eggs. Chemicals, garbage, and other pollutants can also enter estuaries through the river.
Estuaries are called the nurseries of the sea.
Biotic factors – the living components of the estuary, including plants, animals, and microorganisms
Abiotic factors – the non-living factors that affect the organisms in estuaries
“Intertidal zone” sounds like a very scientific term but, actually, if you’ve been to the beach, then you’ve been to an intertidal zone!
Simply, the intertidal zone is that part of the coast that is covered by water during high tide but is exposed or above the water level during low tide.
You probably noticed this while you were at the beach — that rocks that were above water when you arrived gradually became covered by the water as the tide came in, or that the edge of the water was just a few meters from your cottage at first but became farther and farther as the tide went out. That area that was covered by water when the tide was high but got exposed when the water receded (or moved away) is the intertidal zone.
Intertidal zones aren’t just at the beaches we go to. Everywhere the sea reaches — so basically all around every island in the world — there is an intertidal zone.
Intertidal zones can be rocky or sandy. They can also be muddy. They may be seemingly bare, like mud flats; grassy, like in salt marshes; or even be home to an entire forest of mangroves.
The organisms that live in the intertidal zone experience two totally different conditions every day: one when they are submerged in seawater (immersion) and the other when they are exposed to air (emersion). It’s like they live both underwater and on land. For that reason, they need to be able to survive extreme changes in moisture and temperature.
For example, barnacles and mussels keep seawater in their closed shells so that they don’t dry out during low tide. Some sea stars and fish take refuge in tide pools — holes, cracks, or crevices that hold seawater — when the tide goes out. In sandy areas, animals can also bury themselves in the sand so that they can stay cool and moist during low tide.
The inhabitants of the intertidal zone also need to be able to withstand waves. Organisms such as kelp, mussels, and barnacles are able to anchor themselves to keep from getting carried away. Sea stars have tube feet that look like suction cups, isopods have hook-like legs, and mussels produce threads with “glue” that keep them tied to rocks.
The intertidal zone is further divided into three parts:
The intertidal zone is the area that is above water level at low tide and underwater at high tide. It is also known as the foreshore or seashore.
The different habitats found in intertidal zones include:
Coral reefs – provide shelter to thousands of fish
Salt marshes – filled with seawater during high tide and drained during low tide
Mud flats or tidal flats – areas where mud from the seas or rivers is deposited
Rocky shores – areas where solid rocks are found
Mangrove forests – areas filled with mangrove trees
The abiotic factors that affect the organisms in intertidal zones are similar to those in estuaries:
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