- Worksheet on Habitats in the Intertidal Zone
- Worksheet on Estuaries and Intertidal Zones
- Worksheet on Biotic and Abiotic Factors in Estuaries and Intertidal Zones
- Worksheet on Interaction of Biotic and Abiotic Factors in Estuaries and Intertidal Zones
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.
Estuaries
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: Study Notes from the DepEd Module
Estuaries are called the nurseries of the sea.
Biotic factors – the living components of the estuary, including plants, animals, and microorganisms
- Examples are mangrove trees, migratory birds, and small fish.
- Sharks, dolphins, and other big fishes may also be found “visiting” in estuaries.
- Migratory birds stay in estuaries because of the availability of food in the area.
Abiotic factors – the non-living factors that affect the organisms in estuaries
- Waves – the movement of the surface of the water
- Kelp have strong root-like structures that keep them attached to rocks to prevent them from being carried away by the waves.
- Mangroves provide shelter against the waves to marine organisms.
- The combination of seawater and freshwater in estuaries is called brackish water.
- Mangroves and blue crabs adjust to the constantly changing salinity of water.
- Some organisms like fish prefer to breed in places with a low amount of salt.
- The temperature in the estuary differs, depending on the tides and the amount of sunlight.
- Some organisms use plants like mangroves to keep themselves concealed from direct sunlight or away from the coldness of the water.
- Sunlight is the source of energy needed for photosynthesis.
- Estuaries are shallower and receive more sunlight than the sea and so are more conducive for photosynthesis to take place.
- Algae, seaweeds, seagrasses, and other marine plants create their own food and produce oxygen through photosynthesis. These plants support other organisms like marine animals.
- The soil is the upper layer of earth in which plants grow.
- It is the main source of nutrients needed for plants’ growth.
- The type of soil in estuaries depends on the strength of the waves and the kinds of rocks in the area.
- Some areas are full of rocks, sand, pebbles, or clay.
- The topsoil layer in an estuary is composed mostly of peat or salt crust.
- Salt is found in acidic soil; this can be problematic to plant survival.
