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In this lesson, you’ll learn about the biodiversity of ecosystems based on water. You will discover that marine, freshwater and a mixture of salt and freshwater create very different ecosystems, which in turn influence the amount of biodiversity that is found in these habitats.

Definition of Biodiversity

Earth is covered with millions of different species of plants and animals.

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Each ecosystem and habitat on our planet has its own degree of variation of life; we call this biodiversity. The root ‘bio’ means ‘life’ and ‘diversity’ means ‘many different things.’One of the interesting things about biodiversity is that it’s never constant across one type of habitat. For example, the biodiversity of Mount Everest is completely different than the biodiversity of the Rockies! It all depends on where you are on the planet. However, no matter where you are on Earth, one of the areas that has the highest biodiversity is ecosystems based upon water.

Water-Based Ecosystems

We can break water-based ecosystems into three categories.

These are: marine (or saltwater), freshwater and those that are both. Marine ecosystems are habitats like the open ocean, coral reefs and intertidal zones. Freshwater ecosystems include lakes and rivers.

Ecosystems that are considered both marine and freshwater are areas where the salinity (or saltiness) of the water fluctuates between fresh and saltwater due to river input or outgoing tides. These include places like estuaries and salt marshes.

Biodiversity in the Open Ocean

While it’s strange to think about, the open ocean, which makes up about 70% of the planet, has one of the lowest biodiversities we see in an ecosystem! This is because, just like humans, organisms in the ocean need nutrients to grow. The oceans get these essential nutrients from water that has run off from land. So, the further away from land you get, the less diversity you see in the ocean! Another factor that contributes to the biodiversity of the open ocean is light.

Light penetrates water in the ocean to about 200 meters, or 600 feet, a region known as the photic zone.Why is this important? Just like on land, the bottom of the food chain in water is comprised of things that photosynthesize, or get their energy from sunlight. These are called primary producers. In the ocean, these are organisms like algae and plankton, which are very small (smaller than you can usually see with your naked eye!).The larger animals that live in the open ocean eat plankton in order to survive. For example, a whale shark, which can reach over 50 feet long, eats millions of plankton each day! Other organisms that make up the biodiversity of the open ocean are large fish such as tuna, swordfish and sharks; mammals such as whales and dolphins; and invertebrates such as jellyfish and squid.

Biodiversity on Coral Reefs

Coral reefs are one of the most diverse ecosystems on the planet. They provide structure for multiple species to hide in as well as crevices for laying eggs and for living in. Therefore, coral reefs provide a nursery ground for raising young.The majority of organisms found on coral reefs are invertebrates, or animals that don’t have a backbone. The list of invertebrates found on reefs is huge! Corals and sponges make up the largest group of invertebrates found on coral reefs; these are the organisms that make up the structure of the reef itself.Here, primary producers are also the backbone of biodiversity, but not necessarily because of phytoplankton. You may look at a coral or a sponge and think it looks more like a plant, but these are actually considered to be animals! Corals have a special type of algae that live in them called zooxanthellae.

Zooxanthellae photosynthesize and provide nutrients for the coral to grow.Other invertebrates found on reefs include anemones (yes, this is where you can find Nemo!), sea stars and sea urchins, crustaceans such as shrimp and lobster, and mollusks such as giant clams. If you’re lucky, you might even see an octopus! The vertebrates found on coral reefs are sea turtles, eels, fish and sharks.Another interesting part of coral reef diversity is that many of the organisms that live on the reef are nocturnal, meaning they are only active at night. If you visit a reef during the day and then again at night, you will see a totally different reef with a whole new biodiversity!

Biodiversity in the Intertidal Zone

Unlike the open ocean and coral reefs, there are several environmental factors that influence the biodiversity of the intertidal zone, all of which are centered on a tidal cycle, or when the tide is in or out.

When the tide is in, the intertidal can be completely covered with water or waves can splash this zone, possibly harming the organisms that live there. When the tide is out, organisms are exposed to air or live in small pools of water called tide pools. This means these organisms are exposed to predators and direct sunlight and can desiccate, or dry out.All these factors make the biodiversity of the intertidal zone high and low at the same time.

It is considered a low biodiversity overall; very few organisms can handle living in the intertidal zone. However, when you examine the type of organisms that live in the intertidal – invertebrates and algae – the biodiversity is quite high! There are over 1,000 species that can live in the intertidal! This includes organisms such as sea urchins, sea stars, sponges, anemones, mussels, snails, crabs and seaweeds.

Biodiversity in Lakes

Just like the open ocean, light is very important to the biodiversity of a lake. Lakes also have a photic zone, but the depth to which this zone extends is more dependent on the depth of the lake than on anything else.

Another factor that affects biodiversity in lakes is temperature. Because these bodies of water are stagnant, or don’t move, there is always the possibility they will freeze when the weather gets cold. If the lake freezes, no oxygen can get into the water, which makes the lake become anoxic, or oxygen-poor.

If you cover your mouth with your hands and keep breathing, after a minute or so, it becomes hard to breathe, right? Now think about doing that for a few months!Also, like the open ocean, a lake’s biodiversity starts with primary producers. In freshwater, these are mainly plants, phytoplankton and zooplankton. Most lakes typically have quite a few amphibians such as frogs and salamanders, as well as reptiles such as turtles and snakes. Larger lakes have multiple species of fish, and some even have alligators! We also find a number of birds and small mammals such as raccoons and beavers around larger lakes.

Biodiversity in Rivers

Rivers are a bit like the intertidal zone. Because they are constantly moving, organisms that live in them must either be able to swim well or attach to the bottom or sides of the riverbank.

Again, we see the biodiversity of a river dependent upon the primary producers in the river. Fish are, of course, in rivers, as well as macrophytes, which are aquatic plants. As with lakes, you can find amphibians and reptiles in areas where the water moves slowly, and a number of mammals and birds depend upon rivers for feeding grounds.

Biodiversity in Estuaries and Salt Marshes

Species that live in salt marshes and estuaries must endure one of the harshest and most variable habitats on the planet. The salinity in these habitats fluctuates from freshwater to saltwater, which can be problematic for most organisms. Therefore, the biodiversity in these habitats is fairly low for organisms that live in these habitats exclusively. These species typically include plants with special glands that excrete (or get rid of) excess salts, oysters, snails, crabs and small fish.The biodiversity of these habitats increases dramatically when you include the visitors of these specialized ecosystems. Hundreds of species of birds visit estuaries and salt marshes looking for food. Many species even use these habitats to build nests for their young.

Small mammals such as raccoons and skunks find food in these habitats. Another interesting example is the species that have to travel through estuaries or salt marshes to get somewhere else. The incoming tides from the oceans bring fish such as salmon, which spawn in freshwater, even though they live in the ocean for most of their lives.

Lesson Summary

Biodiversity means ‘many different forms of life.’ This includes anything from the smallest phytoplankton, which you can’t see with your naked eye, to seaweeds that are hundreds of feet long, to the largest mammals in the world, the whales.

Ecosystems based upon water can either be highly diverse or have a very low biodiversity. The ecosystems that are marine, or made up of saltwater, include coral reefs and the intertidal zone, which are both highly diverse, as well as the open ocean, which has a fairly low biodiversity. Ecosystems that are based upon freshwater, such as lakes and rivers, also have high biodiversity. Habitats that are both marine and freshwater (estuaries and salt marshes) are some of the harshest areas on the planet to live. Therefore, the biodiversity in these areas is quite low.

Learning Outcomes

Upon completing this lesson, you will be able to:

  • Define biodiversity and explain the various high and low biodiversities of a number of salt and freshwater environments
  • Explain how the photic zone affects the biodiversity of the open ocean and lakes
  • Describe how water movement affects the biodiversity of rivers and intertidal zones
  • Identify why coral reefs are the most biodiverse systems on the planet
  • Summarize why the biodiversity of estuaries and salt marshes is low
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