Marine Ecosystem

Marine Ecosystem Definition

Marine ecosystems can be defined as the interaction of plants, animals, and the marine environment. By “marine,” we mean of, or produced by, the sea or ocean. The term encompasses the salty waters of the Earth, and is also known simply as a salt water ecosystem. As over 70% of Earth’s surface is covered in water, and 97% of that water is salt water, marine ecosystems are the largest types of ecosystems on the planet.

Broadly speaking, the marine ecosystem refers to the oceans and seas and other salt water environments as a whole; however, it can be divided into smaller, distinct ecosystems upon closer inspection. There are various types of marine ecosystems, including salt marshes, estuaries, the ocean floor, the broad ocean, the inter-tidal zones, coral reefs, lagoons, and mangroves.

In accordance with, but not necessarily because of, their large size and wide range, marine ecosystems are also easily the most diverse of all the ecosystems on the planet. Coral reefs alone are home to over 25% of all marine life, despite occupying less than 1% of the ocean floor.

Like all ecosystems, marine ecosystems are finely balanced and highly complex. There are many different parts that make up an ecosystem, and each part plays a role in maintaining balance within the system. Organisms depend on, and are highly influenced by, the physiochemical environmental conditions in their ecosystem.

Marine Ecosystem Food Chain

A food chain refers to a series of organisms that are interrelated in their feeding habits. It is hierarchical in manner, with smaller organisms being fed upon by larger organisms, which in turn feed even larger organisms, and so on. All food chains begin with a producer, which is consumed by a primary consumer, which is consumed by a secondary and then tertiary consumer, and ultimately maps the flow of energy throughout trophic levels. A marine ecosystem food chain is a food chain that is specifically found within marine ecosystems.

Many marine food chains begin with phytoplankton. Phytoplankton are microscopic marine algae no bigger than 20mm. Although seemingly insignificant, phytoplankton provide the foundation, or the first level, of the sea’s food chain in a balanced system. In areas where there is enough light to support photosynthesis, such as the upper parts of the ocean’s surface, these microalgae provide two important services. First, they serve to produce approximately 50% of the world’s oxygen. Second, they support the populations of primary consumers that feed on them and, indirectly, the populations of higher level consumers that feed on them.

Marine system food web diagram
This image depicts a food web with multiple possible food chains with the arrows pointing in the direction of energy flow. In the sea, all arrows lead back to the phytoplankton in the lower right-hand corner. The phytoplankton produce their own energy via photosynthesis. From there, we can see the flow of energy going toward the primary consumers that eat them, such as the school of small fish. The energy is passed on again when those fish are consumed by the larger fish, and passed on again when that fish is consumed by the shark. In this food web, the shark is the top tertiary consumer. The energy in the shark is cycled back into the ecosystem only when the shark dies and its body is consumed by detritivores.

This is just one example of a food chain found in the marine ecosystem. Another food chain might begin with seaweed being eaten by sea urchins. Another still might start with sea grass being eaten by sea turtles.

One thing to remember is that only 10% of energy is passed on from one trophic level to the next, meaning that higher level predators need to consume many lower level prey to sustain themselves. Because of this, there will always need to be a higher number of lower level trophic organisms than higher level predators in an ecosystem.

Basic Marine Ecosystem Facts

Marine Ecosystem Animals

Marine ecosystems support a great diversity of life with a variety of different habitats. They can be categorized into groups based on where they live (benthic, oceanic, neritic, intertidal), as well as by shared characteristics (vertebrates, invertebrates, plankton). Specific examples of marine animals include sea urchins, clams, jellyfish, corals, anemones, segmented and non-segmented worms, fish, pelicans, dolphins, phytoplankton, and zooplankton.

Marine Ecosystem Plants

You can find many types of plants in the ocean, including seaweeds, algae (red, green, brown), sea grasses (the only flowering plants in the marine ecosystem), and mangroves.

Marine Ecosystem Climates

Marine ecosystems are found on many different parts of the Earth, so it shouldn’t be surprising to learn that marine climates can vary from tropical to polar. Other climates found in marine ecosystems include monsoon, subtropical, temperate, and subpolar.


1. What might you find a marine ecosystem?
A. A river shore
B. A pond
C. A beach shore
D. A lake

Answer to Question #1
C is correct. Marine ecosystems are distinguished from freshwater ecosystems by the presence of dissolved compounds, especially salts (Na, Cl). Rivers, ponds, and lakes are all freshwater bodies. A beach shore is subject to the actions of the tide, covered by salty sea water, then exposed again. Shores are considered intertidal zones of the marine ecosystem.

2. Which food chain might you find in a marine ecosystem, assuming arrows are pointing in the direction of energy flow?
A. Grass → grasshopper → mouse → snake → hawk
B. Hawk → snake → mouse → grasshopper → grass
C. Dolphin → sea otters → sea urchins → sea weed
D. Sea weed → sea urchin → sea otters → dolphin

Answer to Question #2
D is correct. Option A is a food chain found on a terrestrial ecosystem. Options B and C has arrows that are pointing in the opposite direction of energy flow. Answer D is a food chain found in marine ecosystems with arrows pointing in the correct direction.

3. How much energy is passed from one trophic level to the next in a food chain?
A. 100%
B. 50%
C. 10%
D. 0%

Answer to Question #3
C is correct. Energy is constantly being lost when moving from one trophic level to the next. Approximately 90% of the energy is used to sustain the life of the organism and is transformed into heat energy, leaving only 10% of the original energy amount available to be passed on.


  • Marine ecosystem. (2017, May 29). Retrieved June 03, 2017, from
  • Basic Facts About Marine Habitats. (2014, October 01). Retrieved June 03, 2017, from
  • Coral Reef Biodiversity. (n.d.). Retrieved June 03, 2017, from
  • Plankton. (2017, May 31). Retrieved June 03, 2017, from
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