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Biomass Pyramid for a Marine Ecosystem

Instructor: Amanda Robb

Amanda holds a Masters in Science from Tufts Medical School in Cellular and Molecular Physiology. She has taught high school Biology and Physics for 8 years.

This lesson will help you understand how to construct a biomass pyramid for a marine ecosystem. First, we'll review what a biomass pyramid is, look at an example for a marine ecosystem and explain why biomass pyramids can become inverted.

What Are Biomass Pyramids?

Think about an ecosystem you're already familiar with. Maybe you're thinking of a forest or grassland. What kind of organisms do you picture in this ecosystem? You might be visualizing grass, rabbits, or even bears.

These organisms can be divided into different levels depending on their ecological role, called trophic levels. Usually, there are lots of producers, or organisms that make their own food. These form the base of the food web. Primary consumers are organisms that eat producers. These form the next level. Secondary consumers eat primary consumers and finally at the top of the food web are tertiary consumers who eat primary and secondary consumers.

These layers can be arranged into a diagram called an ecological pyramid. Ecological pyramids can be based on energy, number of organisms, or biomass. Biomass pyramids show the relative amount of biomass in each of the trophic levels of an ecosystem. Biomass is simply the mass of living things in a particular trophic level.

Terrestrial ecosystems usually have much more biomass in plants, such as trees and grass, and less biomass as you move up in trophic level. This creates a classic pyramid shape in the terrestrial biomass pyramid. However, marine, or ocean-based, biomass pyramids tend to look at little different. These tend to have less biomass at the bottom in the producer level and more biomass at the consumer levels. Let's look at how this plays out next.

Marine Biomass Pyramid

Marine biomass pyramids show the relative level of biomass at each of the trophic levels for ocean ecosystems. Marine biomass pyramids tend to be inverted due to the dynamics of the producers and consumers.

Marine biomass pyramid
marine biomass pyramid

Many marine ecosystems rely on phytoplankton as their primary producer. Phytoplankton are very small, even microscopic. These tiny organisms reproduce and die very quickly. So, at any given moment their biomass is relatively small, even though they supply energy for the entire ecosystem. Their turnover is just very quick!

Microscopic phytoplankton form the base of the marine biomass pyramid
phytoplankton

The following level of primary consumers consists of zooplankton, another microscopic organism, and other small organisms like small fish, sea anemones, krill, or crustaceans. These animals are larger in size than the phytoplankton and thus have more biomass.

Still, the secondary consumers, organisms like larger fish, whales, and other organisms are much larger than the primary consumers and producers. These organisms tend to live for longer periods of time as well, so at any given moment their biomass is quite significant.

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