Phytoplankton and Zooplankton Relationship in Aquatic Ecosystem

Phytoplankton and Zooplankton Relationship in Aquatic Ecosystem

The relationship between Phytoplankton and Zooplankton is very interesting and sophisticated at a time. Zooplankton keep following phytoplankton like a shadow! Do you know how?

Well, the existence or survival of zooplankton solely depends on the number of phytoplankton available in any ecosystem. The more the phytoplankton, the more will be the zooplankton. This is because zooplankton eat up the phytoplankton and maintain sequential growth. But do phytoplankton need zooplanktons any way? And what happens when there is no phytoplankton or no zooplankton? 

In this article, I will be discussing the interaction between phytoplankton and zooplankton in depth. I will also show how they are affected by physical, chemical and biological interactions in any region. So, stay with me and keep reading!

Phytoplankton and zooplankton interrelationship in aquatic ecosystem

To understand the interrelationship between these two tiny organism of aquatic ecosystem- zooplankton and phytoplankton, let’s imagine a model ecosystem first!

Lake- an aquatic ecosystem where phytoplanton and zooplankton present
Lake- an aquatic ecosystem

Distribution of zooplankton and phytoplankton in aquatic ecosystem

Phytoplankton are most abundant in the euphotic zone of aquatic ecosystem.

Euphotic zone– is the uppermost layer of the water column where sunlight penetrates. In this zone, phytoplankton can photosynthesize and convert light energy into organic matter. This zone is rich in nutrients and is where phytoplankton blooms often occur.

Phytoplanktons under microscope
Phytoplankton under microscope


On the other hand, zooplanktons grow nearby phytoplankton, having a variety in distribution range.

Why zooplankton grow nearby phytoplankton?

Zooplankton tend to grow near phytoplankton primarily because of food source. Phytoplankton serve as the primary source of food for zooplanktons. Alongside, many other related complex factors are also there to induce the growth of zooplanktons near phytoplankton. 

Zooplanktons under microscope
Zooplankton under microscope


Here’s why zooplankton are commonly found nearby phytoplankton:

  1. Food source: Zooplankton are heterotrophic organisms. It means, they cannot produce their own food through photosynthesis like phytoplankton. Instead, they rely on organic matter as their source of energy and nutrients. 

Phytoplankton are rich in organic material. It makes them a highly nutritious and readily available food source for zooplankton.

2. Energy Transfer: When zooplankton consume phytoplankton, they assimilate the energy stored in the phytoplankton cells. This energy is then transferred up the food chain when larger organisms, such as fish and other marine creatures, feed on zooplankton. Thus, the energy fixed by phytoplankton is carried throughout the ecosystem. 

Here, zooplanktons acts as a carrier of energy, being the first consumer in trophic level.

3. Chemical Signals: Some zooplankton species can detect chemical cues released by phytoplankton. These cues may include chemical compounds released by phytoplankton when they are under stress or undergoing reproductive events. Zooplankton use these chemical signals to locate areas where phytoplankton are present.

4. Filter Feeding: Many zooplankton species are filter feeders. They have specialized structures, such as fine hairs or appendages, that allow them to filter small particles, including phytoplankton, from the surrounding water. This adaptation enables them to efficiently capture and consume phytoplankton as they passively drift with water currents.

5. Phytoplankton Blooms: Phytoplankton populations often undergo seasonal blooms. During favorable season and condition, the numbers of phytoplankton increase rapidly. These blooms create localized patches of high phytoplankton density.

Blooms are attracting to zooplanktons as an abundant food resource.

Phytoplanktonic bloom
Phytoplanktonic bloom

Importance of Zooplanktonic growth for phytoplankton and the ecosystem

Zooplankton mainly graze near phytoplankton for their food source. But there are many more roles that zooplankton plays to balance the ecosystem. In some cases, they are essential for phytoplankton also!

Zooplankton are important for phytoplankton and the ecosystem for several key reasons:

  1. Control of Phytoplankton Populations: Zooplankton serve as a natural check on the growth of phytoplankton populations. By grazing on phytoplankton, they prevent excessive phytoplankton proliferation. 

Without zooplankton predation, phytoplankton populations could explode recklessly. It ma lead to algal blooms. Algal blooms can disrupt the balance of the aquatic ecosystem, if not checked timely.

2. Nutrient Recycling: When zooplankton consume phytoplankton, they assimilate nutrients contained within the phytoplankton cells. These nutrients are then released back into the water through zooplankton excretion and waste. 

This nutrient recycling is vital for the aquatic ecosystem. Because, it restores essential nutrients like nitrogen and phosphorus for the inhabitants of the ecosystem. Phytoplankton also require N and P for their growth and development. 

As a result, the cycling of nutrients through the food web is facilitated, ensuring a continuous supply of these elements for phytoplankton.

3. Enhance or control of selective species: Some zooplankton species, particularly copepods and other small herbivorous organisms, can selectively graze on specific phytoplankton species or size classes. This selective grazing can lead to a more efficient utilization of available phytoplankton by removing less desirable or competitive species. 

This, in turn, allows certain phytoplankton species to dominate and thrive.

4. Biomass Reduction: The presence of zooplankton can lead to a reduction in phytoplankton biomass. As zooplankton graze on phytoplankton, they reduce the overall quantity of phytoplankton in the water column. 

This control of phytoplankton biomass helps maintain water clarity and prevents the water from becoming overly turbid, which can negatively affect the growth of submerged aquatic plants and other organisms.

Seasonal Dynamics of Phytoplankton and zooplankton interactions

The idea of biomass interaction between phytoplankton and zooplankton in aquatic ecosystems is all about how these tiny organisms affect the total biomass in the ecosystem. It will be easier to understand if we follow how the mass of zooplankton  and phytoplankton is changing in an specific ecosystem, e.g. in the temperate zone.

Now, I am going to deep dive into the change of the growth of both phytoplankton and zooplankton in different seasons in a temperate ecosystem.

January:  Lowest growth

In January, both phytoplankton and zooplankton happens in very low quantity in the temperate zone. You must be thinking why?

Because the growth of phytoplankton depends on several factors including light, temperature and nutrients, etc. Though the nutrient level is highest in January, the light and temperature are low. As a result, the phytoplankton cannot utilize those nutrients! 

April and May: Peak Phytoplankton Growth, followed by zooplanktons

Growth influencing factors i.e. light and temperature increases in the following months after January. These factors help phytoplankton to utilize the available nutrients and keep growing fast! 

A peak in phytoplankton growth happens in April and May. This acceleration in phytoplankton growth is followed by zooplankton. What does that mean?  

It means that the increased phytoplankton will be consumed by the zooplankton. So zooplankton will give the highest growth point in June. 

Let me tell you one interesting part here. The month of zooplankton peak always comes just after the peak months of phytoplankton ends! 

Late May: Decrease in Phytoplankton growth, followed by zooplankton

 The interactions in the ecosystem will lead towards decrease of phytoplankton in two ways: 

  • Reduced nutrients: The utilization of nutrients by phytoplankton results in decreased nutrients. So phytoplankton will face shortage of food and reduce in number!
  • The grazing of zooplankton leads to a decreased phytoplankton biomass. 

By the above two interactions, decrease in phytoplankton happens by late May. 

As a result, zooplankton will now face a shortage of food. So, they move to a new place in search of food. In this way, the number of zooplankton reduces in the earlier place! 

In September: Replenishment of nutrients, increase in phytoplankton growth

After all the reductions, the replenishment of  the growth factors and planktons occur. First, decomposition of zooplankton masses and other organic materials takes place. As a result, large amounts of nutrients are released. 

Newly produced nutrients contribute to the increase in phytoplankton growth. As zooplankton has moved elsewhere, so no phytoplankton consumption happens. In this way, we can see another peak of phytoplankton growth in September! 

After some time, zooplankton will again appear and start consuming phytoplankton. Can you see a small zooplankton peak in October?

Seasonal Dynamics of Phytoplankton and zooplankton interactions
Seasonal Dynamics of Phytoplankton and zooplankton interactions

In December

At the end of the year, many more nutrients will be released by microbial decompositions happening around. As temperature and light are low in winter,

so phytoplankton will not be able to utilize the increased nutrients. And we will again see the peak of nutrients in January!

So, this is how the interaction between phytoplankton and zooplankton keeps happening all the year round. Nutrients, temperature and sunlight keeps regulating this interaction. And we find all of them giving peaks one after another. Consequently the flow of energy in an ecosystem remains balanced.

Saifun Nahar Smriti

Hello, I'm Saifun Nahar Smriti, deeply passionate about plant science and biotechnology. I explore the wonders of nature's green world and the innovative possibilities of biotech. The mysteries of plants and the exciting potential of biotechnology is thrilling to me. Welcome to my world of discovery!