What are Phytoplankton?

Phytoplankton are microscopic organisms that live in salty and fresh water environments. There are hundreds of thousands of species of phytoplankton, each adapted to particular water conditions. Changes in water clarity, temperature, water depth, wind, nutrient content, and salinity change the species that live in a given place and their growth. Some phytoplankton are bacteria, some are protists, and most are single-celled plants. Among the common kinds are cyanobacteria, silica-encased diatoms, dinoflagellates, green algae, and chalk-coated coccolithophores.

Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use photosynthesis to turn it into chemical energy. They consume carbon dioxide on a scale equal to forests and land plants, and release oxygen. Phytoplankton growth depends on the availability of carbon dioxide, sunlight, and nutrients. Phytoplankton use nutrients such as nitrate, phosphate, silicate, and calcium at various levels depending on the species. Some phytoplankton can fix nitrogen and can grow in areas where nitrate concentrations are low. Lack of trace element concentrations can limit their growth or what kinds of predators eat them. Phytoplankton are responsible for the transfer of 10 gigatonnes of carbon dioxide from the atmosphere to the ocean each year.

When conditions are right, phytoplankton populations can grow explosively, a phenomenon known as a bloom. Blooms in the ocean, lakes, and rivers may cover hundreds of square kilometers and are easily visible in satellite images. A bloom may last several weeks, but the life span of any individual phytoplankton is rarely more than a few days.

Winds play a strong role in the distribution of phytoplankton. Their productivity increases during El Niño events from increased rainfall and runoff deliver more nutrients than usual.

Individual phytoplankton are tiny, but when they bloom by the billions, the high concentrations of chlorophyll and other light-catching pigments change the way the surface reflects light and turn the water greenish, reddish, brownish, milky white, or bright blue.

Phytoplankton are the foundation of the aquatic food web, the primary producers, and food for microscopic, animal-like zooplankton to multi-ton whales. Small fish and invertebrates also graze on the plant-like organisms, and then those smaller animals are eaten by bigger ones.

Phytoplankton can also be the harbingers of death or disease. Certain (I believe all) species of phytoplankton produce powerful biotoxins, making them responsible for so-called “red tides,” or harmful algal blooms. These toxic blooms can kill marine life and people who eat contaminated seafood. After a bloom, dead phytoplankton sink to the ocean or lake floor and the bacteria that decompose or break down the phytoplankton deplete the oxygen in the water, suffocating animal life and create a dead zone.

Scientists predict as the ocean surface warms from increasing greenhouse gases, the water column will become more stratified (distinct layers with different temperatures, salinity, or oxygen content in the ocean or other bodies of water), there will be less vertical mixing to recycle nutrients from deep waters back to the surface. As the ocean has warmed since the 1950s, it has become increasingly stratified. About 70% of the ocean is permanently stratified into layers that don’t mix well. This may result in a cascade of negative consequences throughout the marine food web.

Click here to learn more or read an excerpt below.

 

 

Illustrations of types of phytoplankton.

Phytoplankton are extremely diverse, varying from photosynthesizing bacteria (cyanobacteria), to plant-like diatoms, to armor-plated coccolithophores (drawings not to scale). (Collage adapted from drawings and micrographs by Sally Bensusen, NASA EOS Project Science Office.)

 

Derived from the Greek words phyto (plant) and plankton (made to wander or drift), phytoplankton are microscopic organisms that live in watery environments, both salty and fresh.

Some phytoplankton are bacteria, some are protists, and most are single-celled plants. Among the common kinds are cyanobacteria, silica-encaseddiatoms, dinoflagellates, green algae, and chalk-coated coccolithophores.

Like land plants, phytoplankton have chlorophyll to capture sunlight, and they use photosynthesis to turn it into chemical energy. They consume carbon dioxide, and release oxygen. All phytoplankton photosynthesize, but some get additional energy by consuming other organisms.

Phytoplankton growth depends on the availability of carbon dioxide, sunlight, and nutrients. Phytoplankton, like land plants, require nutrients such as nitrate, phosphate, silicate, and calcium at various levels depending on the species. Some phytoplankton can fix nitrogen and can grow in areas where nitrate concentrations are low. They also require trace amounts of iron which limits phytoplankton growth in large areas of the ocean because iron concentrations are very low. Other factors influence phytoplankton growth rates, including water temperature and salinity, water depth, wind, and what kinds of predators are grazing on them.

 

Satellite image of the ocean off the coast of New Zealand on October 11, 2009.
Satellite image of a phytoplankton bloom off the coast of New Zealand on October 25, 2009.

Phytoplankton can grow explosively over a few days or weeks. This pair of satellite images shows a bloom that formed east of New Zealand between October 11 and October 25, 2009. (NASA images by Robert Simmon and Jesse Allen, based on MODIS data.)

When conditions are right, phytoplankton populations can grow explosively, a phenomenon known as a bloom. Blooms in the ocean may cover hundreds of square kilometers and are easily visible in satellite images. A bloom may last several weeks, but the life span of any individual phytoplankton is rarely more than a few days.

Importance of phytoplankton

The food web

Phytoplankton are the foundation of the aquatic food web, the primary producers, feeding everything from microscopic, animal-like zooplankton to multi-ton whales. Small fish and invertebrates also graze on the plant-like organisms, and then those smaller animals are eaten by bigger ones.

Phytoplankton can also be the harbingers of death or disease. Certain species of phytoplankton produce powerful biotoxins, making them responsible for so-called “red tides,” or harmful algal blooms. These toxic blooms can kill marine life and people who eat contaminated seafood.

 

Photograph of fish killed by a red tide on the shore of Padres Island, Texas.

Dead fish washed onto a beach at Padre Island, Texas, in October 2009, following a red tide (harmful algal bloom). (Photograph ©2009 qnr-away for a while.)

Phytoplankton cause mass mortality in other ways. In the aftermath of a massive bloom, dead phytoplankton sink to the ocean or lake floor. The bacteria that decompose the phytoplankton deplete the oxygen in the water, suffocating animal life; the result is a dead zone.

Climate and the Carbon Cycle

Through photosynthesis, phytoplankton consume carbon dioxide on a scale equivalent to forests and other land plants. Some of this carbon is carried to the deep ocean when phytoplankton die, and some is transferred to different layers of the ocean as phytoplankton are eaten by other creatures, which themselves reproduce, generate waste, and die.

 

Diagram of carbon fluxes in the upper ocean.

Phytoplankton are responsible for most of the transfer of carbon dioxide from the atmosphere to the ocean. Carbon dioxide is consumed during photosynthesis, and the carbon is incorporated in the phytoplankton, just as carbon is stored in the wood and leaves of a tree. Most of the carbon is returned to near-surface waters when phytoplankton are eaten or decompose, but some falls into the ocean depths. (Illustration adapted from A New Wave of Ocean Science, U.S. JGOFS.)

Worldwide, this “biological carbon pump” transfers about 10 gigatonnes of carbon from the atmosphere to the deep ocean each year. Even small changes in the growth of phytoplankton may affect atmospheric carbon dioxide concentrations, which would feed back to global surface temperatures.

Long-term changes in phytoplankton

Productivity

Because phytoplankton are so crucial to ocean biology and climate, any change in their productivity could have a significant influence on biodiversity, fisheries and the human food supply, and the pace of global warming.

Many models of ocean chemistry and biology predict that as the ocean surface warms in response to increasing atmospheric greenhouse gases, phytoplankton productivity will decline. Productivity is expected to drop because as the surface waters warm, the water column becomes increasingly stratified; there is less vertical mixing to recycle nutrients from deep waters back to the surface.

 

Graph showing the inverse relationship between temperature and chlorophyll concentration in the stratified oceans.

About 70% of the ocean is permanently stratified into layers that don’t mix well. Between late 1997 and mid-2008, satellites observed that warmer-than-average temperatures (red line) led to below-average chlorophyll concentrations (blue line) in these areas. (Graph adapted from Behrenfeld et al. 2009 by Robert Simmon.)

Over the past decade, scientists have begun looking for this trend in satellite observations, and early studies suggest there has been a small decrease in global phytoplankton productivity. For example, ocean scientists documented an increase in the area of subtropical ocean gyres—the least productive ocean areas—over the past decade. These low-nutrient “marine deserts” appear to be expanding due to rising ocean surface temperatures.

Species composition

Hundreds of thousands of species of phytoplankton live in Earth’s oceans, each adapted to particular water conditions. Changes in water clarity, nutrient content, and salinity change the species that live in a given place.

Because larger plankton require more nutrients, they have a greater need for the vertical mixing of the water column that restocks depleted nutrients. As the ocean has warmed since the 1950s, it has become increasingly stratified, which cuts off nutrient recycling.

Continued warming due to the build up of carbon dioxide is predicted to reduce the amounts of larger phytoplankton such as diatoms), compared to smaller types, like cyanobacteria. Shifts in the relative abundance of larger versus smaller species of phytoplankton have been observed already in places around the world, but whether it will change overall productivity remains uncertain.

 

Graph showing the reduction in proportion of diatoms in the ocean as atmospheric carbon dioxide increases.

As carbon dioxide concentrations (blue line) increase in the next century, oceans will become more stratified. As upwelling declines, populations of larger phytoplankton such as diatoms are predicted to decline (green line). (Graph adapted from Bopp 2005 by Robert Simmon.)

These shifts in species composition may be benign, or they may result in a cascade of negative consequences throughout the marine food web. Accurate global mapping of phytoplankton taxonomic groups is one of the primary goals of proposed future NASA missions like the Aerosol, Cloud, Ecology (ACE) mission.

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