Phytoplankton Blooms in the Northeast Pacific Ocean

Caitlin Dempsey


Phytoplankton are microscopic, photosynthetic organisms that float freely in the world’s oceans and freshwater bodies. Also known as microalgae, they play a foundational role in aquatic ecosystems, forming the base of the marine food web and generating a significant portion of the Earth’s oxygen.

Major producers of the world’s oxygen

As photosynthetic organisms, phytoplankton release oxygen as a byproduct. They are responsible for producing approximately half of the world’s oxygen, making them as crucial as terrestrial plants for maintaining breathable air.

Just as terrestrial plants have their season of growth and proliferation, phytoplankton in the ocean also have their ‘blooming’ seasons. In the northeast Pacific Ocean, these blooms are especially prominent during the summer months.

Phytoplankton play a role in carbon sequestration

Phytoplankton photosynthesize, converting carbon dioxide into organic carbon. When they die or are consumed, some of this carbon can sink to deeper ocean layers or even to the seafloor, sequestering it for extended periods. This process makes phytoplankton blooms vital players in the global carbon cycle and potential mitigators of climate change.

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A view under the microscope showing phytoplankton.
A view under the microscope showing phytoplankton. Photo: Seth Benson, USGS, public domain.

A vital organism in the marine food chain

Phytoplankton are a vital food source for zooplankton, which are tiny marine animals that graze on these microscopic plants. Phytoplankton also serves as a food source for a wide range of sea animals including shrimp, snails, and jellyfish. This abundance, in turn, supports larger predators, including fish, marine mammals, and seabirds. In essence, a phytoplankton bloom can bolster the entire marine food chain.

Why do Phytoplankton Blooms Occur?

Phytoplankton blooms are driven by a combination of factors. The most important of these are light, nutrient availability, water temperature, and physical ocean dynamics.

The northeast Pacific Ocean is rich in nutrients, particularly along the continental shelf and in upwelling zones. The California Current System is a major coastal upwelling system in the Pacific Ocean and one of the world’s eastern boundary currents.

Upwelling is a process where cold, nutrient-rich water from the deep ocean rises to the surface, replacing the warmer, nutrient-depleted surface water. This influx of nutrients acts like fertilizer for the phytoplankton, promoting rapid growth.

The phytoplankton blooms are visible in this satellite image of the ocean waters around Vancouver island. The swirling green waters are a combination of sediment and phytoplankton. The Pacific Northwest (PNW) shelf, a continental shelf, off Vancouver Island is considered one of the

A satellite image of Vancouver island showing puffy clouds and swirling green phytoplankton blooms.
Phytoplankton blooms south of Vancouver Island. Satellite image: NOAA-20, July 26, 2023, public domain.

Toxic blooms

The presence of a phytoplankton bloom isn’t always benign. Some can be harmful algal blooms (HABs), where certain species produce toxins that can harm marine life and even pose risks to human health.

Shellfish contaminated by these toxins can cause illness in humans if consumed. In the northeast Pacific, one such harmful algal bloom is the “red tide” caused by certain dinoflagellates.

A satellite image showing a phytoplankton bloom off the coast of Washington and Oregon.
A harmful algal bloom occurred along the coast of Washington and Oregon in July of 2015. This satellite image shows the green swirls of the bloom. Image: Landsat 8, July 15, 2015.

In July of 2015, a harmful algal bloom was in effect along the west coast of the United States. This bloom included populations of a type of algal called Pseudo-nitzschia. This algal produces domoic acid which is a neurotoxin. As this neurotoxin builds up in shellfish and seafood, it can cause poisoning in humans.


Carlowicz, M. (2015, August 9). Blooms off both North American coasts. NASA Earth Observatory.

Davis, K. A., Banas, N. S., Giddings, S. N., Siedlecki, S. A., MacCready, P., Lessard, E. J., … & Hickey, B. M. (2014). Estuary‐enhanced upwelling of marine nutrients fuels coastal productivity in the US P acific N orthwest. Journal of Geophysical Research: Oceans119(12), 8778-8799.

Perry, M. J., & Eppley, R. W. (1981). Phosphate uptake by phytoplankton in the central North Pacific Ocean. Deep Sea Research Part A. Oceanographic Research Papers28(1), 39-49.

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About the author
Caitlin Dempsey
Caitlin Dempsey is the editor of Geography Realm and holds a master's degree in Geography from UCLA as well as a Master of Library and Information Science (MLIS) from SJSU.