The Effects of Nutrient and Light on Phytoplankton Communities: Implications for Carbon Export

By Delfina Navarro-Estrada and Shannon Burns, oceanography graduate students at the University of South Florida / NORTH ATLANTIC OCEAN /

In the sunlit portion of the ocean exist single-celled microscopic organisms called phytoplankton. They are called the ‘grass of the sea’ because these tiny plants and algae perform many of the same ecological functions as plants on land. As such, they provide energy to the organisms higher up in the food chain that feed on them, forming the foundation of many marine food webs. Through a process called photosynthesis, phytoplankton also remove carbon dioxide from the atmosphere and use it to produce sugars and other organic compounds that they require to live and grow.

Trace metal clean sampling bottles lined up like little soldiers, ready to be deployed on rosette for collection of water from different depths.
Trace metal clean sampling bottles lined up like little soldiers, ready to be deployed on rosette for collection of water from different depths. Credit: Delfina Navarro-Estrada

Because photosynthesis requires sunlight, phytoplankton thrive in the surface layer of the ocean. Eventually some of the carbon in phytoplankton is exported out of the sunlit surface layer to deeper waters where it can become sequestered from the atmosphere for decades to centuries. Export occurs through different mechanisms, including when organisms that get their carbon from eating phytoplankton die and sink, or produce fecal pellets that sink. Most of the sinking carbon ends up being dissolved back into the water column before ever reaching the deep sea or the seafloor. The small fraction of organic carbon that does reach the seafloor, however, ends up being buried and stored for hundreds of thousands of years. This process of carbon export is known as the biological carbon pump.

Incubation set up. In each of these bottles samples of water, brought from different depths, different micro- and macro-nutrients are added to study the response of the phytoplankton community. Credit: Delfina Navarro-Estrada
Incubation set up. In each of these bottles samples of water, brought from different depths, different micro- and macro-nutrients are added to study the response of the phytoplankton community. Credit: Delfina Navarro-Estrada

Net primary production is a term used to describe how much carbon the phytoplankton community incorporates into their cells via photosynthesis, minus the amount of carbon released through respiration. It can be limited by the supply of nutrients and sunlight in the water column, both of which vary over space and time across the ocean. Those needed in large amounts like phosphate, nitrate, and silicic acid, are called macronutrients . Other nutrients are needed in comparatively smaller amounts and are called micronutrients, which include many trace metals like manganese, iron, cobalt, nickel, copper, zinc, and cadmium.

Phytoplankton are so good at gathering required nutrients from seawater that they can run out. When that happens, their growth is slowed or stops and their composition changes in ways that affect export. They can become sticky, aggregate and sink quickly, or become poor food sources for their predators, reducing grazing and the amount of export by zooplankton that eat phytoplankton and produce fecal pellets. In these ways and others, the nutritional status of the phytoplankton can change how efficiently the biological pump moves carbon from the surface ocean to the deep sea.

As part of the EXPORTS program, we are identifying how macronutrient and micronutrient availability affects phytoplankton community composition and their physiological state to understand how these factors drive phytoplankton carbon through the biological pump.

In 2018, our team sailed in the North Pacific, a region with highly stratified waters with minimal nutrient input. Now in 2021, our team is sailing in the North Atlantic, a seasonally stratified region where eddies move nutrients from the ocean depths to the surface through a process called upwelling. We are using a combination of ultra-clean trace metal chemistry combined with the tools of molecular biology to understand how phytoplankton are responding to the changes in nutrients in their environments. 

Our team is trying to understand how different organisms respond to the changing nutrients? Working with the rest of the EXPORTS science team, we will combine measurements of carbon export, and ultimately the amount of carbon dioxide sequestered from the atmosphere.