Dr. Zachary Erickson, Research Oceanographer, National Oceanic and Atmospheric Administration Pacific Marine Environmental Laboratory
Research Interests: Investigating the ocean’s role in the global carbon cycle; using data from ships, underwater robots, airplanes, and satellites to better understand how ocean physics (i.e., currents and the flow of water) affect marine biology and the storage of ocean carbon by phytoplankton.
Research Highlights: Regardless of how far you live from the ocean, phytoplankton, the microscopic plants that live in and serve as the primary building blocks of marine ecosystems, are responsible for almost half the oxygen you inhale. Through the process of photosynthesis, phytoplankton take up carbon dioxide from ocean waters, use it to grow and reproduce, and then release oxygen. In doing so, these tiny plants not only give us the oxygen we need to survive, but also play an important role in the Earth's carbon cycle.
However, unlike plants on land, phytoplankton don’t store much carbon because their populations often expand and contract in boom-and-bust cycles, wherein they quickly build-up in surface waters when nutrients are available or die-off when they aren't. Further, because phytoplankton aren’t anchored to any specific location, their periods of boom and bust depend on where they are in the ocean, and where they are depends on currents and the flow of ocean water.
The fate of the carbon contained within the phytoplankton depends on these currents and flows too, for if phytoplankton die and are re-mineralized at the surface—meaning the carbon within them turns back into carbon dioxide—then the ocean’s physical processes will return it back to the atmosphere. However, if phytoplankton die and then sink below the surface before being re-mineralized, or are consumed by other animals, the carbon they contain will be stored for a longer time. This transfer of carbon away from the surface and into the deeper ocean is called “carbon export,” and among the scientists working to gain a better understanding of the biological and physical ocean phenomena that affect it is Zachary Erickson, a Research Oceanographer with NOAA's Pacific Marine Environmental Laboratory (PMEL).
PMEL’s mission is to conduct innovative oceanographic and atmospheric research, observations, and technology development to address urgent global and regional environmental challenges. Erickson’s research focuses on physical dynamics, bio-physical interactions, and carbon export, which he investigates using long-term in-situ observations of temperature, salinity, oxygen, and other characteristics to understand how the ocean is evolving on decadal time scales. Erickson is also interested in integrating biological and optical oceanography measurements into observational platforms to understand bio-physical interactions in marine ecosystems and how they are responding to our changing environment.
To that end, Erickson says data from NASA and other sources are critical to helping him understand how the ocean’s physical processes impact marine biology and where carbon taken up by phytoplankton is ultimately stored.
“NASA Earth science data are crucial to many aspects of my work,” said Erickson. “I mostly use satellite observations of ocean color, which are provided by several NASA satellite instruments, such as the Moderate Resolution Imaging Spectroradiometer (MODIS) instrument aboard NASA’s Aqua satellite and NASA's Sea-viewing Wide Field-of-view Sensor (SeaWiFS) instrument aboard GeoEye’s OrbView-2 (aka: SeaStar) satellite. NASA also stores in-situ data—data collected from sensors in the ocean—related to satellite remote sensing in its own data archive called the SeaWiFS Bio-optical Archive and Storage System (SeaBASS). I compare these data with those from satellites to help me decide if the equations used to derive information from satellite measurements of ocean color are correct.”
Erickson gets the ocean color and in-situ data he uses in his research from NASA’s Ocean Biology Distributed Active Archive Center (OB.DAAC), which ingests, processes, archives, and distributes data products in NASA’s Earth Observing System Data and Information System (EOSDIS) collection pertaining to ocean biology. Ocean color data are critical inputs in a wide range of ocean research, including studies of the biology and hydrology of coastal zones, changes in the diversity and geographical distribution of coastal marine habitats, biogeochemical fluxes and their influence on Earth's ocean and climate, and the impact of climate and environmental variability on ocean ecosystems and biodiversity.
Currently, Erickson is using NASA data in a research project associated with NASA’s EXport Processes in the Ocean from RemoTe Sensing (EXPORTS), a NASA Earth Expeditions field campaign designed to develop a predictive understanding of the export and fate of global ocean primary production and its implications for the Earth's carbon cycle in present and future climates. The project uses sensors placed directly in the water and aboard ships to understand the life cycle of phytoplankton – how they grow, develop, die, and interact with the rest of the food web and the physical flows of ocean currents – within a limited area of the ocean. Erickson and his colleagues ultimately plan to use these findings to develop new algorithms to connect satellite measurements of ocean color with measurements of carbon export.
Erickson is also investigating the use of space-based ocean color data to develop new methods for estimating phytoplankton concentrations by comparing the accuracy of satellite-derived measurements of phytoplankton with measurements from in-situ sensors.
“If on a research cruise we measure different types of phytoplankton and notice a relationship between the concentrations of different types of phytoplankton and the color of the ocean, we can then go back to the satellite data and generalize this relationship over the whole globe,” he said.
Erickson, along with the study’s lead author Dr. Priscilla Lange and other colleagues, studied this idea in a 2020 publication that appeared in the journal Optics Express. In this paper, Erickson and his colleagues estimated concentrations of picoplankton species (i.e., Prochlorococcus, Synechococcus, and other autotrophic picoeukaryotes) by analyzing multispectral remote-sensing reflectance data. To perform their analysis, the researchers developed models that used correlations between phytoplankton concentrations across the Atlantic Ocean and a combination of satellite-derived ocean color data and sea surface temperatures. The models indicated high Prochlorococcus abundances in the Equatorial Convergence Zone and showed their numerical dominance in oceanic gyres, with decreases in Prochlorococcus abundances toward temperate waters where Synechococcus flourishes, and an emergence of picoeukaryotes in temperate waters. These results performed well when validated against in-situ data, allowing them to conclude that high-resolution satellite instruments can be used to accurately assess phytoplankton concentrations over large areas. This finding bodes well for members of the ocean color research community given the future high-resolution satellite instruments, such as the ocean color instrument aboard NASA's Plankton, Aerosols, Clouds, ocean Ecosystem (PACE) satellite, which is scheduled for launch in 2023.
“In the broadest sense, if a satellite sees more light coming from the green part of the spectrum, we say there is more phytoplankton. But just like how a bumblebee, whose eyes detect different colors than we do, can see vibrant patterns of color in flower petals that look plain white to a human, we can tune satellite sensors to see different wavelengths of color, and potentially get more information out of their measurements,” Erickson said. “For example, we might be able to discern between small changes in green hues of the ocean to detect different species of phytoplankton, which might individually contribute very differently to carbon export.” [Note: Erickson expounds on this concept in a 2020 paper published in Applied Optics. See the Read about the Research section below for citation information.]
In another, earlier study that took place in the North Atlantic, Erickson and his then-Ph.D. advisor, Dr. Andrew Thompson, used data from satellites and an autonomous underwater vehicle known as a Seaglider to determine the effect of ocean instability on phytoplankton concentrations deep in the water column and the export of carbon from the surface layer. Their results, which are detailed in a 2018 paper published in Global Biogeochemical Cycles, revealed carbon export was highest in the spring, when there was periodic instability in the water column and high concentrations of phytoplankton at the surface.
“What we found was that the water column was unstable much more often in the winter, when the amount of phytoplankton in the water was relatively low, meaning there was little potential for carbon export in this season,” Erickson said. “In the summer, when the amount of phytoplankton in the water was relatively high, the ocean column was pretty stable. However, in the springtime when many phytoplankton species are going through their “boom/bust” cycles, we did find instances where there was an unstable water column and also high phytoplankton in the surface waters, pointing to this season as the most likely time when unstable water columns can lead to carbon export.”
Such investigations into the biological and physical factors that impact carbon export are critical given the ocean’s role in removing carbon dioxide from the atmosphere. According to NASA estimates, 48 percent of the carbon emitted to the atmosphere by fossil fuel burning is sequestered into the ocean, yet the ocean’s future as a carbon sink is uncertain due to the potential impacts of climate change on its circulation, biogeochemical cycling, and ecosystem dynamics. Erickson’s efforts to develop new methods for identifying and quantifying the biological and physical factors that affect carbon export to the deep ocean will help researchers better understand how the ocean functions as a carbon sink and sustain it as the effects of climate change unfold.
Representative Data Products Used or Created:
Available through OB.DAAC:
- Ocean Color Level 2 browser (NASA Goddard Space Flight Center)*
- Ocean Color Level 3 browser (NASA Goddard Space Flight Center)*
* These two websites provide access to all the MODIS and SeaWiFS data products Erickson uses.
Other data products used:
- PRISM data, NASA Jet Propulsion Laboratory
- Sea level altimetry data, Copernicus/ European Centre for Medium-Range Weather Forecasts
- Ocean Surface Mixing, Ocean Submesoscale Interaction Study (OSMOSIS), British Oceanographic Data Centre - Natural Environment Research Council, UK
- ERA-interim, European Centre for Medium-Range Weather Forecasts
Read about the Research:
Erickson, Z.K., Werdell, P.J. & Cetinić, I. (2020). Bayesian retrieval of optically relevant properties from hyperspectral water-leaving reflectances. Applied Optics, 59(23): 6902-6917. doi:10.1364/AO.398043
Lange, P.K., Werdell, P.J., Erickson, Z.K., Dall’Olmo, G., Brewin, R.J., Zubkov, M.V., ... & Cetinić, I. (2020). Radiometric approach for the detection of picophytoplankton assemblages across oceanic fronts. Optics Express, 28(18): 25682-25705. doi:10.1364/OE.398127
Erickson, Z.K. & Thompson, A.F. (2018). The seasonality of physically driven export at submesoscales in the northeast Atlantic Ocean. Global Biogeochemical Cycles, 32(8): 1144-1162. doi:10.1029/2018GB005927