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SURFO Special: Ocean Color Optics and Imaging: Phytoplankton in Narragansett Bay

Each summer, the University of Rhode Island Graduate School of Oceanography (GSO) hosts undergraduate students from all over the country to participate in oceanographic research. These Summer Undergraduate Research Fellows (SURFOs) have not only been working with GSO scientists, but they also have spent part of their time learning how to communicate this science to the public. Although their research experience was virtual this summer, they still did a fantastic job. Read on to find out what they have been up to, and why they everyone should be as excited as they are about their work.

Taylor Bowen is a senior at Georgia Gwinnett College, majoring in Biology and Environmental Science, with a minor in Chemistry. Taylor spent the summer working virtually in Dr. Colleen Mouw’s lab, researching phytoplankton imaging and optics in Narragansett Bay.

The Actual Colors of the Ocean 

When you think of the color of the ocean, you probably think of blue, or maybe some green or brown. And while those are the colors we perceive with the naked eye, an entire color spectrum can be seen through a satellite. Satellites are equipped with devices called radiometers, which measure how light reflects and is absorbed by the ocean.

This image shows the overall reflectance of the ocean. The ocean can show an entire rainbow of colors when seen through a satellite radiometer. Image source: NASA










The current satellite radiometry technology is known as Visible Infrared Imaging Radiometer Suite, or VIIRS, and was launched in 2011. While VIIRS has been valuable in collecting satellite imagery, it is limited in scope due to age and out-dated technology. An upcoming NASA mission, known as the PACE mission, will provide new satellite imaging that will allow us to have a greater overall picture of the ecosystems within the ocean. The PACE mission, launching in 2022, will increase the spectral resolution. It will provide a much wider range of wavelengths, more refined spectra, and overall cleaner signals than what VIIRS can currently offer. This jump in technology will allow us to have a better idea of how photosynthetic plankton absorb and reflect light and color. This huge jump in technology requires new algorithms to fully utilize the new potential the PACE mission is giving. The ongoing two and a half year phytoplankton study at the University of Rhode Island Graduate School of Oceanography pier in Narragansett Bay provides data that can be used for creating these algorithms.

Optical signatures of a harmful algal bloom (HAB) and a non-harmful algal bloom (non-HAB) are compared using the current VIIRS system and the upcoming PACE system, showing the difference in data collection that can be expected with the newer technology. Image source: pace.oceansciences.org


The Setup at the Pier

Over the last two and a half years, the University of Rhode Island pier has been equipped  to collect information about environmental factors in the water, such as temperature, salinity, chlorophyll concentration, absorption, light scattering, and colored dissolved organic matter (CDOM) concentration, as well as phytoplankton imaging. These factors can all be compared and analyzed to determine relationships between environmental factors and optical variables, which can then be linked to phytoplankton group and species abundances. 

The Composition of Ocean Water 

The total water absorption can be broken down into four main components: the absorption of the water itself, the absorption of the CDOM in the water, the absorption of the non-algal particles, which are any organic or inorganic particles that are not currently living algae, and the absorption of the phytoplankton themselves. Because of relationships between environmental factors, we can isolate the absorption of phytoplankton and use it  to determine if changes in absorption relate to certain phytoplankton species’ or group’s presence. 

Phytoplankton Imaging 

The Imaging Flow CytoBot (IFCB) collected imagery of phytoplankton cells, which can be identified based on physiological characteristics like size and shape. These images were separated into 54 different groups, each with different physiological characteristics. The number of images per milliliter of water showed the concentration of cells from each group. For the days that were considered, the same groups consistently made up the top contributors: unclassified, which includes all the images that did not fit into other categories; small, round nanoplankton; small, square nanoplankton; and flagellates, which are single-cell phytoplankton that have a whip-like appendage. These top four contributors were consistently seen making up the vast majority of the phytoplankton community, ranging from 67% to 97%. Other types of phytoplankton, such as Skeletonema, Lepticylindrus, Cerataulina pelagica, Dactyliosolen blavyanus, were seen on some days in low amounts (5-8% of the entire amount of phytoplankton). The remaining groups were mainly seen at less than 1% of the phytoplankton community on all days. 

What This Means

When the phytoplankton community fluctuates and different groups become more dominant, the amount of light absorbed by the phytoplankton changes as well. These changes come from the different cell sizes, shapes, and characteristics of the phytoplankton groups. The top four contributors consist of a lot of nanoplankton, which are very small, but they have more surface area, and so they show up much more in the absorption readings. On days when larger phytoplankton cells are more abundant, the absorption is much lower because of lower surface area. Other factors can also influence phytoplankton abundance, but due to time constraints, we were unable to finish these comparisons and analysis. 


The absorption budget shows how the overall absorption is broken down into the four components. The green shows phytoplankton absorption, which changes based on which groups are more dominant. (Left) May 9, 2018 has a lower phytoplankton absorption, but an overall lower (67%) of the top four contributors. Skeletonema is at a much higher concentration (8%) than other days. (Right) July 4, 2018 has a much higher phytoplankton absorption, and also has the highest top four contributor percentage (97%), with other groups remaining in extremely small quantities. (Image by Taylor Bowen)











Why Should We Care?

Phytoplankton make up the foundation of the food web for the ocean ecosystem. Knowing which environmental factors impact phytoplankton diversity and abundance can help maintain the marine ecosystem. With climate change and ocean acidification continuing to impact the ocean’s environmental factors, phytoplankton may be negatively affected by these changes. We can use studies like this one to track these changes and begin working to mitigate these problems before they become devastating to the phytoplankton communities and, by extension, the rest of the ocean’s organisms as well.



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