Researchers “See Red” from Space to Create Remote Sensing Tools for Zooplankton

Researchers "See Red" from Space to Create Remote Sensing Tools for Zooplankton

Almost 50 years ago, scientists figured out that they could detect chlorophyll, a pigment naturally found in phytoplankton, with satellites by measuring changes in ocean color. That provided a proxy for estimating the abundance of phytoplankton in the water, which gave scientists a new way to view the global ocean.

Zooplankton, the tiny animals that eat phytoplankton, don't have the same distinct and consistent pigment. In fact, they're often translucent. So, the remote sensing tools for tracking zooplankton swarms are several steps behind. That's where researchers from Bigelow Laboratory for Ocean Sciences are stepping in.

"When people were able to view chlorophyll from space for the first time, our understanding of the ocean fundamentally changed. Of course, we needed in situ measurements to corroborate everything, but that global view put everything into context," said Senior Research Scientist Karen Stamieszkin. "If we could observe zooplankton in that way, that will, similarly, put things into context and show us a lot we don't currently know at a global scale."

Although zooplankton have proven to be more of a challenge for satellite oceanographers, there is hope. Many species of copepods, one very abundant type of zooplankton, do have a pigment that can be detected with optical sensors. Copepods naturally synthesize astaxanthin, the red pigment that gives salmon its signature color. There are conflicting hypotheses of why. What scientists know for sure, though, is that the reddish tint from a swarm of astaxanthin-containing copepods near the surface is visible from space.

Senior Research Scientist Catherine Mitchell specializes in creating algorithms to translate ocean color into meaningful information on what is in the water. One of her former postdoctoral scientists, Cait McCarry, developed a method for translating red anomalies in satellite color into estimates of the number of copepods in the water. Mitchell then worked with another former postdoc, Rebekah Shunmugapandi, to adapt that method to the Gulf of Maine.

The team wanted to focus on Calanus finmarchicus, a fatty copepod that's a keystone species. Having a remote sensing tool for Calanus would not only be invaluable for monitoring the copepods themselves but also for predicting the movements of the myriad animals that feed on them, including the endangered right whale.

In a study evaluating their new tool, led by Shunmugapandi and published earlier this month, the researchers confirmed that their satellite-based estimates of zooplankton abundance matched real observations of right whale feeding, as hoped. Unfortunately, the tool couldn't distinguish Calanus from the various other sources of astaxanthin in the surface ocean, which has been the fundamental roadblock from the beginning, Mitchell says.

Given those results, the team decided to step back. Rather than develop an algorithm that jumps from ocean color to number of Calanus, they want to tease apart these more foundational equations, starting with how to quantify the amount of astaxanthin in the water from satellite observations.

"I think about it in terms of how we've studied phytoplankton. What we're measuring from space is this pigment chlorophyll, not actually how many phytoplankton are there. We just have a good understanding of the relationship between the green color, chlorophyll concentration, and phytoplankton biomass," Mitchell said. "We don't have the same grasp of the corresponding pieces - red color, astaxanthin, and biomass - when it comes to zooplankton."

This spring, using Bigelow Laboratory kickstarter funding, the researchers went out on the R/V Bowditch, which is outfitted with specialized optical equipment. The timing was such to capture the moment Calanus come out of their winter-long diapause and rise to the surface, numbering in the millions. They took measurements of color and gathered hundreds of samples of individual Calanus. Back in the lab, the team at Bigelow Analytical Services is working to precisely measure how much astaxanthin is contained within each individual.

This kind of preliminary work is exactly what the kickstarter funding was designed for. The hope is that, through a relatively short project, the team can develop viable methods for measuring light absorption and astaxanthin together and begin gathering early data. With these tools and initial insight, they'll be well positioned for follow-on studies and future funding to dig deeper into these relationships - in the Gulf of Maine and beyond.

For example, Centropages is another copepod that's abundant and important locally, especially in the fall. Both Calanus and Centropages contain the same pigment, and satellites should detect that pigment in the same way. But each species contains different amounts of the pigment, and holds it in their bodies in different ways, so it's likely that the relationship between the amount of astaxanthin in the water and the number of copepods will vary.

Future research, Mitchell says, would ideally test these methods for Centropages as well in order to assess whether these relationships between ocean color, pigment concentration, and copepod biomass are species specific. That's the first step to determining whether there's any kind of widely applicable algorithm.

"This kickstarter project is just the start, but it feels like we're opening up new doors to lots of different areas of research, both from the optics point of view to broader questions about zooplankton ecology and forecasting," Mitchell said. "It feels like what I imagine it felt like when they first started using ocean color satellites: there's just so much potential."

Photo Captions

Photos 1 and 4: Micrographs of Calanus finmarchicus, one of the species of copepod that the research team is focused on to understand the relationship between satellite ocean color observations and the pigment astaxanthin (Credit: Karen Stamieskin, Reyes Martín Rodgríguez, and Farley Miller).

Photo 2: Senior Research Scientist Karen Stamieszkin picks out Calanus from a water sample on their spring research cruise to experiment with methods and get some preliminary data (Credit: Cath Mitchell).

Photo 3: Stamieszkin and visiting student Reyes Martín Rodgríguez bring in a net tow with plankton samples aboard the R/V Bowditch during their spring research cruise (Credit: Cath Mitchell).