A Scientist's Life: Moira Décima

Moira Décima is an assistant professor and curator of the Pelagic Invertebrate Collection at the Scripps Institution of Oceanography at UC San Diego. She earned bachelor’s degrees in marine biology and molecular, cellular, and developmental biology from UC Santa Cruz in 2003. She earned a PhD in oceanography from Scripps Oceanography in 2011. She returned to Scripps Oceanography as a professor in 2020.

explorations now: What do you do for a living?

Moira Décima: We study a number of different things in my lab. One of the things we focus on is plankton food webs and the zooplankton community and how they are affected by different environmental perturbations. Those can be natural, such as things like El Niño, but we also have a lot of perturbations that are anthropogenically forced like global warming or ocean acidification or the effect of wildfire smoke on the marine environment. For instance, we study the effect that ocean acidification in the California Current can have on pteropods, which are planktonic gastropods, or snails, that have shells that are susceptible to dissolution. They are what we call the ‘canaries in the coalmine’ for ocean acidification. We are involved in trying to map out their status, and the different states of dissolution at different locations of the California Current, and how that changes seasonally.

en: What are some of the major questions in your field?

MD: We're trying to understand how, as the ocean gets warmer and more acidic, things are going to change and what the implications are for marine food webs, for biochemistry and the absorption of carbon by the ocean and ultimately, and how that's going to affect the human population. The issue with ocean acidification is that the long-term trend in increasing acidification is small compared to what the natural fluctuations are in coastal communities, for instance. We’re putting a lot of effort into trying to monitor it. We don't know how bad it's going to be, but we want to establish baselines now to see how the pteropods are doing now, how that relates to the different ecosystem parameters, how are pteropods doing in those conditions, and moving forward, continue to do that monitoring to see if it gets any worse.

Another question that my lab is interested in is the role of zooplankton in mediating the export of carbon from the surface to depth. That's where certain types of organisms are different - gelatinous zooplankton like salps and pyrosomes. Salps are a very different type of zooplankton because they are gelatinous and they can be large. They can consume lots of phytoplankton really quickly. And because they have this capability of combining swimming and feeding, that allows them to feed fast and produce very large fecal pellets that sink very quickly. Their pellets are nicknamed ‘the expressway to the deep’. Salps can form blooms because of this unique characteristic where they have two stages in their life history, asexual and sexual. The asexual stage is able to, when conditions are favorable, produce many chains of hundreds of individuals that lead to rapid population growth. They can take over communities in the water column and they can graze large portions of the phyto- and microzooplankton, and then export all that carbon to depth via their fast-sinking fecal pellets and their vertical migration. 

It’s hard to sample these events because they happen sporadically. Even though we've known that they have that effect for a while, it's been hard to understand the mechanisms that lead to blooms, or the frequency of these blooms world-wide. Translating salp blooms into their carbon export potential requires us to understand the bloom dynamics, and the environmental conditions that lead to these blooms. Moving forward, we want to be able to incorporate this into global biogeochemistry models that attempt to understand global patterns in carbon export and the role of zooplankton in mediating export flux.

With salps and pyrosomes, another main question concerns the conditions that lead to pyrosomes or salps being present in high abundance in any one environment. What conditions lead them to stay there, what are the implications for the rest of the ecosystem, like their predators or their prey, and what are the implications for carbon export? Does their presence result in there being a lot more carbon that's absorbed from the atmosphere and sequestered to the deep, in the case of salps, or the opposite in the case of pyrosomes? We actually don't know that. There seem to be more reports of gelatinous zooplankton in areas where they were historically rare - in recent years this seems especially true for pyrosomes. One thing that we know is that they're very different from crustacean zooplankton, like krill and copepods. Different fish eat them, they produce different amounts of fecal pellets (and different sizes), they eat different types of phytoplankton. And so as ecosystems change - due to global warming or due to changes in advection, we’re potentially seeing more blooms of gelatinous zooplankton in general (the jury is still out on if this is significant around the globe), which can have dramatic implications for the rest of the ecosystem.

en: What makes the Pelagic Invertebrate Collection so important for research?

MD:This is probably the largest pelagic invertebrate collection in the world. We have over 140,000 lots of organisms, a lot being a mixed plankton sample. We also have thousands of sorted reference specimens. Those are specimens that we have identified to species. We have samples that date back to the early 1900s, and we also have samples from all over the world with special coverage across the Pacific Ocean. Having this type of collection allows us to ask questions that address broad topics like diversity, biogeography, and environmental change. For instance, in regions like the California Current, where we have really good temporal and spatial coverage (70 years + through the CalCOFI time-series), we can ask questions addressing how species distributions, or community assemblages, have changed over space and time.

For example, pyrosomes have been abundant in the California Current since 2014, which is a year that coincided with [a warm ocean water or marine heat wave event called] “the blob,” But when that subsided, and the water was not as warm anymore, pyrosomes are still here. They’re not supposed to be. They've not historically been present in these high abundances in the California Current over long periods of time. However, because we have these samples that go back to the 1950s, we know that every now and then, they do and have come into this system. We know that in the 50s and 60s, there were some blooms of them. We still don't understand why, but we know that it's not completely unprecedented. By going back to those old samples, we can ask questions addressing similarities between now and then, that might be favorable to pyrosomes in the ecosystem.

en: What are some of the tools you use in your research?

MD: I do a lot of seagoing research. We go out on world-class vessels and deploy instrumentation like CTD rosettes that sample the water, sensors that measure lots of different oceanographic parameters like temperature and salinity, nitrate concentrations, transparency of the water, and nets to collect zooplankton. Lots of times, we collect from the surface to 200 meters, but also we use multi-opening-closing nets called MOCNESS. With these we are able to sample deep in the water column and able to sample the different depth stratas discretely. This allows us to know what organisms are living at which depths, how these differ in the daytime vs nighttime, and how these patterns differ across different regions of the global ocean.

en: What inspires your work?
MD: One of the things that really inspires me are my students. I came back to Scripps to train the next generation of ocean scientists. And I want to be a better role model for them, both as a scientist and as a person.

en: Why did you want to come to Scripps Oceanography?
MD: When I finished my PhD, I did not want to be a professor - I loved research but I did not see myself represented in the faculty body. I also didn't think I was good at teaching, and at the time I did not understand that those skills are learned. So I moved to New Zealand to do pure research. In 2016, when the president of the U.S. changed and Trump was elected, I found myself saying on the surface ‘Good thing I’m in New Zealand’, but at a deeper level wishing I could be a part of the resistance I saw in the Women's March. That made me realize that no place is perfect, and if I wanted it to be better I would welcome the chance to be a part of that process. I also really missed the academic environment, fundamentally the students, and I was encouraged by all the changes I saw happening at Scripps, which was why I was happy to return, now as faculty.