By Lyda Harris
Plastic is a human-created pollutant, pervasive across marine systems and is projected to increase in the future. Microplastics (plastic 5 mm) are ubiquitous in marine environments, from surface waters to deep-sea sediments, from sea turtles to plankton, from urban cities to arctic outposts. Sounds depressing, and it is, but there is important information we can gather and differences we can make from studying depressing topics like marine plastic pollution.
Motivated by the need to understand the impacts microplastics have on our environment and our lives, I set out on my dissertation research at FHL. Along the way I convinced three phenomenal undergrads, Harsimran Gill (co-author), Jackson Fennell and Nell Baumgarten, to spend a week with me in the Flume lab (lab 6) at FHL running experiments, counting microscopic particles and learning to operate new (to us) equipment. Little did we know, we would become experts on mussel poop in the process.
Microplastics are likely transported from surface waters to benthic habitats by biotic and abiotic mechanisms similar to those responsible for plankton transportation such as surface currents, wind, and consumption. Due to their small size and presence throughout the water column, microplastics are ingested by numerous animals with different lifestyles, which can negatively impact their physiology (e.g. growth, immune response and reproduction). Ingestion and subsequent digestion and/or excretion can thus affect both the animals and their ecological roles, like benthic-pelagic coupling. In this case, the benthic-pelagic coupling is the exchange of nutrients, energy and mass between benthic (seafloor) sediment and pelagic (mid-column) waters, a crucial ecological role of nutrient cycling and energy transfer in marine food webs.
At this point, it would stand to reason that we could stop here and claim microplastics are “bad.” Bad for animals, bad for their ecosystem roles, bad all around. And while that story is compelling and few would argue, we are several hundred mussel poops shy of any substantial scientific discovery.
To understand how microplastics affect animals and their benthic-pelagic coupling roles, we needed an abundant, charming and well-understood invertebrate to study, which is where our star character The Mussel enters our story (it was also helpful to study the same organism as my advisor, Emily Carrington). As mussels filter and remove particles from the water column, they provide benthic organisms with resources from the water over them, such as food and nutrients, that are otherwise unavailable. As an example: by filtering water, mussels concentrate planktonic particles into biodeposits that are dense and nutrient-rich, thus linking the bottom substrate (benthic) to the water column (pelagic). However, particles pulled into the mussel by their feeding currents are not necessarily ingested — they are either excreted prior to ingestion as pseudofeces or are digested then egested as feces (congratulations, you are now a mussel poop expert as well!). Both types of mussel biodeposits can concentrate nutrients and particles from the water column that may not otherwise be readily available to benthic organisms.
Mussel biodeposits that contain microplastic, which are typically positively or neutrally buoyant, may sink and resuspend at different rates thus changing the benthic-pelagic coupling roles of mussels (previously documented in zooplankton and larvaceans; Cole et al. 2016, Katija et al. 2017).
The goal of our study was to determine how microplastics affect the benthic-pelagic coupling roles of marine mussels. I planned an ambitious week of experiments at FHL, and so with my three amazing co-researchers, we left Seattle for San Juan Island. For one week, we took over the Flume lab: we worked 301 hours (total), completed 132 trials (4 parts each), played 18 holes of disc golf for a break, and recently published our findings (Harris et al. 2021). In the lab setting, we exposed mussels to feeding regimes with and without microplastics and measured four attributes of biodeposits (feces and pseudofeces): 1) morphology; 2) quantity of algal cells and microplastic particles; 3) sinking rate; and 4) resuspension rate. Individual biodeposits were pipetted, photographed and either placed in a water column to sink or placed very carefully in the large laminar flume (it takes up the entire room, and yes, it was as comical as it sounds). We soon became affectionate towards the mussels and their magenta-colored poops.
“What beautiful poops!”
“Wait, I’m making my last poop measurement. Then we can go disc golfing.”
“Can I just leave my poops lying around while we are gone?”
– Quotes from the lab
We found that mussels readily filtered, ingested and egested algae and microplastic, demonstrating their ability to transport particles between pelagic and benthic habitats. Biodeposits from the algae treatment contained more algal cells on average than those from the microplastic treatment. Further, biodeposits from the microplastic treatment sank slower (~35%) and resuspended at slower water velocities (~15%) than biodeposits from the algae treatment; in other words, they were much easier to stir up and transport with low water movement.
Our findings may help explain how floating or mid-pelagic microplastic can be transported across habitats and how the natural biotic “pump” of microalgal communities, particulate organic matter and nutrients may be altered by microplastic. Decreases in sinking rate and resuspension velocity of biodeposits containing microplastic may result in biodeposits spending more time in the water column, thus leading to increased transport of both algal cells and microplastic particles away from mussel beds.
In our mussel case study, two possible ecosystem-level impacts stand out: 1) decreased in-bed nutrient input and 2) increased nutrient and microplastic transport. Mussel beds have high concentrations of carbon and nitrogen due to biodeposit sinking, contributing to more diverse invertebrate communities. As biodeposit transportation distances increase, the concentrations of carbon and nitrogen are likely to decrease in mussel beds, and invertebrate communities may be affected. Further, biodeposits are an important food source for numerous organisms; as transportation increases, biodeposits containing microplastic may become more readily available to benthic communities that do not usually come into contact with positively buoyant particles like microplastic. Other organisms like oysters, barnacles, some fish, and sea urchins contribute to particle and nutrient flux and may also be mechanisms of microplastic transport to deeper depths. This can give fish, zooplankton and other pelagic organisms a greater opportunity to ingest a small, bio-available and compact package of microplastic.
Plastic pollution is devastating at environmental, ecological and physiological levels, and can be inconvenient and depressing to think about, yet it is important to do so. Mussels allowed us a window into potential ecological-level impacts through their unassuming – and as we learned, charismatic – poops. I think the four of us can agree that there are few things that bond you with others like a week at FHL obsessing over mussel poop and microplastics. Harsimran, Jackson, Nell, and I have all since graduated (undergrad and Ph.D.) from UW.
If you are interested in the work discussed here, read Harris L., Gill H., and E. Carrington. 2021. Microplastic changes the sinking and resuspension rates of marine mussel biodeposits. Marine Pollution Bulletin: 165. https://doi.org/10.1016/j.marpolbul.2021.112165.
Lyda Harris (she/her) completed her Ph.D. with Emily Carrington in 2020 in the Department of Biology at UW. For her dissertation — much of it done at FHL — she studied 1) the impacts of microplastic vs silt on marine mussel filtration rates, 2) how microplastics affect the benthic-pelagic coupling role of marine mussels and 3) the growth and spread of marine microplastics research and national plastic policies. Harris is now working as the microplastic postdoctoral fellow at the Seattle Aquarium. For more information about the author Lyda Harris, please visit lydaharris.com.