Stamieszkin, K., Pershing, A. J., Record, N. R., Pilskaln, C. H., Dam, H. G. and Feinberg, L. R. (2015), Size as the master trait in modeled copepod fecal pellet carbon flux. Limnology and Oceanography. doi: 10.1002/lno.10156
The fecal express
Like trees on land, marine algae convert massive amounts of carbon dioxide into oxygen and grow through photosynthesis. However, most of the resulting biomass is quickly decomposed back to CO2, reversing the processes. To store carbon over longer timescales, biomass must sink below about 1000 meters, the depth where water no longer mixes with the surface to exchange gasses with the atmosphere. One of the most important ways organic matter reaches the deep ocean is through fecal pellets—in other words, plankton poop, slowly sinking to these great depths.
Copepods (think the character Plankton from Spongebob) are the most abundant type of zooplankton, and their grazing of smaller organisms effectively packages their biomass into a rapidly-sinking pellet. This process, known as the “fecal express”, is understood to be a key process for trapping carbon in the deep ocean. However, it is very difficult to quantify its importance. While we can count the number of copepods living at the surface and study how much they poop in the lab, these results aren’t always valid in the real ocean. It’s also tough to determine how much of their feces makes it to the deep ocean since they are eaten by bacteria and can fall apart as they sink. To make matters more complicated, copepod abundance varies seasonally, rising and falling with algae blooms, and different species dominate during different seasons. Along with the vast size of the ocean, this makes it nearly impossible to scale up the total amount of fecal pellet carbon sequestration from observations alone.
Modeling carbon sequestration in fecal pellets
This study presents a framework for calculating the total amount of fecal pellet carbon sequestered in the deep ocean based on two simple variables: the number of copepods in the surface water and its temperature. From there, the authors use a series of mathematical relationships to calculate the flux of carbon to the deep ocean (Figure 1). First, the size of the average copepod is related to the sea water temperature, with larger copepods thriving in warmer waters. The easily measured surface temperature can therefore be used to estimate the average copepod size. The body size of copepods in turn, determines the number and size of fecal pellets they produce (Figure 2). Smaller copepods produce more pellets, but larger copepods make bigger pellets that hold more carbon. More importantly, the larger the pellet the faster it sinks and the less time it is exposed to microbial consumption. The authors present a simple equation relating each variable, eventually allowing the calculation of the fecal pellet carbon flux to the deep ocean with only the sea temperature and copepod abundance as inputs.
A surprising result of the model was that even when small copepods were abundant, they didn’t contribute very much to deep-sea carbon export (Figure 3). This is because they produce smaller fecal pellets that sink slower and lose most of their carbon before reaching the deep ocean. The larger copepods contributed a disproportionate amount to carbon sequestration because their larger fecal pellets sink like rocks.
For validation, the authors compared their model results with data from a sediment trap, an upside-down cone suspended in the water column to capture falling particles. They picked out the fecal pellets from the trap, measured the amount of carbon reaching the deep sea, and compared it with their predictions. The sediment trap data is consistent with the modeled results, supporting the accuracy of the model. As further indirect evidence of the model’s accuracy, year-to-year growth of seafloor giant clams was strongly correlated with the abundance of the largest species of copepod at the surface. Clams eat sinking organic matter that reaches the ocean floor, which is dominated by fecal pellets, so this indicates the presence of larger copepods indeed contributes to carbon export to the deep ocean, consistent with model predictions.
Significance and implications
This new model is important because it lets us make large-scale estimates for how much carbon is being stored in the deep ocean from copepod fecal pellets. Sediment traps are useful but are usually only collected every few months and their contents represent an average over that interval. In addition, sediment traps are few and far between and don’t come close to representing the entire ocean. A model based on easily observable parameters allows us to fill in the gaps and work toward a more accurate global estimate of carbon sequestration in the deep ocean, and predict how that flux will vary in response to climate change.