Shannon Doherty, RSMAS: The Biological Pump

COMPASS 2021-03-17: Shannon Doherty, RSMAS
"Towards Quantifying Zooplankton Fecal Pellet Contributions to the Biological Pump"

Zooplankton fecal pellets are a major contributor to the downward flux of particulate organic matter (POM) in the ocean; however, the contribution of fecal pellets to POM is difficult to quantify. Current estimates often rely on visual identification of fecal pellets to determine their relative input to POM pools. However, these methods only capture fecal pellets that are intact or recognizable and potentially underestimate the contribution of fragmented fecal pellets to POM. First, we define the zooplankton fecal pellet chemical end-member to POM using compound-specific stable isotope analysis of amino acids (CSIA-AA). The amino acids threonine and alanine showed distinct nitrogen isotope patterns that distinguished fecal pellets from zooplankton biomass, phytodetritus, and highly microbially reworked detritus. We then apply the threonine-alanine signature and carbon CSIA-AA tools to a water column profile of POM from Monterey Bay, spanning approximately 5 m to 500 m and three particle size classes: 0.7-20 micron, 20-100 micron, and greater than 100 micron. Strong signatures of fecal pellets appear in the mesopelagic around 200 m, suggesting a flux of large fecal pellets from the surface at night into the mesopelagic during the day. Additionally, we examine the relationships between particle composition, microbial reworking, and diel vertical migration in the mesopelagic between 200 and 500 m. We demonstrate the unique potential for CSIA-AA to quantify fecal pellets in detrital POM pools in future studies and examine remaining questions about fecal pellet CSIA-AA signatures.