Small copepods could play a big role in the marine carbon cycle

Abstract

Each year, photosynthetic microbes in the sun-lit upper oceans are responsible for about half of the carbon dioxide that is fixed into organic biomass. Some of these cells die and sink. Others are grazed, and their contents are incorporated into animal tissues and rapidly-sinking fecal pellets. Some of these organic particles sink through the ocean's mesopelagic “twilight” zone (∼100-1000 m deep), sequestering the carbon from the atmosphere. In contrast, most of the sinking organic carbon is remineralized within the mesopelagic zone by microbes or grazing animals. The amounts and proportions of sequestered and remineralized carbon vary greatly across space and time.[1, 2] Understanding the controls on particle sinking and remineralization is critical to quantifying and predicting global carbon budgets. In this issue, Mayor et al. revisit the hypothesis (e.g., [5]) that particle fragmentation by small copepods is a key component in particle flux attenuation, the decrease in the amount of sinking material with depth.[4] This discussion is timely, in part because new observations support high particle fragmentation rates following productive phytoplankton bloom events. Briggs et al. analyzed optical data from robotic profiling floats and reported that particle fragmentation accounted for about 50% of flux attenuation in the mesopelagic following blooms.[1] While Briggs et al. stated that “fragmentation may be the primary process controlling the sequestration of sinking organic carbon”, they did not discriminate among potential sources of fragmentation, such as turbulence, microbial metabolism, and animal activity. These processes can be directly measured, but such studies have usually been restricted to laboratory experiments or the upper ocean due to logistical limitations. Why do feeding activities of small copepods merit such scrutiny? Copepods are among the most abundant animals in the world. In the mesopelagic, small-bodied copepods capture sinking particles that they may entirely ingest or fragment while searching for smaller prey items. This distinction is important because ingested particles might be re-packaged into fast-sinking fecal pellets, while smaller fragments sink slowly and have increased surface area for microbially-mediated remineralization. The idea that small particle-associated copepods could fragment a large proportion of sinking particles is not new, but distributions of these copepods and the specific consequences of their feeding activities are poorly constrained and rarely incorporated into biogeochemical models.[3, 5] Here, Mayor et al. modified an existing marine ecosystem model to explicitly consider the biogeochemical consequences of small particle-associated copepods feeding on fast-sinking particles. They were able to create realistic seasonal patterns and depth distributions of particle-associated copepods and particulate organic carbon flux. A striking output of their model is that copepod feeding activities are responsible for the overwhelming majority of flux attenuation within the upper 250 m, where most of the flux attenuation occurs. The authors openly recognize uncertainties within their model, particularly those resulting from our limited understanding of the distribution, physiology and ecology of small particle-associated copepods. These copepods are too small to be quantitatively sampled in many routine zooplankton surveys, and there is very limited understanding as to how the metabolism and demography of this group varies seasonally and with depth. Mayor et al. highlight these knowledge gaps and issue a call for observational and experimental studies to provide fundamental empirical measurements related to feeding activities of this understudied animal group. Importantly “small particle-associated copepods” is a loose functional term that encompasses species with a wide range of distributions, feeding modes and prey preferences.[3] Thus, an additional future challenge will be to synt