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The aggregation of phytoplankton into marine snow provides a mechanism by which smaller particles can coagulate to form larger particles, which can be consumed at various depths or readily transported to the deep ocean and sequestered from the atmosphere on time scales of a thousand years or more. Zooplankton interacting with these large carbon-rich aggregates can obtain nutrition in environments where the phytoplankton size spectrum is small and not directly available, enhancing the possibility of obtaining adequate nutrition in environments dominated by small cells. In addition, interactions between zooplankton and marine snow can result in fragmentation, thus affecting the particle sinking rate and changing the export of carbon. Unfortunately, these interactions are understudied and poorly understood. This study focuses on how two factors – phytoplankton growth phase and species – affect copepod feeding on marine snow, providing insight into the role of this food source in planktonic trophic dynamics and export of carbon to depth. We conducted a series of grazing experiments using gut pigment and stable isotope methods to quantify the ingestion rate of the copepod, Calanus pacificus, on marine snow aggregates in comparison to their ingestion rate on individual phytoplankton. We also examined how the ingestion of copepods on marine snow was affected by the phytoplankton species and phytoplankton growth phase from which the aggregates were formed. Results demonstrate that marine snow represents a substantial food source for copepods, with ingestion rates comparable to those on individual phytoplankton as measured with both gut pigment and stable isotope analysis. We found that phytoplankton growth phase can significantly affect the ingestion of marine snow. Finally, ingestion of marine snow was affected by phytoplankton species, and while aggregates formed from Thalassiosira weissflogii resulted in consistent patterns of ingestion rate between experiments and methodologies, the same was not the case for aggregates formed from Skeletonema marinoi. These findings suggest that marine snow is likely an important source of nutrition for copepods, but that its role in planktonic food webs may differ depending on the phytoplankton community composition and the stage of phytoplankton blooms.

The effect of phytoplankton properties on the ingestion of marine snow by Calanus pacificus