Phytoplankton photophysiology within zooplankton fecal pellets reveals depth-specific grazing

Abstract Marine particle forensics frequently uncovers information on particle composition, age, size, and ecological history. Zooplankton fecal pellets (FPs) are also interrogated for process-related data, such as grazing rates and carbon sequestration potential. Here, flow cytometric analyses of FP contents revealed intact phytoplankton with photophysiological characteristics mirroring those of free-living cells. Mapping the cytometrically derived properties of Synechococcus cells inside FPs onto vertical profiles from free-living cells revealed the potential to estimate depth-specific grazing by individual zooplankton. An experiment conducted at sea confirmed that the photophysiological characteristics of free-living phytoplankton from multiple depths, consumed by zooplankton, and egested within FPs are retained for at least 24 h after grazing is initiated. These results have implications for high-resolution modeling of individual or group-specific zooplankton grazing dynamics that are critical for accurately linking zooplankton grazing in the surface ocean with the mesopelagic and deep ocean food webs and carbon export.