Investigating Particle Size-Flux Relationships that the particle size-flux relationships may be different within the euphotic zone than in the shallow twilight zone and hypothesize that the changing nature of sinking particles with depth must be considered when investigating the remineralization length scale of sinking particles in the ocean.

Multiple processes transport carbon into the deep ocean as part of the biological carbon pump, leading to long-term carbon sequestration. However, our ability to predict future changes in these processes is hampered by the absence of studies that have simultaneously quantified all carbon pump pathways. Here, we quantify carbon export and sequestration in the California Current Ecosystem resulting from (1) sinking particles, (2) active transport by diel vertical migration, and (3) the physical pump (subduction + vertical mixing of particles). We find that sinking particles are the most important and export 9.0 mmol C m−2 d−1 across 100-m depth while sequestering 3.9 Pg C. The physical pump exports more carbon from the shallow ocean than active transport (3.8 vs. 2.9 mmol C m−2 d−1), although active transport sequesters more carbon (1.0 vs. 0.8 Pg C) because of deeper remineralization depths. We discuss the implications of these results for understanding biological carbon pump responses to climate change.

While the importance of salp grazing has become increasingly apparent in shaping their local ecosystems, little is known about the size ranges of their prey in the field. This study investigated the feeding habits of 7 different species of salp of both solitary and aggregate life stages representing a variety of sizes across subtropical and subantarctic waters east of New Zealand. Scanning electron microscopy was used to examine the gut contents of 58 salps, which were then compared to water column communities sampled via epifluorescence microscopy, FlowCam, and flow cytometry. While most of the gut size spectra and taxonomy resembled ambient waters, substantial differences were found amongst some co-occurring species, such as increased retention of submicron bacteria by Thalia democratica. We find that even for such salps capable of feeding on bacteria efficiently, the majority of gut biomass was still made up by nanoplankton and small microplankton. Larger particles were much rarer in the guts than in the water column, potentially suggesting an upper size-threshold in addition to the lower size-threshold that has been the focus of much previous work. Salp carbon-weighted predator to prey size ratios were variable, with the majority falling between 1000:1 and 10000:1 depending largely on the size of the salp. Taken together our results indicate that despite being able to feed on submicron particles, picoplankton are relatively unimportant to the energetics of most salps in this region compared to nanoplankton such as small dinoflagellates and diatoms.

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