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Abstract
Up to 95% of the oceanic primary production is recycled within the upper few hundred meters of the water column. Marine snow and zooplankton fecal pellets in the upper water column are often recycled at rates exceeding those measured for microbial degradation, suggesting that zooplankton might be important for flux attenuation of particulate organic carbon in the upper ocean. However, direct evidence for interactions between zooplankton and settling aggregates are still rare. We investigated the importance of zooplankton aggregate feeding for carbon flux attenuation in the upper ocean by determining aggregate ingestion rates and feeding behavior on settling aggregates of the dominant Arctic filter-feeding copepods Calanus spp. and Pseudocalanus spp.. Both genera were observed to detect and feed on settling aggregates. Using in situ zooplankton and aggregate abundances in combination with the measured aggregate feeding rates, we calculated that 60-67% of the total carbon flux attenuation at three Arctic locations could be explained by Calanus spp. and Pseudocalanus spp. aggregate feeding alone. When including microbial degradation of the settling aggregates, we could explain up to 77% of the total carbon flux attenuation. Our results suggest that by directly ingesting and fragmenting settling marine snow mesozooplankton are key organisms for flux attenuation in Arctic waters.
Highlights
The export of particulate organic carbon (POC) from the euphotic zone to the deep ocean is an important process in the global carbon cycle, as it governs the oceanic sequestration of atmospheric carbon dioxide (Siegenthaler and Sarmiento, 1993)
In situ Aggregate Feeding Compared to the POC flux attenuation determined from the sediment trap measurements, the attenuation estimated based in situ camera profiles was 55 ± 21%, 46 ± 16%, and 39% at Erik Eriksen Strait, Hornsund, and Storfjorden, respectively (Figure 4)
Our study showed that the majority of the POC flux attenuation in an Arctic shelf sea during the productive season took place in the upper 50 m of the water column
Summary
The export of particulate organic carbon (POC) from the euphotic zone to the deep ocean is an important process in the global carbon cycle, as it governs the oceanic sequestration of atmospheric carbon dioxide (Siegenthaler and Sarmiento, 1993). These results contrast with observations from experimental studies, in which several copepod species have been observed to directly feed on sinking aggregates and fecal pellets (Koski et al, 2005, 2017; Iversen and Poulsen, 2007; Lombard et al, 2013) These experiments are difficult to translate to in situ POC flux attenuation due to the use of laboratory aggregates formed from fresh phytoplankton material or freshly produced appendicularian houses, while in situ aggregates are typically composed of older detritus material and egested compounds with lower organic matter contents (Ploug et al, 2008a). The POC flux attenuation was directly measured from sediment trap deployments and derived from in situ profiles of aggregate abundance and size-distribution using an in situ camera system
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