The effect of distribution on copepod ingestion of marine snow particles

The biological pump is one of the most important pathways within Earth’s carbon cycle, transporting carbon to the deep ocean through biological processes. One of the main components of the biological pump is marine snow, since these particles carry particulate organic carbon out of surface waters as they sink through the ocean. Marine snow aggregates are a known food source for copepods, and these trophic interactions are likely to have implications for the ocean carbon cycle on small scales. Moreover, previous field observations have found that marine snow can be distributed in layers, and some have suggested that these layers may represent particularly important regions of grazing with enhanced levels of ingestion by zooplankton .

We conducted experiments investigating differences in copepod ingestion of marine snow between three different feeding environments: a layer of marine snow, a homogenous distribution of marine snow, and a control treatment without marine snow. Copepods were collected for the experiments off the coast of San Diego, and were then placed in these various feeding environments. Gut pigment analysis was used to estimate ingestion by the copepods for each feeding environment. Contrary to expectations, preliminary results indicated higher levels of ingestion in the homogenous distribution of marine snow aggregates compared to the layered distribution. This finding may be explained by copepods responding to chemical plumes that are left behind by aggregates as they sink through the water column, which was supported by images of copepod behavior that were taken concurrently with our ingestion measurements. Collectively these results suggest that, in regions where aggregates are distributed homogenously and actively sinking, less carbon may be exported to the deep ocean through the biological pump, having important potential implications for marine carbon cycling on larger scales.