The effect of marine snow particle distribution on the foraging behavior of Calanus pacificus

Abstract

Marine snow is a major component of the biological pump, through which carbon is exported to the deep ocean. The sinking of marine snow can be disrupted by organisms, including many species of zooplankton that ingest or break up aggregates. These processes can have important impacts on planktonic food web dynamics and carbon export. Marine snow can have vertically patchy distributions, occurring in thin layers, which may further affect interactions with zooplankton. In this lab-based study, we examined how the presence of a marine snow layer affects copepod behavior and ingestion. We conducted a series of experiments in which copepods of the species Calanus pacificus were exposed to four different feeding environments: a layer of marine snow, a homogenous distribution of marine snow, and two control treatments without marine snow – one with a density gradient and one without a density gradient. Copepod behavior was recorded with two cameras that were set up perpendicular to one another, imaging neighboring sides of the tank. We were able to reconstruct 2D and 3D copepod tracks, allowing us to compare copepod vertical distributions and calculate copepod swimming velocity, jump frequency, and path linearity. Copepod gut fluorescence was measured after the experiments to determine differences in ingestion between treatments. Gut content analysis showed that copepods did ingest marine snow when exposed to the layer and homogenous distributions of aggregates, with potentially higher ingestion seen in the layer treatment. Behavioral analyses show significantly higher residence time of copepods in the middle of the tanks (where the marine snow layer and density gradient were located) in the layer treatment and control with gradient treatment, with substantially higher jump frequency and substantially lower vertical velocity also seen in this region for those two treatments. These findings suggest that marine snow layers may represent regions of enhanced zooplankton foraging, providing insight into how these interactions can influence particle flux.