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

Abstract

Copepods play a critical role in transferring energy up the food web and can often exhibit foraging strategies that allow them to capitalize on patchy prey environments such as marine snow layers. Since marine snow layers co-occur with density gradients in the field, we evaluated how each of these affected copepod foraging behavior by exposing Calanus pacificus to treatments with different distributions of marine snow and with and without density stratification. Swimming behavior was recorded in the lab using imaging in 2D and 3D. We observed significantly higher residence time of copepods in the middle of the tank in the two treatments with a density gradient – both the one with a marine snow layer (which is created using a density gradient) and the one without marine snow – although the accumulation of copepods was more intense when the marine snow layer was present. Copepods also exhibited higher jump frequency and lower vertical velocity in the middle of the tank in the two treatments with a density gradient. Gut content analysis showed that copepods ingested marine snow when exposed to the marine snow layer and the homogenous distribution of aggregates, with possibly higher ingestion in the layer treatment. These results suggest that density gradients may act as a primary physical cue – with the presence of chemical cues having a potential secondary effect. Behavioral responses to these cues allow copepods to find and remain in marine snow layers, providing insight into how these interactions can disrupt the sinking of marine snow, which transports carbon to the deep ocean.