The Feeding Foray Hypothesis - Copepod Foraging Behavior During the Shallow Phase of Dvm

Understanding and predicting the vertical distribution and feeding behavior of planktonic animals is a central theme in aquatic sciences. Current assumptions on planktonic foraging behavior may be a source of error in estimating trophic interactions and cause serious underestimation of vertical material transfer by zooplankton. This has prompted some recent work in an attempt to better understand small spatial- (meters or less), and temporal- (minutes) scale foraging behavior. Observations conducted in Dabob Bay, a fjord in Pugest Sound (WA), have  uncovered the possibility that herbivorous copepods such as Calanus pacificus and Metridia pacifica may conduct short foraging forays during the shallow (in this case nocturnal) phase of DVM. Conceptually, these forays consist in copepods migrating upward to a shallow layer that is  food (phytoplankton)- but also predator- rich, briefly residing in that layer while they feed and fill their gut, after which they would sink out of the shallow layer and down to intermediate layer that is poorer in food but where the predation risk is considerably lower. Once they have sufficiently emptied their gut, these grazers re-ascend into the shallow layer to renew feeding. This cycle would be repeated several times during the course of the night. Here we present some recent work that combine laboratory and field studies with an individual-based model to test the foraging foray hypothesis against alternative such as feeding with little small scale vertical movement or simple random walk behavior. The center piece of our laboratory and field studies is an innovative imaging system that allows the quantification of phytoplankton inside copepod guts, remotely. Analysis of thousands of such images, combined with results from our model suggest that foraging forays do indeed occur, but that the frequency of occurence may strongly depend on the food distribution in the water column.