The Decline of Calanus finmarchicus in the Gulf of Maine: Using Modeling to Investigate the Relative Role of Top-down Verses Bottom-up Processes

Rebecca Tien
Department of Evolution, Ecology, and Organismal Biology, The Ohio State University

(December 9, 2010 10:30 AM - 11:30 AM)

The Decline of Calanus finmarchicus in the Gulf of Maine: Using Modeling to Investigate the Relative Role of Top-down Verses Bottom-up Processes

Abstract

During the 1990s the Gulf of Maine (GOM) underwent an ecosystem regime shift associated with an increase in freshwater inputs. This freshening has been linked to increased phytoplankton abundance, which in turn positively affected the growth of zooplankton and, consequently, many pelagic fish populations. Calanus finmarchicus is one of the most abundant species of zooplankton in the GOM and so is an important prey source for many species higher up the food chain such as herring and the North Atlantic right whale. While reproduction for C. finmarchicus was high during this period, abundance of the later stages of the surface population was paradoxically low. Adult herring preferentially feed on the later copepodid stages; it is therefore possible that increased herring presence exerted top-down control on C. finmarchicus. An alternative hypothesis is that the changes in phytoplankton abundance during the 1990s impacted recruitment of C. finmarchicus into the later stages. Specifically, phytoplankton variability may impact whether C. finmarchicus remain at the surface to reproduce or enter into a resting state until the following year, emerging to take advantage of the spring bloom. Using three simple differential equation models, we examined the interplay of top-down verses bottom-up processes on the observed changes in seasonal patterns of surface populations of late-stage C. finmarchicus.