Seasonal dynamics of phytoplankton and Calanus finmarchicus in the North Sea as revealed by a coupled one-dimensional model

A population dynamics model for Calanus finmarchicus was coupled with a one-dimensional physical and biological upper layer model for phosphate and phytoplankton to simulate the development of the successive stages of Calanus and study the role of these stages in the dynamics of the northern North Sea ecosystem. The copepod model links trophic processes and population dynamics, and simulates individual growth within stages and the changes in biomass between stages. Simulations of annual cycles contain two or three generations of Calanus and indicate the importance of growth of late stages to total population biomass. The spring peak of zooplankton lags that of phytoplankton by a month due to growth of the first cohort. When compared with observations, the simulation shows a broad phytoplankton bloom and a low biomass of Calanus. A higher initial overwintering stock changes the dynamics of Calanus, but not the annual biomass. The timing of the spring ascent of overwintering individuals influences subsequent dynamics. Simulations of spring dynamics compared with data obtained during the Fladen Ground Experiment in 1976 show that grazing by Calanus cannot be the only major cause limiting the phytoplankton bloom because development of the first stages of Calanus is slow, and the last copepodite stages arrive after the bloom. Calanus never attains realistic biomasses feeding on phytoplankton as a single food source. These simulations led us to add a compartment of pelagic detritus, which provides another food source to enable Calanus population growth.