Environmental Modeling and Assessment 2 (1997) 29–36 29 Population dynamics model of interacting copepod species coupled with a 1-D model of phytoplankton dynamics in the Greenland Sea Gyre F. Carlotti a and D. Slagstad b a Laboratoire d’Ecologie du Plancton Marin, URA 716, Station Zoologique, B.P. 28, F-06230 Villefranche-sur-Mer, France b SINTEF Electronics and Cybernetics, Automatic Control, N-7034 Trondheim, Norway The population dynamics of two copepods Calanus hyperboreus and Oithona similis are simulated simultaneously in a 1-D model of phytoplankton and nutrient in the centre of the Greenland Sea Gyre. The copepod model describes the development of cohorts in terms of numbers and biomasses over the year. Effects of competition for food and interactions (predation) between species have been studied. Due to the short period of phytoplankton bloom and the slow growth of large copepods, small but fast growing copepods can play a key role in the ecosystem dynamics: they consume the phytoplankton when the bloom occurs, recycle matter in the upper layer, and serve as preys for larger species. Keywords: population dynamics, copepod interactions, food webs, Greenland Sea, ecosystem model 1. Introduction Numerous models have been used to simulate the dy- namics of upper ocean plankton ecosystems. In these mod- els, the zooplanktonic species are aggregated in a single biological variable. The approach used in the present pa- per is to keep the model as simple as possible concerning the lower trophic levels but to take into account cohort development of the dominant copepod species in the Gree- land Sea Gyre. Such an approach has been successfully used to simulate the development of successive stages of Calanus finmarchicus in the North Sea and to study their role in the dynamics of the northern North Sea ecosystem [2]. Carlotti and Radach [2] concluded that a too simple representation of the pelagic ecosystem with nutrients, phy- toplankton and the population dynamics of Calanus could not be complete enough to explain the dynamics of the sys- tem. The slow growth of large copepod species does not permit the late copepodite stages of a cohort to match the phytoplankton bloom. Carlotti and Radach [2] concluded that another biological component, with a higher growth rate, grazing on phytoplankton and serving as prey for the late stages of large copepods is needed. Microzooplank- ton and small copepods are known to play this role in the pelagic ecosystem. The knowledge about microzooplank- tonic activity in the Greenland Sea is limited, but it is likely to play a significant role (Nielsen, pers. com.) as it does in other cold areas [17]. An indirect proof of the microbial activity is the very abundance of omnivorous small cope- pods. Richter [18] mentions that Oithona similis (O.s.) is by far the most abundant species in the Greenland Sea Gyre (GSG), but contributes to only 2% to 4% of the total bio- mass. O.s. has a high development rate and can quickly ex- ploit different food resources: phytoplankton, detritus and microzooplankton [8,12]. Calanus hyperboreus (C.h.) is the most important species in the GSG in terms of biomass [18]. Richter suggests that C.h. has a two-year cycle, and that late copepodites are found all year round in the water column, with variations in the vertical distribution. In this paper, we intend to study the respective roles of both species in the ecosystem dynamics in the central part of GSG (75 ◦ N, 3 ◦ W) with different scenarios of interaction between these species. 2. Model description Figure 1 shows a conceptual schema of the different physical and biological components. The 1-D submodel for the physics of the upper layer uses temperature and vertical turbulent diffusion profiles that are the output of a 3-D model of the Greenland Sea [21,24] for the position 75 ◦ N, 3 ◦ W. Depth levels are 10, 5, 5, 5, 5, 5, 5 and 10 me- ters in the first 50 meters and 25, 25, 50, 50 from 50 to 200 meters depth. The model was driven by data for wind and calculation of heat flux supplied by the Norwegian Meteo- rological Institute (MI) in Oslo. These data were sampled every 6 hours. Figure 2A shows the temperature profiles in the 100 upper meters from 15 March to 31 October, as well as the mean sea surface temperature (figure 2B) between 0 and 40 meters. Dissolved inorganic nitrogen, phytoplankton and detritus are vertically resolved and described by partial differential equations. The units are in mmol N m −3 . Copepods are considered to be evenly distributed in the 40 upper meters, and the growth and development of individuals in stages are described by ordinary differential equations. The two populations of C.h. and O.s. are modelled in a way similar to the one described by Carlotti and Radach [2] for both model structure and formulae of processes. The  Baltzer Science Publishers BV