Regional dynamics of Calanus in the Norwegian Sea in response to ocean climate in 1997

Abstract

A coupled 3D hydrodynamic and ecological model has been set up for the Norwegian Sea (i.e. SINMOD). The hydrodynamic model uses the z-coordinates in the vertical direction. The horizontal grid point distance is 6.67 km and covers the northern North Sea, Mid Norwegian Shelf, Lofoten Basin, Faroes Shelf. The western limit is just at the eastern boarder of the Icelandic Shelf. The open boundaries are taken from a regional model using 20 km grid resolution. The models have been forced with atmospheric input from 6 hourly wind and pressure fields. The ecological model has eight state variables in addition to a structured model of Calanus finmarchicus. The state variables are nitrate, ammonium, silicate, diatoms, flagellates, microzooplankton, slow and fast sinking detritus. The Calanus model uses developmental structure for the nauplii and weight structure for the copepodites. Data from Station M (66°N, 2°E) has been used to adjust the mortality rates of Calanus. 10% of the CV’s are allowed to develop into females and start a new generation and the remaining 90% is transferred into an overwintering stage having a depth distribution between 500 and 1500m. The overwintering stock of Calanus at Station M below 600m was taken as an initial distribution for the whole model domain (north of the Scotland-Iceland ridge). The model simulates the pattern of phytoplankton growth, which is in accordance with observational data from several cruises in the region during the year of Calanus. An early start (March/April) in the Coastal Norwegian waters and at the front between the coastal water and the Atlantic waters was found. The spring bloom starts gradually later West and North in the Norwegian Sea. The earliest spawning took place near the shelf break off Norway where ascending Calanus met the spring bloom in surface waters. At Station M the timing of the spring bloom and the successive copepodite stages are simulated correctly. The simulated second generation is two weeks earlier than observed in field data. However, there is a large heterogeneity around this position in the Norwegian Sea on the southwestern slope of the Vsring Plateau. Coastal water and perhaps water from the East Iceland Current may ‘change the property (vertical mixing and nutrient supply) and the history of the Calanus stock entering this geographical region. Two generations are of Calanus simulated in the southeastern part of the Norwegian Sea. Along the shelf break the second generation is advected as far north as the entrance to the Barents Sea. In the Northern North Sea the temperature and the supply of food is sufficient for the model to produce three generations.