Modelling the population dynamics of Calanus in the Fair Isle current off northern Scotland

The population dynamics of a marine zooplankton species in the Fair Isle Current off northern Scotland have been investigated by modelling and field study. An age- and weight-structured model of a population of the copepods Calanus finmarchicus and Calanus helgolandicus was embedded in a biomass based ecosystem model comprising nutrients, phytoplankton, and other non-Calanus zooplankton. The model was configured to represent a Lagrangian water column drifting in the Fair Isle Current off the north of Scotland during June 1988, with physical characteristics derived from the results of a three-dimensional hydrodynamic model of the northwest European shelf. The time-series results from the model were compared to data from a semi-synoptic field study by assuming the system to be short-term steady state and transposing the spatially resolved field observations into pseudo-time series along the modelled column drift track. The hydrodynamic model correctly reproduced the general physical characteristics of the system which were destratification of an initially stratified water column as a result of advection through a tidally energetic mixing zone, and subsequent re-establishment of stratification with distance downstream. The biological components of the model were broadly successful at reproducing the main features of the phytoplankton biomass response to the physical processes. The field data indicated that, despite the short-term changes in phytoplankton abundance along the drift track, the stage composition and biomass of the Calanus population was relatively stable. However, the model revealed that the main diagnostic features of the response were at the individual level, reflected in the weight at age distribution and reproductive output. The study highlights the difficulty of obtaining adequate data for testing complex models of zooplankton responses to short-term spatio-temporal variations in physical forcing