Modeling the growth dynamics of Antarctic krill

A time-dependent, size-structured, bioenergetically-based model was developed to examine the growth dynamics of Antarctic krill, Euphausia superba. A system of coupled, ordinary differential equations was developed to describe the growth of krill between 2 and 60 mm. The metabolic processes included in the model were ingestion, assimilation efficiency, a baseline respiration. respiratory losses due to feeding and digestion, and an activity-based respiration factor. Positive net production resulted in the transfer of individuals to the next highest size class (growth), whereas negative net production resulted in transfer to the next smallest size class (shrinkage). Size-dependent parameterizations of model coefficients were constructed from an analysis of field and laboratory measurements provided in the current literature. The model was forced with an environmental time series of food (pelagic phytoplankton concentration) that was derived from data sets collected west of the Antarctic Peninsula. Three time series were created to represent high, low, and intermediate food conditions with the high and low conditions representative of phytoplankton concentrations observed on the inner and outer shelf, respectively. Simulated growth rates during the spring and summer for all size classes were consistent with published growth rates; however, winter shrinkage rates were too large. Although the use of a seasonally-varying respiration activity factor (reduced winter respiration rates) resulted in winter shrinkage rates of adults that were consistent with observations of experimentally starved individuals. the annual change in length of specific size classes was still inconsistent with observations. Subsequent simulations were designed to examine the effect of the ingestion of sea-ice algae in the late winter and early spring. The annual growth cycle best matched observations with reduced winter respiration rates and ingestion of sea ice algae, particularly for larval and subadult krill (< 35 mm).