Mechanisms relating ocean-scale distribution of Calanus finmarchicus to environmental and hydrographic heterogeneity

Douglas C. Speirs
Statistics & Modelling Science
University of Strathclyde

The copepod Calanus finmarchicus dominates the mesoplanktonic fauna of the North Atlantic and is an important prey for commercially-exploited fish. Because of challenges associated with the interaction between a complex life history and physical transport over a large and heterogeneous environment, previous attempts at modelling its demography have been spatially and temporally limited. This talk describes a new discrete-time-space Eulerian model which is spectacularly more efficient than established methods. The biological model is driven by a yearly cycle of temperature and transport from global ocean circulation model and by a food field derived from satellite sea-surface colour observations. Its numerical efficiency permits for the first time a full population model of C. finmarchicus spanning its entire geographic range. The model can be rapidly run to a quasi-stationary annual cycle and the results assessed against a variety of field data. Confrontation with data from times series and from the Continuous Plankton Recorder (CPR) allows the optimization of unmeasured parameters and the exploration of structural variants in relation to poorly-understood aspects of biology. The model is most successful when a constant fraction of each generation enters the overwintering diapause state at the end of the fifth copepodite stage, and when the onset of reproduction in the spring is cued by photoperiod. The results also indicate that an association between temperature and mortality may be responsible for observed differences in seasonality between regions of the North Atlantic. The demographic impact of transport is shown to be limited, but there is nonetheless a high degree of population connectivity over the whole North Atlantic.