Biological/physical simulations of Calanus finmarchicus population dynamics in the Gulf of Maine

A site-specific, coupled biological/physical model is described for the dominant winter-spring copepod in the Gulf of Maine. The biological portion describes temperature- and food-dependent progression through 13 life stages in an Eulerian (concentration-based) framework. The population is transported in a flow field depicting the climatological mean conditions in 2 mo 'seasons'. Behavioural assumptions account for depth selection and 2 limiting cases are contrasted: dispersal throughout the water column, and aggregation in the surface layer. Simulations are inspired by MARMAP observations, with an emphasis on the mid-winter initiation of the annual bloom by diapausing populations, and their role in supplying reproducing populations to Georges Bank during spring. Passive tracer simulations illustrate the role of the circulation. Georges Bank itself is an open system and depends on resupply from external sources. All 3 deep basins of the Gulf are capable of contributing populations to the Bank. The Scotian Shelf is capable of populating the Southern Flank. In the case where the organisms aggregate in the surface layer, the effect of convergence in downwelling zones is shown to be a significant contributor to population distribution. Baseline population dynamics are initiated on January 1 by activating a diapausing population (G(0)) based on 10 yr mean abundance and distribution from the MARMAP program. The abundance and distribution of G(0) adults is reproduced with a 3-layer model, spatially variable mortality, an extended period of activation, no food limitation, and a large, heretofore, unobserved source of diapausing C5s near-bottom. Reproduction modeled in this system shows significant development of generation 1 (G(1)) over the Gulf of Maine in February-March, which is not observed. Delay of reproduction over the Gulf, and/or severe early-stage mortality, is required to conform with the data. The space-time pattern of this effect is consistent with the observed chlorophyll distribution and timing, and reasonable food-limitation thresholds. Inclusion of this effect initiates the spring Calanus bloom in the correct space-time pattern, with significant cohorts of G(0) females and G(1) nauplii over important cod and haddock spawning grounds on Georges Bank. The implication is that G(1) is locally spawned in food-rich waters over Georges Bank by females advected in the food-poor Gulf of Maine surface layer.