A site-specific, coupled biologicaVphysica1 model is described for the dominant winterspring copepod in the Gulf of Maine. The biological portion describes temperature-and fooddependent 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 colun~n, and aggregation in the surface layer. Simulations are inspired by MARMAP observat~ons, 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 sirnulations illustrate the role of the circulation. Georges Bank itself is an open system and depends on resupply from external sources. A l l 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 &stribution. Baseline population dynamics are initiated on January 1 by activating a diapausing population (Go) based on 10 yr mean abundance and distribution from the MARMAP program. The abundance and distribution of Go adults is reproduced with a 3-layer model, spatially vanable 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,) 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 hming, and reasonable food-limitation thresholds. Inclusion of this effect initiates the spring Calanus bloom in the correct space-time pattern, with significant cohorts of Go females and G, nauplii over important cod and haddock spawning grounds on Georges Bank. The implication is that G, is locally spawned in food-rich waters over Georges Bank by females advected in the food-poor Gulf of Maine surface layer.
This paper comes from a 3 yr study of environmental effects on egg and larval mortality of snapper Pagrus auratus (Sparidae), during their spring-summer spawning season in the Hauralu Gulf In the first 2 yr, the salps Thalia democratjca and Salpa fuslformls dominated mesozooplankton biomass in November and December, average chl a concentrations In the rn~xed layer were reduced and chl a maxima were well below the mixed layer. In the third year, salps were very rare, and the mixed layer was richer in chl a and the chl a maxima were at the bottom of the mixed layer. Diatoms dominated the phytoplankton community in the first 2 yr, especially below the mixed layer, but a more even mix of diatoms, dinoflagellates and flagellates was present in the third year. Averaged mixed layer depth and stratification intensity varied little across the 3 yr, suggesting that variation in vertical mixing rates did not drive the phytoplankton contrasts. Salp population grazing rates were high enough to overtake phytoplankton population growth at many sites. However, most sites had lower salp biomass than this, yet the reduction of chl a concentration and deepening of chl a maxima were widespread and temporally persistent. The vertical distributions of chl a were simulated in a nutrient-phytoplankton-zooplankton model which contrasted a salp-dominated community that had high sedimentation of organic wastes with a microzooplankton community within which wastes were recycled in situ. This model, and calculated rates of grazing, sedimentation, and water column mixing, showed that rates of nutrient incorporation into salp mass and sedimentation of waste material from salp grazing would be much greater than mixing of remineralized nutrient back up through the pycnocline. This causes formation of deep chl a maxima which can persist well after significant grazing pressure has dissipated. The variation in phytoplankton taxonomic composition is explained in terms of growth rates of the taxa under different grazing, nutrient and light regimes. The major effects of salp grazing in the coastal environment appear to be to deepen phytoplankton distributions and reduce biomass, rather than to remove phytoplankton biomass from the euphotic zone completely, as can occur in slope and oceanic waters.
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