A numerical biogeochemical model was applied to the North West Shelf to investigate the primary productivity dynamics of the region. A subsurface chlorophyll a maximum (SCM) of 1 to 1.5 mg Chla m-3 was found below the mixed layer at depths of approximately 70 m. Surface concentrations of chlorophyll and nitrate were low. The SCM is maintained by a balance between nitrate uptake by phytoplankton, fed by nitrate fluxes into the SCM, and nitrate export in particulate form. The depth of the SCM is where phytoplankton can minimise growth by minimising light and nitrate limitation. The SCM will relocate closer to the surface as light availability decreases or the nitrogen flux increases, and vice versa. Also, any change in the grazing of zooplankton on phytoplankton results in a change in the SCM depth, where increased grazing leads to a shallower SCM. The flux of nitrogen into the SCM was primarily due to vertical processes, with vertical diffusion responsible for the largest background flux of nitrate. Vertical advection can locally increase nitrate supply into the SCM. Horizontal fluxes of nitrate do not directly contribute to the supply of nitrate into the SCM, but are important in maintaining a high concentration pool of nitrate at depth. Variability of the SCM occurs on timescales of the spring neap tide. Surface chlorophyll concentration is highest in an offshore band and during spring tides when mixing is more vigorous the chlorophyll concentration in this band increases and its position moves further offshore. This is a combination of the SCM becoming shallower due to higher turbidity decreasing light availability, and larger bottom boundary layers creating deeper zones of surface to bottom mixing. Variability of the SCM is also observed on seasonal timescales. The SCM is more distinct with a surface signature closer inshore in the wet season, and more dispersed with surface signature offshore in the dry. The seasonal variability is attributed to changes in mixed layer depth resulting from atmospheric forcing. The impact of the passage of a tropical cyclone (Tropical Cyclone Bobby, February, 1995) on the SCM was investigated. Primary productivity only increases by small amounts during such an event. Increased, deeper mixing due to the large wind stress and upwelling near the centre due to divergence competes with a deepening nutricline, resulting in little new nitrate brought to the surface. Subsequent to the cyclone passage, vertical motion of the nutricline at the near inertial period can lead to increases in nitrate concentration and productivity above and within the SCM. Greater increases are observed further offshore.
