Biogeochemical modelling on Australias North West Shelf

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.

本研究将数值生物地球化学模型应用于西北陆架（North West Shelf），以探究该区域的初级生产力动态。研究在约70米深度的混合层（mixed layer）下方，观测到浓度为1~1.5 mg Chla·m⁻³的次表层叶绿素a最大值（subsurface chlorophyll a maximum，简称SCM）。表层叶绿素与硝酸盐浓度均处于较低水平。SCM的维持依赖于浮游植物（phytoplankton）对硝酸盐的摄取、由输入至SCM的硝酸盐通量所支撑的养分供给，与颗粒态硝酸盐输出之间的动态平衡。SCM所处深度为浮游植物可通过弱化光限制与硝酸盐限制来优化生长的区域。当光可获得性降低或氮通量升高时，SCM会向表层迁移；反之则会向更深层移动。此外，浮游动物（zooplankton）对浮游植物的捕食压力发生任何变化，都会引起SCM深度的改变：捕食压力增强会导致SCM变浅。输入至SCM的氮通量主要源自垂直过程，其中垂直扩散（vertical diffusion）是硝酸盐背景通量的主要贡献者。垂直平流（vertical advection）可在局地提升输入至SCM的硝酸盐供给量。硝酸盐的水平通量不会直接为SCM提供硝酸盐，但对维持深层高浓度硝酸盐库具有重要作用。SCM的变化周期与大小潮（spring neap tide）尺度一致。表层叶绿素浓度在离岸带（offshore band）达到峰值；在大潮（spring tides）期间，由于混合作用更强，该离岸带的叶绿素浓度会升高，且其位置会向更远的海域迁移。这一现象由两方面因素共同导致：一是浊度（turbidity）升高降低了光可获得性，使得SCM变浅；二是更厚的底边界层（bottom boundary layer）形成了更深的表层-底层混合区间。SCM的变化同时存在季节尺度（seasonal timescales）上的波动。在湿季，SCM特征更为显著，其表层信号更靠近近岸区域；而在旱季，SCM特征更为弥散，表层信号向远海偏移。这种季节变化可归因于大气强迫（atmospheric forcing）引起的混合层深度变化。本研究还探究了1995年2月的热带气旋鲍比（Tropical Cyclone Bobby）过境对SCM的影响。在该热带气旋过境期间，区域初级生产力仅出现小幅提升。强风应力引发的更强更深层混合，以及气旋中心附近由辐散导致的上升流，与加深的营养跃层（nutricline）产生拮抗作用，最终几乎没有新的硝酸盐被输送至表层。气旋过境后，营养跃层在近惯性周期（near inertial period）内的垂直运动，可使SCM上方及内部的硝酸盐浓度与生产力水平得到提升。在更远的近海区域，这种提升效果更为显著。