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Physical-Biological Interaction |
Franks and Chen (1996) coupled a Nutrient-Phytoplankton-Zooplankton (NPZ) model into a primitive equation model and applied it to examine the summertime plankton dynamics on GB. That was the first modeling effort to study the biological process under the “realistic” physical environment in the GoM/GB region.
The NPZ model describes a simple food web system in which nitrogen is used as a tracer for the stable variables (Fig. 1): dissolved nutrients are taken by the phytoplankton following Michaelis-Menten kinetics, phytoplankton are grazed by zooplankton with an Ivlev functional response and nutrients are regenerated from the source of the mortality of phytoplankton and zooplankton and egestion from zooplankton. A detailed description of NPZ model can be seen in Franks et al. (1986) and Franks&Chen (1996).
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Numerical experiments were carried out for 2-D and 3-D cases. The 2-D experiments were conducted on a south-north transect across the center of George Bank. The model is driven by tidal forcing only with an assumption that the cross-bank distribution of temperature, phytoplankton and nutrients on Georges Bank is mainly related closely to the tidal mixing front. The 3-D experiments were conducted with emphasis on the role of tidal mixing and advection in the spatial and temporal distributions of temperature, phytoplankton, zooplankton, and nutrients on GB. Similar to the 2-D case, the initial fields of temperature and biological variables are specified to be uniform in the horizontal with assumption that the spatial and temporal variations of physical and biological variables are caused by tidal mixing and advection. A detailed description of 2-D and 3-D model results can be seen in Franks and Chen (1996, 2001). A brief description of model results is given below.
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Tidal mixing front. Driven by the M2 tidal forcing, the model generates a well-defined tidal mixing front which separates the well-mixed water in the shallow region from stratified water off the bank. Northern tidal mixing front is located at about the 40-m isobath, while the front on the southern flank is between the 50- and 60-m isobaths. Strong horizontal gradient forms within the frontal zone. This front moves fore and back in the cross-bank direction due to tidal variation. This structure of model-predicted tidal mixing front is in good agreement with the summertime observation.
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Phytoplankton. Phytoplankton field becomes vertically well-mixed on the top of the bank with slightly decreasing concentration from south to north. A subsurface maximum of  develops at about 18-20 m depth in the stratified water off the bank. A patch of high phytoplankton biomass  forms in the northern and southern tidal mixing front, stretching from the surface at the front to the depth of the subsurface phytoplankton maximum layer off the bank. Inside the tidal mixing fronts, the phytoplankton concentrations were high down to the bottom. This model-predicted distribution closely resemble the observations shown in O’ Reilly et al. (1987) and Horne et al. (1989).
Click here or image on the right to view the animation
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Nutrients. Nutrients show very low values in the well-mixed water on the bank and a sharp nutricline off the bank at about 25-30 m depth. Tongues of higher nutrient concentration  extends toward the surface in the front. Very strong horizontal gradients of dissolved nutrients develops below the euphotic zone and also within the nutricline on both sides of the bank. Nutrients are advected into the frontal during tidal cycles.
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On the steep northern slope, deep internal depressions of isotherms formed at the transition time from off- to on-bank current, which produced a sharp spatial gradient of tidal currents and led to strong mixing throughout the water column in the upper 50 m below the surface. As a result, the phytoplankton concentration was advected downward during a M2 tidal cycle, which could reach ~100 m on the northern slope. This model results suggest a physical mechanism for food required for scallops on the northern slope of GB.
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Distributions of Near-surface Temperature and Phytoplankton.
To test if the model could capture a right physics of tidal mixing, the initial condition of water temperature was specified by a linear function of z, with a value of 15 degree C at the surface and 6 degree C at a depth of 300 m. Tidally generated mixing in the model created well-defined tidal mixing fronts around GB, Brown Bank (BB) and over Nantucket Shoals (NS). Over GB, the front is characterized by a narrow zone at the 40-m isobath on the northern flank and a relatively wider region between the 50- and 60-o isobaths on the southern flank. The frontal zone on the northeastern flank of GB extends eastward to about 80-m isobath. The model-predicted distribution of tidal mixing front is in excellent agreement with previous observations.
Correspondingly, a donut pattern of higher phytoplankton concentration forms near the tidal mixing front, with maximum at the northeast frank and southwestern flank. A high concentration patch is also found on NS and BB related to shallow tidal mixing front. This model-predicted distribution of phytoplankton is consistent with the donut structure of nutrients measured during the US GLOBEC/GB nutrient survey conducted by David Townsend, and also in good agreement with previous Chl-a measurements made on GB.
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Distribution of Near-surface Zooplankton and Nutrients.
Zooplankton concentration is low within the frontal zone and relatively high in the well-mixed region on the top of the bank and off the bank. A beautiful donut structure of nutrients forms along the tidal mixing front, with maximum concentration on the northeastern and southwestern flanks of GB. This donut pattern was recently demonstrated in the nutrient measurement made by David Townsend.
It should be pointed here that the distribution of zooplankton probably does not have too much meaning here, because it acts like an energy flux transformation term in NPZ model to close the simplest lower trophic level food web system.
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Cross-bank Distribution of Temperature and Phytoplankton.
Cross-bank distributions of temperature and phytoplankton are very similar to those shown in the 2-D experiments. The model created well-defined tidal mixing front across the bank and also strong internal tidal waves on the slope. Phytoplankton becomes vertically well-mixed on the top of the bank with slightly decreasing concentration from south to north. A subsurface maximum develops at about 18-20 m depth in the stratified water off the bank. A patch of high phytoplankton biomass forms in the northern and southern tidal mixing front, stretching from the surface at the front to the depth of the subsurface phytoplankton maximum layer off the bank. Inside the tidal mixing fronts, the phytoplankton concentrations were high down to the bottom.
Click here or image on the right to view the animation. |
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Cross-bank Distribution of Zooplankton and Nutrients
Cross-bank distribution of zooplankton and nutrient concentrations are very similar to those shown in the 2-D experiments. Model creates a very mixed region of zooplankton concentration on the top of the bank and high concentration in the upper euphotic layer. Nutrients are low on the top of the bank, with two tongues of the peaks toward the surface within the frontal zone on both sides of the bank.
Click here or image on the right to view the animation. |
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