Franks Report

### U.S. GLOBEC: Modelling Studies of Coupled Biological/Physical Processes Affecting Recruitment on Georges Bank

    INVESTIGATORS:

    Peter J.S. Franks
    Marine Life Research Group
    Scripps Institution of Oceanography
    University of California, San Diego
    La Jolla, CA 92093-0218
    pfranks@ucsd.edu

    Changsheng Chen
    Department of Marine Sciences
    University of Georgia
    Athens, GA 30602-2206
    chen@whale.marsci.uga.edu

    GRANT PERIOD:  July 1, 1993 - June 30, 1996

### STATEMENT OF OBJECTIVES:

The objectives of this study were to develop a hierarchy of coupled biological/physical models to:

- Examine the effects of wind events on plankton production at fronts

- Examine the relationship of plankton production to physical forcing in tidal fronts on Georges Bank

- Examine the relationship of larval cod and haddock growth to biological patchiness, turbulence and tidally forced flows on Georges Bank

- Examine the interactions of swimming organisms and tidally forced dynamics in creating biological patchiness on Georges Bank

### STATEMENT OF WORK:

#### Wind-Forced Production At Fronts

In this series of models we examined the influence of transient wind forcing on phytoplankton production at oceanic fronts. The first product of this study was a paper examining the relationship of phytoplankton photoresponse and productivity to wind-induced mixed-layer turbulence (Franks and Marra, 1994). A Lagrangian model of turbulent mixing was coupled with a novel model of phytoplankton photoadaptation, and showed that under certain conditions of surface wind stress and diffuse attenuation coefficient, euphotic-zone production could be enhanced by wind mixing.

An unexpected spin-off of this study was a model examining the formation of thin (10's of cm) layers of phytoplankton (Franks, 1995). This model supported the hypothesis that these thin layers could form by the interaction of along-isopycnal patchiness of phytoplankton with vertical shear caused by near-inertial waves. The thickness and duration of the layers could be predicted from the model given the initial horizontal scale of the patchiness and the shear. One significant prediction of the model, that the layers should be inclined across isopycnals, is quite testable, and is being explored in field programs.

A significant portion of this work was devoted to developing a fully coupled primitive-equation/mixed-layer/ecosystem model to study phytoplankton production at fronts. A slab mixed-layer model was coupled to the Franks et al. (1986) nutrient- phytoplankton-zooplankton (NPZ) model. In a study of the effects of transient wind events on phytoplankton production at fronts (Franks and Walstad, submitted ms.), we found that significant patchiness of phytoplankton could develop around and within a front after a wind event. The formation of patches depended on cross-frontal nutrient gradients and the relationship of the diffuse attenuation coefficient to the depth of the front. Wind forcing excited inertial oscillations at the front, which generated vertical oscillations of the pycnocline. This nutrient-pumping mechanism was found to be less important in the formation of patches than the changes in vertical stratification driven by cross-frontal mixing. Winds forcing an Ekman flux to the cold side of the front suppressed vertical mixing by increasing stratification at the front. Winds with an Ekman flux to the warm side of the front caused enhanced cross-frontal mixing and decreased stratification in the front. These asymmetries were reflected in the phytoplankton patchiness. Strong, persistent patches formed under winds with an Ekman flux to the cold side. Phytoplankton patches tended to erode under winds with Ekman fluxes to the warm side of the front.

This study led to the development of the first fully coupled primitive-equation/mixed-layer/ecosystem model in use in biological oceanography. The results of the study have increased our understanding of the mechanisms influencing phytoplankton patchiness at fronts, and the effects of transient wind events on phytoplankton production in these dynamic physical regimes.

#### Plankton Production At Tidal Fronts

In this study we used a primitive-equation/turbulence- closure/ecosystem model to examine the relationship of planktonic production on Georges Bank to the tidally forced dynamics (Franks and Chen, submitted ms.). After initializing the model with a stratified temperature field similar to that of Georges Bank in the summer, and a steady-state solution to the NPZ model, we allowed the model to evolve for 20-30 tidal cycles. Tidal-average biological fields from the 25th tidal cycle showed striking modifications of the initial conditions (Fig. 1). The phytoplankton field became vertically homogeneous on the top of the bank, with slightly decreasing concentrations from south to north. A subsurface maximum developed in the stratified waters off the bank. A patch of high phytoplankton biomass formed in the northern tidal front. Inside the tidal fronts, the phytoplankton concentrations were high down to the bottom, with a tongue of high biomass extending down the northern flank of the bank. These distributions closely resemble those found during field programs on and around Georges Bank during the summer.

The phytoplankton distributions were mirrored by the dissolved nutrient distributions, which showed very low values on the bank, and a sharp nutricline off the bank. On the flanks of the bank, the nutricline became horizontal, following the tidal fronts. Tongues of higher nutrient concentration could be seen extending toward the surface in the fronts. Inshore of these upward-extending tongues, the low nutrient concentrations extended down the flanks of the bank, reaching ~100 m on the northern flank. Very strong horizontal gradients of dissolved nutrients developed between the waters on top of the bank, and off the bank, particularly below the euphotic zone.

The zooplankton populations, like the phytoplankton and dissolved nutrients, were homogenized on the bank, but their concentration increased from south to north, in the opposite sense of the phytoplankton gradient. The highest zooplankton concentrations were found in the unperturbed waters off the bank, separated from the well-mixed waters on the bank by a region of very low zooplankton concentration.

The highest nutrient uptake rates were found at the surface, in the well-mixed waters of the bank, with tongues of high uptake rates extending downward along the two tidal fronts. Regeneration rates of nutrients were quite low, with maximal rates in the subsurface phytoplankton maximum layers off the bank.

An f ratio can be calculated from the fraction of total production not accounted for by recycled production, i.e. (uptake - regeneration)/uptake. This ratio of production supported by non- regenerated nutrients to total production showed values up to 0.7 at the surface of the southern front. Values of 0.5 - 0.8 were found in the surface waters of the bank and the fronts, with values of 0.1 - 0.2 off the bank. These values suggest a system on the bank which is strongly supported by the flux of new nutrients, surrounded by regions in which production is maintained largely by recycled nutrients. The high values of the f ratio at the surface of the fronts indicates that the phytoplankton patches in these regions are growing largely on nutrients supplied from the aphotic zone by the frontal dynamics.

These results were compared to field data from Georges Bank, and found to be accurate descriptions of the planktonic system. Detailed analysis of the physical-biological couplings in the tidal fronts revealed features that would be difficult to sample in the field due their transient nature and small spatial scale. This study has given us significant new insights into the physical-biological couplings on Georges Bank, and generated a new tool for the exploration of physical-biological interactions in the ocean: the primitive-equation/turbulence-closure/ecosystem model.

#### Larval Cod And Haddock Model

To investigate the physical-biological interactions affecting cod and haddock larvae on Georges Bank, we developed a detailed metabolic model of these fish. In addition to the usual features of a metabolic model (temperature-dependent respiration, excretion, etc.), the model includes an array of prey sizes, the effects of turbulence on prey capture, and the effects of temperature on the capture and ingestion of prey. The model gives good agreement with available data on growth and development of cod and haddock larvae. Sensitivity of the model to various parameters and forcings has given us insight into the response of fish larvae to changes in food concentration, prey size distribution, swimming speeds, turbulence, and temperature (Franks and Leising, in prep.).

The larval fish model is now ready to be incorporated into the two- and three-dimensional physical/ecosystem model described above. We intend to examine the influence of tidally induced biological and physical patchiness on larval development at tidal fronts on Georges Bank (two-dimensional model), and the effects of tidally rectified and wind-driven flows on larval retention on Georges Bank (three-dimensional model).

#### Sinking And Swimming In Tidal Fronts

This portion of the study is still in its infancy. To date, we have examined the influence of sinking behavior of phytoplankton in the generation of patchiness on Georges Bank. We intend to expand this study to explore the interactions of more complicated swimming behaviors with tidally generated flows on Georges Bank, using the primitive-equation/turbulence-closure/biological model described above.

### SUMMARY OF KEY FINDINGS:

#### Wind-Forced Production At Fronts

- Wind mixing can enhance integral production under certain conditions of mixed-layer depth, diffuse attenuation coefficient and wind stress

- Transient wind events can enhance phytoplankton production at fronts. Wind stresses from different directions lead to very different scales of phytoplankton patchiness at the fronts.

- Thin layers of phytoplankton can form by the interaction of along-isopycnal patchiness of phytoplankton with vertical shear caused by near-inertial waves.

#### Plankton Production At Tidal Fronts

- The planktonic ecosystem on Georges Bank during the summer behaves like a steady-state ecosystem strongly perturbed by physical forcing.

- High f ratios in the tidal fronts on Georges Bank result from transient cross-frontal mixing of nutrients.

- High production in the well-mixed region of Georges Bank is maintained by recycled nutrients, and an excess of new nutrients.

- Important events in the transport of nutrients occur over a few hours in a tidal cycle, and have a spatial scale of < 10 km. Such features would be difficult to sample in the field.

#### Larval Cod And Haddock Model

Larval cod and haddock are extremely sensitive to the ambient temperature at a given prey concentration. Turbulence appears to have only a small effect on larval growth. Only a small fraction of the initial pool of larvae survives to the juvenile stage.

#### Sinking And Swimming In Tidal Fronts

Sinking or swimming behavior is not necessary for the formation of a phytoplankton patch in the northern-flank tidal front.

### MANUSCRIPTS REFERENCING THIS GRANT:

Franks, P.J.S. 1995. Coupled physical-biological models in oceanography. U.S. National Rept. to IUGG. In press.

Franks, P.J.S. 1995. Thin layers of phytoplankton: a model of formation by near-inertial wave shear. Deep-Sea Research I 42:75- 91

Franks, P.J.S. and C. Chen. Plankton production in tidal fronts: a model of Georges Bank in summer. Submitted ms.

Franks, P.J.S. and A. Leising. A metabolic-ecosystem model of larval cod and haddock. In prep.

Franks, P.J.S. and J. Marra. 1994. A simple new formulation for photoadaptation and an application in a wind-driven mixed-layer model. Marine Ecology Progress Series 111:143-153.

Franks, P.J.S. and L.J. Walstad. Phytoplankton response to transient wind forcing at a front. Submitted ms.