Modeling Studies of the Physical-biological Processes

1. Modeling Studies of the Physical-biological Processes Controlling Spring Bloom Dynamics on Georges Bank: 1-D and 2-D experiments R. Ji, C. Chen, P.J.S. Franks, D.W. Townsend, E.G. Durbin, R.C. Beardsley, and R.W. Houghton

2. Chlorophyll a (g/l) Jan Jan • Inter-annual variability of the spring bloom: Timing Magnitude Location • “Second” bloom in June Near the front? Feb Mar Apr May Jun Compiled from Dr. Townsend’s data 1997 1998 1999

3. Nitrate+Nitrite (M) Silicate (M) Jan Feb Mar Apr May Jun 1997 1998 1999 1997 1998 1999

4. Phytoplankton Composition Diatom Dinoflagellate Other Townsend&Thomas, 2002, MEPS

5. SeaWiFS data from GOMOOS website By Dr. Andrew Thomas, UMaine

6. What we learned from data? • Inter-annual variability of seasonal variation and spatial distribution of phytoplankton and nutrients during spring-early summer time • Seasonal succession of phytoplankton community • Depletion of silicate earlier than nitrogen. Implication of nutrients limitation • “Second” bloom near front area in June, may be corresponding to silicate recovery. • Advection importance. GOM water, SSW, slope water, exchange between central bank and flanks.

7. Model structure Nitrate Silicate Ammonia Uptake Uptake Uptake Remineralization Dissolution Small Large Phytoplankton Phytoplankton Grazing Grazing Mortality Mortality Small Large Predation Zooplankton Zooplankton Mortality Mortality Fecal Grazing Mortality Detritus Detritus Nitrogen Silica

8. The central bank in which water is shallow, vertically well-mixed, and relatively self-contained; • The mid-bank region characterized by a seasonal tidal mixing front; • The outer-flank between the seasonal tidal mixing front and the permanent shelf break front. A 1 B 2 3 ECOM-si ECOM-si FVCOM 1-D 2-D 3-D • Seasonal dynamics • Sensitivity • Model behavior • Stratification • Frontal structure • Cross-section variation • Advection • Event level

9. 1-D

10. Biological variables 1-D Model Site A Site B

11. 1-D Model Model-Data Comparison Site A Site B

12. 1-D Model Site A Light

13. 1-D Model Site A N/Si in diatom N/Si = 1.5 N/Si = 0.8

14. 1-D Model Site A Sensitivity analysis

15. What is learned from 1D • Light environment controls the onset of the bloom in the shallow region, while stratification plays a more significant role in the deep region. • Magnitude of bloom is modified by both light and nutrients. • N/Si ratio is an important parameter for the nutrients limitation process and succession of phytoplankton community. What is missing in 1D • Second bloom in shallow area is not reproduced in the 1D model, although silicate is recovered from later spring depletion with increased water temperature. • The site with deep water didn’t capture the basic pattern of springtime phytoplankton bloom, indicating the significance of advection process.

16. 2-D 2-D model grid

17. Sectional view light Wind PMA • The early spring bloom occurring in the central bank: controlled by biological process associated with light intensity. • The recurrent bloom event during late spring and summer is mainly driven by the seasonal tidal mixing front. • The timing of the bloom on the southern flank is determined by (1) timing of the formation of the tidal mixing front and onset of vertical stratification, and (2) front-ward nutrient flux from the deeper region. Vertical mixing Vertical mixing Critical depth Before Stratification TMF Vertical mixing Vertical mixing Tide excursion Nutrients flux Critical depth High Nutrients Deep water After Stratification

18. Biological field 2-D Model

19. 2-D Model Sensitivity of bloom to heat flux

20. What we learn from 2D • The basic pattern of lower level trophic food web dynamics in shallow and deeper area mirrors the site A and site B in 1D model. A unique pattern is developed in tidal mixing frontal zone. • If no impact from advection process, the development of weak stratification is critical for the springtime bloom; wind and heat flux can regulate this process. • The frontal zone is the possible area for the “second” diatom bloom What is missing in 2D • Like 1D, the deep-water site didn’t reproduce the basic pattern of springtime phytoplankton bloom, indicating the significance of advection process.