Modeling Phytoplankton Seasonal Variation and Nutrients Budget of a Semi-Arid Region Ecosystem in the Southern Mediterra

1. Modeling phytoplankton seasonal variation and nutrients budget • of a Semi-Arid region ecosystem in the Southern Mediterranean Sea: • -Case of the Bizerte Lagoon- • BéchirBEJAOUI1, Ali HARZALLAH1, NoureddineZAABOUB1, • Annie CHAPELLE3, MahmoudMOUSSA2 • 1Institut National des Sciences et Technologie de la Mer, Tunisia. LMM Laboratory. • 2National Engineering School of Tunis - LMHE Laboratory • Institut National des Sciences et Technologies de la Mer. • 28 rue du 2 Mars 1934, 2025, Salammbô. Tunisie. • E-mail : bejaoui.bechir@instm.rnrt.tn

2. Objectives: •  Develop a three-dimensional coupled Dynamic-Bio-Geo-Chemical model for the Bizerte Lagoon, •  Model the phytoplankton and nutrients seasonal variation, • Estimate the water and nutrient budgets of the Lagoon. • Methodology: •  Development of bio-geo-chemical, •  Coupling to hydrodynamic model, • Construction of a dataset for model validation, 2

3. SECTION I MODEL EQUATIONS AND THEORETICAL ASPECTS

4. I.1 Presentation of Bio-geo-chemical model Respiration Death & Feces Zoo O2 Graze Photosynthesis O2 Death Phyt Ndet Diffusion Excretion Uptake NO3 Sedimentation Nitrification Sedimentation O2 Mineralization O2 NO3 NH4 T O2 T Dénitrification Diffusion Diffusion Diffusion NO3 NH4 Ndet O2 T O2 Ndet T O2  Conceptual diagram Model Presentation The bio-geo- Chemical model of the Bizerte lagoon is 3D based on nitrogen (Norg NO3 and NH4) and phosphorus (PO4, Porg) forced by temperature and oxygen. The model of Bizerte lagoon is Based on IFREMER model. Uptake NH4 4

5. I.1 Presentation of Bio-geo-chemical model 1. Ammonia 2. Nitrates 3. Organic-N 4. Phytoplankton 6. Oxygen 5. Zooplankton  Model Equations  Internal Kinetic 5

6. I.1 Presentation of Bio-geo-chemical model Zooplankton P h y t o p l a n k t o n Mineral  Bio-geo-chemical model functions and rates 6 Oxygen

7. I.2 Boundary conditions of bio-geo-chemical Model River’s input map 7

8. SECTION II MODEL SIMULATIONS

9. II. Bio-geo-chemical model simulations  Simulated variables in : Water Ammonium • The trend of ammonia measurements is reproduced by the model. • The model underestimatesthe ammonia concentration measurements . 9

10. II. Bio-geo-chemical model simulations  Simulated variables in : Water Nitrate • The high nitrate concentration is simulated in winter whereas the low one in summer when phytoplankton blooms. 10

11. II. Bio-geo-chemical model simulations  Simulated variables in : Water Phosphorus • The high phosphorous concentration is simulated in winter whereas the low concentration in summer when phytoplankton blooms. • The model over estimates the phosphorous concentration. 11

12. II. Bio-geo-chemical model simulations  Simulated variables in : Water Chla (g.m-3) Phytoplankton Simulated Chla (g.m-3) • The phytoplankton variation is well reproduced by the model, • Spatial distribution shows higher concentrations inside the lagoon and relatively low concentrations in the inlet, • Vertical homogeneity of phytoplankton concentrations. 12

13. II. Bio-geo-chemical model simulations  Simulated variables in : Water O2 (g.m-3) Simulated Oxygen O2 (g.m-3) Observed • A seasonal cycle well reproduced with high concentrations in winter and low in summer, • Consistency between simulated and observed spatial distributions. 13

14. II. Bio-geo-chemical model simulations  Simulated variables in : Sediment Ammonia • The simulated ammonia and nitrate concentration are in the same order as measurements. • The ammonia average concentration is about 3 mmolN/m3. • The average nitrate concentration is about 1.2 mmolN/m3. Nitrate 14

15. SECTION III LAGOON WATER BUDGET - LWB & LAGOON NUTRIENT BUDGET - LNB

16. III. Bio-geo-chemical model simulations  Lagoon Water Budget - LWB • Lagoon Water Circulation: LWC • - Surface waters are subjected to wind stress blowing from North-West moving towards the eastern sector where they drop to the bottom and then move back to the North-West through the lagoon centre. • - Current intensity in the inlet and in the northern and southern coasts of the lagoon is relatively high. • Lagoon Water Budget: LWB • The annual fresh water coming into the lagoon (factories, urban sewage, Tinja channel, rivers and rainfall) is estimated to 119.27 Mm3year-1, • The annualvolume of thewater lost by evaporationreached - 75.54Mm3 year-1 • Waters flowed into the Mediterranean Sea is estimated to -38.47 Mm3 year-1 • - Annual water budget of the lagoon is 5.25 Mm3year-1 Water dynamics scheme and water budget are deduced from observations and numerical simulations in the Bizerte lagoon. 16

17. III. Bio-geo-chemical model simulations Exchange with the Mediterranean Sea NO3 0.003 NH4 6.5 NH4 0.3 NO3 31.4 NH4 40.1 ZOO -28.6 NDET 11.8 PHY -166.0 NO3 -9.5 Ishkeul Lake inputs Urban inputs River inputs NH4 1.8 NDET 0.8 NDET 8.0 NO3 8.4 NDET 1.0 ZOO 6.0 78.5 13.8 NDET 1.4 PHY 31.8 73.4 394.3 291.4 41.5 NO3 12.7 NH4 6.4 162.3 154.0 10.3 6.4 77.7 58.9 0.1 NDET 8.4 NH4 0.02 NO3 0.01 504.6 0.06  Lagoon Nutrient Budget - LNB Annual nitrogen budget for the whole lagoon (Fluxes are given in tN year-1, amount of nitrogen in tN). • The PP in the lagoon is sustained by the NPP and the RPP with same proportions, • The Phytoplankton natural mortality and the Zooplankton grazing flows are equivalent in this ecosystem, • The dissolved NDET constitutes the main source of ammonia, • The nitrification flow is considered to be important in comparison to the flows between different compartments, 18

18. Conclusions-MODEB • The model was reproduced the major ecosystem variables in water column and interstitial water (Nitrogen, Phosphorus, Oxygen, Plankton), • The phytoplankton bloom is well reproduced by the model, • Based on a modeling approach water and nitrogen budgets are estimated for the lagoon of Bizerte. • Further improvements should be made : • Dissociation of phytoplankton in to -micro (diatoms), -pico, -nano-phytoplankton (dinoflagelates) and zooplankton in to -micro and -meso-zooplankton. • The macrophytes compartment should be developed as it reacts with the nitrogen and oxygen evolution. 19