1. Modeling phytoplankton seasonal variation and nutrients budget of a Semi-Arid region ecosystem in the Southern Mediterranean Sea: -Case of the Bizerte Lagoon- Béchir BEJAOUI 1, Ali HARZALLAH 1, Noureddine ZAABOUB 1, Annie CHAPELLE 3, Mahmoud MOUSSA 2 1 Institut National des Sciences et Technologie de la Mer, Tunisia. LMM Laboratory. 2 National 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.tnbejaoui.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. 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. 4 I.1 Presentation of Bio-geo-chemical model PhytNdet NH 4 NO 3 Ndet NH 4 NO 3 O2O2 T O2O2 T O2O2 T O2O2 T O2O2 O2O2 O2O2 Mineralization Nitrification Dénitrification Death Respiration Death & Feces Graze Uptake NO3 O2O2 Zoo Ndet Photosynthesis Sedimentation Diffusion Excretion Diffusion Uptake NH4

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

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

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

8. SECTION II MODEL SIMULATIONS

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

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

11.  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 II. Bio-geo-chemical model simulations Phosphorus Simulated variables in : Water

12. Phytoplankton  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 II. Bio-geo-chemical model simulations Chla (g.m-3) Simulated Simulated variables in : Water

13.  A seasonal cycle well reproduced with high concentrations in winter and low in summer,  Consistency between simulated and observed spatial distributions. 13 II. Bio-geo-chemical model simulations Simulated variables in : Water Oxygen O 2 (g.m -3 ) Simulated O 2 (g.m -3 ) Observed

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

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

16. Lagoon Water Budget - LWB 16 III. Bio-geo-chemical model simulations Water dynamics scheme and water budget are deduced from observations and numerical simulations in the Bizerte lagoon.  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 Mm 3 year -1, - The annual volume of the water lost by evaporation reached - 75.54Mm 3 year -1 - Waters flowed into the Mediterranean Sea is estimated to -38.47 Mm 3 year -1 - Annual water budget of the lagoon is 5.25 Mm 3 year -1

17. Lagoon Water Renewal Time - LWRT 17 III. Bio-geo-chemical model simulations Day  Lagoon Water Renewal Time: LWRT - Spatial distribution of water renewal time (in day), - LWRT in the inlet is very low (about some days) in comparison to waters inside the lagoon where it is about 8 months.  The difference is due to : - The significant length of the inlet preventing the sea waters to reach the lagoon inside rapidly, - Low circulation in the Centre of the lagoon and the Eastern sector

18. Lagoon Nutrient Budget - LNB 18 III. Bio-geo-chemical model simulations ZOO 6.0 41.5 73.4 154.0 394.3 291.4 162.3 0.1 6.410.3 PHY 31.8 NDET 1.4 NH 4 6.4 NO 3 12.7 NH 4 0.02 NO 3 0.0 1 NDET 8.4 0.06 58.9 13.8 77.7 Exchange with the Mediterranean Sea Ishkeul Lake inputs NH 4 6.5 NO 3 31.4 River inputs NH 4 0.3 NO 3 0.003 Urban inputs NH 4 1.8 NO 3 8.4 NDET 1.0 NDET 8.0 NDET 0.8 NDET 11.8 PHY -166.0 ZOO -28.6 NH 4 40.1 NO 3 -9.5 504.6 78.5 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,

19. Conclusions-MODEB 19 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.