An attempt to evaluate the biogeochemical budget of the Nador Lagoon (Morocco)

Firdaous Halim 1, Gianmarco Giordani. 2, Maria Snoussi 1

1 University Mohamed V, Faculty of Sciences, Department of Earth Sciences, Avenue Ibn Battota, B.P. 1014, Rabat, Morocco. Firdaous.halim@gmail.com 2 Department of Environmental Sciences, University of Parma, Italy. Giordani@nemo.unipr.it

INTRODUCTIONCoastal lagoons are among the most productive ecosystems but also the most exposed aquatic ecosystems to current and foreseen changes. The Nador lagoon, which is classified as RAMSAR site represents important sources of food and habitats for many species.However, the demand for space and natural resources has increased tremendously over the last decades due to increasing population, expanding agriculture, rapid urbanization and economic development around the lagoon.

This paper aims to attempt a preliminary estimation of the biogeochemical budget of water, salt and nutrients (N and P) in the Nador lagoon.

Final workshop - START Project Maghlag. Tunisia / Bizerte 25-27 June 2012

ConclusionIn accordance with the assumptions of the model, the Nador lagoon acted as sink for DIN as DDIN was negative, and as source for DIP since DDIP was positive.

The rates of nonconservative DIP and DIN fluxes can be used to estimate the apparent rate of nitrogen fixation minus denitrification (nfix-denit) as the difference between observed and expected DDIN. The annual Estimated N fixation-denitrification was estimated to be approximately: - 1,2 mmol m-² d-1.Thus the system appears to be denitrifying at a substantial rate.

The calculation of the Net Ecosystem Metabolism (NEM), that is, the difference between organic carbon production (p) and respiration (r) within the system (p-r), is calculated assuming again that organic oxidation is the primary source of nonconservative DIP flux. This rate is estimated as the Redfield ratio of the reacting organic matter (C:P = 106:1) (p-r) = -3,97 mmol m-² d-1. This negative value indicates a net mineralization of organic matter and that the Nador lagoon is a net heterotrophic system.The LOICZ budgeting model is a very useful tool to state the trophic status of the lagoon and foresee its evolution in the context of climate change. However, in the case of the Nador lagoon, the lack of time series of the basic variables didn't allow to have reliable results. The budgeting should then be considered as very preliminary and interpreted with caution. 

STUDY SITEThe lagoon of Nador is located on the Mediterranean coast of Morocco (2º 45'-2º 55', 35º 10' N). It has a NW-SE oval shape and covers an area of about

115 km², with a depth not exceeding 8 m. The lagoon is separated from the sea by a narrow sandy barrier and opens into the Mediterranean through a single inlet which has undergone several modifications. In fact, during the last fifty years, the inlet evolved along the barrier in various positions along the sandy bar and with a changing cross-section (Guelorget et al., 1987). Very recently, an artificial inlet has been created in order to improve the water circulation and quality within the system.

The Nador system is microtidal. Tidal amplitudes range from about 0.1 m at neap tide to about 0.5 m at spring tide (Erimesco, 1961; Brethes and Tesson, 1978). The dominant direction of the winds is W-SW from November to May, and E-NE from May to October (Tesson, 1977). Average salinity varies from 36.2 to 39 psu over the year.

The climate is semi-arid with two pronounced seasons: the summer condition from May to October with an average temperature of 22.5°C and the winter season from November to April with an average temperature of 14°C (Hilmi, 2005). The rainfall is irregular and varies from 150 to 450 mm/year. The average evaporation ranges between 840 mm in winter and 1130 mm in summer (Lefebvre et al., 1997). The lagoon receives drainage mainly from Selouane and Bou Areg streams that flows into the lagoon only in rainy season and from several canals used for the irrigation of the Bou Areg plain, which represents the most important agricultural area of the Mediterranean coast of Morocco. Frisoni et al. (1982) have estimated the mean continental runoff between 40 to 200.106 m3/year and groundwater inputs to 18.106 m3/year.

 About 800 000 people inhabit the area around the lagoon (RGPH 2004), concentrated mainly in the Nador city. The main activities are agriculture and livestock. Besides, the lagoon sustains an important fish activity for the local communities. The main sources of pollution are from agriculture and urban areas. Apart from the urban effluents of the city that are partly (70%) treated, all the sewage is directly discharged into the lagoon without any treatment. The total discharge was estimated from 8 000 to 13 000 m3 per day (RADEEN-NADOR).

METHODOLOGYIn order to establish budgetary calculations, data for water and salinity are available from various sources (Tesson,1977; Lefebvre,1997 ; Frisoni et al.,1982; Guelorget et al.,1987; Kharmiz,1989; Abouhala et al.,1995; DAFIR, 1997 ; Hilmi,2003; Hilmi,2005 and Bloundi,2005).

The dissolved N and P data in the lagoon and the near Sea are available from (Abouhala et al.,1995 and Bloundi,2005), based on samples collected twice a month between June 1991 and May 1992 at 5 stations (Abouhala et al.,1995); and 69 water surface samples collected between July 2000 and September 2004 at about fifty stations (Bloundi,2005). Data on groundwater are obtained from (Frisoni et al., 1982 and Lefebre et al.1997). DIN represents NO3 + NO2 + NH4. The LOICZ Toolbox for the calculation of the BGC budget has been applied to estimate the fluxes of water, salt, P and N using a simple box model.

RESULTS

VE = -150

VSystem = 575106 m3

S System = 37 psu

t = 33 days

VP = 20.3

VQ = 146.5

VO = 5

VR = -31.83VR SR = -1084

VG = 10.0S Ocean = 36.4 psuSR = 36.67 psu

VX (S Ocean S System) = 1084.4VX = 6216.5

Fluxes in 106 psu-m3 yr-1

DIPSystem = 1.83 Mm

DDIP = +1574

DIPatm = 0 (assumed)

VQ DIPQ = 2743

VO DIPO = 0 (assumed)

VR DIPR = -39.79VR SR = -1084

VG DIPG = 0DIP Ocean = 0.66 mMDIPR = 1.25 mM

VX (DIP Ocean -DIP System) = -7263.3Fluxes in 103 mole yr-1

WATER & SALT BALANCE CALCULATION DIP BALANCE CALCULATION

DINSystem =5.75 mM

DDIN = -26695

DINatm = 0 (assumed)

VQ DINQ = 62532

VO DINO = 0 (assumed)

VR DINR = -91.51

VG DING = 0DIN Ocean = 0

DINR = 2.87

VX (DIN Ocean -DIN System) = -35744.87Fluxes in 103 mole yr-1

DIN BALANCE CALCULATION

VQ, VG,VO

LandOcean

VPVE

SystemV sys, A sys

VR = -(VQ+VP+VE+VG+VO)

NUTRIENT (Y) BUDGETSALT BUDGETWATER BUDGET

VR

VX

VRSR

VQ SQ, VG SG,VOSO = 0

LandOceanS ocean

VESE =0

SystemV sys, S sys

VX = -(VRSR- VGSG ) /( SOcn -SSys)

VPSE =0

SR = ( SOcn +SSys) /2

VPYE =0

YR = ( YOcn +YSys) /2

VRYR

VQ YQ, VG YG,VOYO = 0

LandOceanY ocean

VEYE =0

SystemV sys, Y sys

DY = -VQYQ - VGYG - VOYO - VRYR - VX(Yocn - Ysys)

VX = ( YOcn -YSys)

RemerciementsThis preliminary work was carried out thanks to the scientific support of Gianmarco Giordani and then financial support of START.

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