Figure 1.  Map of Mandovi estuary.

The estuary is navigable round the year up to about 45 km from the mouth upstream and is one of the two main waterways in Goa used for transporting iron ore by barges loaded at jetties along its northern margin in the upstream region.  Iron ore mined in the hinterland is transported by trucks and staked at sites along the riverbank from where it is loaded onto the barges (capacity 1000 tons) and transported to the Mormugao Port for loading on ships for export.  There are some iron ore beneficiation plants situated on the riverbank, where the ore is washed with river water and the wash water is discharged directly into the estuary.  This discharge contains high quantities of sediment rich in iron and also NH₄NO₃ used as explosive in the mining operations (de Sousa, 1983 & 1999).  These materials cause high turbidity in the estuarine water in the upstream region.  The ammonium nitrate undoubtedly also raises the DIN content of the river water.

The catchment area of the estuary (1,150 km²) is densely populated.   However, the villages are not connected to any sewage system and depend on individual closed septic tanks; hence no waste load apparently enters the estuary from these areas.  The urban population of the city of Panaji is connected to a sewage system where the sewage is treated (primary and secondary treatment) and the treated sewage waste is discharged in Mandovi estuary (Outer Box on the figures) at Campal (3 km from the mouth) at a rate of about 7x10³ m³ d^-1.  The sewage effluent contains phosphate concentration averaging 5 mg/l (~160 mM) and ammonium concentration of  ~50 mg/l (~3,600 mM).

 The estuary is connected to the neighbouring Zuari estuary through a narrow natural canal --the Cumbarjua Canal.  This canal is flanked along both the margins by agricultural lands locally known as “Khazan” lands where rice is grown during the monsoon season.  These agricultural lands are low-lying, and during the spring high tide get inundated with the water from the canal which attains high salinity during the summer months.  However, the heavy freshwater runoff during the monsoon months washes away the salts from these lands into the canal.  During this season the canal is totally dominated by freshwater.

 Data on salinity and nutrients DIN (nitrate + nitrite; NH₄ data are not available during the sampling period used here, but concentrations are known from other measurements in the system to be low) and DIP were collected at 4 stations in the estuary - one each at the mouth and at the extreme freshwater-end, and the remaining two stations at intermediate salinities.  A coastal station in the Arabian Sea just outside the mouth of the estuary defines the properties of the coastal ocean water.  Similarly, a station at Colem in the freshwater region of the Khandepar river defines the characteristics of the freshwater.  Two stations in the Cumbarjua Canal give the properties of the Canal.  Since the Canal is dominated with freshwater derived from the surrounding agricultural lands during the monsoon, the properties of the Canal water have been used to define the properties of the surface runoff during monsoon.

 Monthly observations of chlorinity (from which salinity was calculated) and nutrients were made at all the stations during 1980.  Surface, mid-depth and bottom water samples were collected at the three estuarine stations in the lower estuary while only surface and bottom samples were collected at the uppermost station.  In the case of the coastal station and the Canal stations, only mid-depth samples were collected while only surface sample was collected from the freshwater station Colem.

 All the estuarine data were pooled together by season and averaged.  Similarly, during the monsoon data from the two Canal stations were pooled together and averaged to give the average properties of the surface runoff during the monsoon.  Data from the coastal station were pooled by season and averaged to give average conditions in the coastal sea.  Rainwater was found to contain nitrate in the range 0.3-3.8 mM (average ~ 2 mM) and practically no phosphate (de Sousa, 1997).

 Water and salt budgets

 Table 1 summarizes the size of the 3 boxes used for budgeting purposes; boundaries based on salinity gradients are shown on Figure 1. 

 Rainfall in this area is seasonal, about 90% occurring during the southwest monsoon (June to September).  The annual rainfall averages 3 m.  However, during the year 1980 when the present data were collected, the precipitation was only 2.5 m during the monsoon and another 0.1 m during the dry seasons.  Based on the rainfall and runoff patterns, the year is divided into 3 4-month seasons: pre-monsoon (February through May), monsoon (June-September), and post-monsoon (October-January). The monsoon is the dividing factor; the choice of January—February as the boundary between post- and pre-monsoon seasons is arbitrary.  Evaporation from the water surface has been calculated using the evaporation rates of 1.2 m/year (average ~ 3 mm/d^-1), derived from Baumgartner and Reichel (1975) for this area.

 The combined annual runoff from both the tributaries was about 1.8 x 10⁹ m³ (Shetye et al., 1995).  The surface runoff (Table 4) was calculated by Dr. Laura David (personal communication) based on the monthly temperature and rainfall data for this region and using the relationship given at http://data.ecology.su.se/MNODE/Methods/runoff.htm.  Total surface runoff is that runoff downstream of the river gauging station.

 Figures 2 to 4 present the daily water and salt budgets for the three seasons.   During the monsoon season, water exchange is rapid (t_syst = 2 days) because of high river flow.  The post-monsoon, with intermediate flow, also has intermediate water exchange (t_syst = 19 days), and the pre-monsoon period is characterized by both low freshwater flow and long exchange time (t_syst = 31 days).

Budgets of nonconservative materials

 Figures 5 to 10 illustrate the nutrient budgets.  In general, Mandovi estuary apparently takes up both DIP and DIN except in the monsoon season when the system releases DIN. The monsoon behavior, with respect to both DIN and DIP, must be regarded with some caution because of the very rapid water exchange. Slight errors in the exchange calculations at the estuary mouth (Figures 3, 6, 9) could alter the calculated fluxes substantially.  The system (average of the three seasons) appears to take up approximately equivalent amounts of DIP and DIN: –23 x 10³ mole P d^-1 (–0.8 mmole P m^-2 d^-1) and  –24 x 10³ mole N d^-1 (–0.8 mmole N m^-2 d^-1), respectively. If the averages were restricted to the pre-monsoon and post-monsoon season, because of uncertainties in the hydrographic fluxes, the average fluxes would still indicate uptake, but the DIN:DIP uptake ratio would change from 1 to 10: be –4 x 10³ mole P d^-1 (–0.14 mmole P m^-2 d^-1) and   –41 x 10³ mole N d^-1 (–1.4 mmole N m^-2 d^-1).

 Stoichimetric calculations of aspects of net system metabolism

 Table 6 summarizes the non-conservative nutrients and stoichiometrically derived net apparent biogeochemical processes.  The whole system appears to be net autotrophic and net nitrogen fixing for all seasons.  Apparent net production during the monsoon is very high if all the DDIP is assumed to have assimilated by phytoplankton: (p-r) +83 mmol C m^-2 d^-1.  Similarly using phytoplankton N:P ratios, the calculated value for (nfix-denit) for data from the 3 seasons is +12 mmol m^-2 d^-1. If the calculations are restricted to the data for the pre-monsoon and post-monsoon seasons, (p-r) drops to a value of +15 mmol C m^-2 d^-1.   Similarly restricting the (nfix-denit) calculations to the two dry seasons, the average rate of (nfix-denit) declines to +0.8 mmol N m^-2 d^-1.