Camboriu River Estuary, Santa Catarina State

Jurandir Pereira Filho and C.A. Schettini

Study area description

The estuary of the Camboriú River is a small system in Santa Catarina State, southern Brazil, which flows to the Camboriú Bight, and on to the inner shelf (Figure 1). The area of its drainage basin is about 200 km² and it is used mainly for agricultural activity.  The regional climate is subtropical, with mean precipitation of about 1,600 mm yr^-1 and evaporation of about 1,000 mm yr^-1.  The mean annual temperature is about 19  Although the system is very small, the combined estuary and bight are economically very important.  Balneário Camboriú City is the major tourist resort in southern Brazil.  The resident population is nearly 50,000, but the population can increase up to 800,000 during the summer season and holidays.

Figure 1.  Map and location of the Camboriú River Estuary.

The sewage system is precarious, and the sewage treatment plant is not enough during the population peaks.  The sewage treatment plant effluent flows to the estuary and there are many other small effluents along the beach.  This scenario results in poor water quality both in the estuary and in the bight (Kuroshima et al. 1996).

Schettini et al. (1996) and Siegle et al. (1999) described the hydrological and morphological characteristics of the system.  The local tide is semi-diurnal, with a mean range of 0.8 m and a maximum of 1.2 m.  The meteorological influence over tide height is important and can raise tides up to 1 m above the astronomical tide (Schettini et al. 1996; Carvalho et al. 1996).   The estimated freshwater inflow to the system is about 500,000 m³ day^-1, on average.  Siegle et al. (1996, 1998) classified the Camboriú River estuary as a shallow and partially mixed estuary.   The water column stratification is greater during neap tide conditions, whereas during spring tide condition the water column is vertically almost homogeneous.  The estuary is the main source of materials to the bight.  Its channel is about 120 m wide near the mouth and has a mean depth of 2 m.  There are a few mangrove patches around the inlet, but they are severely degraded.

 Most of the bight has an homogeneous water column, but close to the estuarine mouth there is a buoyant plume that indicates local stratification.  The Itajaí-açu River mouth is just 15 km to north, but its plume goes north and does not play an important role to the bight water quality, although it does influence the inner shelf salinity (~ 33 psu).

 A major project was carried out to evaluate the bight water quality, with 16 sampling stations distributed over the bight, surveyed monthly over a year in 1994 and 1995.  This characterization was summarized by Morelli (1997).  Some minor projects were carried out in the estuary, and there is an ongoing project with fortnightly sampling along the estuary (Kuroshima, unpublished data).   Information from experiments over tidal cycles is also available after 1998 and was used in this work to characterize the estuary (Pereira Filho et al., unpublished data).  The Camboriú River estuary budget was calculated using some of these data (Tables 1 and 2).

Source: 
CamboriúRiver-Kuroshima (unpublished data); Schettini 
Camboriú Estuary- Pereira Filho  (in preparation)
Camboriú Bight- Morelli (1997).

Water and salt balance

Figure 2   illustrates the water and salt budget for Camboriú River estuary.  The budget was calculated using LOICZ Biogeochemical Modelling Guidelines (Gordon et al. 1996).   The river discharge (V_Q) is based on Schettini et al. (1996).  There are no data on groundwater discharge (V_G) and we assume it to be zero.  Direct rainfall and evaporation over the estuary are close to zero.  The residual water flux to the Camboriú Bight (V_R) is 500 x 10³ m³ day^-1. The average salinity in the inner estuary and in the Camboriú Bight is based on Schettini et al. (1996) and on Morelli (1997) respectively.  The mixing flux (V_X) calculated the Camboriú estuary and bight is 3,628x10³ m³ day^-1.  Water exchange time (V_syst/(V_X + |V_R|) is about 0.24 day.

Figure 2.  Steady-state water and salt budget for Camboriú River estuary, based on a 2-box model.  Water fluxes in thousand m3 day-1; salt fluxes in thousand psu m3 day-1.

Budgets of nonconservative materials

The nutrient budgets have been calculated, although experience in other systems (e.g., Mamberamo Estuary, Indonesia; MaeKlong River, Thailand) indicates that estimates of nonconservative fluxes for systems with such short exchange times are unreliable because of insufficient time to develop a reliable nonconservative signal in the water composition and extreme sensitivity of the results to estimates of loading and exchange.

DIP balance

Figure 3 shows the DIP budget.  The Camboriú River delivers about 250 mol day^-1 of DIP to the estuary.  There are no data from atmospheric DIP input by precipitation, but it is probably small.  We do not have data on sewage loading, so it was estimated using a mean per capita waste production.  Considering a per capita daily discharge of DIN and DIP as 0.3 mol and 0.04 mol respectively (from Von Sperling 1996), the sewage loading was obtained using the resident population: 58,000 (Censo IBGE in Morelli 1997).  The estimated DIP input from sewage loading is about 2,300 mol day^-1.  Residual DIP flux is 275 mol day^-1 and exchange flux is 1,088 mol day^-1, resulting in an estimated DDIP of about -1,200 mol day^-1 (-2.4 mmol m^-2 day^-1).

Figure 3.   Steady-state DIP budget for Camboriú River estuary; two-box model.  Arrows indicate directions of hydrographic fluxes. Fluxes in mol day-1.

This very rapid rate of DIP flux is unreasonably rapid for biotic uptake and might indicate either abiotic uptake or an erroneous estimate of DDIP.  Let us consider the possibilities for uncertainty in this estimate.  Besides uncertainty in the actual per capita production of waste, there is uncertainty in how much of the actual waste production reaches the system.  Halving the sewage delivery of DIP (through some combination of a lower estimate of per capita waste production and only partial delivery of wastes to the estuary) would decrease DDIP to zero.  This seems extreme, but it demonstrates sensitivity of the actual uptake to waste load estimates.   The population can increase considerably in some periods of the summer, resulting in a large increase of sewage loading.  With a population of 800,000 (e.g., approximating the peak of tourist season), the DIP loading from sewage would be 32,000 mol day^-1 and the new DDIP estimate would be about -31,000 mol day^-1.  This extreme apparently does not describe the budget, because the nutrient data set used to construct this budget was obtained in the beginning of the autumn, at the end of the holiday season.

The prudent conclusion from these results is that rapid water exchange and uncertainties in nutrient loading preclude quantification of DDIP for this system.  It does seem likely that the system is taking up DIP.

DIN balance

The DIN budget is shown in Figure 4.  The riverine DIN load is about 9,000 mol day^-1, and the estimated sewage load is about 17,000 mol day^-1 (see discussion of waste production, above).  The main N form is ammonium, which represents almost 90% of DIN in the system. This probably results from the high sewage loading, for which ammonium is the main N form.   The residual DIN flux from the estuary is 11,500 mol day^-1 and the exchange flux results in a DIN transport from the estuary of about 79,800 mol day^-1.  The estimated nonconservative DIN flux is thus about +65,000 mol day^-1 (+130 mmol m^-2 day^-1), and the estuary apparently represents a source of DIN.

As discussed above for DIP, there is too much uncertainty in the loading to be confident of the actual DDIN for this system.  Considering the magnitudes of uncertainty, it seems likely that the system is, indeed, a DIN source.  The magnitude of the source remains unresolved.

Figure 4.  Steady-state DIN budget for Camboriú River estuary; two-box model.  Arrows indicate directions of hydrographic fluxes. Fluxes in mol day-1.

Stoichiometric estimates of aspects of net system metabolism

It has been observed that neither the DDIP nor the DDIN estimate can be considered quantitatively reliable.  Nevertheless, it appears possible to derive some qualitative understanding of net metabolism in this system.  The system appears to be a net sink for DIP, qualitatively indicating the likelihood that (p-r) is positive; that is the system appears to be a net autotrophic system.  Net autotrophy would indicate that DIN should be taken up by net production, yet this system appears to be a DIN source.  It therefore seems likely that nitrogen fixation exceeds denitrification; that is, (nfix-denit) appears to be positive.

Back to [Node Introduction] [Budgets][South America] [LOICZ]

Last Updated 21 May 2006 by DPS