Bahía de Altata-Ensenada del Pabellón, Sonora

by F.J. Flores-Verdugo and G. de la Lanza-Espino

Study Area Description

The Bahía de Altata and Ensenada del Pabellón (EP) lie in Region B (as modified from Lankford, 1977; see Appendix I of Smith et al., 1997). Within the organization of this report, it is considered among the systems of the "humid Pacific Coast," because, unlike the systems to the north and west, rainfall plus runoff clearly exceeds evaporation. An important aspect of this system, again in contrast to the arid coast systems, is that it sees the clear influence of agricultural activities on inflowing water composition. Agricultural development around coastal lagoons is increasing without enough consideration of the environmental implications with respect to biodiversity and other aspects of ecological change. River diversions by dams and the artificial channelization for the agricultural development increase problems of siltation inside the lagoons, erosion in the sand barriers and eutrophication and pesticides from agriculture waste waters toward drainage channels as non-point pollution sources. The estuarine complex of Bahía de Altata-Ensenada del Pabellón lies near 107° 38' N and 24° 25' W. Bahía de Altata is a long, narrow lagoon running parallel to the coast, with sandy sediments, mean water depth of 5 meters with mainly marine conditions (32 psu). Ensenada del Pabellón is joined to this bay and is wider than Altata, with silt and clay sediments and a mean depth of 1 m and display mainly estuarine conditions wit salinities between 10 to 28 psu. The annual mean salinity for the whole system is approximately 28 psu. The adjacent marine water averages 35 psu. Water temperature varies from 20° C in January to 32° C in August. The area of the complex is approximately 460 km ² , including 100 km ² of mangrove swamps. The Culiacan River discharge to the lagoon has a mean annual flow of (VQ) about 3,400 x 10 ⁶ m ³ . The depth of the system varies from less than 25 cm in the mangroves to 15 meters in La Tonina inlet. We estimate a mean depth of 3 meters for the whole system, giving a total volume of approximately 1,400 x 10 ⁶ m ³ . The system is separated from the sea by a narrow sand barrier interrupted with two inlets: a small and relatively recent one (called La Palmita) and the main one (La Tonina). The estuarine complex is located in the valley of Culiacan and received the agriculture discharges of the district by several drainage channels. This district comprises more than 2,700 km² of irrigated agricultural lands used mainly for horticultural production. The production of this district comprises one third of the total national horticultural export. Also three sugar cane industries and a paper mill discharges their waste into ponds connected to the system. Several authors have reported the presence of pesticides and heavy metals in the lagoon. Relatively recent the system is receiving a new impact from the waste waters of shrimp farms. The lagoon sustains an important fishing activity of shrimp (Penaeus spp), oyster (Crassostrea corteziensis), clam (Chione subrugosa) and fish such as snappers, mullets, etc. The borders of the lagoon and interior islands are covered by mangroves (Rhizophora mangle, Laguncularia racemosa and Avicennia germinans). The latter is the dominant mangrove species with 86% of the total density (from 4,800 to 7,600 trees ha ^-1 ). Landward of the mangroves predominates a belt of seasonal floodplains with high salinity soils with no vegetation at all or with patches of the terrestrial halophytes (saltworts) Salicornia spp and Batis sp. locally known as "marismas". There is a special place where an extensive (100 km ² ) freshwater marsh of cattail (Thypha spp) occurs (Chiricahueto) located in the SE of Pabellón and where several hundreds thousands ducks from Canada and the United States of America arrive during the winter. This freshwater swamp has increased in area as consequences of the agricultural waste waters displacing what use to be a seasonal flood plain The mean annual rainfall of the region is 670 mm and the mean annual evapotranspiration is 1,500 mm. Concentrations of DIP vary in Pabellón from 3.1 (August) to 13 mmol m ^-3 (April) and in Altata from 2.5 to 5.8 mmol m ³ . We estimate a mean annual values for the whole system of 7.2 mmol m ^-3 . In the adjacent ocean a mean value of 0.6 mmol m ^-3 can be observed and in the Culiacan river a mean value of 7.5. For DIN (nitrate + nitrite + ammonia) the values vary in Pabellón from 4.4 (February) to 6.3 mmol m ^-3 (August) and in Altata from 0.6 (February) to 0.8 (August). A DIN mean annual value of 3.7 mmol m ^-3 was estimated for the entire system, 0.6 for the ocean, 40 in the river. Nutrients are always lower in Altata compared to Pabellón as consequences of a higher oceanic water influences in Altata. In Ensenada del Pabellón there is more freshwater influence, mainly from the agricultural drainage. Nutrients cycling from the sediments to the water were estimated to be as high as 0.6 mmol m ^-2 day ^-1 for ammonium and of 0.05 mmol m ^-2 day ^-1 for phosphate. In lagoon sediments influenced by sugar cane waste water values as high as 16 mmol m ^-2 day ^-1 for ammonium and 2,2 mmol m ^-2 day ^-1 for phosphate were detected, demonstrating that eutrophication is a clear problem in some parts of this system. Plankton net annual productivity (p) was estimated to be of 267 g C m ^-3 year ^-1 , equivalent to about 70 mol C m ^-2 year ^-1 The water column has a p/r ratio of 2.0 describing this part of the system as autotrophic in general.

Water and Salt Budgets

Figure 1 summarizes the water and salt budgets for this system. Runoff (V_Q) + precipitation (V_P) substantially exceed evaporation (V_E), and groundwater input (V_G) is assumed to be zero. In order to balance the water budget, residual flow removes water and salt from the system (V_R = -3,000 x 10 ⁶ m ³ year ^-1 ; V _R S _R = -94,500 x 10 ⁶ psu m ³ year ^-1 ). In order to maintain a steady state salinity (that is V_systemdS_system/dt = 0), salt must mix into the system (V_X[S_ocean-S_system] = +94,500 x 10 ⁶ psu m ³ year ^-1 ). S_ocean and S_system are known, so we can solve for mixing (V_X = 13,500 x 10 ⁶ m ³ year ^-1 ). The volume of the system is 1,400 x 10 ⁶ m ³ , so water exchange time can be calculated as t = V_system/(|V_R| + V_X) = 0.08 year. That is, the water exchange time is about 1 month.

Figure 1. Water and salt budgets for Bahía Altata-Ensenada del Pabellón, annual average. System volume in 10 ⁶ m ³ . Water fluxes in 10 ⁶ m ³ year ^-1 . Salt fluxes in 10 ⁶ psu m ³ year ^-1 .

Budgets of Nonconservative Materials

Figure 2 summarizes the DIP and DIN budgets for the system.

P Balance

Concentration and flux of DIP in river water flowing into this system are high, apparently reflecting the product of agricultural drainage into the system. System concentrations are also very high, and outward transport of DIP occurs via both residual flow and mixing. These outward fluxes greatly exceed the estimated river inflow of DIP, so there apparently is an internal source of DIP (DDIP = +75 x 10 ⁶ mol year ^-1 = +0.19 mol m ^-2 year ^-1 ). We have observed that there is very high release of DIP, especially from the sediments associated with sugar cane wastes, so this and other organic discharges into the system are assumed to support the high nonconservative flux of DIP.

N Balance

Concentration and flux of DIN in river water are also high. While there is export of DIN both in the residual flow and in the mixing, this outward flux is substantially lower than the river DIN import. There must therefore be a substantial sink of DIN in this system (DDIN = -118 x 10 ⁶ mol year ^-1 = -0.46 mol m ^-2 year ^-1 ). There is thus a clear discrepancy between the nonconservative fluxes of DIP and DIN.

Stoichiometric Calculations of Aspects of Net System Metabolism

Net nitrogen fixation minus denitrification in this system (nfix-denit) is calculated as the difference between observed and expected DDIN. Expected DDIN is DDIP multiplied by the N:P ratio of the reacting particulate organic matter. We do not know that N:P ratio. If this material were plankton, the expected ratio would be near the Redfield Ratio of 16:1. Waste from sugar cane or other terrestrial plant material might have a higher ratio, while animal wastes might be somewhat lower. Lacking a definitive value, we assume that the appropriate N:P ratio of decomposing organic matter is near the Redfield Ratio, and therefore that the expected value for DDIN is 16 x (75 x 10 ⁶ ) mol year ^-1 . Thus: (nfix-denit) = -118 x 10 ⁶ - 16 x (75 x 10 ⁶ ) mol year ^-1 = -1,082 x 10 ⁶ mol year -^-1 (-2.4 mol N m ^-2 year ^-1 over the area of the system). Thus, the system appears to be denitrifying at a substantial rate. We can also estimate net ecosystem metabolism (NEM), that is the difference between primary production and respiration (p-r), as the negative of the nonconservative DIP flux multiplied by the C:P ratio of the reacting material. Again, we do not know the C:P ratio of the reacting material, but it seems likely to equal or exceed the C:P ratio of plankton. Thus:

(p-r) = -106 x (75 x 10 ⁶ mol year ^-1 ) = -7,950 x 10 ⁶ mol C year ^-1 (-17 mol m ^-2 year ^-1 over the system area). This is a substantial rate of net respiration, especially when compared with the estimated of primary production (17 mol C m ^-2 year ^-1 ). Moreover, if the reacting material is terrigenous plant organic matter, the likely rate of (p-r) may well exceed what is estimated here. Nevertheless, we believe that these numbers make sense in view of the discharge of sugar cane and other agricultural waste products into this system.

Figure 2. DIP and DIN budgets for Bahía Altata-Ensenada del Pabellón, annual average. Fluxes in 10 ⁶ mol year ^-1.

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Last Updated 14 Jan 2000 by DPS