Teacapan-Agua Brava- Marismas Nacionales, Sinaloa and Nayarit

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

Study Area Description

The region of Teacapan-Agua Brava-Marismas Nacionales (TAB, 22° 08’ N, 105° 32’ W) has an ancient story for human settlements. A brief historical and natural history perspective is presented in the next paragraph. There are evidences of considerable human populations over more than 1,500 years, in the forms of small hills of shells (Tivela sp.). Near the town of Mexcaltitan a tribal group migrated to the upper lands and founded the city of Tenochtitlan (now Mexico city), the capital of the Aztec empire. When the Spaniards arrived in the region in the 16th century, they estimated a population of more than 300,000 inhabitants; this can be considered extremely high for rural standards of those days. This high population density was sustained by the fertility of the land in the alluvial plain but also to the richness of seafood as oyster, shrimp and fishes. The region has been a used by several generations for multiple uses. Mangroves where mainly used for fuel and rustic construction, during the colonial age, an important industry of tannins for cow hide treatment was developed until the 19th century. The region was an important hunting ground up to the 1950’s for jaguar and crocodile. In the winter, more than the 20% of the total migratory birds of the Pacific arrive enroute from Alaska, Canada and the United States. It is also possible to find areas with subhumid tropical forest and low human impact between the mangroves on some islands. The region indirectly supports one of the most important shark fishery in Mexico in the adjacent marine waters. Shrimp fishing is an important economical activity and recently shrimp farming is developing. In the beginning of the 1970’s an artificial inlet was opened (Cuautla Channel) with the idea to allow the shrimp post-larvae get inside the Agua Brava lagoon more directly, but the channel widened uncontrollably from 40 m wide 3 m depth, as was planned, to more than 1,200 m wide and 18 m depth. The drastic change in the hydrological pattern killed more than 50 km ² of mangroves. The aquaculture development without planning with an environmental sound management, the river dam construction programs together with the agricultural activities and the opening of artificial inlets, will put at risk the basic structural functions of the ecosystem and this actual and potential resources as important cultural values.

The area comprises approximately 2,000 km ² of tidal channels, seasonal flood plains, more than 150 semi-parallel coastal lagoons formed by stranded beach ridges, big estuarine water bodies and mangrove swamps. The mangroves, small tidal channels and the semi-parallel coastal lagoons occupies approximately 1,100 km ² . The waterways occupy 500 km ² (total area of the mangrove-estuarine complex 1,600 km ² ). Water depths of the waterways vary from below 1 m to 18 m near the inlets, with a mean depth of 2 m. The mangrove water depth varies according to the species, being 0.40 m above mean sea level for the dominant species, the white mangrove (Laguncularia racemosa) and 0.7 m for the black mangrove (Avicenia germinans) we assume that during the flooding periods (flows) a mean value of 0.5 m is reasonable. Obviously 0.0 m depth is assumed during low ebb tides. A mean value (flows and ebbs) of 0.25 m is considered here to be appropriate. The total volume (V1) of the mangrove-estuarine complex comprises 1,266 x 10 ⁶ m ³ . There are 5 stream flows that discharge to the system: The Cañas, Rosamorada, Bejuco, Acaponeta and San Pedro rivers. But only the last two have flow all year around, with an annual flow of 3,000 x 10 ⁶ m ³ year ^-1 for the Acaponeta and 2,456 x 10 ⁶ m ³ year ^-1 for the San Pedro river. The other rivers are seasonal with flows below 180 x 10 ⁶ m ³ year ^-1 . Precipitation is estimated to be about 1,459 mm year ^-1 and evapotranspiration of 1,991 mm year ^-1 (mean annual values of more than 10 years). Data on groundwater flow and non-point agriculture waste-waters that discharge to the system are not available.

Salinity: Salinities of freshwater from rivers and rainfall was assumed to be 0 psu, estuarine annual average salinity is about 20 psu, and adjacent coastal marine water (Pacific ocean) averages 34 psu. Nutrients and other non conservative components: Data are available for DIP, DIN (nitrate, nitrite and ammonia) in this system Table 1. The freshwater nutrient concentration data have been weighted according to the relative flow rates of the two major rivers.

Other: Mangrove litter-fall in three sites averages about 1,200 g m ^-2 year ^-1 . If 40% of this material is considered carbon it is equivalent to approximately 40 mol C m ^-2 year ^-1 . Leaf degradation rate varied from about 2 in the soil to 5 year ^-1 in the water. Humid substances (DOC) can be greater than 100 mg l ^-1 with a mean value of 30 mg l ^-1 . Mean daily net plankton productivity in the Agua Brava lagoon is estimated to be of 0.4 g C m ^-2 d ^-1 equivalent to 12.5 mol C m ^-2 year ^-1 . In general the respiration rate in the water column is higher than the productivity so the water column is heterotrophic for most of the year (Gross Productivity/24 hour Respiration = 0.64), a reflection of the influence of high organic matter from rivers and mainly the adjacent mangroves swamps.

From these data we present the budgetary analysis as follows:

Table 1. Average nutrient concentration (mmol m ^-3 ) for components of the Teacapan-Agua Brava-Marismas Nacionales mangrove-estuarine complex.

Water and Salt Budgets

Figure 1 illustrates the average annual water and salt budgets for Teacapan-Agua Brava-Marismas Nacionales. Freshwater flow (V_Q ) is estimated by adding the flows of the Acaponeta and San Pedro rivers (5,456 x 10 ⁶ m ³ year ^-1 ). Direct precipitation (V_p) and Evaporation (V_E) are estimated as 2,412 x 10 ⁶ m ³ year ^-1 and 3,223 x 10 ⁶ m ³ year ^-1 , respectively, considering the area of the complex (1,619 x 10 ⁶ m ² ). The other components (V_G and V_o) are assumed to be small. The system shows substantial net residual outflow of water, as expected from the freshwater inputs to the system. It can be seen from these calculations that the water exchange time in this system is about a month.

Figure 1. Water and salt budgets for Teacapan-Agua Brava-Marismas Nacionales, annual average. System volume is in units of 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 illustrates the DIP and DIN budgets for these systems. The data are also listed in the regional comparison tables.

P Balance

Substantial DIP comes in from terrigenous runoff into this system, but very little P exchanges with the ocean with either residual flow or exchange flow. Clearly the system takes up most of the DIP delivered to it. The system is a sink for virtually all of the land derived DIP (Figure 2). Thus, DDIP = -170 x 10 ⁶ mol year ^-1 = -0.11 mol m ^-2 year ^-1 over the mangrove-estuary area.

N Balance

Similarly to DIP, DIN comes in from land and is mostly trapped in the system (Figure 2). Thus, DDIN = -452 x 10 ⁶ mol year ^-1 = -0.28 mol m ^-2 year ^-1 over the mangrove-estuary area.

Stoichiometric Calculation of Aspects of Net System Metabolism

We can calculate net nitrogen fixation minus denitrification (nfix-denit) as the difference between observed and expected DDIN. Expected DDIN is DDIP multiplied by the N:P ratio of the reacting particulate organic matter. In a mangrove-dominated system, it is not entirely obvious what the dominating reactive organic matter is, so we use two N:P values. First, we employ the Redfield N:P ratio of plankton (16:1). Then we use a reactant which has an N:P ratio of 30:1, more typical of various land plants. Thus, with plankton: (nfix-denit) = -452 x 10 ⁶ – 16 x (-170 x 10 ⁶ ) = +2,268 x 10 ⁶ mol N year ^-1 (+1.4 mol N m ^-2 year ^-1 over the estuary-mangrove area).

With land plants: (nfix-denit) = -452 x 10 ⁶ – 30 x (-170 x 10 ⁶ ) = +46,448 x 10 ⁶ mol N year ^-1

(+2.9 mol N m ^-2 year ^-1 over the estuary-mangrove area). Note that, with either material, the system appears likely to be fixing nitrogen in excess of denitrification.

Similarly, we can estimate net ecosystem metabolism (NEM = p-r) as the negative of the nonconservative DIP flux multiplied by the C:P ratio of the reacting organic matter. If the net reacting material is plankton, the particulate C:P ratio is about 106:1. If it is mangrove litter, then the ratio may be as high as 1000:1. With plankton:

(p-r) = -106 x (-170 x 10 ⁶ ) = +18,020 x 10 ⁶ mol C year ^-1 (+11 mol C m ^-2 year ^-1 ).

With mangroves dominating the net production: (p-r) = -1,000 x (-170 x 10 ⁶ ) = +170,000 x 10 ⁶ mol C year ^-1 (+106 mol C m ^-2 year ^-1 ).

This latter figure is about double the rate of mangrove litter production, and that production should approach an upper limit of net primary production. We therefore suggest that the lower figure more accurately reflects the likely rate of net ecosystem production. In either case, if the DIP uptake primarily represents net organic metabolism, rather than inorganic sorption or precipitation of P, this system is strongly net autotrophic. We assume that the lower value more accurately reflects (p-r). Primary production was estimated above to be about 30 mol C m ^-2 year ^-1 , implying that respiration is approximately 30. These numbers suggest that the p/r ratio of the system is about 1.3.

Figure 2. DIP and DIN budgets for Teacapan-Agua Brava-Marismas Nacionales, annual average. Fluxes in 10 ⁶ mol year ^-1.

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Last Updated 14 Jan 2000 by DPS