Laguna Madre, Tamaulipas

by S. Ibarra-Obando and F. Contreras-Espinosa

Study Area Description

Laguna Madre (LM, 23-25 ^º N, 97 ^º W) is located on the Gulf of Mexico shoreline. Its northern limit is the Rio Bravo delta, and its southern limit the mouth of the Soto La Marina river. It occupies a shallow basin that has an average depth of 0.7 m, and an area of about 2,000 km ² ; it is the largest Mexican lagoon. It is separated from the sea by a sand bar. The San Fernando river mouth divides the lagoon into two sub-basins, known as North and South. The lagoon presents 13 mouths that communicate with the sea only on a temporal basis, as sediment accumulation after cyclones and hurricanes promotes their closure (Contreras, 1993). Climate is arid with evaporation being far more important than freshwater input from rivers. As a consequence, the lagoon is drying out, with increasing salinity in the remaining water body, and salt deposition in its margins. Average evaporation is 1,900 mm year ^-1 , and average precipitation 600 mm year ^-1 . River freshwater input which might flow into the system is diverted for agricultural and urban purposes, so none gets into the lagoon (Estado de Tamaulipas, 1996).

Dune vegetation exists at the lagoon margins, being represented by the following species: Uniola paniculata, Ipomoea pescaprae and Croton punctulatus. Halophytes like Spartina spartinae, Suaeda nigrica, Salicornia ambigua and Distichlis spicata, are also found. Upland areas present both spiny shrubs and low forest spiny species, like mesquite and ebony. Submerged vegetation consist of algae and seagrasses, which have been greatly reduced by the desiccation process and hypersaline conditions. Only after heavy rains and mouth openings, salinity decreases, but this effect is only temporary (Contreras, 1993). Halophytes are also disappearing from the lagoon margins, promoting erosional areas that also contribute to lagoon infilling (Estado de Tamaulipas, 1996).

The icthyofauna comprises 78 species, among which prawns, croakers, snooks, mullets and houndsharks are the most representative (Contreras, 1993). Oyster and prawn aquacultural projects have failed due to poor water quality conditions (lack of freshwater input, and lack of a free water exchange with the ocean). For the same reasons, crab harvest has declined. The lagoon is also well known for being a resting, feeding and reproductive site for migratory waterfowl, mainly ducks: Aythya americana and Anas discors (Estado de Tamaulipas, 1996). Urban wastewater, as well as pesticides, herbicides and fertilizers residues represent the main pollution source. Boat traffic contributes to pollution by oil and fuel spill, as well as boat washing.

Pollutant products are spilled in the open ocean without any regulation. All these products are carried away by currents and end up in the lagoon (Estado de Tamaulipas, 1996). About 80,000 people inhabit the area around the lagoon. The coastal plain represents one of the most important agricultural areas of the country, being among the most important crops: cotton, cereals, leguminous and oily crops. Cattle raising is also an economically important activity. Besides the artesanal fishery, large scale fisheries are represented by tuna and prawn fisheries. For the Laguna Madre area, fisheries represent the most important economic activity, although the volume captured is small (Estado de Tamaulipas, 1996).

Phytoplankton primary production for this system has been estimated to be about 600 g C m ^-2 year ^-1 (i.e., about 50 mol C m^-2 year ^-1 ).

Water and Salt Budgets

Figure 1 summarizes the water and salt budgets for Laguna Madre. Like the systems for the arid Pacific and unlike the humid Pacific, Laguna Madre is a net evaporative system and shows penetration of water from the ocean. This seawater penetration delivers salt, so salt mixing is outward in order to maintain a steady-state salinity in the system. The estimated water exchange time (t) is determined as has been discussed in other sections and is about 0.09 year, that is, about 1 month.

Figure 1. Water and salt budgets for Laguna Madre, 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 summarizes the DIP and DIN budgets for this system.

P Balance

This system is a clear net source for DIP. Even though the rate of nonconservative DIP flux is relatively small (DDIP = +31 x 10 ⁶ mol year^-1 = 0.02 mol m ^-2 year^-1 ), this rate results in a strong concentration gradient between the system and the coastal ocean. The first consideration was that this DIP production is consistent with the production seen for the various arid systems of the arid Pacific coast (section 2.1). That analogy seems to be an unlikely explanation for this phenomenon in Laguna Madre, because the oligotrophic coastal ocean waters seem unlikely to be a major source of oceanic organic matter delivery to the lagoon. The next alternative considered was direct discharge of human wastes. This source would deliver both organic and inorganic P; as long as the organic P decomposed to inorganic material, the two would be indistinguishable in the budget. If we consider the human population living adjacent to the lagoon (80,000) and use an approximate per capita P discharge of 20 mol person^-1 year^-1 , then this P source would amount to only approximately 2 x 10 ⁶ mol DIP year^-1 . It therefore seems certain that there are other sources of either direct discharge of DIP into this system or organic P discharge and decomposition of that organic matter. Hydrocarbon pollutants and agricultural waste seem the most plausible sources. In either case, the decomposition of these materials would constitute DIP sources to sustain the apparently net heterotrophic conditions of this system.

N Balance

This system is also a source of DIN (DDIN = +61 x 10⁶ mol year^-1 = 0.03 mol m ^-2 year^-1 ). Both NO3 and NH4 are elevated well above oceanic values (NO3System = 2.8; NO3Ocean = 0.3; NH4System = 4.6; NH4Ocean = 1.5; all units in mmol m -3 ). The relatively high concentrations of NH4 also tend to suggest that the nutrient source is at least partially the result of organic decomposition.

Stoichiometric Calculations of Aspects of Net System Metabolism

Estimation of nitrogen fixation - denitrification (nfix-denit) is made from the difference between observed and expected DDIN, where the expected value is given by DDIP x N:P ratio of the decomposing organic matter. Although we do not know the source of this material, we assume that it has an N:P ratio approximating the Redfield Ratio (16:1). Therefore: (nfix-denit) = +61 x 10 ⁶ - 16 x (+31 x 10 ⁶ ) = -435 x 10⁶ mol year^-1 (-0.2 mol m ^-2 year^-1 averaged over the entire lagoon). This value is obviously uncertain, because we cannot readily predict the nature of the decomposing organic matter. Nevertheless, the calculated rate seems reasonable. Net ecosystem metabolism (NEM = [p-r]) can also be estimated from DDIP, again with the uncertainty as to the nature of the primary organic matter which is apparently decomposing. This quantity is estimated as the negative of DDIP multiplied by the C:P ratio of the reacting organic matter. For the sake of this calculation, we assume that the reacting organic matter has an approximately Redfield C:P ratio of 106:1. Thus:

(p-r) = -106 x (+31 x 10 ⁶ ) = 3,286 x 10 ⁶ mol year^-1 (-2 mol m ^-2 year^-1 over the lagoon area). That is the system appears to be net heterotrophic by approximately 2 mol m ^-2 year^-1 . This value is about 4% of the estimated annual primary production; thus the p/r ratio of this system is about 0.96.

Figure 2. DIP and DIN budgets for Laguna Madre, annual average. Fluxes in 10 ⁶ mol year ^-1 .

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Last Updated 14 Jan 2000 by DPS