Environmental Quality Analysis using Hydrobiological Indicators and an Updated Hydrodynamic Model of the Terminos Lagoon

The presented study had the main objective of identifying temporal and spatial variability of the environmental quality of the Terminos Lagoon. Updated information on the Terminos Lagoon system was required to guide management decisions, due to its important ecological role, socio-economical implications and importance of the health of this ecosystem as an indicator of the general condition of the area. Hydrobiological indicators were used and a hydrodynamic model of the system was developed.

The working strategy consisted of a series of field sampling campaigns throughout the year 2001 (January to December), to determine assess aspects of the system:

I) Description of water quality and sediments (Chapter I).

II) Hydrological characterization (ChapterII).

III) Characterization of the connecting mouths (Chapter III).

IV) Hydrodynamic model (Chapter IV).

CONCLUSIONS:

The variables that explain the general hydrological behavior of the lagoon are: salinity, inorganic forms of Nitrogen and Chlorophyll-a.
The variability of inorganic Nitrogen shows the importance of keeping the connections of the mouths and the stability of the central basin.
The spatial temporal variations of Chlorophyll-a and suspended sediments reflect the trend towards eutrophication of the water column of the Terminos Lagoon, mainly in the areas of "Bocas del Carmen", "Puerto Real" and the Palizada river, thus reinforcing the initiative of improving the hydrodynamic circulation of the lagoon.
The trophic status of the Terminos Lagoon varies spatially, with totally eutrophic locations (Boca del Carmen and the Panlau system) and regions basically oligotrophic (Puerto Real mouth).
The most vulnerable zones are those that present a eutrophic state (Boca del Carmen y Sistema Panlau) and a meso-eutrophific one (inner coast and the West continental margin).
The sediments in the Terminos Lagoon are spatially variable: terrigenous from Carmen Island Westward and carbonated towards the North East.
The lagoon is hydrologically zoned into six regions:
I-ZAH-Puerto Real mouth; II-ZAH-Inner coast; III-ZAH-Panlau fluvial-lagoon system; IVZ-AH-Central basin; V-ZAH-West continental margin and VI-ZAH-Carmen mouth.
The mouths of Puerto Real and Carmen are fundamental for water quality and circulation in the Terminos Lagoon, and special care must taken to avoid the increase of water residency and nutrient concentration, both of which entering through both mouths connection to the Gulf of Mexico.

The distortion induced by the tide in the Terminos Lagoon is important not only to bodies of water that are far from the mouths (e.g. Pom Lagoon), but also in the Terminos Lagoon. This is due mainly to the facts that: a) In this sector of the Gulf of Mexico the oceanic tide propagates from East to West, b) The surface of the lagoon is big, and c) The lagoon is shallow.
The distortion of the tide is influenced and turns more complex by the existence of the significant discharge of the main rivers of the system (Palizada, Chumpan, Candelaria and Mamantel). Such distortion is in part responsible for generating the residual discharge in the lagoon, which makes part of the water volume that comes from the Puerto Real mouth and is distributed in such a way that it goes out through the two adjacent mouths.
The residence time seems high in the remote lagoons of the system, due to the strong friction in the upper areas of the system. According to the results, the time it takes a given volume to go from the Pom Lagoon to the Carmen mouth is around the order of ten days. This time is also required for the water to travel in the opposite direction, thus the effect of a storm (which increases the height of the sea level for approximately 12 hours), can not be appreciated in these areas.
According to the bathymetric information available at the river mouths, the speed through minimum transversal sections is very low (of the order of 0.1 m.s-1 at Balchacan and Pargos and of 0.2 m.s-1 at Chica and San Francisco), indicating that the rate of sedimentation of material eroded from the basin and transported by the rivers is high.