Coral Reefs of the Southern Gulf of Mexico

Chapter 12. Environmental Impacts

6 Environmental Impacts Guillermo Horta-Puga The coral reefs in the southern Gulf of Mexico are subject to many natural and anthropogenic environmental stressors (Tunnell 1985, 1992; Chávez 1989; Botello et al. 1992; Chávez and Tunnell 1993; Lang et al. 1998). Natural threats include hurricanes, winter cold fronts, freshwater inflow carrying suspended sediments, bleaching, red tides, and massive die-offs. Coral reefs, especially those of the Campeche Bank, are in the path of hurricanes. The meteorological phenomenon known as norte (winter front) induces strong winds over the whole southern Gulf of Mexico and can be as destructive as hurricanes. The Gulf of Mexico is a semi-enclosed oceanic basin that receives the outflow of many river systems carrying a high load of suspended solids that settle down on the continental shelf. In the flood plumes of riverine systems, seawater turbidity increases and salinity decreases periodically with increased river flow volume. Human activities, sometimes in concert with natural impacts, are an increasing threat to the natural environment. In general, the consequences of anthropogenic disturbances are decreased biodiversity, changes in community structure, increased concentrations and varieties of chemical pollutants, and landscape (reefscape) modification. The degree of environmental degradation is usually associated with distance from disturbance sources. Thus, the nearer the source, the greater the impact. Fortunately, the reefs of the Campeche Bank lie more than 50 km away from shore, and the primary threats to them are impacts from oil and gas exploration and production and overfishing. The Tuxpan Reef System (TRS), although located near the coast and the city of Tuxpan, is not heavily impacted, and anthropogenic threats are restricted to oil and gas activities on Isla de Lobos, the occasional presence of tourists, and overfishing. The reverse is true at the Veracruz Reef System (VRS) because it is offshore from the largest city in the southern Gulf of Mexico and the largest port in all of Mexico. Additionally, because it is under the influence of the flood plumes of at least two large river systems that drain municipal and industrial sewage waters from various inland cities, as well as from agricultural lands and paper pulp mills, the sources of environmental impact are numerous. Consequently, most impacts are in the vicinity of Veracruz city. Sediments and Low Salinity The VRS evolved on a terrigenous shallow continental shelf (Morelock and Koenig 1967). The main terrigenous sediment source was eroded materials transported by river flow from the nearby mainland. This condition does not apply on the Campeche Bank reefs (CBR), because they developed on a carbonate platform where river drainage is almost absent. There are many natural river systems that flow into the southwestern Gulf, including Grijalva-Usumacinta, Coatzacoalcos, Papaloapan-Blanco, Jamapa, Antigua, Nautla, Tuxpan and Candelaria, Cazones, and Pánuco (see Fig. 4.1). The joint 12 Natural Disturbances A4339.indb 126 A4339.indb 126 7/6/07 11:19:31 AM 7/6/07 11:19:31 AM Environmental Impacts 127 hydrological basin of these rivers extends over ~263,000 km2 (CNA 2000), and the mean annual precipitation is 1,500 mm (INEGI 1995). The mean annual discharge of these rivers to the ocean is ~5,200 m3 /s (CNA 2000). The waters of these rivers have a high sediment load derived from eroded adjacent lands. In the Papaloapan River, which drains into the Gulf approximately 100 km south of the VRS, the mean annual load of suspended solids is 29 mg L–1 , with 695 mg L–1 of dissolved solids (CNA 2000). These values probably are similar for the other rivers. With this high sediment load, turbidity increases, and consequently , visibility can be lowered from 10–12 m to less than 1 m (PEMEX 1987; Tunnell 1988). During the rainy season (June–September), the amount of eroded particles in river waters increases (Plate 56), thus increasing turbidity of nearshore seawater and in the VRS. During this season of increased riverine discharge, live corals are covered by a thin layer of fine sediments (Plate 57), surface salinity is lowered , and at times, masses of freshwater plants such as water hyacinths, as well as terrestrial wood debris, can be found on the reefs (Plate 58). Tunnell (1988) recorded a surface salinity of 14 psu near in Antón Lizardo, 24 psu at Enmedio Reef, and 28 psu at Chopas Reef. Salinities of 28–32 psu are common in summer at the VRS (Secretaría de Marina 1978). A massive die-off of the coral Acropora cervicornis on Topatillo reef...