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Investigaciones marinas

versión On-line ISSN 0717-7178

Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002 

Analyzing the Effect of the 1992
"El Niño", and a Power Plant,
on Benthic Polychaetes from the
Mexican Tropical Pacific

N.A. Hernández-Guevara,
J. A. Rodríguez-Valencia

Wadden Sea Station Sylt.,
Alfred-Wegener-Institute Foundation,
Hafenstraße 43, 25992 List, Germany,

The effect of "El Niño" events on biotic variables and physicochemical properties of the water has frequently been analyzed at wide spatial scales and in the open sea, but not in small coastal areas. We describe the patterns of water temperature, concentration of dissolved oxygen, and salinity in the water column of Bahia Petacalco; during and after the 1992 "El Niño" (March-September 1992, and October 1992-April 1994, respectively), and relate them to the abundance and species number of benthic polychaetes from this area.

Bahia Petacalco is a small bay located in the Mexican Tropical Pacific. It has some interesting features: i) The major trench of the submarine canyon Petacalco, which reaches the Central-America Trench 110 km off the coast at 4600 m depth, begins there; ii) It is located near to the large delta of the Balsas River; iii) Its productivity is high, due to the combined effect of nutrients discharged at the delta and the occurrence of seasonal deep water intrusions through the submarine canyon. Additionally, since 1993, a 2100 MW power plant discharges warm freshwater from its cooling system into the bay.

During the 1992 "El Niño", the mean water temperature of the water column reached 29.2°C, after that it decreased significantly to 27.4°C. The mean water temperature during this "El Niño" was 2°C higher than that of all non-"El Niño" years occurring between 1952 and 1999 in this area. The mean concentration of dissolved oxygen in the water column did not changed significantly during and after the "El Niño" (6.5 and 6.4 mg•l-1, respectively). Even those concentrations measured inside the bay during the 1992 "El Niño" were higher than the mean concentration measured at the bordering oceanic zone between 1968 and 1983 (5.9 mg•l-1). Salinity (PSU) increased significantly after the warm event (29.5 during the 1992 "El Niño" and 31.7 after it). Vertical stratification of the water column was absent during the "El Niño", but was afterwards established in all measured parameters.

The mean density of polychaetes during the 1992 "El Niño" was 959 organisms*m-2, increasing significantly to 3400 organisms*m-2 afterwards. The mean number of species detected during the "El Niño" was 29.5, increasing significantly to 58.3 after the "El Niño". Highly significant relationships were found between polychaete densities and temperature, and species number and temperature, but not with dissolved oxygen concentration and salinity. Increased water temperatures during the "El Niño" caused the low densities and low species numbers detected during the sampling period.

These data illustrate the negative effect of increased water temperature on polychaetes during "El Niño" conditions, but also their recovering capacity, even shortly after an anthropogenic impact (operation of the power plant) began.

According to the literature, the typical effect of a power plant such as that operating at Bahia Petacalco should be very negative, because of the discharge of warm freshwater into the sea. Nevertheless, this effect was better represented by "El Niño" conditions than by the operation of the power plant itself (namely, benthos affected by the contact with warm, low salinity water, due to the non-stratified water column). Stratification of the water column was established during the operative phase of the power plant, and only the surface of the bay became warm and brackish. This allowed the recovery of the benthos, because the superficial warm and brackish layer was never mixed with the rest of the water column, and never reached the bottom of the bay.

The productivity of the bay was surely enhanced as the "El Niño" conditions gradually receded, but the warm-brackish layer established at the surface of the bay by the power plant could also have promoted the flocculation of suspended organic matter and sinking of plankton by thermal and osmotic shock. This may have increased the amount of organic matter in the sediment, and may constitute the main impact of the power plant. Temporal analyses of numerical dominance among trophic guilds bring evidence of this: Significant temporal changes were detected in those of sedentary polychaetes (from Filter-Feeders to Sub-Surface Deposit Feeders) and errant polychaetes (from Carnivores to Sub-Surface Deposit Feeders).

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