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Investigaciones marinas

versión On-line ISSN 0717-7178

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

Structure and Population Changes of
the Marine Benthos during La Niña
1999-2001 in the Coastal Zone near
Tumbes, Perú

J. K. Robles R., K. A. Coronado L.,
A. N. Robles P., D. Cruz C.

CONCYTEC _ UNT, FCS - Dpto. B. and BQ., DRP -
DPAPP. P.O. BOX 157, Tumbes-Perú,

Within the Project OEA-CONCYTEC and agreement between El Consejo Nacional de Ciencia y Tecnología (CONCYTEC) with the National University of Tumbes (UNT) investigations into the structure and population changes caused by the impact of the "LN"event in the marine benthos were coordinated and planned for the coastal zones of three predetermined stations of the littoral of Tumbes, opposite the district of "Punta La Cruz".

The stations are considered hinges, taking the presence of a platform plunged of petroleum founds at 25 metres depth as a point of reference, with the other stations in 13 and 40 metres depth respectively. Geographical coordinates were determined with GPS.

Sampling was carried out using a van Veen grab, with seven replies on every fixed station and often monthly, during the period 1999-2001.

Bulk and surface temperatures were measured using an environmental thermometer, calibrated from 0º C to + 32 ºC. Similarly, for measurements of the temperature of the submarine bed (bottom) a Niskin bottle with a built-in reversible thermometer was used (calibrated from 2ºC to + 35 ºC). For the determination of dissolved oxygen, the (modified) Winkler method was usedThe volumétric method of Mohr-Knudsen was used to determine salinity.

All samples that were obtained at the three fixed stations were analysed, and average values of number of individuals per sample calculated for each taxonomic group.

The structure and population changes caused by natural phenomena in the marine benthos were investigated. Also, the impact of these events on the marine macrozoobenthos and it consequences for the population of demersal fish was analyzed. The temperatures of the submarine bed affected the structure, abundance and biodiversity of the macrozoobenthos at all three fixed stations. Temperatures appeared to decrease from 20.0 ºC in July, 1999 to 15.0 ºC in October 2000, then increased to 22.0 ºC in the summer of 2001, and finally decreased to 18.0 ºC in April of the same year and stabilized at that temperature until the end of December, 2001.

The oxygen levels followed the same trend as temperature; beginning to decrease during September, 1998 from 5.5ml/l to 2.2ml/l in May, 2001. From this date, the values started increasing, ending up at 5.7ml/l in December of the same year. Salinity was lowest during the period of the "LN" event at the shallow station of 13m depth, at 33.94‰, probably influenced by the rains across the gullies and the river Tumbes. At the second station of 25m depth it averaged 34.48‰ and for the third station of 40 meters depth the salinity was 34.65‰. This wide zone, influenced by falling salinity, affected the populations of demersal fish, driving them from zones in which they would traditionally be expected and changing migration patterns in zones and traditional areas of development of benthic fish populations.

The taxonómic groups more representative of the benthos which appeared at all three fixed stations during the "LN" event were very abundant only for two taxonomic groups, the Polychaeta and the Ophiuroida. The abundances of these groups reached maximum values during the winter and spring seasons of 2001, with 45 and 40 ind/sample respectively. This sensibilization of these two groups of benthic species in the year 2001 coincides with a drastic decrease in temperature at the submarine bed (bottom) at the end of 2001. Other taxonómic groups mentioned had minimal presence of 10 ind/sample. This biological phenomenon of low availability of the marine macrozoobenthos drastically affected the availability of the artesanal fishing in the littoral of Tumbes.

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