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Investigaciones marinas

versión On-line ISSN 0717-7178

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

http://dx.doi.org/10.4067/S0717-71782002030100054 

Impacts of El Niño/La Niña on the
Population Dynamics of the Tropical
Bivalve Donax dentifer from Bahía
Málaga, Colombian Pacific

José M. Riascos V.1, H.-Jörg Urban2

1INCIVA - Museo de Ciencias Naturales "Federico C.
Lehmann" Diagonal 28 # 30-11, Cali - Colombia,
E-mail: pesarnoso@hotmail.com
2Alfred Wegener Institute for Polar and Marine
Research, Section for Comparative Ecosystem
Research, Postfach 12 01 61, 27515 Bremerhaven,
Germany, E-mail: jurban@awi-bremerhaven.de

Objectives

The effects of the climate phenomenon El Niño/Southern Oscillation (ENSO) on the population dynamics of a Donax dentifer population (Bivalvia, Donacidae) from the Colombian Pacific coast are to be studied. Based on data from August 1997 to July 1998 and from December 1999 to November 2001, we studied growth, the reproductive cycle and somatic production of this species, which is dominant in sandy beaches at many sites of the Colombian Pacific.

Results & Discussion

Comparing the three periods El Niño (EN, 1997/98), La Niña (LN, 1999/2000) and Post-El Niño (Post EN, 2000/2001), growth during EN was lowest, and higher and similar during LN and Post EN (Fig. 1) During EN we observed lower temperature and higher salinity (Fig. 2) giving a possible explanation for the recorded growth patterns. Interesting to note is the fact that temperatures were lower during EN than during LN. This is in contrast to other regions as for example the upwelling system off Peru, and most likely due to different regional rainfall and river discharge patterns. Several studies state that hydrological variability is probably the most important forcing function for the tropical coastal marine ecosystems of the South American Pacific, since the temperature variability is small.


Fig. 1 Donax dentifer. Growth curves of the von Bertalanffy growth function from tagging-recapture experiments (EN 1997/1998, LN 1999/2000, Post-EN 2000/2001) from Bahía Málaga-Colombia. Growth parameter are given.

Gametogenic stages (= Gonad stages, Fig. 2a), Gonado-Somatic Index (GSI) and Condition Index (CI = shell free dry weight cycle, Fig. 2c) were plotted in overlay graphs in order to analyze the reproductive cycle. During EN 1997/1998 principally developing/ripe stages are present, except for October 1997 and January 1998 when spent/indifferent stages dominate. Low GSI and high CI values during these months seems to be related to abnormal spawning events. Thus, under stress caused by environmental conditions, energy was allocated to metabolic activities, leading to gamete reabsorption and reflecting high CI values during weak spawning events.


Fig. 2 (A) Reproductive cycle based on gametogenic stages determined from fresh gonad material of Donax dentifer from the Colombian Pacific. (B) Temperature and salinity cycle for the study period in Bahía Málaga, Colombian Pacific. (C) Overlay of Condition Index (shell free dry weight) for a 25 g wet weight standard individual and Gonado-Somatic Index for the study period.

During LN 1999/2000, developing/ripe stages fluctuated and low values correlate well with low GSI and CI values, indicating prolonged strong spawning periods in November/December and a short spawning period in May.

Table 1 Results of the production estimation of Donax dentifer for EN, LN and
Post-EN periods. (AFDW= Ash-free dry weight).

Parameter

El Niño

La Niña

Pos-EN

Mean annual abundance [n m-2 yr-1]

64.0

126.3

62.73

Mean annual biomass (B) [g AFDW m-2 yr-1]

1.111

0.230

0.621

Mean annual somatic production (P )

0
0
0

[g AFDW m-2 yr-1]

1.137

0.613

1.150

Somatic productivity (P/B)

1.023

2.663

1.851

During the first semester of 2001 (Post EN) a high percentage of developing/ripe stages was present, which correlates with high GSI and CI values. Nevertheless, by the end of the year these variables decreased dramatically. This pattern could be explained by unusually low salinity values and high temperatures.

Although during LN lower somatic production was observed, somatic productivity was highest compared to all three periods. This is related to a strong increase of small individuals with low mean annual biomass. EN also affected somatic productivity negatively, as shown by the low P/B value. This result is in agreement with the growth results.

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