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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/S0717-71782002030100061 

Indirect El Niño Effects on
Reproductive Strategies of the
Carribbean Bivalves Pteria colymbus,
Pinctada imbricata
and Pinna carnea*

H.-Jörg Urban

Alfred 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

Possible effects of oceanographic changes caused by El Niño on reproduction and recruitment of three tropical bivalve species from the Colombian Caribbean (11° 20' N, 74° 10' W and 12° 10' N, 72° 20' W) are to be studied. Thus, cyclic reproductive patterns are investigated and reproductive strategies are discussed.

Results & Discussion

Time series data (spat abundance, temperature, salinity and organic matter were smoothed with moving averages. Series were then normalized according to "(x-mean)/standard deviation" and the positive values surpassing the mean, i.e. the positive peaks, were plotted.

El Niño effects on oceanographic factors show less temperature and salinity and more POM peaks from 1997 on (= beginning of El Niño, Fig. 1), indicating a strong salinity and a slight temperature decrease as well as a strong POM increase. In order to correct the El Niño effects which mask the salinity and POM peaks, these two factors were normalized for two periods (pre-El Niño: March 1994 to December 1996, El Niño: January 1997 to August 1998).


Fig. 1 El Niño effects on temperature, salinity and particulate organic matter. Decreasing or increasing tendencies are indicated by arrows

During the entire study period (54 months), Pteria colymbus spat abundances exhibited only four annual short peaks between December and March of each year, whereas Pinctada imbricata and Pinna carnea spat had seven peaks in the same time (Fig. 2).


Fig. 2 Spat abundance of Pteria colymbus, Pinctada imbricata and Pinna carnea compared with temperature, salinity and particulate organic matter. (Periods of spat peaks are indicated as gray bars).

The effects of El Niño on environmental factors and spat variability are summarized in Fig. 3. Clearly El Niño had an effect on salinity and POM, as salinity on average decreased from 36.25 to 34.75 ‰ and POM increased from 0.25 to 0.6 mg l-1. However, no conclusive results were obtained indicating that these oceanographic changes had any significant influence on spat abundances, though Pteria colymbus typically seems to have a lower spat abundance. Given the fact that this study was carried out under tropical conditions, one would assume that reproduction depends on salinity and food availability rather than on temperature because compared to temperate and polar regions, in the tropics annual temperatures are less variable. However, this assumption could not be confirmed. At high latitudes seasonality is principally governed by the large annual temperature variability, while at low latitudes (i.e. the tropics) temperatures are much more constant. A clear seasonality of environmental factors other than temperature, can be observed in tropical regions. In the study area a combination of precipitation and wind patterns seem to be the principal driving force. Annually, the rainy season (normally between June and November) is followed by the dry season (normally between December to May). Precipitation decreases salinity and the run off of nutrients increases POM.


Fig. 3 Effects of El Niño on salinity, particulate organic matter andd spat abundance of Pteria colymbus, Pinctada imbricata and Pinna carnea. 6 months means of original data.

El Niño increases precipitation in the semi-arid region of the Colombian Caribbean close to Venezuela. Thus, decreasing salinity, increasing POM and negligible effect on temperature during El Niño can be postulated, which is contrasting to El Niño effects in the South American Pacific where temperature increases, and (in the pelagic regions), nutrients decrease. In the Caribbean, salinity variability owing to El Niño most likely has little influence on the ecosystem. However, taking into account that POM increased almost three fold (Fig. 3), it could be assumed that suspension feeding secondary producers such as bivalves benefited from the increased POM pool, gaining more energy for production. But except for Pinctada imbricata, no differences in spat abundance that could be attributed to El Niño-induced changes of environmental factors were observed in this study. Moreover, Pinctada imbricata rather showed a reduction of spat abundance during El Niño. This finding might be explained by noting that filtration rates of filter feeding bivalves are positively related to particle concentration only up to a certain optimum concentration. Should the POM concentration surpass this optimum no extra surplus energy would be assimilated.

Regarding the reproductive strategies, all three species are epifaunal filter feeders distributed in the same study area. The principal differences in their niches seem to be substrate preference (Pteria colymbus: octocorals, Pinctada imbricata: sea grass community, Pinna carnea: buried semi-infaunal). All display continuous reproductive cycles, typical for tropical regions with spent gonads during at least 7 months of the year, and spat throughout most of the year. Apart from common characteristics, this study revealed the following differences: The two closely related pearl oysters (Pteria colymbus and Pinctada imbricata) show several similarities such as high correlations between spat cycles. In comparison, spat cycles of Pinna carnea are not correlated with either of the oyster spat cycles. On the other hand, Pteria colymbus had only half of the spat peaks, and thus less mean spat abundance clearly displaying differences from Pinctada imbricata. Thus, on a scale from discontinuous reproduction of polar species to continuous reproduction of tropical species as the extremes, we would find Pteria colymbus in a more intermediate position inclined towards the "tropical group".


* This contribution is based on the paper "Reproductive Strategies in Tropical Bivalves (Pteria colymbus, Pinctada imbricata and Pinna carnea): Temporal Coupling of Gonad Production and Spat Abundance Related to Environmental Variability" published in "Journal of Shellfish Research Vol. 20, No. 3".

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