versión On-line ISSN 0717-7178
Investig. mar. v.30 n.1 supl.Symp Valparaíso ago. 2002
Structure of the Microphytoplanktonic
Community in the Coastal Zone of
Iquique (20°17'S-70°08'W) during the
1997-98 El Niño
Liliana Herrera Campos,
Edgardo Santander Pulgar
Depto. Ciencias del Mar, Universidad Arturo Prat.
Casilla 121, Iquique, Chile,
The structure of the microphytoplanktonic community present in Bajo Molle Bay (20°17'S-70°08'W), Iquique, and its relationship to the variability of the physical and chemical environment was analyzed during the period between December 1996 and December 1997.
The physical conditions observed in December 1996, January and February 1997, were coincident with those associated with the presence of "La Niña", which characterized 1996. The gradual increase in temperature toward April 1997, and the high values observed in May, July and December 1997 are consistent with the periods in which the pulses of high temperatures of the "El Niño" (EN) 97-98 were detected.
The nutrients nitrate, phosphate and silicate, responded in different ways to the environmental fluctuations observed from May 1997. Nitrate concentrations showed a higher variability, and the lowest concentrations during the whole period of study coincided with the first pulse (May-July) of EN 97-98, being similar to the values observed in Peru during EN 82-83. In the following months, nitrate concentrations increased and fluctuated within normal ranges.
Phosphate and silicate maintained relatively homogeneous concentrations, with minimum values coincident with the months in which microphytoplanktonic concentrations were highest.
The microphytoplanktonic community was dominated by diatoms, with species whose occurrence continued through almost the whole study period, revealing the presence of a typical upwelling ecosystem community instead of an EN period community, in which a gradual invasion of oceanic and warm water diatoms and dinoflagellates occurs, and the cellular abundance and biomass decrease.
A relatively fast response of the microphytoplankton community to the prevailing conditions was noted, with an alternation of groups and species according to variations in the physical structure. From the variability observed in the microphytoplanktonic community it could be concluded that changes in community structure followed environmental gradients which induced species succession.
The fluctuations of microphytoplanktonic cellular abundance showed a similar pattern to that described for a specific component, with alternation between highest and lowest diatom and dinoflagellate abundances in response to cold and warm periods respectively. Chlorophyll-a concentrations varied together with the fluctuations of cellular abundance of diatoms and dinoflagellates. During most of the study period, diatoms accounted for the observed chlorophyll-a values, except in April, when dinoflagellates showed the highest cellular concentration.
In July, when diatom and dinoflagellate cellular abundances were lowest, being absent at many sampling stations, the chlorophyll-a concentration, which reached values as low as 0,5 µg/L, could be explained by the presence of pico- and nanophytoplankton, groups which acquire importance in the upwelling regions during the development of positive anomalies caused by the presence of EN.
A rank correlation analysis revealed a better correlation of the dinoflagellates than diatoms with the physical and chemical variables, yielding a positive association with temperature and negative with the rest of the parameters. The results of an ANOVA test applied to the physical and chemical variables showed the existence of differences among all months of the study period, which were more significant for salinity and density.
Considering the characteristics of microphytoplanktonic structure during the study period, and that the observed changes were associated with the prevailing environmental conditions, it is possible to conclude that the microphytoplankton responded to the influence of EN 97-98, but not with the magnitudes observed during the development of other events.
This could be due to the fact that, in the coastal zone (considering the spatial scale used), the local conditions were dominated by upwelling processes, and these processes overcame the effects that EN can have on them.