Resumen

RAMIREZ-R, JOHN J; GONZALEZ-B, ROIS  y  GUTIERREZ-V, FERNANDO LEÓN. Establishment of limiting nutrient based on the changes of phytoplankton assemblage structure in a tropical Colombian reservoir. Caldasia [online]. 2012, vol.34, n.2, pp.421-441. ISSN 0366-5232.

To establish the limiting nutrient for phytoplankton assemblage, in an experiment of artificial eutrophication three concentrations of Nitrogen and four of Phosphorus were added to samples of surface water from the reservoir La Fe. The enriched water was placed in clear two-liter plastic bags. The collections were made biweekly (n = 12) during six months. Values were calculated from the series of Hill's numbers, richness and evenness. Then, range-abundance curves were built and their slope was estimated using a linear transformation. To test the hypothesis concerning the variability between treatments, we used a block design with sampling times as a blocking variable. The taxa found corresponded to 2 cyanobacteria, 21 chlorophyta, 13 diatoms, 1 dinophyta and 2 criptophyta. Fertilization caused that groups such as Cryptophyta, Dinophyta, Zygnematales and Desmidiales had very little density and that Euglenophyta was never present. Changes in phytoplankton assemblage evenness compared with eutrophication were highly significant (α = 0.0309). The richness did not show significant changes with enrichment (α = 0.4940). The response rate expressed general agreement with that shown by the evenness; the significance of changes in slopes among treatments was significant (α = 0.0315). The overall response of the Chlorococcales to the enrichment was to increase its density as eutrophication prospered, and then the decline gradually until the last treatment; the response was significantly high (α = 0.0000). Although Cyanobacteria of the order Chroococales behaved similarly to Chlorococales, its behavior was not significant between treatments (α = 0.4347). The Bacillariophyta presented a highly significant response in front of enrichment (α = 0.0002), but initially they showed higher density in the water treatment of the reservoir without any enrichment, and then decreased and increased occasionally in N₁P₁ and N₂P₁ treatments. In front of increasing of N and P, the total density of assemblage showed a highly significant response (α = 0.0064). The most abundant taxa and the abundant ones showed the best response compared with that submitted by the rare taxa. As increased the number of taxa very abundant and increased eutrophication, the slope tended to increase too. In the case of abundant and rare taxa, the behavior was reversed, i.e., their number decreased as the slope decreased and where eutrophication increased. We conclude: 1) that the artificial enrichment produced a differential increase of different phytoplankton assemblages, the effect being more evident on the most abundant taxa; 2) that during the period the limiting nutrient was always nitrogen; and 3) that if in La Fe Reservoir began a process of eutrophication on the date which started this research, the consequences would be: a) the increase in algal biomass and density, b) the disappearance of Euglenophyta, and the ostensible decline of Cryptophyta, Dinophyta, Desmidiales and Zygnematales; c) an alternating dominance between Chlorophyta, Cyanobacteria and Bacillariophyta; d) sporadic occurrence of filamentous cyanobacteria heterocitated); e) decrease in diatoms, especially at high nitrogen concentrations, but increases in same periods in the mixed layer; and f) increasing the number of most abundant taxa and decreased of abundant and rare ones with the increasing of eutrophication.

Palabras clave : Tropical reservoir; restrictive nutrient; temporary variability; phytoplankton.

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