Artificial destratification of a small tropical reservoir: effects upon the phytoplankton |
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Authors: | P R Hawkins D J Griffiths |
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Institution: | (1) Department of Botany, James Cook University of North Queensland, 4811 Townsville, Queensland, Australia |
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Abstract: | Seasonal changes in the phytoplankton community of a small tropical reservoir were monitored over a four year period comprising
of an initial two seasonal cycles during which the water column stratified strongly for extended periods each year, and two
further seasonal cycles after installation of a mechanical aeration system to induce artificial destratification.
In the unmanaged reservoir, the concentration of chlorophyll a at 0.5 m reached maximum values (on one occasion > 90 mg m−3) when the water column was stratified and the epilimnion was very shallow (ca 2 m depth). The hypolimnion at this time was anoxic (less than 2% oxygen saturation) and had a high concentration of bacteriochlorophyll
(100–200 mg m−3).
The phytoplankton community of the unmanaged reservoir was generally dominated by cyanobacteria (Cylindrospermopsis raciborskii, Anabaena tenericaulis) during the warmer months of the year (November–March) (but replaced by chlorophyta, dinophyceae and euglenophyceae after
periods of intense rain) and by bacillariophyceae (Synedra ulna var. chaseana, S. tenera) during the cooler, dry months.
In the artificially destratified reservoir (8 h aeration day−1), the phytoplankton community was largely dominated by diatoms except after depletion of the silica content of the water
column which caused diatoms to be replaced by cyanobacteria (dominated by A. tenericaulis) and a range of chlorophytes.
The changing pattern of stratification and circulation of the water column in the unmanaged reservoir caused repeated disruption
of the established phytoplankton assemblage with peaks of high biomass associated with transient cyanobacterial blooms. Continuous
aeration and the consequent increase in the ratio mixed: euphotic depth provided conditions suitable for dominance of the
phytoplankton by diatoms, as long as silica was available, and resulted in average chlorophyll levels higher than in the unmanaged
reservoir (120 ± 10 v. 64 ± 9 mg m−2).
Hierarchical fusion analysis based on the biomass of species differentiated the phytoplankton samples into cluster groups
that could be related primarily to stratification or mixing of the water column. |
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Keywords: | phytoplankton population dynamics artificial destratification tropical reservoir |
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