Journal of Plankton Research, Vol 21, 859-876, Copyright © 1999 by Oxford University Press
R Sommaruga, B Sattler, A Oberleiter, A Wille, S Wograth-Sommaruga, R Psenner, M Felip, L Camarero, S Pina, R Girones and J Catalan
We studied the impact of ambient levels of solar ultraviolet B (UVB)
radiation on the planktonic microbial food web (viruses, heterotrophic
bacteria, heterotrophic nanoflagellates and ciliates) of a high-mountain
lake (2417 m above sea level) under in situ conditions
for 16 days. Enclosures of 1 m3 receiving either the
full sunlight spectrum or sunlight without UVB radiation were suspended at
the lake surface. We found that the abundance of heterotrophic
nanoflagellates was always lower in the +UVB treatment than in the -UVB
one. In addition, bacterial consumption, measured by the disappearance of
fluorescently labelled bacteria, was significantly (P < 0.05)
reduced in the +UVB treatment. The abundance of non-filamentous bacteria
(<10
ORIGINAL ARTICLES
An in situ enclosure experiment to test the solar UVB impact on plankton in a high-altitude mountain lake. II. Effects on the microbial food web
University of Innsbruck, Institute of Zoology and Limnology, Technikerstrasse 25, A-6020 Innsbruck, Austria; Department of Ecology and Department of Microbiology, University of Barcelona, Avgda. Diagonal 645, E-08028 Barcelona, Spain
m long) was also lower in the
+UVB treatment, suggesting a direct effect of UVB on their growth. This was
supported by the significantly (P < 0.05) lower cell-specific
activity ([3H])thymidine incorporation) found on the
fifth day of the experiment. In contrast, UVB radiation had no effect on
filamentous bacteria (>1 
m long), which represented only a
small fraction of the total abundance (<4%), but up to 70% of the
total bacterial biovolume. Ciliates, mainly Urotricha
pelagica and Urotricha furcata, were less
impacted by UVB radiation, although the net growth rate during the first
week of the experiment was lower in the +UVB treatment than in the -UVB one
(0.22 and 0.39 day-1, respectively). The abundance
of virus-like particles during the first week of the experiment was higher
in the -UVB treatment. After reaching the maximum value for the interaction
viruses x bacteria, viral abundance decreased dramatically (by 85%) in both
treatments with a decay rate of 0.017 h-1. This
study illustrates the complexity in assessing the impact of UVB radiation
when more than one trophic level is considered and indicates the existence
of different sensitivity to UVB radiation among components of the microbial
food web.
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