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

Abstract

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 <it>in situ</it> 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 &lt; 0.05) reduced in the +UVB treatment. The abundance of non-filamentous bacteria (&lt;10 <IMG SRC="/math/mu.gif">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 &lt; 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 (&gt;1 &mgr;m long), which represented only a small fraction of the total abundance (&lt;4%), but up to 70% of the total bacterial biovolume. Ciliates, mainly <it>Urotricha pelagica</it> and <it>Urotricha furcata</it>, 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.