Organic carbon and microbial food web assemblages in an oligotrophic alpine lake
Picoplankton, both autotrophic (APP) and heterotrophic (HPP), ciliates and heterotrophic nanoflagellates (HNF) were counted and their biovolume measured monthly over a 3 year period in Lake Paione Superiore (LPS), a high mountain lake in the Italian alpine region. Analyses of organic carbon, particulate and dissolved, were performed at the same time. APP were negligible and picocyanobacteria almost absent. HPP showed seasonal variations, with low numbers in winter/spring and maxima of nearly 106 cell ml-1 in August/September, corresponding to 60μg C l-1. Free-living, non- pigmented flagellates showed a density range from 104 l-1 to 106 l-1 with a prevalence of cells <3 μm. Their carbon ranged between 0.1-9 μg C l-1. Ciliate numbers ranged from 0.02 to 11 103 l-1. For much of the year different species of Urotricha were found. Conversely, Strombidium appeared during the ice-free period and Halteria grandinella under the ice, indicating a strict dependence on temperature. Carbon in the microbial loop of LPS (near the bottom) was mainly confined to bacteria (73%), with 20% in HNF and only 7% in ciliates. Total organic carbon (TOC) concentration, measured after the removal of net plankton, ranged from 0.26 to 1.77 mgC l-1 with a prevalence of the dissolved form (87% av.). The average particulate organic carbon (POC) concentration was 0.24 mg C l-1. All the components of the microbial loop showed a decline under the ice-cover. Bacterial carbon concentration was three times lower under the ice than in the ice-free season (7.9-24.4 μgC l-1, respectively); protozoa carbon too declined under the ice-cover (3.1-5.8 μgC l-1 for HNF and 0.4-1.7μgC l-1 for ciliates in the ice-cover and ice-free periods, respectively). The drop in the microbial-loop carbon occurring in late summer may be related to the presence of a Daphnia population peak. At that moment, the structure of the microbial loop is transformed by a top-down control of Daphnia.
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