Original paper

Structure of pelagic microbial assemblages in European mountain lakes during ice-free season

Straškrábová, Vera; Bertoni, Roberto; Blažo, Martin; Callieri, Cristina; Forsström, Laura; Fott, Jan; Kernan, Martin; Macek, Miroslav; Stuchlík, Evzen; Tolotti, Monica

Advances in Limnology Volume 62 (2009), p. 19 - 53

93 references

published: Dec 23, 2009

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ArtNo. ESP143006200010, Price: 29.00 €

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Pelagic food webs, with particular emphasis on microbial loops, were investigated in European alpine lakes. During two ice-free seasons (1996-1997) investigations of biomass structure in pelagic food webs were performed at 7 lakes in low-alkalinity mountain regions (Lake Districts). In 2000 (in some cases, 2001), late-summer (autumn) samples from 304 lakes from 10 Lake Districts were analysed for pelagic bacterial biomass (BAC), chlorophyll a (CHL) and zooplankton (ZOO) abundance. In two districts (Northern Finland-NF and Tyrolian Alps-TY), phytoplankton (PHY) species structure was also determined, together with ciliate (CIL) species structure in NF. BAC abundances (0.02 – 2.7 x 106 ml-1) generally corresponded to values found in other oligotrophic lakes, and to winter values from lowland meso- to eutrophic lakes. Values from two regions, the Julian Alps (JA) and Western Greenland (SS), however, were exceptionally high (0.10 – 25x106 ml-1). Here, bacterial cells often were large, elongated and even filamentous. Mean cell volumes (per sample) reached values up to 0.91 μm3 and cell lengths up to 4.1 μm, thus surpassing the values known from lowland eutrophic lakes. Consequently, BAC biomasses frequently reached 40–50 μg C L-1, and in lakes with filamentous bacteria up to 100 – 150 μg C L-1. In Greenland lakes values of more than 200 μg C L-1 were observed. Autotrophic picoplankton (APP) were scarce (<500 ml-1) or absent in 6 of the 7 low-alkalinity lakes studied. APP abundances regularly surpassing 103 ml-1 (biomass 15–150 μg C L-1) were only observed in Lochnagar (Scotland), which is characterised by a high sodium and chloride concentration. APP biomass at this site surpassed that of BAC during late summer and autumn. Biomass of heterotrophic flagellates regularly reached 5–20% (exceptionally 50 %) of BAC biomass, whereas CIL biomass usually reached only 1–8 % of BAC biomasses, except for NF lakes (8 – 50 % of BAC biomass). The most abundant CIL species belonged to five groups: large mixotrophic oligotrichs, algivorous prostomatids, large hymenostomatids, gymnostomatids and minute omnivorous oligotrichs. Besides heterotrophic protists, mixotrophic (containing Chl) bacterivores were always present. PHY biomass varied considerably among lakes and within individual lakes, and sometimes reached values found in meso- to eutrophic lakes. The smallest variation (0.3–153 μg C L-1) was observed in lakes within a pH range of 5.3–5.9. In lakes with either lower or higher pH values (ranges 4.5–5.2 and 5.9–9.4), PHY biomass varied from 1.8 up to 1000 μg C L-1. Among PHY species, motile and potentially phagotrophic bacterivorous taxa dominated from the Cryptophyceae, Chrysophyceae and Dinophyceae families. Representatives of Chlorophyceae and Bacillariophyceae usually occurred in lower abundance. In Northern Finland Conjugatophyceae were frequent. Redundancy Analysis was applied to all lake districts except SS to test the response of BAC to chlorophyll a (CHL) and to the abundance of aggregated ZOO groups. 12.8 % of variation in BAC abundances is explained by Daphnidae (mostly), Cyclopidae and Diaptomidae. Partial Redundancy Analysis proved that, though the BAC response to ZOO plus CHL is statistically significant, the majority of the explained variation is accounted for by proximal variables attributed to the physico-chemical environment of the lakes, such as nitrate, chloride and littoral organic substrate. In the set of 7 intensely studied low-alkalinity lakes, the share of BAC in the total pelagic microbial biomass was significantly inversely correlated with trophy


food webbiodiversitybacteriaprotozoansphytoplanktonzooplanktonlakes above timber line