Recurrent seasonal variations in abundance and composition of filamentous SOL cluster bacteria (Saprospiraceae, Bacteroidetes) in oligomesotrophic Lake Mondsee (Austria)

Abstract

The spatial and temporal variation of SOL cluster bacteria was assessed in oligomesotrophic Lake Mondsee and adjacent lakes by fluorescence in situ hybridization over two annual cycles. The filamentous SOL bacteria were present in Lake Mondsee throughout the study period, and the seasonal dynamics of the SOL community were remarkably similar with respect to both abundance and composition in the two consecutive years. Only two of the three SOL subclusters were detected in Lake Mondsee and four connected lakes. These two populations significantly differed in size distribution and demonstrated pronounced but recurrent differences in seasonality and length of period of appearance in Lake Mondsee. Extensive sampling of the lakes in September 2003 revealed low horizontal variation in the composition of the SOL community within Lake Mondsee but marked variations with depth. Between connected habitats pronounced differences in the composition and abundance of the SOL community were detected. The interaction of SOL bacteria with bacterivorous protists, mesozooplankton, and phytoplankton was investigated in order to reveal variables controlling the structure and dynamics of SOL communities. No strong indication for a bottom-up influence of phytoplankton was found, while the estimated community grazing rates of mesozooplankton on SOL bacteria indicated a top-down control of SOL abundance during mesozooplankton peaks in spring and early autumn. Furthermore, species-specific differences in grazing of mesozooplankton on SOL bacteria were observed. In general, the overall composition of SOL communities was controlled by abiotic factors (water chemistry), while their dynamics seemed to be controlled by abiotic and biotic interactions.

FIG. 1.

Biweekly sampling of Lake Mondsee in the years 2002 and 2003. All samples were taken from a depth of 1 m at the main sampling station. (A) Water temperature. (B) Total bacterial numbers and SOL bacteria numbers. The arrow marks the date of extensive sampling in September 2003 (see data presented in Fig. S1 in the supplemental material and Fig. 4). (C) Composition of SOL cluster community.

FIG. 2.

(A) Estimation of SOL bacteria mortality (as a percentage of the population per day) by mesozooplankton community grazing and SOL bacteria abundances in Lake Mondsee in 2002. (B) Estimated contribution of different metazooplankton groups to SOL mortality. Light gray bars (top) represent the mortality caused by cladocerons with coarse filter meshes (Daphnia hyalina and Eubosmina coregoni), the dark gray bars (bottom) represent the contribution of cladocerans with fine filter meshes (Diaphanosoma brachyurum and Daphnia cuculata), and the black bars depict the contribution of the calanoid copepod Eudiaptomus gracilis. (C) Length distribution of SOL filaments. Symbols: •, median length in samples exclusively containing LD2 filaments; ○, median length in samples containing LD2 and HAL filaments. The upper and lower lines depict the 75 and 25% percentiles, respectively, of the SOL length distributions.

FIG. 3.

Box plots depicting length distributions of LD2 and HAL populations in Lake Mondsee. Pooled data from 2002 and 2003 are presented. The 10th, 25th, 75th, and 90th percentiles and the medians are shown.

FIG. 4.

Depth profiles of Lake Mondsee from September 2003. Samples were taken from 0 to 40 m (A, B, and C) and 0 to 60 m (D, E, and F), respectively. (A and D) Temperature, pH, and conductivity. (B and E) Total bacterial numbers, SOL bacteria numbers, and chlorophyll a. (C and F) Composition of SOL cluster community.

FIG. 5.

Abundance and composition of the SOL cluster community in Lake Mondsee and neighboring lakes on 3 and 4 September 2003. All samples were taken at a depth of 1 m. Sampling positions of Lake Mondsee are marked with gray lines. The names of sampled lakes and rivers connecting the lakes (underlined names with distances specified) are given. Arrowheads mark the directions of river flows. The sizes of the pie charts correspond to the total number of SOL bacteria. The scale bar (1 km) is only valid for Lake Mondsee. Sampling sites of Lake Attersee were 20 km apart.