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In two reservoirs of different trophic state, the abundance of autotrophic picoplankton was continuously assessed for 7 and 11 years, respectively. Simultaneously, other important environmental and biotic factors (nutrients, temperature, phyto- and zooplankton) were analysed. The mesotrophic Saidenbach Reservoir receives five- to sevenfold higher phosphorus imports, and exhibits twice higher phosphorus contents, and tenfold higher phytoplankton concentrations than the oligotrophic Neunzehnhain Reservoir. Despite these differences, the quantities and dynamics of autotrophic picoplankton (APP) showed remarkable similarities in both reservoirs. The APP consisted almost exclusively of Phycoerythrin-rich cyanobacteria. A notable growth of cyanobacteria never started before the onset of stratification and finished no later than the beginning of the autumnal turnover. Although the soluble reactive phosphorus (SRP) concentrations in the epilimnion were always near the limit of detection, there was no evidence for growth restriction of APP by nutrients in either of the reservoirs. However, the data reveal a strong grazing impact of all potential grazers, especially of the daphnids. This zooplankton group always showed a strict behaviour opposite of the APP during its growth periods. The grazing influence of rotifers and protozoans is less pronounced but also obvious, notable by the frequently observed inverse relationships between their quantities and those of APP. It is assumed that during the stratification period in the summer the APP dynamics is mainly top-down controlled.

In Colgada Lake, one of the 15 lakes belonging to Lagunas de Ruidera Natural Park, the components of the lineal food chain (phytoplankton and metazooplankton) and the microbial loop phytoplankton, metazooplankton, ciliates, autotrophic picoplankton, and bacterioplankton) were studied from June 2003 to December 2004 with a monthly sampling frequency. This lake has monomictic and mesotrophic characteristics and a mean depth of 8 m. Sixty-two species of the phytoplankton community, 27 species of metazooplankton community, and 12 species of ciliates were identified. Phytoplankton and metazooplankton integrated biomass followed seasonal patterns with higher values in summer, up to 105 mg WW/m2 and 2 × 104 mg WW/m2, respectively. Autotrophic picoplankton did not exceed 500 mg WW/m2. Microbial loop components did not show seasonality, and its biomass concentration fluctuated between 500-2500 mg WW/m2 for ciliates and 100- 2000 mg WW/m2 for bacterioplankton. Centric Bacillariophyceae, Cryptophyceae, and Dinophyceae represented most of the phytoplanktonic biomass throughout the studied period, although an increase of cyanobacteria was also observed. Cladocerans and calanoid copepods dominated during summer stratification, rotifers and cyclopoid copepods were present during the winter mixing period. The microbial loop biomass in relation to total planktonic biomass was higher in winter and spring 2004. Interannual changes in the presence of planktonic groups were observed in the different periods of the year: i) the algal composition included larger and non-edible species (Cyanobacteria, Dinophyceae), ii) a reduction in the clearance function by cladocerans was produced, which were substituted by a cyclopoid predator that iii) can feed on rotifers and ciliates, favouring in this way bacterioplankton and autotrophic pico-nanoplankton. This interannual variation could be related to the ongoing eutrophication process in the Lake. All of this may change the way the lake looks: if spring primary production is not strongly controlled by herbivory, this could threaten the annual recruitment of submerged macrophytes that significantly contribute to improve the water quality of the lake. En la laguna Colgada, una de las 15 lagunas que componen el Parque Natural de Las lagunas de Ruidera, se han estudiado los componentes de la red trófica planctónica (fitoplancton, metazooplancton, ciliados, picoplancton autotrófico -PPA- y bacterioplancton), desde junio de 2003 a diciembre de 2004, con una frecuencia de muestreo mensual. Esta laguna posee un carácter monomíctico y mesotrófico y una profundidad media de 8 m. Se identificaron 62 especies de la comunidad fitoplanctónica, 27 especies de la comunidad de metazooplancton y 12 especies de ciliados. La biomasa integrada de fitoplancton y metazooplancton mostró un patrón estacional con valores más elevados en verano, hasta 105 mg PF/m2 y 2 × 104 mg PF/m2, respectivamente. El PPA no superó los 500 mg PF/m2. Bacterias y ciliados no presentaron estacionalidad, y su concentración fluctuó entre 500-2500 mg PF/m2 en el caso de los ciliados y 100-2000 mg PF/m2 en el bacterioplancton. Poblaciones de Bacillariophyceae de tipo centrales, Cryptophyceae y Dinophyceae constituyeron la mayor parte de la biomasa fitoplanctónica durante todo el periodo de estudio, aunque se observó un incremento de cianobacterias. Cladóceros y copépodos calanoides fueron dominantes durante la estratificación estival, rotíferos y copépodos ciclopoides estaban presentes en la mezcla invernal. La biomasa relativa del bucle microbiano frente al total de biomasa planctónica fue superior en inverno y primavera de 2004. Se han observado cambios interanuales en la presencia de los grupos planctónicos de los diferentes periodos del año: i) la composición algal incluye especies más grandes y menos comestibles (Cyanobacteria, Dinophyceae), ii) se produce una reducción de la función de aclarado de los cladóceros que son sustituidos por un depredador ciclopoide que iii) se pue de alimentar de rotíferos y ciliados, favoreciendo así a las bacterias y el pico-nanoplancton autotrófico. Esta variación interanual podría estar relacionada con el proceso de eutrofización que está sufriendo la laguna. Todo ello puede llegar a cambiar el aspecto de la laguna: si la producción primaria en primavera no está fuertemente controlada por la herbivoría, se puede impedir el reclutamiento anual de los macrófitos sumergidos que contribuyen significativamente a mejorar la calidad de la aguas de esta laguna.

We conducted a 1-year survey of the surface waters of two deep peri-alpine lakes, and investigated the abundances and community structure of picocyanobacteria and co-occurring cyanomyophages. Picocyanobacterial abundances ranged between 4.5 × 10(4) and 1.6 × 10(5) cells mL(-1) in Lake Annecy vs. 2.2 × 10(3) and 1.6 × 10(5) cells mL(-1) in Lake Bourget. Cyanomyoviruses ranged between 2.8 × 10(3) and 3.7 × 10(5) copies of g 20 mL(-1) in Lake Annecy vs. between 9.4 × 10(3) and 9.4 × 10(5) copies of g 20 mL(-1) in Lake Bourget. The structures of picocyanobacteria and cyanomyoviruses differed in the two lakes, and a more pronounced dynamic pattern with greater seasonality was observed in Lake Bourget. At the annual scale, there was no relationship between cyanomyovirus and picocyanobacterial abundances or structures, but we could observe that abundances of the two communities covaried in spring in Lake Bourget. We showed that (i) the changes of picocyanobacteria and cyanomyoviruses were caused by the combined effect of several environmental and biological factors the importance of which differed over time and between the lakes, and (ii) the viral control of the picocyanobacterial community was probably relatively weak at the scale of the investigation.