The microstratification of the microbial community at the chemocline of Lake Cadagno and the associated inorganic carbon fixation activity was studied by fine layer sampling. A deep chlorophyll maximum caused by diatoms overlying Cryptomonas was found at the upper edge of the chemocline. A high population density of phototrophic sulphur bacteria, mainly Amoebobacter cf. purpureus, occurred closely below the oxic-anoxic boundary. Despite the small fraction of total lake volume represented by the chemocline, half of the total carbon photoassimilation of the lake occurred within the chemocline with approximately equal contributions by oxygenic and anoxygenic phototrophs. Rates of dark carbon fixation in the chemocline were even higher than rates of photoassimilation, especially at the depths where oxygen and sulphide coexisted during part of the day. These results indicate a substantial contribution by chemolithotrophic organisms to the carbon cycle in Lake Cadagno. Analysis of stable carbon isotopes suggests that zooplankton may obtain as much as half of its carbon at the chemocline, indicating a strong link between production in anoxic waters and the food web in the oxic part of the lake.
This introductory article of the special GAP issue gives an overview on general limnological characteristics of the prealpine Lakes Zürich and Lucerne and the alpine Lake Cadagno and reports on the specific situation of primary production parameters during the international GAP Workshop in mid September 1999. Furthermore, it describes methods used for water analysis and fieldwork in these lakes.A comparison of data related to primary production in the three lakes in September 1999 during stratification shows that (i) phytoplankton community structure varied considerably between the lakes. The dominating algae were Planktothrix rubescens in Lake Zürich, various chrysophytes and diatoms in Lake Lucerne, and Echinocoleum elegans in Lake Cadagno, (ii) the euphotic zone in Lake Lucerne was considerably deeper (app. 15 m) than in the other two lakes (app. 10 m), (iii) chlorophyll a standing crop was highest in mesotrophic Lake Zürich (August: 121 mg m -2 ), followed by oligotrophic Lake Lucerne (August: 75, September: 34 mg m -2 ) and mesotrophic Lake Cadagno (August: 33, September: 25 and 14 mg m -2 ), and (iv) areal primary production was highest in Lake Zürich (August: 105, September: 124 mg C m -2 h -1 ), followed by Lake Cadagno (August: 102, September: 52 mg C m -2 h -1 ) and Lake Lucerne (August: 90, September: 52 mg C m -2 h -1 ). Physiological parameters, determined in situ from P versus I relationships, showed a lower initial slope a in Lake Lucerne (August: 0.03, September: 0.02 mg C mg -1 chl a h -1 mmol -1 m 2 s) than in the other two lakes (Lake Zürich in August: 0.05, in September: 0.11; Lake Cadagno in August: 0.05, in September: 0.11 and 0.28 mg C mg -1 chl a h -1 mmol -1 m 2 s). Lake Zürich showed the lowest AN max (August: 2.6, September: 3.2 mg C mg -1 chl a h -1 , as compared to 5.9 -7.4 mg C mg -1 chl a h -1 in the Lakes Lucerne and Cadagno), while in Lake Cadagno the highest inhibitory effects of Cassimilation were found (highest slopes of inhibition b, 0.007 -0.011, as compared to 0.0003 -0.0026 in the other two lakes), due to a higher UV-exposure in this alpine lake.
In late summer and autumn, before the vertical circulation reaches the thermocline, the phytoplankton population of Lake Zürich is dominated by the red-coloured filamentous cyanobacterium Planktothrix rubescens, which stratifies in the metalimnion at depths close to the photosynthetic compensation point. The filament volume concentration reached a maximum of 12 cm 3 m -3 ; the depth of the maximum varied from 10.5 to 12.5 m. Changes in the depth distribution were attributed to a combination of (1) seiche movements, which raised or lowered the thermocline by up to 2 m over 36 h, and (2) flotation by the buoyant filaments relative to the isotherms, by up 0.4 m d -1 . These changes caused a 2-fold change in insolation at the Planktothrix peak. Estimates were made of the daily integral of photosynthetic O 2 -production, SS(NP), by the population of P. rubescens over a period of four cloudless days. The estimates were calculated from measurements of surface irradiance (at 5-min intervals), vertical light attenuation, temperature, filament volume concentration and the photosynthesis/irradiance (P/I) curves of filaments concentrated from the metalimnion. Despite the similar, high insolation on each of the four days, the calculated values of SS(NP) varied from 9 to 53 mmol m -2 d -1 , owing to the changing depth distribution of the filaments. Measurements of P/I curves of lakewater samples incubated at a depth of 11 m showed changes in the photosynthetic coefficients during the day. These also generated large changes in calculated values of SS(NP). The computer spreadsheet used to calculate SS(NP) was modified to incorporate timebased changes in the photosynthetic coefficients and vertical distribution of the organism. These refinements provide a more accurate description of photosynthesis by the deep-living P. rubescens, which adjusts its position by buoyancy regulation to exploit the light field in the metalimnion, where it outcompetes other phytoplankton.
The sensitivity of photosynthesis to ultraviolet radiation (UV) was assessed for phytoplankton assemblages in two Swiss lakes, pre-alpine Lake Lucerne (Vierwaldstättersee) and alpine Lake Cadagno, using both in situ and laboratory incubations. Biological weighting functions for UV inhibition of photosynthesis (BWFs) were determined in the laboratory using polychromatic exposures in a Xe-lamp based incubator. Samples were concurrently incubated in situ under UV exposed and protected bottles (profiles 0 -5 m), while additional spectral treatments were carried out at the 50 % UV-B penetration depth: full spectrum, UV-A only (Mylar protected) and UV protected quartz tubes. Both particulate (> 0.2 mm) and total organic carbon incorporation were measured. Measured attenuation coefficients and incident UV spectral irradiance data was used to evaluate a BWF/photosynthesis-irradiance model (BWF/P-I) for in situ exposure conditions and compared with measurements. The BWFs showed sensitivity across the UV spectrum at similar, though somewhat lower, levels than an average BWF for marine assemblages. Relative photosynthesis in situ (UV exposed/UV excluded) was about 40 % at the surface and about 60 % at the 50 % UV-B penetration depth. Similar inhibition was predicted by the BWF/P-I model. Generally, full spectrum (UV-B and UV-A) exposure had little additional effect compared to UV-A only exposure. Reciprocal transfer of samples between lakes showed enhancement of UV effects in L. Cadagno compared to incubation of the same sample in L. Lucerne, consistent with increased UV sensitivity due to the 5°C cooler water temperature in L. Cadagno. Similarly, BWF prediction of in situ response in L. Cadagno was improved by increasing UV sensitivity according to a Q 10 of 2. Aquat.sci.63 (2001) 265-285 1015-1621/01/030265-21 $ 1.50+0.20/0 © Birkhäuser Verlag, Basel, 2001 Aquatic Sciences * Corresponding author, e-mail: Neale@serc.si.edu.Full profile calculations using the BWF/P-I model suggest stronger effects of UV on L. Lucerne compared to L. Cadagno phytoplankton due to greater sensitivity of the assemblage combined with higher overall transparency to UV relative to PAR in L. Lucerne. The BWF/P-I model was a good overall predictor of UV-dependent photosynthetic performance in these lakes.
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