The performance and methodological limits of the Phyto-PAM chlorophyll fluorometer were investigated with laboratory grown algae cultures and natural phytoplankton from the rivers Saar and Saale. The Phyto-PAM is a 4-wavelength chlorophyll fluorometer with the functional combination of chlorophyll (Chl) estimation and assessment of photosynthetic activity, both differentiated into the main algal groups. The reliability of fluorescence-based Chl estimation strongly depends on the group specific calibration of the instrument and the resulting chlorophyll/fluorescence (Chl/F) ratios in reference algal cultures. A very high reliability of the Chl estimation was obtained in the case of constant Chl/F-ratios. Algae grown at different light intensities showed marked differences in Chl/F-ratios, reflecting differences in pigment composition and Chl a specific absorption (a*). When the Phyto-PAM was calibrated with laboratory grown diatoms, the Chl a in river grown diatoms was underestimated, due a lower content of accessory pigments and stronger pigment packaging. While this aspect presently limits the application of PAM fluorometry in limnology, this limitation may be overcome by future technical progress in the detection of dynamic changes in Chl/F-ratio via fluorescence-based measurements of the functional PS II absorption cross-section. Practically identical Chl/F-ratios were found for the diatom-dominated waters of the rivers Saar and Saale, suggesting that the same instrument calibration parameters may be applied for hydrographically similar surface waters. For this particular case, despite of the present methodological limitations, the potential of PAM fluorometry in limnology could be demonstrated. Light response curves were measured to estimate primary production with a spectrally resolved model in daily courses at two sampling sites. Fluorescence based primary production was closely correlated with measured oxygen evolution rates until midday. In the afternoon, at the water surface the fluorescence approach gave higher rates than the measured oxygen evolution. Possible explanations for the observed differences are discussed.
The River Saar is a heavily impounded river with an average discharge of 80 m 3 s À1 . The German reach of the River Saar, i.e. the lower 90 km, was gradually impounded from 1977-2000, resulting in a doubled average water depth (today 4.2 m). In parallel to river development, water pollution was decreased strongly, relieving the critical oxygen budget in the river. Nowadays, oxygen concentrations may still fall below 4 mg O 2 l À1 during low-flow periods in summer, when thermal stratification and depth gradients of oxygen occur. In August 2005, high-resolution measurements of temperature, conductivity, turbulence, as well as oxygen-levels were carried out over 48 hours. These data were used to develop and validate a 'quasi-two-dimensional', depthresolving modelling approach with the deterministic water quality model QSim. This model includes the mathematical description of the influence of flow velocity and solar radiation on thermal stratification, on which the exchange between water layers depends. Three data sets of continuous measurements were compared with the model outputs. Thermal stratification shows diurnal rhythms and longitudinal variability depending on solar radiation, water depth, and flow velocity. In the course of the day, measurements and model outputs showed best agreement during the strongest stratification in the evening. The modelled effects of thermal stratification on oxygen-budget rates were quantified and showed that the reduction of atmospheric re-aeration is partly compensated by an increase in algal oxygen production. For the River Saar, the high temporal and spatial variability of oxygen concentrations documented here is of major ecological significance.
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