Lothar Paul

Identifying cardinal dates in phytoplankton time series to enable the analysis of long-term trends

Phenology and seasonal succession in aquatic ecosystems are strongly dependent on physical factors. In order to promote investigations into this coupling, methods of characterising annual time series of phytoplankton were derived and applied to a 31-year data set from Saidenbach Reservoir (Saxony, Germany). Field data are often scarce and irregularly sampled, particularly in the transition period from winter to spring, so reliable methods of determining cardinal dates in the time series are necessary. The proposed methods were used to determine the beginning, maximum and end of the spring mass development of phytoplankton by estimating the inflexion points (A), fitting a Weibull-type function (B) and fitting linear segments to the logarithmic values (C). For the data set from Saidenbach Reservoir, all three methods proved to be relevant to the analysis of long-term trends. Differences between the maxima determined by the different methods seemed small, but there were deviations when the maximum was related to physical factors such as ice-out. The Weibull-type fit gave the most reliable and comprehensible results and is recommended for trend analyses. For all methods, long-term analysis of the duration of the spring mass development and the duration of the spring full circulation revealed a period of consistently low values (1975–1990) followed by a period of higher values (1990–2005). These periods were also identified for the date of ice-out, although in this case there was a period of high values followed by a period of low values. A sensitivity analysis that compared results from subsampled time series with increasing time intervals indicated that a minimum of one sample every three weeks is needed to obtain reliable results.

Reservoirs as sentinels of catchments: the Rappbode Reservoir Observatory (Harz Mountains, Germany)

Reservoirs can be viewed as sentinels of their catchments and a detailed monitoring of reservoir systems informs about biogeochemical and hydrological processes at the catchment scale. We developed a comprehensive online monitoring system at Rappbode reservoir, the largest drinking water reservoir in Germany, and its inflows. The Rappbode Reservoir Observatory comprises of a set of online-sensors for the measurement of physical, chemical, and biological variables and is complemented by a biweekly limnological sampling schedule. Measurement stations are deployed at the four major inflows into the system, at the outlets of all pre-reservoirs, as well as in the main reservoir. The newly installed monitoring system serves both scientific monitoring and process studies, as well as reservoir management. Particular emphasis is paid to the monitoring of short-term dynamics and many variables are measured at high temporal resolution. As an example, we quantitatively documented a flood event which mobilised high loads of dissolved organic carbon and changed the characteristics of the receiving reservoir from eutrophic to dystrophic within a few days. This event could have been completely missed by conventional biweekly sampling programs, but is relevant for biogeochemical fluxes at the catchment scale. We also show that the high frequency data provide a deeper insight into ecosystem dynamics and lake metabolism. The Rappbode Reservoir Observatory; moreover, offers a unique study site to apply, validate, and develop state of the art lake models to improve their predictive capabilities.

Comparing in situ particle monitoring to microscopic counts of plankton in a drinking water reservoir

In a one-year study, the multispecies assemblages of phytoplankton (picoplankton to microplankton) within a drinking water reservoir were counted, determined and evaluated in their size fractions using microscope enumeration (MC). The manual counts were compared with the size evaluation obtained by a light obscuring particle counter (PC) in order to evaluate its use for the monitoring practice of a drinking water reservoir. With this multispecies comparison we present a novel approach for the evaluation of automated counting systems. The picoplankton clearly remained uncounted by the PC even though its lower size limits imply an adequate match. The highest and most consistent count numbers of plankton (nano- and microplankton) and particles were obtained during the spring mass development. However, from the middle of the year onwards, the measured particle concentration surpassed the counted plankton abundances by two- to threefold indicating the rise of seston within the water column. This fraction would be missed if counted solely by MC. Further, the PC consistently undersized the biological counts, but not the minerogenic fraction represented by the manganese oxidising bacteria. Consequently, the rise and decline of Metallogenium bacteria was reliably monitored with the PC. The PC provides additional size information compared to other bulk optical sensors (turbidity, chlorophyll-a). The correlation of particles with probe measurement always exceeded the plankton coefficient, but all combinations of plankton, particle and probe measurement revealed significant linear regressions. However, the redundancy of the chlorophyll-a probes was also shown in order to explain plankton abundances. Our results indicate that background knowledge of the monitored system and cautious interpretation of data is required to allocate and understand automated particle counts. Therefore, only in combination with MC, the PC enables phytoplankton or minerogenic particle counts under frequent real-time monitoring conditions. As such it may serve as a helpful tool for example in critical situations in the management of drinking water reservoirs.

Fixation of manganese and iron in freshwater sediments through electrochemically initiated processes I: Principles and laboratory studies

Abstract.Most existing in-lake methods used to minimize the remobilization of redox-sensitive compounds from sediments are costly and limited in their effectiveness. Therefore, new approaches for this problem are needed. We describe electrochemically initiated transformation processes of iron and manganese, resulting in their immobilization in the sediment. This process could possibly be applied to prevent remobilization of phosphate from lake sediments. With electrodes positioned 3 cm in the sediment and 7 cm above the sediment-water interface, an electrochemically controlled redox- and pH-”barrier” was constructed. By means of laboratory experiments in columns filled with sediment and water from a reservoir, we demonstrated the possibility to fix and concentrate Fe and Mn under specific conditions in the sediment, or if required, to mobilize and drain them in concentrated solution. The behaviors of these metals were studied by simultaneous measurements of their concentrations, pH-, Eh-values, and O2-concentration at the interface and in the water with and without current influence. The results show that soluble species can be transported and concentrated in an electric field. Iron and manganese can be prevented from being released from the sediments by sustainable in-situ fixation.

Wie zeigt sich der Klimawandel in den deutschen Talsperren

Langzeitdaten von deutschen Talsperren zeigen eine Erhöhung der Temperaturen der oberen Wasserschichten vor allem im Winter, Frühling und Frühsommer. Die Trends sind regional ähnlich. Da sich schon geringe Temperaturerhöhungen auf die biotische und abiotische Struktur und somit auf die Wasserbescha enheit auswirken, ergibt sich für Trink wassertalsperren die Notwendigkeit, die Bewirtschaftung an die Veränderungen anzupassen.