Rainer Koschel

Deep-layer autotrophic picoplankton maximum in the oligotrophic Lake Stechlin, Germany: origin, activity, development and erosion

Autotrophic picoplankton (APP) abundance, primary production and vertical distribution were studied in the oligotrophic Lake Stechlin (northeastern Germany) in 1994 and 1995. Within the euphotic zone APP contributed 34% of phytoplankton biomass and 35% of primary production. Annual average APP cell number was 209 × 103 cells ml−1, of which 95% were unicellular cyanobacteria, 2% were colonial cyanobacteria and 3% were eukaryotes. Three ecologically and/or morphologically different groups of APP were recognized: (i) unicellular cyanobacteria belonging to the genus Cyanobium, (ii) eukaryotic species growing in early spring under isothermal conditions and (iii) cyanobacteria, partly colonial species, growing in the stratified period in the euphotic zone. Three species of eukaryotic green algae were identified: Choricystis minor, Neocystis diplococca and Pseudodictyosphaerium jurisii, the latter two being colonial. This is the first record of the occurrence of colonial eukaryotes potentially of APP size in fre...

Interactions between calcite precipitation (natural and artificial) and phosphorus cycle in the hardwater lake

The influence of calcite precipitation on the phosphorus cycle in stratified hardwater lake was studied before and during experiments with a new restoration technique. Surveys of the chemical composition of water column and monitoring of settling particles of Lake Luzin (North–East) showed that calcite precipitation occurs each year over 2–3 periods during spring and summer. The change of the phosphorus content influenced the calcite precipitation intensity. The sedimentation fluxes of phorphorus and the calcite precipitation were closely associated. Based on the hypothesis that calcite precipitation acts as an improvement to the trophic state by enhancing the internal phosphorus sink, this new technology for lake restoration was developed. The hypolimnetic Ca(OH)2 addition during summer stratification in 1996–1997 induced the calcite precipitation in the deep water layer of Basin Carwitz of Lake Schmaler Luzin. The treatment also supported the natural calcite precipitation in the epilimnion. The annual total phosphorus content decreased from 0.46 tons in 1995 to 0.35 tons in 1997. The annual SRP content decreased from 0.02 tons in 1996 to 0.01 tons in 1997 after beginning the artificial calcite precipitation in 1996. The decrease of the annual Chl-a concentration in 1998 on 38% compared with that in 1996 pointed out the lake recovering. According to the one box model, the artificial calcite precipitation affected the P cycle in the lake by suppressing the P release from the sediments.

Long term study on the influence of eutrophication, restoration and biomanipulation on the structure and development of phytoplankton communities in Feldberger Haussee (Baltic Lake District, Germany)

Feldberger Haussee provides a classic example of eutrophication history of hardwater lakes in the Baltic Lake District (Germany) and of changes in their algal flora during the 20th century. The lake originally was regarded as slightly eutrophic. A process of drastic eutrophication from the 1950s until the end of the 1970s caused mass developments of blue-green and green algae. A restoration program was started in the 1980s to improve the water quality of the lake using both diversion of sewage outside the catchment area, and biomanipulation by altering the fish community. This restoration program led to positive changes in the lake ecosystem. Direct effects of biomanipulation resulted in an increase of herbivorous zooplankton, a decrease of phytoplankton biomass, and an increase of water transparency. The recovery of Feldberger Haussee also may have been indirectly enhanced by an increase in nutrient sedimentation as a consequence of intensified calcite precipitation, decrease in phosphorus remobilization due to a pH-decrease, increased NIP-ratio, and recolonization of the littoral zone by macrophytes. This paper concentrates on the long term development of the phytoplankton community as a response to changes in the food web structure as well as to alterations in the chemical environment of the algae. Both are reflected in four major stages passed by the algal assemblage between 1980 and 1994: (1) From 1980-summer 1985 dense green algal populations were found indicating similar conditions as in the 1970s during the period of maximum eutrophication. (2) A diverse phytoplankton community during summer 1985–1989 showed the first effects of a recovery. (3) From 1990–1992 the phytoplankton was characterized by ungrazeable filamentous blue-green algae first of all as a response to increased herbivory of zooplankton on edible species and to increasing N/P-ratios. (4) Finally, the algal species diversity increased in 1993 and 1994 whereas the phytoplankton biomass decreased showing the success of the combined restoration measures.

Long-term phytoplankton changes in an artificially divided, top-down manipulated humic lake

Lake Große Fuchskuhle (Brandenburg, Germany) is a naturally acidic bog lake that was artificially divided into four basins by large plastic curtains for biomanipulation experiments in 1990. Different numbers of perch were added to each compartment beginning in the spring of 1993. The species composition and abundance of phytoplankton, pH, nutrient concentrations, dissolved organic carbon (DOC) and chlorophyll a content were analyzed at regular intervals during 1991 and 1998. The division of the lake resulted in divergent developments in the physical and chemical environment of the compartments. This study compared the phytoplankton assemblages of the Northeast- (NE) and Southwest- (SW) basins which differed strongly in chemistry during the investigation period. Divergent developments in phytoplankton species composition in both basins can be explained by changes in physical and chemical conditions (bottom-up effects). Increased pH values and DOC concentrations probably favoured mass developments of the dinoflagellate Gymnodinium uberrimum since 1993, while increased nutrients (dissolved inorganic carbon, total nitrogen and especially total phosphorus) as well as further changes in pH and DOC led to the dominance of the raphidophyte Gonyostomum semen in 1998. This bloom was characterized by extreme biomasses of up to 143 mg l−1 wet weight, corresponding with high chlorophyll a concentrations of up to 413 μg l−1 at the same time. In contrast, no significant relationship between experimental manipulations by piscivorous fish stocking (top-down effects) and phytoplankton biomass were observed.

Lake restoration by hypolimnetic Ca(OH)2 treatment: impact on phosphorus sedimentation and release from sediment.

A whole-lake hypolimnetic Ca(OH)(2) addition, that induced calcium carbonate precipitation, combined with deep water aeration has been applied to eutrophic Lake Luzin, Germany during 1996-1998. In this study we investigated the dynamic of phosphorus and its binding forms in seston and sediment before and during the treatment. The sedimentation rates of phosphorus increased within three years of induced calcite precipitation. The phosphorus binding forms shifted to the calcite-bound phosphorus in the settling matter. The increase of calcite-bound P in the settling material did not coincide with the maximum induced CaCO(3)-precipitation caused by the hypolimnetic addition of Ca(OH)(2). An impact of chemicals additions and pH on phosphorus binding forms in seston and surface sediments has been studied in laboratory experiments with sediment core incubations and slurry experiments. Laboratory studies showed that the lowest phosphorus flux from sediment was related to the experiment with pH=7 in overlaying water adjusted with Ca(OH)(2). The adjusting of pH with Ca(OH)(2) leads to a lower P flux of 2.3 mg Pm(-2)d(-1), while the highest P-flux is attributed to the experiment with the pH which was adjusted with NaOH. Phosphorus fraction which reflects phosphorus binding on carbonates in surface sediments increased within one year of treatment, enhancing the phosphorus retention capacity of sediments.

Phytoplankton succession in the oligotrophic Lake Stechlin (Germany) in 1994 and 1995

Phytoplankton samples were taken weekly from January to December of 1994 (epilimnion) and 1995 (0–25 m, euphotic zone) from the deep, stratified, alkaline, oligotrophic Lake Stechlin, Baltic Lake District, Germany. The purpose of the study was to gain detailed information about phytoplankton changes including those of picophytoplankton, to relate these changes to stratification patterns and nutrient chemistry of the lake and to compare them to results from other lakes of similar character.During 1994–1995, a total of 142 phytoplankton taxa was encountered in quantitative samples, most being common in deep, oligotrophic lakes. Seasonal development of phytoplankton is characterized by a definite spring peak followed by a moderate summer peak. Autotrophic picophytoplankton made the largest contribution to the annual total biomass. This is probably true for other, temperate, non-acidic, oligotrophic lakes.Development of the spring assemblage (autotrophic picophytoplankton and centric diatoms) starts in February–March and is terminated by the onset of stratification when diatoms sink to the hypolimnion. Picophytoplankton, especially Synechococcus sp., assembles in a narrow deep-layer maximum in the upper hypolimnion.Our data show that neither deep circulations nor decreased incident radiation under winter ice and snow cover prevent the development of some specially adapted low-light – high-nutrient species. Our views about the length of vegetation period for phytoplankton need to be revised with respect to winter and isothermal conditions.

Restoration of a Stratified Lake (Feldberger Haussee, Germany) by a Combination of Nutrient Load Reduction and Long-Term Biomanipulation

A long-term biomanipulation has been performed in the stratified Feldberger Haussee since 1985. Prior to manipulation, nutrient load to the lake had declined due to waste water removal. Planktivorous fish were reduced by seining and by enhancement of piscivorous fish. Changes in transparency, nutrients, phytoplankton, zooplankton and fish were documented for both the premanipulation period (1978-1985) and the manipulation period (1986-1998). Transparency increased in response to the manipulation (+54%), but strong year-to-year fluctuations were observed. These fluctuations were correlated to chlorophyll a, primary production and the proportion of piscivores in the fish community. We conclude that the success of the restoration was predominantly attributed to bottom-up forces as a result of the declining nutrient load and an intensified co-precipitation of phosphorus with calcite. However, the increased predation impact by the piscivorous fish may have caused a reduced nutrient recycling by the planktivorous fish thus contributing also to the improvement in water quality.

Reduction of nutrient loading and biomanipulation as tools in water quality management: Long-term observations on Bautzen Reservoir and Feldberger Haussee (Germany)

Abstract Long-term (1976–1999) biomanipulation in Bautzen Reservoir (BR) revealed that a combination of piscivore stocking and catch restrictions for piscivores led to the desired effects of low planktivorous fish biomass and enhanced biomass of large filter feeders (Daphnia galeata). Despite the hypertrophic status of BR, fisheries management shifted the planktivore-dominated fish community into a piscivore-dominated community. High winter (Jan-Mar) Daphnia biomass was a sensitive indicator of reduced planktivory. Although edible phytoplankton was suppressed by elevated Daphnia biomass, mean seasonal (May-Oct) total phytoplankton biomass remained unchanged due to growth of large inedible algae and cyanobacteria. Inedible and total phytoplankton biomass was primarily controlled by phosphorus availability. However, during clear water periods a reduction of total phytoplankton was achieved with drastically increased Secchi readings. In Feldberger Haussee (FH), despite intensive long-term manual removal of cyprinids (1985–2002) and stocking of piscivorous fish (1988–2002), biomanipulation only had restricted, delayed, or transient effects on the ecosystem. Mean proportion of piscivores within total yield increased but was below 20% in most years. Planktivore cyprinid yields dropped until 1990 and remained constant thereafter. Daphnia biomass slightly increased after biomanipulation became effective (1987–1989) but declined to pre-biomanipulation levels later in the experiment (1996–2005). Beginning in 1997, both edible and inedible phytoplankton biomass started to decrease. Finally, when biomanipulation had been implemented after a delay of 8 years water clarity increased significantly. Although external loading reductions and biomanipulation in both lakes resulted in moderate phytoplankton biomass reduction and Secchi depth enhancement, the reasons for the observed changes were different. Even though strong cascading effects were detected at the top of the food web in BR, the trophic cascade was largely decoupled between phyto- and zooplankton. External plus internal phosphorus loading still exceeded a critical threshold below which a top-down-induced indirect effect of phosphorus sedimentation and finally limitation could have reduced phytoplankton biomass. In constrast to BR, the critical phosphorus loading threshold in FH has probably been approached. Nonetheless, cascading effects were weak due to insufficient reduction of planktivorous cyprinids. Improved water quality was primarily a result of resource-related effects. Thus, the 2 long-term experiments reveal that (1) biomanipulation cannot be applied successfully without reducing nutrient loading below a critical threshold (BR), and (2) for successful biomanipulation, 30–40% piscivores within total fish standing stocks are required (BR and FH). Without optimum piscivory, manual removal of planktivores will hardly produce sustained cascading effects (FH).

Reduction of nutrient loading, planktivore removal and piscivore stocking as tools in water quality management: The feldberger haussee biomanipulation project

The Feldberger Haussee, a highly eutrophic stratified hard-water lake located in the eastern part of Germanys Baltic lake district, was selected for a restoration programme combining external nutrient loading reduction and long-term biomanipulation. In 1980 the external phosphorus loading (1.8 g TP m -2 a -1) decreased by 90%, but water quality did not improve significantly within the following 6 years. In 1985 biomanipulation was initiated, with manual removal of cyprinid fish coupled with piscivore introductions as the principal measures. The expected changes in the pelagic community and improvements of water quality occurred after a delay of several years. Despite intensive manual removal of cyprinids and stocking of piscivores, standing stocks of cyprinid fish remained relatively high (130-260 kg ww ha -1) after some years of decline. Compared to the pre-biomanipulation period, mean seasonal (May-September) Daphnia spp. biomass roughly doubled (0.037 g C m -3 vs. 0.084 g C m-3). However, the predominance of small (< 1 ram) D. cucullata throughout the whole investigation period indicated that planktivory was still substantial. Paired observations between edible phytoplankton biomass and Daphnia spp. indicated that a significant decline in algal stocks would only occur if herbivorous biomass was above a certain threshold (0.2 g C m 3). Reduced external and internal loading in concert with pelagic calcite precipitation were most likely responsible for the decline in lake phosphorus concentrations, thereby substantially improving the water quality of Feldberger Haussee. Although this is not supported by quantitative evidence, we hypothesize that resource-related water quality improvements were caused by changes in the structure of the pelagic community leading to increased calcite precipitation. In agreement with the results of other investigations, we conclude that because stabilising mechanisms such as macrophyte growth were lacking in Feldberger Haussee, biomanipulation in stratified lakes may not be as successful as has been observed in shallow lakes. However, in hard-water lakes, calcite precipitation may act as another stabilising resource-related mechanism. Phosphorus associated with sedimenting calcite particles is insensitive to redox-conditions and may therefore not be re-mobilised from lake sediments even if hypolimnetic oxygen is depleted.

On the occurrence of Phacotus lenticularis (Chlorophyta) in lakes of different trophic state

The occurrence of the calcite-loricated chlamydophycean alga Phacotus lenticularis was investigated in relation to the trophic state of inland waters. The Phytoflagellate was observed in deep, stratified dimictic lakes, in shallow polymictic lakes, oxbow-lakes and ponds. Habitats of Phacotus are rich in lime and vary from oligo- to hypertrophic. Mass developments with cell numbers of 500,000–5.5 × 106 individuals 1−1 were found in lakes with properties in the ranges: TP 34–484 µg 1−1, SRP 2–88 µg 1−1, NO2 − 0–32 µg 1−1, NO3 − 0–820 µg 1−1 and NH4 + 10–740 µg 1−1. The most consistently co-incidental site factors were water temperature and pH values. High abundances were observed at temperatures between 15.8 and 24.7 °C and at pH values from 8.3 to 9.6. The results are discussed in the context of numerous records of Phacotus published in the literature. The morphometric and ecological data come mainly from the main investigated waters of the Baltic Lake District of northeastern Germany.