Jürgen Benndorf

Conditions for effective biomanipulation; conclusions derived from whole-lake experiments in Europe

Since indirect effects, feedback mechanisms and time lag play an important role in top-down manipulated food webs, conclusions regarding the reliability of biomanipulation as a management tool should be derived only from whole-lake studies, the results of which were followed for at least three to five years. There are only a few experiments and applications of that type from which the following conclusions can be drawn: (1) Food web manipulation should be combined with resource limitation measures (nutrients or light) if a ‘biomanipulation-efficiency threshold of the phosphorus loading’ is exceeded. (2) The critical biomass of undesirable fish shows a very wide range of variation (20–1000 kg ha−1) and depends on several factors, like species and age of fish, total food assemblage, refugia etc. (3) The maximum possible stability of food web manipulation is probably achieved at a biomass of undesirable fish a little below the critical biomass rather than by extermination of these fish. (4) High piscivore diversity increases stability and reliability of food web manipulations. (5) Various techniques of food web manipulation should be combined, except poisoning the entire fish community.

Food web manipulation without nutrient control: A useful strategy in lake restoration?

The results of long-term, full-scale experiments in biomanipulation (in the closer sense of top-down control of the food web) in water bodies having extremely different phosphorus loads reveal that there is a close connection between the efficiency of food web manipulation and the nutrient situation in the particular water body. Top-down control provides a high probability of improving water quality with respect to Secchi depth and algal biomass only if it is connected with a decrease in the in-lake phosphorus. If that decrease in phosphorus is not achieved for different reasons, the high probability of an efficient top-down control is limited to zooplankton and to structural changes in the phytoplankton. A lake-specific ‘biomanipulation-efficiency threshold’ of the phosphorus load should not be exceeded. The results show that integrated water quality control, i.e. a combination of the strategy of load reduction with the strategy of ‘ectotechnology’, seems to be a very promising way.

WAX AND WANE OF MICROCYSTIS (CYANOPHYCEAE) AND MICROCYSTINS IN LAKE SEDIMENTS: A CASE STUDY IN QUITZDORF RESERVOIR (GERMANY)1

Benthic stages of the annual life cycle of the meroplanktonic cyanobacterium Microcystis spp. in relation to microcystin (MCYST) dynamics in sediments of a shallow lake (Quitzdorf Reservoir, Germany) were investigated. Based on changes in the absolute abundance of benthic Microcystis, the annual life cycle was subdivided into four phenological stages: reinvasion, pelagic growth, sedimentation, and overwintering. Habitat‐coupling processes, such as reinvasion of the pelagic zone in spring as well as autumnal sedimentation, were particularly triggered by changes in water temperature. During reinvasion substantial losses of Microcystis were detected. Only a minor part of benthic Microcystis (about 3%) formed the inoculum for pelagic growth. Between 65% and 85% of the benthic Microcystis stock disappeared during the reinvasion phase. Because these colonies were neither detected within the sediments nor in the pelagic inoculum, it was concluded that they were subjected to decay. The occurrence of extracellular MCYSTs in the pelagic zone during this period, which cannot solely originate from the pelagic Microcystis population, supports this conclusion. Dynamics of benthic Microcystis and MCYSTs were characterized by almost identical successions with a decrease during reinvasion, an increase during sedimentation, and remarkable invariability throughout pelagic growth and overwintering. It can be deduced that MCYSTs are preserved within benthic resting stages of Microcystis because they could play a role during overwintering or reinvasion.

Problems of application of the ecological model salmo to lakes and reservoirs having various trophic states

Abstract A short description is given of a dynamic ecological model (SALMO) of the pelagic zone of reservoirs and lakes. The model involves only three state-variables (two groups of phytoplankton, zooplankton, orthophosphate), but nevertheless a great number of internal control mechanisms are considered. The applicability of this model to quite different water bodies is checked by comparing simulation results with observations. Four water bodies are used for these comparisons: Lake Stechlin (deep, oligotrophic), the Saidenbach reservoir (deep, mesotrophic), the Bautzen reservoir (shallow, hypereutrophic), and the Bleiloch reservoir (deep, hypereutrophic, high light extinction). Despite numerous deviations between the simulations and observations, the general result of this validation justifies the use of SALMO in water-quality management. A relative comparison of simulated scenarios is regarded to be the most adequate method in such an application because it minimizes the influence of inaccuracies in the model. An example of a scenario analysis for decision-making in the management of a multipuroose reservoir is given in detail.

Impact of inorganic carbon availability on microcystin production by Microcystis aeruginosa PCC 7806.

ABSTRACT Batch culture experiments with the cyanobacterium Microcystis aeruginosa PCC 7806 were performed in order to test the hypothesis that microcystins (MCYSTs) are produced in response to a relative deficiency of intracellular inorganic carbon (Ci,i). In the first experiment, MCYST production was studied under increased Ci,i deficiency conditions, achieved by restricting sodium-dependent bicarbonate uptake through replacement of sodium bicarbonate in the medium with its potassium analog. The same experimental approach was used in a second experiment to compare the response of the wild-type strain M. aeruginosa PCC 7806 with its mcyB mutant, which lacks the ability to produce MCYSTs. In a third experiment, the impact of varying the Ci,i status on MCYST production was examined without suppressing the sodium-dependent bicarbonate transporter; instead, a detailed investigation of a dark-light cycle was performed. In all experiments, a relative Ci,i deficiency was indicated by an elevated variable fluorescence signal and led to enhanced phycocyanin cell quotas. Higher MCYST cell quotas (in the first and third experiments) and increased total (intracellular plus extracellular) MCYST production (in the first experiment) were detected with increased Ci,i deficiency. Furthermore, the MCYST-producing wild-type strain and its mcyB mutant showed basically the same response to restrained inorganic carbon uptake, with elevated variable fluorescence and phycocyanin cell quotas with increased Ci,i deficiency. The response of the wild type, however, was distinctly stronger and also included elevated chlorophyll a cell quotas. These differences indicate the limited ability of the mutant to adapt to low-Ci,i conditions. We concluded that MCYSTs may be involved in enhancing the efficiency of the adaptation of the photosynthetic apparatus to fluctuating inorganic carbon conditions in cyanobacterial cells.

Control of eutrophication of lakes and reservoirs by means of pre-dams—I. Mode of operation and calculation of the nutrient elimination capacity

Abstract Pre-dams are small reservoirs having an average theoretical water retention time of a few days. They are situated immediately above the larger main reservoir (or lake), whose nutrient loading it is their purpose to reduce. Since nutrient uptake by algae and sedimentation are the dominating processes of the nutrient removal in pre-dams, the following demands to the design, construction, and operation should be taken into consideration: (1) relatively low mean depth, (2) surface release, and (3) optimum size. A calculation procedure for the mean monthly rates of orthophosphate elimination is described, which makes it possible to predict the orthophosphate elimination of designed pre-dams of a particular size. Furthermore, this procedure is the basis for the optimization of the size of the pre-dam, which will be shown in Wat. Res. 21, 839–842. The elimination capacity with respect to inorganic nitrogen can be roughly estimated from the mean residence time of the water and the N:P ratio. The described method involves 10 years experience with its application.

Phosphorus reduction in a shallow hypereutrophic reservoir by in-lake dosage of ferrous iron

In Bautzen Reservoir (Germany), a technique of internally dosing iron compounds in combination with a local water column destratification was developed in order to control Microcystis blooms. In this paper, experimental results concerning the phosphorus (P) precipitation by iron application are reported. First, preliminary studies were conducted serving the choice of an appropriate precipitant. Subsequently, a whole-lake experiment was carried out. Fe2+ salts displayed a delayed oxidation and flocculation behaviour when injected into an ascending water jet. Nevertheless, the efficiency of soluble reactive phosphorus (SRP) elimination was equal to experiments with Fe3 + compounds both in laboratory and field experiments. During Fe2+ application, a large P-binding potential remained during the horizontal spreading of the treated water, since the proportion of the dissolved iron fraction near the dosage point was still high. Thus, measurements at a greater distance from the dosage point revealed higher amounts of P eliminated by Fe2+ than by Fe3+ dosage. Compared to the preceding year 1995, during the treatment periods in May-August 1996 and 1997 the SRP contents in the whole water body dropped by 72% and 54%, respectively, while the total phosphorus contents dropped by 45% at each period. We conclude that in-lake dosage of Fe2+ is an appropriate method to reduce the P loading of hypereutrophic reservoirs.

Food web manipulation by extreme enhancement of piscivory: an invertebrate predator compensates for the effects of planktivorous fish on a plankton community

Abstract By enhancing the stock of piscivorous fish in a whole-lake experiment in Grafenhain (Germany) since 1981 to such an extent that almost all planktivorous fish were exterminated, we examined the concept of “over-biomanipulation”. This hypothesis predicts that (a) extremely strong piscivory will allow uncontrolled development of large invertebrate predators and (b) these invertebrates can exert the same strong predation pressure on large herbivorous zooplankton as planktivorous fish. The hypothesis is tested the first time by a cross-comparison of the long-term response of the plankton community structure in the experimental lake (Piscivore L.) with (1) the intermediate response in the same lake and with (2) that of the long-term state in a nearby reference lake (Planktivore L.) densely inhabited by planktivorous fish ( Leucaspius delineatus , a small cyprinid). The intermediate (1989–1992) response in Piscivore L. revealed a strong increase of the abundance of the invertebrate predator Chaoborus flavicans . Large daphnids were able to coexist with C. flavicans so that edible phytoplankton were suppressed and water transparency increased. As part (a) of the hypothesis predicts, the long-term response in Piscivore L. was characterized by the immigration of the larger predator Chaoborus obscuripes which displaced the smaller C. flavicans completely. The results support also part (b) of the hypothesis of “over-biomanipulation”: C. obscuripes -dominated Piscivore L. showed not much difference in biomasses of daphnids and total and edible phytoplankton as well as Secchi depth compared with fish-dominated Planktivore Lake. On the other hand, C. obscuripes -dominated Piscivore L. was characterized by distinctly lower biomass of daphnids, mean body volume of all crustaceans and Secchi depth as well as by higher biomass of edible phytoplankton compared with C. flavicans -dominated Piscivore Lake. We conclude that long-lasting success of biomanipulation cannot be achieved by extremely high piscivory leading to the almost complete extermination of planktivorous fish.

The importance of pre-reservoirs for the control of eutrophication of reservoirs

Pre-reservoirs are small reservoirs, with a water-retention time of a few days, that reduce the phosphorus input in main reservoirs. The process of phosphorus removal involves the biochemical conversion from the dissolved to the particulate form (mainly phytoplankton) and the sedimentation of this particulate matter. The input variables are light, orthophosphate concentration, temperature of the inflowing water and discharge. The phytoplankton activity plays the most important role among the various processes. The maximization of orthophosphate elimination depends on adequate design, construction and operation of prereservoirs. A simple calculation procedure for the removal rates of orthophosphate-P has been developed. The efficiency of pre-reservoirs is limited, because the light intensity and the temperature during the winter period are low; on the other hand, discharge is often high in March and April (snowmelt). Although prereservoirs are an important tool for reservoir water-quality management, but they cannot substitute remedial action in the catchment area.

Initiation of the midsummer decline of Daphnia as related to predation, non-consumptive mortality and recruitment: a balance

The midsummer decline (MSD) of daphnids has been attributed to a range of factors related to either resource availability or predation. However, the relative magnitude and importance of each of these factors remained unknown. Therefore, we quantified simultaneously, but independently, both consumptive mortality due to predation and non-consumptive mortality of Daphnia galeata due to senescence, disease, food limitation as well as limiting physical or chemical factors between May and July 1998 in Bautzen Reservoir, Germany. These losses were balanced by the population gains by recruitment. The predation pressure exerted by age-0 fish, by older zooplanktivorous fish and by carnivorous zooplankton did not completely account for the observed mortality pattern of D. galeata that caused the MSD. Non-consumptive mortality in the field was calculated from recruitment patterns estimated from field samples and from age-specific survival probabilities of Daphnia offspring according to lifetable experiments. The results suggest a high non-consumptive mortality during the clear-water phase which is comparable to the magnitude of consumptive losses and may give a possible explanation for the remaining part of Daphnia mortality. By integrating all data concerning the initial abundance in early May as well as population gains and losses of daphnids, a balance sheet was drawn up for four years of investigation (1995-1998). The differences between the cumulated positive and negative book values corresponded well to the actually observed Daphnia abundance in Bautzen Reservoir at the transition period to an MSD of every year. Generally, the hypothesis is supported that an MSD should be expected if the period with bottom-up limitation during the clear-water phase overlaps strongly with top-down effects whereby in detail the following combination of conditions should be fulfilled: (i) a high biomass of age-0 percids in May/June, (ii) an early start of Daphnia consumption by carnivorous zooplankton, (iii) a quick increase in Daphnia density leading to the dominance of a peak cohort during the clear-water phase and (iv) a long lasting clear-water period characterized by reduced longevity and low recruitment of Daphnia. [KEYWORDS: AGE-0 PERCIDS; ROACH; CARNIVOROUS ZOOPLANKTON; POPULATION BUDGET; DAPHNIA; LONGEVITY]