Annekatrin Wagner

Predation by piscivorous fish on age-0 fish: spatial and temporal variability in a biomanipulated lake (Bautzen reservoir, Germany)

Predation by perch (Perca fluviatilis L.) and zander (Stizostedion lucioperca L.) on their new-hatched fish was studied in the littoral and pelagic zone in the biomanipulated Bautzen reservoir (Germany). Sampling was conducted from the middle of May to the middle of August in 1997. To investigate predation, we compared the littoral and pelagic zone, because the littoral zone could provide a refuge area for age-0 fish. This study focused on: 1. timing of predation, 2. selectivity of predation and 3. feeding pressures. In the littoral zone, age-1 perch fed first on age-0 percids at the beginning of June, when age-0 percids reached a mean length of 20 mm. Age-2 and older perch and zander started feeding on age-0 percids in the middle of June in both habitats. At this time, age-0 percids reached a mean length of 30 mm. In both habitats, species selective feeding on age-0 perch and age-0 zander was found. Most of the sampling time zander fed selectively on their own descendants. In general, mean lengths of age-0 fish found in the predator stomachs were smaller than those in situ. The strongest decrease of age-0 fish abundance started in both areas before percids began to feed on age-0 fish. In the middle of June, predation pressure on age-0 fish was higher in the pelagic zone, whereas in July and August it was higher in the littoral zone. After a strong initial decline due to unknown factors, age-0 fish abundance was controlled by piscivores. Predation started at the time when age-0 fish gape sizes were big enough to feed on large daphnids. As age-1 perch started first feeding on age-0 fish, at high densities they could be an important source of early mortality on age-0 zooplanktivores. This would be advantageous for the daphnid population in two ways, because the efficiency of two different predators would be reduced. Age-0 fish are consumed by age-1 perch, and age-1 perch feed on age-0 fish and not on daphnids. Consequently, the feeding behaviour of age-1 perch could be a key to achieve a stable long-term biomanipulation.

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]

Phytoplankton responses to grazing by Daphnia galeata in the biomanipulated Bautzen reservoir

We studied the response of phytoplankton to grazing by Daphnia galeata in the hypertrophic Bautzen reservoir (Dresden, Germany) from January 1995 to May 1996 during a long-term whole-lake biomanipulation experiment. The correlation between clearance rate of D. galeata and total phytoplankton biomass was negative only if biomass of Microcystis aeruginosa was excluded. This suggests that M. aeruginosa is the main grazing resistant phytoplankton species in the Bautzen reservoir. Except for M. aeruginosa and grazing-resistant Staurastrum quadridentatum spec. nov. (Scharf, 1995) no other phytoplankton species was able to reach a biovolume above 1 mm3 L-1 when the clearance rate of Daphnia exceeded 0.1 L L-1 d-1. There was significant positive correlation between mean cell or colony size of phytoplankters and clearance rate of D. galeata, showing an advantage of bigger cells or colonies at high grazing pressure. Cross-correlation indicated a time lag of about one month between changes in grazing pressure and a change in phytoplankton mean size. The phytoplankton species were divided into edible and inedible fractions, depending on their width and length. No edible species were able to reach high biovolumes during high biomasses of D. galeata but a positive correlation was found between the edible fraction of phytoplankton biovolume and the clearance rate of D. galeata. However, this relationship disappeared when the ‘ingestible edible’ fraction of M. aeruginosa was excluded, suggesting a rejection of ‘edible’ Microcystis colonies by D. galeata. A negative correlation was found between the inedible fraction of phytoplankton biovolume without M. aeruginosa and the clearance rate of D. galeata which might be due to superior competition of M. aeruginosa. We could clearly show that biomanipulation might not work well with respect to the reduction of total phytoplankton biomass under hypertrophic conditions and finally discuss a theoretical threshold of phosphorus (probably around 80 μg L-1), below which biomanipulation should become effective.

The feeding behaviour of large perch Perca fluviatilis (L.) in relation to food availability: a comparative study

The aim of the present study was to analyse and compare the feeding behaviour of large perch (>25 cm total length) in two lakes of different state and corresponding differences in food availability. Investigations were conducted in the Bautzen Reservoir, Germany, (biomanipulated, macrophyte cover <1%) and in Lake Ring, Denmark (non-biomanipulated, macrophyte cover ca. 20%). Fish were sampled regularly using trawl nets and gill nets. We focused our study on seasonal diet patterns. Further, we investigated size-related feeding and predation on both the juvenile fish and invertebrate communities. The availability of Chironominae was high in both lakes (e.g. ca. 1500 ind. m−2 in Lake Ring), but age-0 fish abundance was most probably higher in Bautzen Reservoir (6.4 individuals m−3 in the beginning of June 1998). Invertebrates and age-0 fish were the main food components of large perch of both populations. Mean lengths of prey fish were consistently smaller than those in situ. Large perch in Bautzen Reservoir started earlier to feed on age-0 fish, which was not related to prey fish sizes. Predation on the age-0 fish community was distinctly higher in Bautzen Reservoir than in Lake Ring. Generally, invertebrates represented the most important food of large perch in Lake Ring, whereas the by far dominant food resource of large perch in Bautzen Reservoir was the age-0 fish stock. In conclusion, it can be stated that the prey fish availability was an important factor in determining the feeding behaviour of large perch.

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).

The Roach Population in the Hypertrophic Bautzen Reservoir: Structure, Diet and Impact on Daphnia galeata

The structure and diet of the roach (Rutilus rutilus) population in the hypertrophic Bautzen Reservoir was examined from April to November 1998. Under the long-term impact of high predation pressure by piscivorous fish, a very heterogeneous population structure of roach had developed. Only a few age classes were dominant while other age classes were nearly absent. The proportion of males decreased with increasing age to 4% of the total abundance of one age class, which nevertheless seemed to have no negative effect on reproductive success. Food analysis revealed that the diet consisted of a high proportion of algae and macrophytes. The collapse of the Daphnia galeata population in early summer 1998 forced the roach to switch to benthic food resources [macroinvertebrates and fish: chironomids, molluscs and ruffe (Gymnocephalus cernuus)] in early June. Total consumption of age-2 and age-4 roach, the two most dominant year classes, was calculated by a bioenergetics model. Additionally, consumption of age-0 roach was estimated by assuming a fixed daily food consumption rate. The results suggest that daily consumption by these age groups, which never exceeded 0.2% of total biomass of the D. galeata population, had a negligible impact on the population of daphnids in Bautzen Reservoir during the period studied.

Reduction of the filtration rate of Daphnia galeata by dissolved photosynthetic products of edible phytoplankton

The filtration rate of Daphnia galeata was determined in in situ experiments in Bautzen Reservoir and in laboratory experiments, where daphnids were exposed to filtrates that previously contained either natural phytoplankton or cultured eukaryotic algae (Scenedesmus obliquus or Asterionella formosa), respectively. Individual filtration rate (FR) was measured using fluorescent beads, taking into account ingested beads in the gut only. Compared to heated control treatments (100 °C), dissolved compounds released by the nutritious cultured algae during the preconditioning phase or by the natural phytoplankton assemblages from Bautzen Reservoir strongly reduced the filtration rate of D. galeata (down to 60%). Heating deactivated these dissolved compounds. A significant correlation was found between primary production measured in situ and the reduction of FR in the filtrate of reservoir water, indicating that extra-cellular products released during photosynthesis triggered the reduction of the filtration rate. The ratio of ingested to collected beads was used to quantify the proportion of food, which was not only collected but passed the mouth of D. galeata. The ratio of ingestion to collection was compared between filtered and unfiltered reservoir water both media identical with respect to the concentration of dissolved compounds, whereas other factors (e.g. food concentration, temperature, filtration rate) were different. The changes in this ratio between filtered and unfiltered reservoir water suggest that D. galeata is capable of a chemosensory control of the ingestion behaviour by detecting external metabolites.

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.

Piscivory as a factor controlling invertebrate predators (Leptodora kindtii, Chaoborus flavicans, Piona spp.) in a biomanipulated reservoir

The dominance of invertebrate predators can exert predation pressure on herbivorous zooplankton equal to that of planktivorous fish. Although this observatian was supported by several whole lake experiments ( e.g. GLIWICZ & PREJS 1977, BENNDORF e t al. 2000), a determination o f the relative importance o f vertebrate and invertebrate predation have not yet been demonstrated. Our study focuses on quantifying changes in predation pressure on Daphnia by planktivorous fish and carnivorous zooplankton along a continuous gradient of increasing piscivory in Bautzen Reservoir (Germany). As a consequence of long-term biomanipulation (1981-1998), the proportion ofpiscivorous fish increased to about 60% ofthe total fish biomass in this reservoir. Since 1981 Daphnia galeata has become the dominant species within the zooplankton community (BENNDORF et al. 200 l). Based on our long-term ecological observation in Bautzen Reservoir, we depict how predator-prey interactions between invertebrate predators and D. galeata are affected by the increasing piscivorous biomass.