Biomanipulation-induced reduction of sediment phosphorus release in a tropical shallow lake

Biomanipulation via fish regulation combined with submerged plant introduction is an effective measure to restore eutrophic shallow lakes. Improved water quality and clarity promote growth of benthic algae, which with submerged plants may limit sediment phosphorus (P) release, thereby reinforce lake recovery. Our study sought to evaluate the effect of such a biomanipulation on water quality, benthic algal development and sediment P release in a shallow, tropical lake by (1) comparing porewater and lake water quality, light intensity and benthic algal development in restored and unrestored sections; (2) conducting a ³²P radiotracer experiment to track P release from sediment cores sampled from both sections. The biomanipulation led to lower total P, total dissolved P, and soluble reactive P concentrations in lake water, lower phytoplankton biomass, and increased light intensity at sediment surface, stimulating benthic algal development. Moreover, sediment ³²P release was lower in the restored than unrestored section. Concurrently, dissolved oxygen levels in upper layers of the sediment cores were higher in the restored section. Our study indicates that the biomanipulation improved water quality and enhanced growth of benthic algae, thereby reducing sediment P release, which may be one of the main mechanisms to create successful restoration.     

Sediment phosphorus cycling in a large shallow lake: spatio-temporal variation in phosphorus pools and release

Sediment and water column phosphorus fractions were recorded monthly for one year (April 2004–April 2005) in a shallow lake recovering from nutrient pollution (Loch Leven, Scotland). Equilibrium phosphate concentration (EPC0) and gross sediment phosphorus (P) release rates were estimated from laboratory experiments. Pore water and organic P pools were lowest during warm water periods whereas bottom water P was lowest during cold water periods. Reductant-soluble, organic, metal oxide-adsorbed, residual and sediment total phosphorus pools all varied significantly with overlying water depth. Short-term, high magnitude, redox initiated P release events occurred in late summer and winter as a result of anoxic sediment conditions. Lower magnitude long-term release conditions were maintained for most of the year, most likely as a result of organic P cycling and maintenance of high concentration gradients between the pore and bottom water P pools. Estimates of summer P uptake/release rates, across an intact sediment-water interface, suggested that maximum gross internal release was ~12 mg SRP m⁻² lake surface area d⁻¹ with EPC0 values ranging between 180 and 270 μg P L⁻¹. This study highlights the biological mediation of internal loading in shallow eutrophic lakes, and in particular, the role of sediment algae in decreasing, and sediment bacteria in enhancing, sediment P release.     

Benthic shear stress gradient defines three mutually exclusive modes of non-biological internal nutrient loading in shallow lakes

Assessment of the importance of internal nutrient loading is essential for managing and restoring eutrophic shallow lakes. To date, studies of internal loads have tended to focus on one of two abiotic processes, either molecular diffusion or sediment/nutrient entrainment (resuspension). This study presents a new approach to determining the non-biological fluxes of nitrogen (N) and phosphorus (P) from the sediment to the water column of shallow lakes. Three mutually exclusive flux processes: (i) molecular diffusion, (ii) turbulent diffusion (eddy diffusivity) and (iii) wind-induced resuspension of N and P, were related to a gradient of benthic shear stress. A model presented here allowed the durations and magnitudes of different non-biological fluxes to be calculated over time, based on benthic shear stress. Two site-specific critical shear stress thresholds determined which of the three flux processes dominated for any benthic shear stress value. The model was calibrated for a shallow lake and the continuous flux of nutrient from the sediment to the overlying water generated by each process during that period was calculated, enabling the estimation of the relative importance of each of the three flux processes over a one-year period. Wind-induced resuspension dominated the internal nutrient flux, operating for 38% of the time and contributing 0.9 T P year⁻¹ and 10.2 T N year⁻¹ to the internal nutrient load. In contrast, molecular diffusion only contributed 0.01–0.02 T P year⁻¹ and 0.12–0.20 T N year⁻¹ to the water column, while turbulent diffusion provided up to 0.6 T P year⁻¹ and 6.2 T N year⁻¹. Our model suggests that turbulent diffusion is a neglected and potentially important process contributing to internal nutrient loading in shallow lakes, whereas molecular diffusion appears to be relatively unimportant in lakes that experience turbulence at the sediment–water interface. Handling editor: L. Naselli-Flores     

Quantifying the impact of periphytic algae on nutrient availability for phytoplankton

SUMMARY.

• 1 Recent laboratory studies demonstrate that periphytic algae growing on the sediment surface reduce nutrient availability in the overlying water. Consequently, periphytic algae may competitively reduce growth of phytoplankton.
• 2 The aim of this study was to quantify the competitive impact of sediment-attached periphytic algae on phytoplankton in the presence of all other factors simultaneously affecting nutrient dynamics in natural systems.
• 3 In enclosure experiments, performed in three lakes of different productivity, the periphytic algal biomass was manipulated. When compared to enclosures with high biomass of periphytic algae, those with reduced biomass showed an increase in total phosphorus concentration in the water of 32–44%. Extrapolation of the experimental results to whole lakes predicts an increase in original total phosphorus concentration of between 1.5% and 8.0%. According to existing regressions between total phosphorus and phytoplankton chlorophyll, the potential increase in original phytoplankton biomass will be between 2.5% and 12.6%.
• 4 With respect to the shallow parts of lakes, my results support the conclusions revealed from laboratory studies that periphytic algae have a significant impact on the phosphorus concentration in the overlying water. However, when considering whole-lake dynamics, the competitive impact of periphytic algae on phytoplankton biomass development is probably of minor importance.
• 5 Rather, the main competitive advantage of growing on the sediment surface, compared to in the water, may be the exclusive access to nutrients in the sediment.

     

Chara beds acting as nutrient sinks in shallow lakes—a review

The capability of Chara beds to act as nutrient sinks in shallow lakes is reviewed. Under favorable conditions charophytes form dense meadows. Biomass and nutrient content in such beds are comparable or even higher than in beds of vascular aquatic macrophytes. As some Chara species are capable of overwintering, the nutrient storage in plant biomass may extend beyond the growing season. Some commonly observed phenomena in vascular plants (nutrient uptake and mobilization of nutrients from the sediment) appear to be unlikely or negligible in Characeae. Charophytes have been reported to decompose slower than their vascular counterparts prolonging nutrient storage in plant biomass.Charophytes may also indirectly affect nutrient cycling in lakes. Utilization of bicarbonate is accompanied by precipitation of calcite during periods of intensive photosynthesis, favoring immobilization of P by binding in the crystal structure or sorption on sedimenting mineral particles. Charophytes are able to deliver oxygen to the sediment, thus potentially enhancing nitrification/denitrification processes and preventing iron-bound sediment phosphorus from being released to the overlying water. Furthermore, dense Chara meadows restrict sediment resuspension, consequently blocking an important internal source of nutrients to planktonic algae. We conclude that Chara meadows probably are an efficient nutrient trap in shallow lakes.     

Simulation study on water quality based on sediment release flume experiment in Lake Taihu, China

Sediments have a significant influence on the overlying water, and nutrient release from sediments is an important source for lake eutrophication, particularly in shallow lakes. Sediment resuspension is primarily driven by wind-induced currents. In this research, the correlation between release rate of suspended sediment and flow velocity was studied, and an experiment on hydrodynamic forces was conducted in a rectangle flume using water and sediments collected from three sites in Lake Taihu, a eutrophic lake in China. It was shown that the starting velocities of sediment in Lake Taihu at three different incipient standards gained from the experiment were 15, 30, and 40 cm s⁻¹ and the release rate of suspended sediment could reach up to 643.4, 5377.1, and 13980.5 g m⁻² d⁻¹, respectively. Based on the experiment, a water quantity and quality numerical model of wind-induced current with sediment pollution for Lake Taihu was developed. The model was calibrated and validated by applying it to the study of the water quality of Lake Taihu. The calculated values were generally in good agreement with field observations, which indicated that the developed model could represent the dynamics of sediment resuspension to a certain extent. This study provides a new approach and a practical tool for planning and management policy and operations to protect the water quality and ecosystems of shallow lakes.     

The importance of sediment phosphorus release in the restoration of very shallow lakes (The Norfolk Broads,England) and implications for biomanipulation

Phosphorus release from the sediments of very shallow lakes, the Norfolk Broads, can be as high as 278 mgP m^-2 d^-1. These high rates are associated with high total sediment Fe:P ratios and occur when sulphide from sulphate reduction removes Fe(II) from the pore water. There is also evidence that bioturbation from benthic chironomids can enhance phosphorus release rates, particularly in sediments low in total iron. The release of phosphorus from the sediments of these lakes is delaying restoration following the control of phosphorus from sewage discharges. Biomanipulation is being used in these lakes to create clear water and re-establish aquatic macrophytes. This removal of fish has allowed larger populations of benthic chironomid larvae to develop which may result in an increase in the rate of phosphorus release and changes to the pore profiles of dissolved phosphorus, soluble iron and free sulphide.     

Development and application of a technique for estimating nutrient deficiency in soft sediments

A diffusion enrichment technique is presented which allows for chemical enrichment of soft surficial and shallow subsurface sediments and subsequent measurement of O₂ production. The sediment is enriched by inserting a perforated tube containing dialysis tubing filled with a nutrient/agar mixture. O₂ production by surficial sediment is measured using an inverted, translucent, polyethylene chamber over the sediment. The inside of the chamber contains a collapsible bag connected to the water outside the chamber. When water overlying the sediment is withdrawn from a sampling port, it is displaced with water from outside the chamber, thus preventing contamination of water samples with pore water from below. The technique was tested by enriching near-shore sediments in a large oligotrophic lake with inorganic N and P. NHinf4/^p+ additions significantly stimulated benthic primary production as measured by 0₂ production, whereas enrichment with POinf4/^3- had no effect.     

Nutrient uptake and benthic regeneration in Danube Delta Lakes

We investigated the nutrient uptake capacity of three lakes (Uzlina, Matita and Rosu) within the Danube Delta during high water level in June and low water level in September 1999. Special emphasis was placed on nutrient cycling at the sediment-water interface and on the self-purification capacity of the lakes in the Danube Delta. In order to estimate the nutrient uptake of selected lakes we present in this paper the results of water analyses, benthic flux chamber experiments and deck incubation experiments of ¹⁵N-labeled sediment cores at the inflow and the outlet of the lakes. The external input of dissolved inorganic nitrogen and silica into the lakes decreases with increasing distance to the main Danube branches whereas the total dissolved phosphorus input is independent of the hydrological distance to the main branches. The nutrient loading is highest in the inflow channels, and decreases towards the outflow of the lakes. In June, the uptake of NO₃ ^–, TDP and Si(OH)₄ in the lakes was higher than in September. In contrast, NH₄ ⁺ uptake was more intense in September, when benthic release was more intense as well. On average, about 76% of the external plus internal nitrogen and phosphorus input into the lakes was taken up by macrophytes and phytoplankton during the growing season, whereas the uptake of external nutrient input amounted to about 43%. The benthic release of ammonia and silica increases from June to September and indicates, that part of the nutrients taken up during the growing season might be released during winter. We estimate the net impact of the Delta on the nutrient reduction of the Danube during the growing season is about 4.3%, assuming 10% of the Danube water is flowing through the Delta.     

Distribution and release of sedimentary phosphorus in Lake Ladoga

Sedimentary phosphorus fractions and phosphorus release from the sediments were studied in Lake Ladoga at altogether 46 sampling sites, representing the full range of sediment types encountered in the lake. Determination of P fractions and physico-chemical analyses were made of surface sediment cores (10–20 cm long, each sampled at 3–4 levels) and in the overlying water. The range of total phosphorus per dry weight of sediment was 0.2–3.3 mg g^–1, and that of inorganic P 0.1–2.5 mg g^–1. The levels of interstitial soluble phosphorus, range 2–613 µg 1^–1 for total P and 1–315 µg 1^–1 for inorganic P, were higher than those of dissolved P concentrations in the overlying water. Diffusive fluxes of phosphate from sediment to the overlying water were estimated using three independent methods. The estimated range was 4–914 µg P m^–2 d^–1; the mean value for the whole bottom area, 0.1 mg P m^–2 d^–1, is lower than previously published estimates. The estimated annual contribution of sedimentary inorganic P flux to Lake Ladoga water is equal to 620 tons of P per year, which amounts to more than 10% of the estimated external P load into the lake. 68% of the total diffusive flux emanates from deep water sediments, which are not exposed to seasonal variation of conditions. In deep lakes, such as Lake Ladoga, phosphorus release from the sediments is controlled primarily by diffusive mechanisms. Wave action and currents as well as bioturbation are probably of importance mainly in shallow near-shore areas. Phosphorus release by gas ebullition and macrophytes is considered negligible.     

Effects of Terrestrial Organic Matter on Aquatic Primary Production as Mediated by Pelagic–Benthic Resource Fluxes

Flows of energy and matter across habitat boundaries can be major determinants of the functioning of recipient ecosystems. It is currently debated whether terrestrial dissolved organic matter (tDOM) is a resource subsidy or a resource subtraction in recipient lakes. We present data from a long-term field experiment in which pelagic phosphorus concentration and whole-ecosystem primary production increased with increasing tDOM input, suggesting that tDOM acted primarily as a direct nutrient subsidy. Piecewise structural equation modeling supports, however, a substantial contribution of a second mechanism: colored tDOM acted also as a resource subtraction by shading benthic algae, preventing them from intercepting nutrients released across the sediment–water interface. Inhibition of benthic algae by colored tDOM thus indirectly promoted pelagic algae and whole-ecosystem primary production. We conclude that cross-ecosystem terrestrial DOM inputs can modify light and nutrient flows between aquatic habitats and alter the relative contributions of benthic and pelagic habitats to total primary production. These results are particularly relevant for shallow northern lakes, which are projected to receive increased tDOM runoff.     

背角无齿蚌(Anodonta woodiana)对浅水系统底栖和浮游藻类竞争关系的影响

底栖藻类和浮游藻类之间的竞争关系对浅水生态系统的结构、功能具有重要的影响,双壳类可通过滤食控制浮游藻类,从而改变底栖藻类与浮游藻类之间的竞争结果。论文通过比较放养背角无齿蚌(Anodonta woodiana)(蚌处理组)与不放养背角无齿蚌(对照组)系统中底栖藻类、浮游藻类的生物量和优势种等的变化,研究了滤食性双壳类对底栖藻类和浮游藻类间竞争的影响。结果表明,背角无齿蚌可显著降低浮游藻类生物量,提高水体透明度和沉积物表面光照条件,从而显著提高底栖藻类的生物量;背角无齿蚌也改变了浮游藻类的优势种,使优势种由蓝藻转变成硅藻。因此,滤食性双壳类有利于促进浅水生态系统从混水态向清水态转变,本研究结果对富营养化浅水湖泊修复与管理具有一定的参考意义。     

In situ phosphorus release experiments in the Warnow River (Mecklenburg,northern Germany)

Results are presented of in situ benthic phosphorus release experiments in an undercut bank of an impounded river. Due to high sedimentation of phytoplankton biomass high oxygen consumption rates between 259.4 and 947.0 mg O₂ m^–2 d^–1 developed, leading to almost anaerobic conditions and phosphorus releases between 175.2 and 236.3 mgP m^–2 d^–1 over a period of 18 days.In a second series of experiments the water column overlying the sediment was aerated, resulting in much lower P release rates (1.1 to 32.9 mgP m^–2 d^–1) over a period of 30 days. The influence of pH and nitrate was studied by adjusting pH and adding NO₃ ^– to the overlying water. Increasing pH positively affected P release rates and enhanced NO₃ ^– levels led to an increase of benthic P release, too.     

Undesirable side-effects of water hyacinth control in a shallow tropical reservoir

1. Based on a comprehensive data set collected monthly during 8 years (1997–2004), we evaluated the effects of mechanical removal of Eichhornia crassipes on the limnological characteristics and algal biomass of a polymictic shallow tropical reservoir. 2. Interrupted time series analyses indicated that the limnological responses to macrophyte removal can be classified as an ‘abrupt permanent impact’ implying that the overall mean of the time‐series shifted promptly after intervention. These analyses indicated a significant increase for pH, total phosphorus, total phytoplankton and cyanobacterial biomass, and a decrease in water transparency and CO₂ concentrations in the surface water; also, the increase in water stability, increase of bottom soluble reactive phosphorus (SRP) and decrease in bottom oxygen levels. 3. Cyclic anoxic periods previously observed during springs and summers were replaced by a persistent period of anoxic conditions in the sediment overlying water. Anoxic conditions were suitable for SRP release from sediments. Heavy cyanobacterial blooms became more persistent, maximum biomass (4229 mm³ L⁻¹) was 30 times larger, the blooms frequently reached 2 m and sometimes the bottom of the reservoir, contrasting to the preremoval period in which it reached at most 1 m deep. 4. The long‐term P dynamics in the system, initially driven by allochthonous nutrient loadings were replaced by internal ecological processes. Water hyacinth removal markedly accelerated the process of eutrophication due to internal feedback mechanisms, leading to a switch to a more turbid state. Biological feedback mechanisms were driven by cyanobacterial blooms by enhancing water stability, oxygen anoxia at the bottom and by increasing suitable conditions for P internal loading. These data support the hypothesis of the role of cyanobacterial blooms as an important factor impairing water quality and driving the ecosystem towards a stable degraded state. 5. These findings have important implications for the restoration of shallow stratifying eutrophic lakes, as the alternative degraded state is most likely to occur when compared with their non‐stratifying counterparts. Moreover, feedback mechanisms in tropical and subtropical shallow lakes seem to be stronger than in temperate ones, as stratification events are more likely to occur over the year, intensifying system resilience to restorative strategies.     

Phosphorus cycling in a dense Potamogeton pectinatus L. Bed

Summary An analysis of phosphorus concentrations and standing stocks in the various components of a dense Potamogeton pectinatus L. community in a brackish lake showed that the amounts of P per unit area varied in the following order: Sediments > Above ground macrophytes > Detritus > Benthic Invertebrates > Below ground macrophyte tissue > Epiphytic algae > Water soluble P > Water borne particulate P. Seasonal changes in the variability of these stocks are described, and shown to be controlled by the annual growth and decomposition pattern of the Potamogeton. The sediments were suggested as the major source of P for the plant biomass. Studies using ³²P tracer showed that of a given input of P to the water, 32% went to large epiphytic algae, 17% to the Potamogeton, 16% to the benthic fauna (mostly filter feeding bivalves), 28% to the sediments (almost all incorporated in the top 1 cm), and the remaining 7% was adsorbed on to, or absorbed by microorganisms associated with detritus. Analysis of ³²P uptake curves indicated that of the P absorbed by the Potamogeton, a significant proportion went to the complex adnate periphyton on the leaf surface. We were unable to separate this fraction. Movement of P in the community was shown to be a closed cycle, and any release of P from decaying macrophytes would be rapidly reabsorbed by epiphytic algae. It is unlikely that phosphorus, once cycling in the macrophyte community, would become transferred to the circulation in the open lake.     

Mesocosm experiments on nutrient and fish effects on shallow lake food webs in a Mediterranean climate

1. Nutrient and fish manipulations in mesocosms were carried out on food‐web interactions in a Mediterranean shallow lake in south‐east Spain. Nutrients controlled biomass of phytoplankton and periphyton, while zooplankton, regulated by planktivorous fish, influenced the relative percentages of the dominant phytoplankton species. 2. Phytoplankton species diversity decreased with increasing nutrient concentration and planktivorous fish density. Cyanobacteria grew well in both turbid and clear‐water states. 3. Planktivorous fish increased concentrations of soluble reactive phosphorus (SRP). Larger zooplankters (mostly Ceriodaphnia and copepods) were significantly reduced when fish were present, whereas rotifers increased, after fish removal of cyclopoid predators and other filter feeders (cladocerans, nauplii). The greatest biomass and diversity of zooplankton was found at intermediate nutrient levels, in mesocosms without fish and in the presence of macrophytes. 4. Water level decrease improved underwater light conditions and favoured macrophyte persistence. Submerged macrophytes (Chara spp.) outcompeted algae up to an experimental nutrient loading equivalent to added concentrations of 0.06 mg L^?1 PO₄‐P and 0.6 mg L^?1 NO₃‐N, above which an exponential increase in periphyton biomass and algal turbidity caused characean biomass to decline. 5. Declining water levels during summer favoured plant‐associated rotifer species and chroococcal cyanobacteria. High densities of chroococcal cyanobacteria were related to intermediate nutrient enrichment and the presence of small zooplankton taxa, while filamentous cyanobacteria were relatively more abundant in fishless mesocosms, in which Crustacea were more abundant, and favoured by dim underwater light. 6. Benthic macroinvertebrates increased significantly at intermediate nutrient levels but there was no relationship with planktivorous fish density. 7. The thresholds of nutrient loading and in‐lake P required to avoid a turbid state and maintain submerged macrophytes were lower than those reported from temperate shallow lakes. Mediterranean shallow lakes may remain turbid with little control of zooplankton on algal biomass, as observed in tropical and subtropical lakes. Nutrient loading control and macrophyte conservation appear to be especially important in these systems to maintain high water quality.     

Contrasting responses of phytoplankton and benthic algae to recent nutrient enrichment in Arctic tundra ponds

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The role of phosphorus release by roach [Rutilus rutilus (L.)] in the water quality changes of a biomanipulated lake

SUMMARY 1. Following fish removal, the water quality in biomanipulated lakes often improves concomitant with decreased phosphorus (P) levels. Because the decrease in P concentrations derives most probably either directly or indirectly from fish, which are the main target of biomanipulation, this study examined the P release of 0+, 1+ and 2+ roach [Rutilus rutilus (L.)] and changes in the P release during summer in a shallow eutrophic lake in Finland. 2. The P release was separated into P derived from benthic and littoral food items and into recycled P derived from feeding on zooplankton, to estimate the contribution of net P additions to the water column by the fish to the increase in P concentrations of the lake water (75–110 mg P m^?3) in summer 1991–96. 3. Individual P release of roach by both egestion and excretion was estimated with a bioenergetics model. The size of the roach population was estimated with a depletion method and the proportions of different age groups from catch samples, using a programme separating mixtures of normal distributions. The sensitivity of the release estimates to variation in the growth data was estimated with the jackknife technique. 4. The biomass‐specific P release by 0+ roach (0.36–0.54 mg P g^?1 day^?1) was higher than that by older roach (0.07–0.16 mg P g^?1 day^?1) throughout the summer. The P release by the whole roach population deriving from benthic and littoral food items (0.7–2.7 mg m^?3 during July to August, representing a net addition to the water column) was 5–19 times lower in 1991–96 than the recycled P release deriving from zooplankton (8.9–25.7 mg m^?3), and too low to explain the increase in the P concentration of the lake water during the summer. Because the biomass‐specific P release and roach diet composition vary with fish age, it is important to consider the age structure of fish populations to obtain correct estimates of P release and net additions to the water column. 5. The removal of roach by fishing diminished the roach stock greatly, but the fish‐mediated P release to the water column changed little. This effect was because of the high compensation capacity of the roach population, leading to high recruitment of young fish with higher biomass‐specific P release rates. 6. External loading is very low during summer months and therefore it cannot explain the increase in the P concentration of water during that time. Internal loading from the sediment might be as high as 10.2 mg P m^?2 day^?1, i.e. 50 times higher than the maximum net P addition by the total roach population.     

Climate drivers of diatom distribution in shallow subarctic lakes

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Influence of diet shifts in underyearling fish on phosphorus recycling in a hypertrophic biomanipulated reservoir

1. In addition to effects of direct predation by planktivorous fish, nutrient recycling by fish may also contribute to structuring foodwebs in lakes. There is little evidence, however, about whether underyearling fish undergoing several ontogenetic diet shifts may have a comparable bottom-up impact. 2. This study examined seasonal patterns of phosphorus (P) concentration and external load, phytoplankton, zooplankton and benthos, and diet shifts in three underyearling fish [perch (Perca fluviatilis), roach (Rutilus rutilus) and ruffe (Gymnocephalus cernuus)] in the shallow, hypertrophic biomanipulated Bautzen reservoir, Germany. Phosphorus metabolism of fish was calculated by a balanced bioenergetics model on the basis of fish diet, growth and water temperature. 3. The fish showed several shifts from planktivory to other food sources during the sampling period from May to September. These shifts were probably caused by the seasonal succession of the zooplankton community, mainly the midsummer decline of Daphnia galeata. 4. The diet shifts in fish also had consequences for the amount of P consumed and released. During periods of dominant zooplanktivory, the excretion of P did not exceed the removal of P stored in pelagic prey. By contrast, if benthivory dominated, fish subsidized the pelagic P pool by excreting more P from benthic prey than had been removed from the pelagic area. This occurred predominantly in perch and ruffe during periods of low zooplankton biomass, whereas the roach ate more algae and therefore excreted less P of benthic origin. 5. Phosphorus release by underyearling fish was estimated at a maximum of 0.1 mg m^–3 JY day^–1. This value was negligible compared with both the external load of P to Bautzen reservoir and the concentration of P in the pelagic area during summer. It is therefore concluded that both the predominance of underyearling zooplanktivorous fish and the high Daphnia biomass during certain periods of the year in the Bautzen reservoir may be the reason that nutrient release by the fish structured the foodweb only marginally. 6. This study suggests that biomanipulation has altered both top-down and bottom-up impacts of fish in Bautzen reservoir. The highest efficiency of foodweb manipulations may be obtained after reduction of the external P loading below a certain threshold. In turn, if external restoration of eutrophied lakes is not accompanied by changes in fish community, then the combined forces of strong zooplanktivory and high P recycling of dense stocks of zooplanktivorous and benthivorous fish may hold the water in a eutrophic-like stage, even if external load has been significantly reduced.