SUMMARY1. This synthesis examines 35 long-term (5-35 years, mean: 16 years) lake re-oligotrophication studies. It covers lakes ranging from shallow (mean depth <5 m and/or polymictic) to deep (mean depth up to 177 m), oligotrophic to hypertrophic (summer mean total phosphorus concentration from 7.5 to 3500 lg L )1 before loading reduction), subtropical to temperate (latitude: 28-65°), and lowland to upland (altitude: 0-481 m). Shallow northtemperate lakes were most abundant. 2. Reduction of external total phosphorus (TP) loading resulted in lower in-lake TP concentration, lower chlorophyll a (chl a) concentration and higher Secchi depth in most lakes. Internal loading delayed the recovery, but in most lakes a new equilibrium for TP was reached after 10-15 years, which was only marginally influenced by the hydraulic retention time of the lakes. With decreasing TP concentration, the concentration of soluble reactive phosphorus (SRP) also declined substantially. 3. Decreases (if any) in total nitrogen (TN) loading were lower than for TP in most lakes. As a result, the TN : TP ratio in lake water increased in 80% of the lakes. In lakes where the TN loading was reduced, the annual mean in-lake TN concentration responded rapidly. Concentrations largely followed predictions derived from an empirical model developed earlier for Danish lakes, which includes external TN loading, hydraulic retention time and mean depth as explanatory variables. 4. Phytoplankton clearly responded to reduced nutrient loading, mainly reflecting declining TP concentrations. Declines in phytoplankton biomass were accompanied by shifts in community structure. In deep lakes, chrysophytes and dinophytes assumed greater importance at the expense of cyanobacteria. Diatoms, cryptophytes and chrysophytes became more dominant in shallow lakes, while no significant change was seen for cyanobacteria. 5. The observed declines in phytoplankton biomass and chl a may have been further augmented by enhanced zooplankton grazing, as indicated by increases in the zooplankton : phytoplankton biomass ratio and declines in the chl a : TP ratio at a summer mean TP concentration of <100-150 lg L )1 . This effect was strongest in shallow lakes. This implies potentially higher rates of zooplankton grazing and may be ascribed to the observed large changes in fish community structure and biomass with decreasing TP contribution. In 82% of the lakes for which data on fish are available, fish biomass declined with TP. The percentage of piscivores increased in 80% of those lakes and often a shift occurred towards dominance by fish species characteristic of less eutrophic waters. 6. Data on macrophytes were available only for a small subsample of lakes. In several of those lakes, abundance, coverage, plant volume inhabited or depth distribution of submerged macrophytes increased during oligotrophication, but in others no changes were observed despite greater water clarity. 7. Recovery of lakes after nutrient loading reduction may be confounded by concomitant environmental cha...

1. The effect of benthivorous bream and carp on sediment resuspension and the concentrations of nutrients and chlorophyll a were studied in sixteen experimental ponds (mean depth lm, mean area 0.1 ha, sandy clay/clay sediment), stocked with bream or carp at densities varying from 0 to 500kgha~\ Planktivorous perch {Perca fluviatilis L.) were added to some ponds to suppress zooplankton. 2. Suspended sediment concentrations increased linearly with biomass of benthivorous fish. Bream caused an increase of 46g sediment m"^ day"' per 100kg bream ha~' and a reduction of 0.38 m~' in reciprocal Secchi disc depth, corresponding to an increase in the extinction coefficient of 0.34m~V 3. No relationship was found between size of fish and amount of resuspension, but the effect of bream was twice as great as that of carp. Benthivorous feeding was reduced in May because alternative food (zooplankton) was available. 4. Assuming a linear relationship, chlorophyll a level increased by 9.0 [igl"', total P by 0.03mgr' and Kjeldahl-N by 0.48mgr' per 100kg bream ha^V Silicate, chlorophyll a, total P and total N were all positively correlated with fish biomass, but orthophosphate showed no correlation.

No abstract

During the unusually long European drought between 2000 and 2003, the water level of the large and shallow Lake Balaton, Hungary (area 5 596 km 2 , mean depth 5 3.25 m), decreased by 28%. Although food availability for zooplankton remained unchanged, and the fish stock declined more than the water mass, the density of populations of several planktonic rotifers, cladocerans, calanoid copepodes, and veligers decreased by 60-90% simultaneously with the water-level decrease and regenerated only after the drought. The generally strong turbulence of the lake was intensified during the four consecutive years of low water, as verified by instrumental monitoring of the turbulence intensity and by the estimation of the turbulent kinetic-energy dissipation rate. In our tank experiments, turbulence conditions similar to those that existed in the lake during low water were simulated, but mineral suspended material was minimized and food was regularly resupplied. Under these experimental conditions, zooplankton taxa showing the highest mortality were the same as those that were most susceptible in situ. Increased turbulence coupled with the water-level decrease is especially unfavorable for rotifer Keratella, the cladoceran Daphnia, Bosmina, and copepodit and adult stages of the calanoid copepod Eudiaptomus gracilis in this lake.Moderate turbulence intensity is beneficial for zooplankton, because it increases the producer-consumer encounter rate, and improves consumers' capture efficiency, feeding, and growth rates (Saiz and Alcaraz 1991; Visser and Stips 2002;Visser et al. 2009). High turbulence, however, interferes with the feeding current of the suspension feeders, erodes the remote detection of prey by their predators, inhibits their food ingestion, and increases their metabolic energy expenditures Kiørboe and Saiz 1995;Peters and Marrasé 2000). And finally, very high turbulence could disorientate the movement, lead to uncontrolled vertical drifting, and could even injure the largest bodied organisms (Reynolds 1992;Visser et al. 2009).Earlier we proved experimentally and quantified the relationship between frequently resuspended fine mineral sediment (mineral turbidity) of Lake Balaton and inhibition of the effective food collection and development of cladocerans, especially Daphnia, even if food abundance exceeded threshold concentrations (G.-Tó th 1984; G. -Tó th et al. 1986). However, the direct physical effect of turbulence on the zooplankton was never established in Lake Balaton or any other shallow lake.Lake Balaton is the largest shallow lake in Central Europe. Because the lake is very shallow, the vertical space for the dissipation of the kinetic energy content of the water mass induced by the wind is very limited, and so dissipation per unit of volume is very high. Higher than 4 m s 21 wind velocity occurs, on average, every third day and causes a turbulence sufficient to resuspend sediments. Wind faster than 10 m s 21 occurs , 12-15 times annually, resulting in sediment resuspension up to 600 mg dry wei...