Stabilizing the clear-water state in eutrophic ponds after biomanipulation: submerged vegetation versus fish recolonization

Biomanipulation through fish removal is a tool commonly used to restore a clear-water state in lakes. Biomanipulation of ponds is, however, less well documented, although their importance for biodiversity conservation and public amenities is undisputed. In ponds, a more complete fish removal can be carried out as compared to lakes and therefore a stronger response is expected. Fish recolonization can, however, potentially compromise the longer term success of biomanipulation. Therefore, we investigated the impact of fish recolonization on zooplankton, phytoplankton, and nutrients for several years after complete drawdown and fish removal in function of submerged vegetation cover in 12 peri-urban eutrophic ponds situated in Brussels (Belgium). Fish recolonization after biomanipulation had a considerable impact on zooplankton grazers, reducing their size and density substantially, independent of the extent of submerged vegetation cover. Only ponds with <30% cover of submerged vegetation shifted back to a turbid state after fish recolonization, coinciding with an increase in density of small cladocerans, rotifers, and cyclopoid copepods. In ponds with >30% submerged vegetation cover, macrophytes prevented an increase in phytoplankton growth despite the disappearance of large zooplankton grazers. Our results suggest that macrophytes, rather than by providing a refuge for zooplankton grazers, control phytoplankton through other associated mechanisms and confirm that the recovery of submerged macrophytes is essential for biomanipulation success. Although the longer term effect of biomanipulation is disputable, increased ecological quality could be maintained for several years, which is particularly interesting in an urban area where nutrient loading reduction is often not feasible.     

A comparison of zooplankton densities and biomass in Lakes Peipsi and Võrtsjärv (Estonia): rotifers and crustaceans versus ciliates

The abundance and biomass of ciliates, rotifers, cladocerans and copepods were studied in Lake Peipsi and Lake Võrtsjärv, both of which are shallow, turbid and large. Our hypothesis was that in a large shallow eutrophic lake, the ciliates could be the most important zooplankton group. The mean metazooplankton biomass was higher in Peipsi than in Võrtsjärv (mean values and SD, 1.8 ± 0.7 and 1.3 ± 0.6 mg WM l^?1). In Peipsi, the metazooplankton biomass was dominated by filtrators that feed on large-sized phytoplankton and are characteristic of oligo-mesotrophic waters. In Võrtsjärv, the metazooplankton was dominated by species characteristic of eutrophic waters. The planktonic ciliates in both lakes were dominated by oligotrichs. The biomass of ciliates was much greater in Võrtsjärv (mean 2.3 ± 1.4 mg WM l^?1) than in Peipsi (0.1 ± 0.08 mg WM l^?1). Ciliates formed about 60% of the total zooplankton biomass in Võrtsjärv but only 6% in Peipsi. Thus, the food chains in the two lakes differ: a grazing food chain in Peipsi and a detrital food-chain in Võrtsjärv. Consequently, top-down control of phytoplankton can be assumed to be much more important in Peipsi than in Võrtsjärv. When the detrital food chain prevails, the planktonic ciliates become the most important zooplankton group in shallow, eutrophic and large lake. Neglecting protozooplankton can result in serious underestimates of total zooplankton biomass since two-thirds of the zooplankton biomass in Võrtsjärv comprises ciliates.     

Dust mediated transfer of phosphorus to alpine lake ecosystems of the Wind River Range,Wyoming, USA

Alpine lakes receive a large fraction of their nutrients from atmospheric sources and are consequently sensitive to variations in both the amount and chemistry of atmospheric deposition. In this study we explored the spatial changes in lake water chemistry and biology along a gradient of dust deposition in the Wind River Range, Wyoming. Regional differences were explored using the variation in bulk deposition, lake water, sediment, and bedrock geochemistry and catchment characteristics. Dust deposition rates in the Southwestern region averaged 3.34 g m^?2 year^?1, approximately three times higher than deposition rates in the Northwestern region (average 1.06 g m^?2 year^?1). Dust-P deposition rates ranged from 87 µg P m² day^?1 in the Northwestern region to 276 µg P m² day^?1 in the Southwestern region. Subalpine and alpine lakes in the Southwestern region had greater total phosphorus (TP) concentrations (5–13 µg L^?1) and greater sediment phosphorus (SP) concentrations (2–5 mg g^?1) than similar lakes elsewhere in the region (1–8 µg L^?1 TP, 0.5–2 mg g^?1 SP). Lake phosphorus concentrations were related to dissolved organic carbon (DOC) across vegetation gradients, but related to the percent of bare rock, catchment area to lake area, and catchment steepness across dust deposition gradients. Modern phytoplankton and zooplankton biomasses were two orders of magnitude greater in the Southwest than in the Northwest, and alpine lakes in the Southwest had a unique diatom species assemblage with relatively higher concentrations of Asterionella formosa, Pseudostaurosira pseudoconstruens, and Pseudostaurosira brevistriata. These results suggests that catchment controls on P export to lakes (i.e. DOC) are overridden in dominantly bare rock basins where poor soils cannot effectively retain dust deposited P.     

Does high nitrogen loading prevent clear‐water conditions in shallow lakes at moderately high phosphorus concentrations?

1. The effect of total nitrogen (TN) and phosphorus (TP) loading on trophic structure and water clarity was studied during summer in 24 field enclosures fixed in, and kept open to, the sediment in a shallow lake. The experiment involved a control treatment and five treatments to which nutrients were added: (i) high phosphorus, (ii) moderate nitrogen, (iii) high nitrogen, (iv) high phosphorus and moderate nitrogen and (v) high phosphorus and high nitrogen. To reduce zooplankton grazers, 1⁺ fish (Perca fluviatilis L.) were stocked in all enclosures at a density of 3.7 individuals m^?2. 2. With the addition of phosphorus, chlorophyll a and the total biovolume of phytoplankton rose significantly at moderate and high nitrogen. Cyanobacteria or chlorophytes dominated in all enclosures to which we added phosphorus as well as in the high nitrogen treatment, while cryptophytes dominated in the moderate nitrogen enclosures and the controls. 3. At the end of the experiment, the biomass of the submerged macrophytes Elodea canadensis and Potamogeton sp. was significantly lower in the dual treatments (TN, TP) than in single nutrient treatments and controls and the water clarity declined. The shift to a turbid state with low plant coverage occurred at TN >2 mg N L^?1 and TP >0.13–0.2 mg P L^?1. These results concur with a survey of Danish shallow lakes, showing that high macrophyte coverage occurred only when summer mean TN was below 2 mg N L^?1, irrespective of the concentration of TP, which ranged between 0.03 and 1.2 mg P L^?1. 4. Zooplankton biomass and the zooplankton : phytoplankton biomass ratio, and probably also the grazing pressure on phytoplankton, remained overall low in all treatments, reflecting the high fish abundance chosen for the experiment. We saw no response to nutrition addition in total zooplankton biomass, indicating that the loss of plants and a shift to the turbid state did not result from changes in zooplankton grazing. Shading by phytoplankton and periphyton was probably the key factor. 5. Nitrogen may play a far more important role than previously appreciated in the loss of submerged macrophytes at increased nutrient loading and for the delay in the re‐establishment of the nutrient loading reduction. We cannot yet specify, however, a threshold value for N that would cause a shift to a turbid state as it may vary with fish density and climatic conditions. However, the focus should be widened to use control of both N and P in the restoration of eutrophic shallow lakes.     

Lake responses to reduced nutrient loading – an analysis of contemporary long-term data from 35 case studies

1. 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 μg L^?1 before loading reduction), subtropical to temperate (latitude: 28–65°), and lowland to upland (altitude: 0–481 m). Shallow north‐temperate 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 μg 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 changes such as global warming. However, effects of global change are likely to run counter to reductions in nutrient loading rather than reinforcing re‐oligotrophication.     

Precipitation-Induced Alternative Regime Switches in Shallow Lakes of the Boreal Plains (Alberta,Canada)

This study focused on unraveling the natural mechanism for the frequent shifts in alternative regimes in pristine shallow lakes of the Boreal Plains, Alberta, Canada. The lakes tend to be clear and dominated by submerged aquatic vegetation (SAV) or turbid and dominated by phytoplankton. We report on the inter-annual response of 23 lakes from 2001 to 2007. We explore the effect of fluctuations in annual precipitation on the lake response including water depth, total phosphorus (TP) concentration, turbidity, phytoplankton biomass, SAV biomass, and the proportion of clear and turbid lakes. The regime switches appear driven by the transient dynamics of phytoplankton, and dilution of nutrients, phytoplankton biomass, and turbidity during wet years, and evapoconcentration during dry years. Increased precipitation was correlated with decreased phytoplankton biomass, TP concentration, chloride concentration, and turbidity. In 2005, the wettest year, no phytoplankton-dominated lakes were observed. During the driest year (2002), the phytoplankton-dominant regime (>18 μg chl-a L^?1) occurred in 22% of lakes, which was higher than the study period average. SAV biomass was not directly affected by precipitation, but was negatively associated with phytoplankton biomass and positively associated with the previous year’s SAV growth. SAV biomass was carried over from year-to-year, and the occurrence of SAV-dominated (>25% cover) lakes was significantly higher in 2007 (90%) following 3 years of high precipitation levels.     

Submerged macrophytes as indicators of the ecological quality of lakes

1. We analysed submerged macrophyte communities from 300 Danish lakes to determine the efficacy of different species, maximum colonisation depth (C_max) of plants as well as coverage and plant volume inhabited (PVI) as indicators of eutrophication. 2. Most species occurred at a wide range of phosphorus and chlorophyll a (Chla) concentrations, but some species of isoetids (Lobelia, Isoëtes) and Potamogeton (Potamogeton gramineus, Potamogeton alpinus and Potamogeton filiformis) were mainly found at low nutrient concentrations and hence may be considered as indicators of nutrient poor conditions. However, species typically found in nutrient‐rich conditions, such as Elodea canadensis and Potamogeton pectinatus, were also found at total phosphorus (TP) <0.02 mg P L^?1 and Chla <5 μg L^?1 and therefore cannot be considered as reliable indicators of eutrophic conditions. 3. Submerged macrophyte coverage, PVI and the C_max were negatively correlated with TP and Chla. However, variability among lakes was high and no clear thresholds were observed. At TP between 0.03 and 0.07 mg P L^?1 plant coverage in shallow lakes ranged from nearly 0 to 100%, whilst at concentrations between 0.10 and 0.20 mg P L^?1 only 29% of the lakes had coverage >10%. C_max was found to be a useful indicator only in deep lakes with unvegetated areas in the deeper part, whereas the use of coverage was restricted to shallow lakes or shallow areas of deep lakes. 4. Overall, submerged macrophytes responded clearly to eutrophication, but the metrics investigated here showed no well‐defined thresholds. We developed a simple index based on species richness, presence of indicator species, coverage and C_max, which might be used to track major changes in macrophyte communities and for lake classification.     

The role of solid waste materials as habitats for macroinvertebrates in a lowland dam reservoir

Eight hypereutrophic phytoplankton dominated ponds from the Brussels Capital Region (Belgium) were biomanipulated (emptied with fish removal) to restore their ecological quality and reduce the risk of cyanobacterial bloom formation. Continuous monitoring of the ponds before and after the biomanipulation allowed the effects of the management intervention on different compartments of pond ecosystems (phytoplankton, zooplankton, submerged vegetation and nutrients) to be assessed. Fish removal resulted in a drastic reduction in phytoplankton biomass and a shift to the clear-water state in seven out of eight biomanipulated ponds. The reduction in phytoplankton biomass was associated with a marked increase in density and size of large cladocerans in six ponds and a restoration of submerged macrophytes in five ponds. The phytoplankton biomass in the ponds with extensive stands of submerged macrophytes was less affected by planktivorous fish recolonisation of some of the ponds later in the summer. The two non-vegetated ponds as well as one pond with sparse submerged vegetation showed a marked increase in phytoplankton biomass associated with the appearance of fish. Phytoplankton biomass increase coincided with the decrease in large Cladocera density and size. One pond lacking submerged macrophytes could maintain very low phytoplankton biomass owing to large Cladocera grazing alone. The results of this study confirmed the importance of large zooplankton grazing and revegetation with submerged macrophytes for the maintenance of the clear-water state and restoration success in hypereutrophic ponds. They also showed that large Cladocera size is more important than their number for efficient phytoplankton control and when cladocerans are large enough, they can considerably restrain phytoplankton growth, including bloom-forming cyanobacteria, even when submerged vegetation is not restored. The positive result of fish removal in seven out of eight biomanipulated ponds clearly indicated that such management intervention can be used, at least, for the short-term restoration of ecological water quality and prevention of noxious cyanobacterial bloom formation. The negative result of biomanipulation in one pond seems to be related to the pollution by sewage water. Guest editors: B. Oertli, R. Cereghino, A. Hull & R. Miracle Pond Conservation: From Science to Practice. 3rd Conference of the European Pond Conservation Network, Valencia, Spain, 14–16 May 2008     

Mechanisms regulating abundance of submerged vegetation in shallow eutrophic lakes

Shallow eutrophic lakes tend to be either in a turbid state dominated by phytoplankton or in a clear-water state dominated by submerged macrovegetation. Recent studies suggest that the low water turbidity in the clear-water state is maintained through direct and indirect effects of the submerged vegetation. This study examined what mechanisms may cause a recession of the submerged vegetation in the clear-water state, and thereby a switch to the turbid state. The spatial distribution of submerged vegetation biomass was investigated in two shallow eutrophic lakes in the clear-water state in southern Sweden. Biomass of submerged vegetation was positively correlated with water depth and wave exposure, which also were mutually correlated, suggesting that mechanisms hampering submerged vegetation were strongest at shallow and/or sheltered locations. The growth of Myriophyllum spicatum, planted in the same substrate and at the same water depth, was compared between sheltered and wave exposed sites in two lakes. After 6 weeks the plants were significantly smaller at the sheltered sites, where periphyton production was about 5 times higher than at the exposed sites. Exclosure experiments were conducted to evaluate the effects of waterfowl grazing on macrophyte biomass. Potamogeton pectinatus growth was decreased by grazing, whereas M. spicatum was not affected. The effects were greater at a sheltered than at a wave-exposed site, and also negatively related to distance from the reed belt. These results suggest that competition from epiphytes and waterfowl grazing hamper the development of submerged vegetation at sheltered and/or shallow locations. An increased strength of these mechanisms may cause a recession of submerged vegetation in shallow eutrophic lakes in the clear-water state and thereby a switch to the turbid state. Received: 24 June 1996 / Accepted: 8 September 1996     

Above- and belowground competition between two submersed macrophytes

Several studies have shown that submerged macrophytes provide a refuge for zooplankton against fish predation, whereas the role of emergent and floating-leaved species, which are often dominant in eutrophic turbid lakes, is far less investigated. Zooplankton density in open water and amongst emergent and floating-leaved vegetation was monitored in a small, eutrophic lake (Frederiksborg Slotssø) in Denmark during July–October 2006. Emergent and floating-leaved macrophytes harboured significantly higher densities of pelagic as well as plant-associated zooplankton species, compared to the open water, even during periods where the predation pressure was presumably high (during the recruitment of 0+ fish fry). Zooplankton abundance in open water and among vegetation exhibited low values in July and peaked in August. Bosmina and Ceriodaphnia dominated the zooplankton community in the littoral vegetated areas (up to 4,400 ind l^?1 among Phragmites australis and 11,000 ind l^?1 between Polygonum amphibium stands), whereas the dominant species in the pelagic were Daphnia (up to 67 ind l^?1) and Cyclops (41 ind l^?1). The zooplankton density pattern observed was probably a consequence of concomitant modifications in the predation pressure, refuge availability and concentration of cyanobacteria in the lake. It is suggested that emergent and floating-leaved macrophytes may play an important role in enhancing water clarity due to increased grazing pressure by zooplankton migrating into the plant stands. As a consequence, especially in turbid lakes, the ecological role of these functional types of vegetation, and not merely that of submerged macrophyte species, should be taken into consideration.     

Phosphorus retention of forested and emergent marsh depressional wetlands in differing land uses in Florida,USA

The translocation of phosphorus (P) from terrestrial landscapes to aquatic bodies is of concern due to the impact of elevated P on aquatic system functioning and integrity. Due to their common location in depressions within landscapes, wetlands, including so-called geographically isolated wetlands (GIWs), receive and process entrained P. The ability of depressional wetlands, or GIWs, to sequester P may vary by wetland type or by land use modality. In this study we quantified three measures of P sorption capacities for two common GIW types (i.e., emergent marsh and forested wetlands) in two different land use modalities (i.e., agricultural and least impacted land uses) across 55 sites in Florida, USA. The equilibrium P concentration (EPC₀) averaged 6.42 ± 5.18 mg P L^?1 (standard deviation reported throughout); and ranged from 0.01–27.18 mg P L^?1; there were no differences between GIW type or land use modality, nor interaction effects. Significant differences in phosphorus buffering capacity (PBC) were found between GIW types and land use, but no interaction effects. Forested GIWs [average 306.64 ± 229.63 (mg P kg^?1) (µg P L^?1)^?1], and GIWs in agricultural settings [average 269.95 ± 236.87 (mg P kg^?1) (µg P L^?1)^?1] had the highest PBC values. The maximum sorption capacity (S_max) was found to only differ by type, with forested wetlands (1274.5 ± 1315.7 mg P kg^?1) having over three times the capacity of emergent GIWs (417.5 ± 534.6 mg P kg^?1). Classification trees suggested GIW soil parameters of bulk density, organic content, and concentrations of total P, H₂O-extractable P, and HCl-extractable P were important to classifying GIW P-sorption metrics. We conclude that GIWs have high potential to retain P, but that the entrained P may be remobilized to the wetland water column depending on storm and groundwater input P concentrations. The relative hydrologic dis-connectivity of GIWs from other aquatic systems may provide sufficient retention time to retain elevated P within these systems, thereby providing an ecosystem service to downstream waters.     

Grazing effects of a freshwater bivalve (Corbicula leana Prime) and large zooplankton on phytoplankton communities in two Korean lakes

This study examined the effects of a freshwater filter feeding bivalve (Corbicula leana Prime) and large zooplankton (>200 μm, mostly cladocerans and copepods) on the phytoplankton communities in two lakes with contrasting trophic conditions. A controlled experiment was conducted with four treatments (control, zooplankton addition, mussel addition, and both zooplankton and mussel addition), and each established in duplicate 10-l chambers. In both lakes there were significant effects of mussel grazing on phytoplankton density and biomass. The effects were greater in mesotrophic Lake Soyang than in hypertrophic Lake Ilgam. Effects of zooplankton grazing did not differ between these lakes, and zooplankton effects on phytoplankton were much less than the effects of mussels. Although mussels exerted a varying effect on phytoplankton according to their size, mussels reduced densities of almost all phytoplankton taxa. Total mean filtering rate (FR) of mussels in Lake Soyang was significantly greater than that in Lake Ilgam (p=0.002, n=5). Carbon fluxes from phytoplankton to mussels (977–2,379 μgC l^?1d^?1) and to zooplankton (76–264 μgC l^?1 d^?1) were always greater in Lake Ilgam due to the greater phytoplankton biomass (p<0.01, n=6). Based on the C-flux to biomass ratios, the mussels consumed 170–754% (avg. 412%) of phytoplankton standing stock in Lake Soyang, and 38–164% (avg. 106%) in Lake Ilgam per day. The C-flux to biomass ratio for mussels within each lake was much greater than for large zooplankton. Mussels reduced total phosphorus concentration by 5–34%, while increasing phosphate by 30–55% relative to the control. Total nitrogen also was reduced (by 9–25%), but there was no noticeable change in nitrate among treatments. The high consumption rate of phytoplankton by Corbicula leana even in a very eutrophic lake suggests that this mussel could affect planktonic and benthic food web structure and function by preferential feeding on small seston and by nutrient recycling. Control of mussel biomass therefore might be an effective tool for management of water quality in shallow eutrophic lakes and reservoirs in Korea.     

The Impact of Nutrient State and Lake Depth on Top-down Control in the Pelagic Zone of Lakes: A Study of 466 Lakes from the Temperate Zone to the Arctic

Using empirical data from 466 temperate to arctic lakes covering a total phosphorus (TP) gradient of 2-1036 mg L-1, we describe how the relative contributions of resource supply, and predator control change along a nutrient gradient. We argue that (a) predator control on large-bodied zooplankton is unimodally related to TP and is highest in the most nutrient-rich and nutrient-poor lakes and generally higher in shallow than deep lakes, (b) the cascading effect of changes in predator control on phytoplankton decreases with increasing TP, and (c) these general patterns occur with significant variations--that is, the predation pressure can be low or high at all nutrient levels. A quantile regression revealed that the median share of the predator-sensitive Daphnia to the total cladoceran biomass was significantly related unimodally to TP, while the 10% and 90% percentiles approached 0 and 100%, respectively, at all TP levels. Moreover, deep lakes (more than 6 m) had a higher percentage of Daphnia than shallow (less than 6 m) lakes. The median percentage of Daphnia peaked at 0.15 mg L-1 in shallow lakes and 0.09 mg L-1 in deep lakes. The assumption that fish are responsible for the unimodality was supported by data on the abundance of potential planktivorous fish (catch net-1 night-1 gill nets with the different mesh sizes [CPUE]). To elucidate the potential cascading effect on phytoplankton, we examined the zooplankton phytoplankton biomass ratio. Even though this ratio was inversely related to CPUE at all TP levels, we found an overall higher ratio in oligotrophic lakes that declined toward low values (typically below 0.2) in hypertrophic lakes. These results suggest that planktivorous fish have a more limited effect on the grazing control of phytoplankton in oligotrophic lakes than in eutrophic lakes, despite similar predator control of large-bodied zooplankton. Accordingly, the phytoplankton yield, expressed as the chlorophyll a-TP ratio, did not relate to CPUE at low TP, but it increased significantly with CPUE at high TP. We conclude that the chances of implementing a successful restoration program using biomanipulation as a tool to reduce phytoplankton biomass increase progressively with increasing TP, but that success in the long term is most likely achieved at intermediate TP concentrations.     

Growth and turion formation of Potamogeton crispus in response to different phosphorus concentrations in water

The vegetative growth and turion formation of Potamogeton crispus, a submersed aquatic macrophyte, was investigated under a range of phosphorus (P) concentrations (0.025, 0.25, 2.5 and 25 mg P L^?1) in the ambient water free of algae, aiming to identify the responses of submersed aquatic macrophytes to nutrient enrichment, a common eutrophication problem in China and worldwide. Plant growth was not affected by different P concentrations in terms of biomass accumulation of stems and leaves. However, the contents of chlorophyll a and starch in plants decreased with increasing water P levels, whereas chlorophyll b and carotenoids declined with P level ranging from 0.025 to 2.5 mg P L^?1. The soluble sugar content decreased when water P concentration increased up to 2.5 mg L^?1. The P content in plants increased with increasing water P levels, whereas plant N content decreased and soluble protein increased when water P concentration increased over 0.25 mg L^?1, implying that P. crispus may have modified its metabolism to adapt to water P availability. When P concentration increased to 25 mg L^?1, the number and dry matter production of turions per plant decreased significantly. Meanwhile, there was a significant reduction in turion weight and the accumulations of soluble sugar and starch in turion, when water P concentration was over 0.25 mg L^?1. The results suggest that turion formation in P. crispus is sensitive to P concentration in the ambient water, and high P levels may lead to decreases in P. crispus populations due to the decline in turion production.     

Variability of bio-optical parameters in two North-European large lakes

The bio-optical properties of some North-European large lakes were examined during 1995–2005 using field data and laboratory measurements. The key variables were optically active substances (OAS: chlorophyll, total suspended matter and dissolved organic matter), Secchi depth, and the “spectrometric” and diffuse light attenuation coefficients. Our main study sites were Lake Peipsi and Lake Võrtsjärv in Estonia, both eutrophic with mean Secchi depth below 3 m. The measured water parameters were compared with those obtained from two clear-water Swedish lakes, Lake Vänern and Lake Vättern. This comparison describes the bio-optical differences of the water in eutrophic and oligotrophic lakes. The variability of water parameters in the turbid Estonian lakes was rather high, e.g. the chlorophyll content varied from 1.8 to 102 mg m^?3 and the diffuse light attenuation coefficient from 0.92 to 6.5 m^?1. The change in water properties depends on the season and the biological activity of phytoplankton. We found no apparent long-time trend in water properties. Regression analysis showed that in the turbid Estonian lakes the optical properties were well correlated with chlorophyll and suspended matter, but not with dissolved organic matter. The highest determination coefficients (between 0.73 and 0.89) were obtained when the optical parameters were correlated with all three OAS together (multiple regressions). Our results concerning the variability and interconnections among bio-optical parameters in two Estonian large lakes illustrate the effect of OAS and light field on the ecological conditions of lakes in general.     

Remote sensing of phytoplankton-macrophyte coexistence in shallow hypereutrophic fluvial lakes

We investigated with remote sensing (APEX images) the coexistence of phytoplankton and macrophytes in three interconnected shallow and hypereutrophic fluvial lakes (Mantua Lakes, Northern Italy). High concentrations of chlorophyll-a, up to 60 mg m^?3, were determined in the open water between well-developed stands of floating-leaved, submerged, and emergent macrophytes. Our data suggest a general inhibition of phytoplankton by macrophytes, evidenced by decreasing chlorophyll-a concentrations in proximity of macrophyte stands. Chlorophyll-a concentrations halved in the proximity of emergent stands (~6 mg m^?3 within 21 m from the stand border) when compared to the outer zones (~13 mg m^?3). Contrasting trends were observed for submerged stands, where concentrations decreased inwards from ~8 to ~3 mg m^?3. Floating leaved stands had a neutral effect, chlorophyll-a being nearly constant in both inner and outer zones. Overall, remotely-sensed data allow evaluation of quantitative and spatially defined interactions of macrophytes and phytoplankton at the whole ecosystem scale.     

THE INFLUENCE OF SUBMERGED MACROPHYTES ON SEDIMENTARY DIATOM ASSEMBLAGES1

Submerged macrophytes are a central component of lake ecosystems; however, little is known regarding their long‐term response to environmental change. We have examined the potential of diatoms as indicators of past macrophyte biomass. We first sampled periphyton to determine whether habitat was a predictor of diatom assemblage. We then sampled 41 lakes in Quebec, Canada, to evaluate whether whole‐lake submerged macrophyte biomass (BiomEpiV) influenced surface sediment diatom assemblages. A multivariate regression tree (MRT) was used to construct a semiquantitative model to reconstruct past macrophyte biomass. We determined that periphytic diatom assemblages on macrophytes were significantly different from those on wood and rocks (ANOSIM R = 0.63, P < 0.01). A redundancy analysis (RDA) of the 41‐lake data set identified BiomEpiV as a significant (P < 0.05) variable in structuring sedimentary diatom assemblages. The MRT analysis classified the lakes into three groups. These groups were (A) high‐macrophyte, nutrient‐limited lakes (BiomEpiV ≥525 μg · L^?1; total phosphorus [TP] <35 μg · L^?1; 23 lakes); (B) low‐macrophyte, nutrient‐limited lakes (BiomEpiV <525 μg · L^?1; TP <35 μg · L^?1; 12 lakes); and (C) eutrophic lakes (TP ≥35 μg · L^?1; six lakes). A semiquantitative model correctly predicted the MRT group of the lake 71% of the time (P < 0.001). These results suggest that submerged macrophytes have a significant influence on diatom community structure and that sedimentary diatom assemblages can be used to infer past macrophyte abundance.     

Changes in physicochemical and biological factors during regime shifts in a restoration demonstration of macrophytes in a small hypereutrophic Chinese lake

Shallow, eutrophic lakes are usually characterized by a turbid state devoid of submerged vegetation subject to human-induced eutrophication. In most cases, it is rather hard to restore a vegetated clear state due to reduced resilience caused by a blend of complicated factors. In this study, we successfully reestablished a plant community in a small hypereutrophic lake over a certain period. In winter and spring with transparency of >55 cm and temperature of <20 °C, a submerged stands bed formed gradually under strong human interventions. The reestablished plant bed displayed obvious seasonal succession and prolonged the clear-water stage until July 2005, when it collapsed. The regime shift to a turbid state was mainly attributed to the decreasing biomass of stands bed and mechanical damage brought about by the elimination of Spirodela polyrhiza, increasing water temperature, P concentration as well as periphyton biomass, etc. The reestablishment also changed the aquatic ecosystem greatly. A ‘clear-water’ stage was characterized by higher NO₃^?–N, NH₄⁺–N, electrical conductivity, transparency and TN/TP level and more cladocerans (mainly Daphnia pulex), while the turbid state was characterized by higher temperature, chlorophyll a and TP level and more abundant rotifers. It is thus viable to restore submerged macrophytes in such lakes in winter and spring, when transparency is relatively high while temperature and water level are low. Nevertheless, to obtain a long-term, vegetated clear state, control of internal nutrient loading by means of obstruction, purification, dredging or solidification, is extremely necessary since nutrients play an important role in regime shifts as evidenced by the present case, too.     

Long-term nutrient trends and harmful cyanobacterial bloom potential in hypertrophic Lake Taihu,China

Rapid economic development in China’s Lake Taihu basin during the past four decades has accelerated nitrogen (N) and phosphorus (P) loadings to the lake. This has caused a shift from mesotrophic to hypertrophic conditions, symptomized by harmful cyanobacterial blooms (CyanoHABs). The relationships between phytoplankton biomass as chlorophyll a (Chla) and nutrients as total nitrogen (TN) and total phosphorus (TP) were analyzed using historical data from 1992 to 2012 to link the response of CyanoHAB potential to long-term nutrient changes. Over the twenty year study period, annual mean Chla showed significantly positive correlations with both annual mean TN and TP (P < 0.001), reflecting a strong phytoplankton biomass response to changes in nutrient inputs to the lake. However, phytoplankton biomass responded slowly to annual changes in TN after 2002. There was not a well-defined or significant relationship between spring TN and summertime Chla. The loss of a significant fraction of spring N loading due to denitrification likely weakened this relationship. Bioavailability of both N and P during the summer plays a key role in sustaining cyanobacterial blooms. The frequency of occurrence of bloom level Chla (>20 μg L^?1) was compared to TN and TP to determine nutrient-bloom thresholds. A decline in bloom risk is expected if TN remains below 1.0 mg L^?1 and TP below 0.08 mg L^?1.     

Can artificial plant beds be used to enhance macroinvertebrate food resources for perch (<Emphasis Type="Italic">Perca fluviatilis</Emphasis> L.) during the initial phase of lake restoration by cyprinid removal?

Restoration of shallow lakes to a clear-water state, often characterized by high submerged macrophyte cover and a high proportion of piscivores such as perch, Perca fluviatilis L., frequently involves removal of a large proportion of the zoobenthivorous fish, such as bream, Abramis brama L., and roach, Rutilus rutilus L. (i.e. biomanipulation). However, establishment of submerged macrophytes is often delayed following fish removal. This is unfortunate because plant beds typically host high densities of the macroinvertebrates constituting the diet of small perch and thus help perch to go through the bottleneck from feeding on macroinvertebrates to feeding on fish. Establishment of artificial plant beds may be a useful tool to enhance macroinvertebrate population growth and thus food resources for small perch until the natural plants have established. To investigate this restoration option, we studied during two growing seasons (June–October) the composition and abundance of the macroinvertebrate community in artificial plant beds installed in shallow Lake Væng (Denmark) comprising the initial phase of a biomanipulation effort by fish removal. Lake areas with artificial plant beds exhibited substantially higher macroinvertebrate densities than the lake bottom. This suggests that artificial plant beds may be used as feeding grounds for small perch, similarly to the well-known refuge effect for zooplankton against fish predation. In this way, artificial plant beds could help maintain a clear-water state during the transient period when natural submerged vegetation is not yet established in the lake.