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Many urban ponds experience detrimental algal blooms as the result of eutrophication. During a two year field experiment, the efficacy of five in situ treatments to mitigate eutrophication effects in urban ponds was studied. The treatments targeted the sediment phosphorus release and were intended to switch the ponds from a turbid phytoplankton-dominated state to a clear-water state with a low phytoplankton biomass. Two eutrophic urban ponds were each divided into six compartments (300 -400 m(2); 210-700 m(3)). In each pond the following treatments were tested: dredging in combination with biomanipulation (involving fish biomass control and the introduction of macrophytes) with and without the addition of the flocculant polyaluminiumchloride, interception and reduction of sediment phosphorus release with lanthanum-modified bentonite (Phoslock (R)) in combination with biomanipulation with and without polyaluminiumchloride; biomanipulation alone; and a control. Trial results support the hypothesis that the combination of biomanipulation and measures targeting the sediment phosphorus release can be effective in reducing the phytoplankton biomass and establishing and maintaining a clear-water state, provided the external phosphorus loading is limited. During the experimental period dredging combined with biomanipulation showed mean chlorophyll-a concentrations of 53 and 6.2 mu g L-1, compared to 268.9 and 52.4 mu g L-1 in the control compartments. Lanthanum modified bentonite can be an effective alternative to dredging and in combination with biomanipulation it showed mean chlorophyll-a concentrations of 5.9 and 7.6 mu g L-1. Biomanipulation alone did not establish a clear-water state or only during a limited period. As the two experimental sites differed in their reaction to the treatments, it is important to choose the most promising treatment depending on site specific characteristics. In recovering the water quality status of urban ponds, continuing attention is required to the concurrent reduction of external phosphorus loading and to maintaining an appropriate fish community. (C) 2015 Elsevier Ltd. All rights reserved.

Small lakes and ponds are common features in urban areas and they contribute to the quality of citizens’ life. A poor water quality, however, can easily give rise to nuisance. A major cause for a poor water quality is a high concentration of plant-nourishing nutrients, eutrophication. In (semi-)standing waters, eutrophication often results in a high biomass of blue-green algae (cyanobacteria), turbid water and the disappearance of submerged aquatic plants. The cyanobacterial blooms can be accompanied with fish kills due to anoxia, the development of unpleasant surface scums and malodors. As cyanobacteria can produce potent toxins, they impose a serious risk for citizens’ health, pets and wildlife. The cyanobacterial blooms hamper the anthropogenic use of the water and can have negative economic impacts. Water managers experience that the reduction of cyanobacterial nuisance is arduous. As long-term positive effects of management interventions are not often achieved, there is need for effective approaches. The objective of this study was to investigate the efficacy of promising methods to reduce cyanobacterial nuisance in city waters, targeting the clear water state and promoting the growth of aquatic plants. Various methods were tested, in the laboratory, in small and large compartments and were eventually applied in whole ponds and lakes. It is widely accepted that the reduction of nutrient inputs is essential for long-term positive effects. This study focused on the reduction of the input and the availability of the key-nutrient phosphorus. It was shown that cyanobacterial nuisance was wide spread in urban ponds and lakes in the Dutch province of North Brabant. The phosphorus inputs of four urban lakes in this province were addressed. The study lakes differed greatly in the phosphorus sources and loads, depending on site-specific characteristics. These differences affected the selection of measures. It was shown that in-lake measures were effective in realizing the long-term abatement of the cyanobacterial nuisance, provided the external phosphorus input was limited. If the external phosphorus input could not be limited sufficiently, in-lake measures did not result in the long-term reduction of cyanobacterial nuisance. To reduce the bioavailable phosphorus stock in the lake with in-lake measures, sediment capping with a phosphorus-binding agent (lanthanum-modified bentonite, LMB) can be effective and cheaper than sediment removal by dredging. The additional use of a flocculant may have added value and suppressed cyanobacterial blooms quickly and effectively. Aquatic plants and macroinvertebrates responded positively to the achieved improvement of the water quality. Accumulation of lanthanum was shown in aquatic plants and fish, following LMB exposure. No toxic effects of lanthanum from LMB were observed. Depending on site-specific characteristics, dredging or LMB did not suffice to limit the available phosphorus stock in the lake. For this situation, the additional capping of the sediment with sand was tested and subsequently applied in a lake. Management of the fish biomass and lake reconstruction can support rehabilitation. The results of this study underpin the importance of a site-specific diagnosis (water system analysis). The diagnosis clarifies the underlying causes of cyanobacterial nuisances and is essential for a site-specific tailored set of measures. This study showed that a site-specific set of measures reduced cyanobacterial nuisance effectively for a long term. As eutrophication control is not always feasible or might be effective only in the long run, curative measures are needed for symptom relief. Several curative end-of-pipe measures that are often suggested were evaluated: effective microorganisms (EM®), golden algae, plant extracts, ultrasound and artificial mixing of non-stratifying waters. No strong support for the efficacy of these measures could be shown. Next to the above mentioned application of flocculant, the use of freshwater quagga mussels is promising. The efficacy of the mussels was experimentally tested and it was shown that the introduction of mussels in a hypertrophic urban pond reduced the phytoplankton biomass, including cyanobacteria, and induced a clear water state. The quagga mussel is an invasive alien species and new introductions should be considered carefully. Based on the results from this study, the thesis provides a road map for water managers for the reduction of cyanobacterial nuisances in urban ponds and lakes. Deze studie had als doel te onderzoeken welke kansrijke aanpak effectief blauwalgenoverlast bestrijdt en de waterkwaliteit langdurig verbetert, met helder water en goede groeimogelijkheden voor waterplanten.

Last century, Lake Taihu (China) became serious eutrophic due to excessive nutrient input. During the 1980s, the first algal blooms emerged in the lake, reaching disastrous proportions in 2007. During that year, the intake of drinking water had to be shut down and millions of people had to look for an alternative source of drinking water. This raises the question whether such problems can be avoided. Of crucial importance in avoiding and reducing toxic algal blooms is the identification of the maximum nutrient load ecosystems can absorb, while remaining in a good ecological state. In this thesis, I aim to determine the critical nutrient load for Lake Taihu. I approach the search for critical nutrient loads of Lake Taihu in five steps with diversity as an overarching topic throughout this thesis: diversity in lakes, diversity in models, diversity in spatial distribution of nutrient and water sources, diversity in the development of lakes around the earth and finally diversity within specific lakes. From the long list of available models I chose the model PCLake to use in my analysis because it is the most extensively used food web model applied for bifurcation analysis of shallow aquatic ecosystems. The approach has resulted in a range of critical nutrient loads for different parts of Lake Taihu. Furthermore, critical nutrient loads depend on management goals, i.e. the maximum allowable chlorophyll-a concentration. According to the model results, total nutrient loads need to be more than halved to reach chlorophyll-a concentrations of 30-40 μg.L-1 in most sections of the lake. To prevent phytoplankton blooms with 20 μg.L-1 chlorophyll-a throughout Lake Taihu, both phosphorus and nitrogen loads need a nearly 90% reduction. This range contrasts to the single point of recovery that is often found for small shallow lakes. The range in critical nutrient loads found for Lake Taihu can be interpreted as providing a path of recovery for which each step leads to water quality improvement in certain parts of the lake. To reach total recovery, nutrient reduction seems to be the most promising management option.

The hypothesis that the combination of the flocculent polyaluminium chloride (PAC) with the lanthanum-modified bentonite Phoslock ® (Flock & Lock) could sink effectively a water bloom of cyanobacteria and could shift a turbid, cyanobacteria infested lake to a clear water lake was tested in a controlled laboratory experiment and a whole lake experiment. In the laboratory, a relatively low dose of the flocculent PAC (2.2 and 4.4 mg Al l −1 ) was insufficient to sediment positively buoyant cyanobacteria ( Microcystis aeruginosa ). Similarly, the lanthanum modified clay (dosed at 390 mg l −1 ) was insufficient to sediment the positively buoyant cyanobacteria. However, the combination of PAC and Phoslock ® effectively sedimented cyanobacteria flocks. Likewise, a combined treatment of 2 tons PAC and 18 tons Phoslock ® in Lake Rauwbraken in April 2008 effectively sedimented a developing cyanobacteria bloom of Aphanizomenon flos-aquae. The average chlorophyll- a concentration in the two years prior to this Flock & Lock treatment was 19.5 (±36.5) μg l −1 , while it was as low as 3.7 (±4.5) μg l −1 in the years following the treatment. The combined treatment effectively reduced the amount of total phosphorus (TP) in the water column from on average 169 (±126) μg P l −1 before the application to 14 (±15) μg P l −1 after the treatment. Based on mean summer chlorophyll- a and TP concentrations, the lake was shifted from a eutrophic/hypertrophic state to an oligo/mesotrophic state. From directly after treatment in April 2008 until and including 2013, Lake Rauwbraken remained in an oligo-mesotrophic clear water state with TP reduced to less than 10% of the pre-treatment. This result shows that eutrophication in relatively small, isolated, stratifying lakes can be restored by targeting both water column and sediment P using a combination of flocculent and solid phase P-sorbent.

Organohalide-respiring bacteria (OHRB) are able to breathe natural and anthropogenically produced organohalides persistent in a broad range of oxygen-depleted environments. Therefore, these microorganisms are of high interest for organohalide-contaminated site bioremediation and natural halogen and carbon cycle. Nevertheless, to assess and adjust in situ bioremediation strategies and to enhance current understanding about the role of OHRB in natural habitats, thorough understanding of their ecophysiology and interaction with surrounding biotic and abiotic forces is necessary. To this end, this thesis focused on exploring ecophysiology and environmental distribution of OHRB in pristine and contaminated sites and unraveling their interactions with the co-existing microbial guilds in the community and geochemical parameters by application of a suite of physiological, molecular and geochemical analyses. Based on a comprehensive overview of currently known organohalide-respiring isolates and their environmental distribution, the presence of yet unknown OHRB in extreme environments was proposed as the known organohalide-respiring isolates survive/thrive at a moderate range of pH and salinity in laboratory culture. Therefore, the OHRB were surveyed in alkaline and hypersaline sediments collected from Lake Strawbridge, Western Australia, that was known to emit organohalides. As a result, for the first time, the dechlorination of chloroform and perchloroethene (PCE) to dichloromethane and trichloroethene, respectively， was documented from an alkaline hypersaline pristine environment. Corrinoids are essential cofactors for the activity of reductive dehalogenase enzymes. Ironically, some OHRB are reported to be corrinoid auxotrophs. Using transcriptional analysis and shotgun proteomics, here we show corrinoid auxotrophy in Dehalobacter restrictus PER-K23T. This detrimental deficiency seems to be compensated by up-regulation of relevant cobalamin salvaging and transport pathways to ensure sufficient corrinoid supply under partial corrinoid starvation. Hence, such OHRB incapable of de novo corrinoid synthesis will be dependent on non-dechlorinating community members to fulfill their nutritional needs indicating paramount importance of syntrophic interactions in supporting robust growth and activity of OHRB. Bacterial community analysis of chlorinated benzene dechlorinating consortia derived from contaminated harbour sludge suggested members of the Bacteroidetes phylum and Clostridiales order as well as sulfate-reducing Deltaproteobacteria as putative stimulating guilds that provide electron donor and/or organic cofactors to OHRB i.e. D. mccartyi and Dehalobacter. However, despite well-controlled lab condition, syntrophic interactions could be influenced by geochemical parameters under field settings. Accordingly, analysis of geochemical and microbial determinants of OHR at a site biostimulated by glycerol injection further verified supportive role of fermenters and sulfate reducers under highly reduced condition following biostimulation. However, towards the end of field experiment, reducing condition faded and sulfate increased concurrent with the appearance of Epsilonproteobacteria and Deferribacteres as putative oxidizers of reduced sulfur compounds. The latter guilds might serve as detoxifiers of sulfide and thereby stimulate D. mccartyi, but could also be inhibitory as successors of the more important syntrophic fermenting and sulfate reducing bacteria. In conclusion, this thesis expands our understanding of ecophysiology and environmental distribution of OHRB, addressing their presence in pristine environments as well as providing further evidence for their dependencies on other microbial community members in order to meet their nutritional requirements. Hence, research described here strengthens the scientific foundation for evaluating and optimizing strategies for the bioremediation of organohalide-contaminated sites and expands the natural niche of OHRB to extreme pristine environments.

Global warming and eutrophication in fresh and coastal waters may mutually reinforce the symptoms they express and thus the problems they cause.

It has been suggested that shallow lakes in warm climates have a higher probability of being turbid, rather than macrophyte dominated, compared with lakes in cooler climates, but little field evidence exists to evaluate this hypothesis. We analyzed data from 782 lake years in different climate zones in North America, South America, and Europe. We tested if systematic differences exist in the relationship between the abundance of submerged macrophytes and environmental factors such as lake depth and nutrient levels. In the pooled dataset the proportion of lakes with substantial submerged macrophyte coverage (430% of the lake area) decreased in a sigmoidal way with increasing total phosphorus (TP) concentration, falling most steeply between 0.05 and 0.2mgL � 1 . Substantial submerged macrophyte coverage was also rare in lakes with total nitrogen (TN) concentrations above 1‐2mgL � 1 , except for lakes with very low TP concentrations where macrophytes remain abundant until higher TN concentrations. The deviance reduction of logistic regression models predicting macrophyte coverage from nutrients and water depth was generally low, and notably lowest in tropical and subtropical regions (Brazil, Uruguay, and Florida), suggesting that macrophyte coverage was strongly influenced by other factors. The maximum TP concentration allowing substantial submerged macrophyte coverage was clearly higher in cold regions with more frost days. This is in agreement with other studies which found a large influence of ice cover duration on shallow lakes’ ecology through partial fish kills that may improve light conditions for submerged macrophytes by cascading effects on periphyton and phytoplankton. Our findings suggest that, in regions where climatic warming is projected to lead to fewer frost days, macrophyte cover will decrease unless the nutrient levels are lowered.

Both nitrogen (N) and phosphorus (P) can limit primary production in shallow lakes, but it is still debated how the importance of N and P varies in time and space. We sampled 83 shallow lakes along a latitudinal gradient (5 degrees 55 degrees S) in South America and assessed the potential nutrient limitation using different methods including nutrient ratios in sediment, water, and seston, dissolved nutrient concentrations, and occurrence of N-fixing cyanobacteria. We found that local characteristics such as soil type and associated land use in the catchment, hydrology, and also the presence of abundant submerged macrophyte growth influenced N and P limitation. We found neither a consistent variation in nutrient limitation nor indications for a steady change in denitrification along the latitudinal gradient. Contrary to findings in other regions, we did not find a relationship between the occurrence of (N-fixing and non-N-fixing) cyanobacteria and the TN:TP ratio. We found N-fixing cyanobacteria (those with heterocysts) exclusively in lakes with dissolved inorganic nitrogen (DIN) concentrations of100 microg/L, but notably they were also often absent in lakes with low DIN concentrations. We argue that local factors such as land use and hydrology have a stronger influence on which nutrient is limiting than climate. Furthermore, our data show that in a wide range of climates N limitation does not necessarily lead to cyanobacterial dominance.

Uit een proef met een maatschappelijke kosten-batenanalyse voor het Friese merengebied blijkt de waardestijging van woningen aan het water de meeste baten op te leveren als gevolg van de verbetering van de waterkwaliteit door KRW-maatregelen. Op de tweede plaats staan de baten uit recreatie. Natuur, landschap, visserij, vastlegging van de emissie van kooldioxide en de afname van kosten voor Natura 2000-gebieden leveren eveneens baten op. De totale kosten zijn echter hoger dan de totale baten: voor een tussenvariant van het maatregelenpakket respectievelijk 159 en 42 miljoen euro en voor een maximumvariant 770 en 325 miljoen euro. De onzekerheid in deze bedragen is groot en de baten zijn meestal minder 'hard' dan de kosten. De kosten-batenbalans kan gunstiger worden door het schrappen van enkele niet-kosteneffectieve maatregelen

In deze studie evalueren we gegevens van 28 Nederlandse en 50 Deense meren waar effectgericht maatregelen zijn toegepast, waaronder voornamelijk actief biologisch beheer door afvissen van brasem en blankvoorn. Daarnaast zijn in sommige meren ook andere maatregelen toegepast, zoals baggeren en chemische fosfaatfixatie in het sediment. In meer dan de helft van de gevallen was actief biologisch beheer succesvol op de korte termijn. In sommige gevallen nam niet alleen het chlorofyl-, maar ook de fosfor- en stikstofgehaltes sterk af. Op de lange termijn was de heldere toestand in de meeste gevallen echter niet stabiel

De monitoringsstudies naar de vervuilingsgraad van Chinese Wolhandkrab (WHK) die vanaf 2010 in Nederlandse wateren worden uitgevoerd tonen aan dat de WHK sterk vervuild kan zijn met polychloordibenzo-p-dioxines en -furanen (PCDD/F's, verderop aangeduid als 'dioxines'), met dioxine-achtige polychloorbifenylen (dl-PCB's) en met niet-dioxine achtige PCB’s (ndl-PCB’s). Deze contaminanten bevinden zich vooral in het vlees met hoge vetgehalten, de hepatopancreas (= middendarmklier) en gonaden (= geslachtsklier), vaak aangeduid met het 'bruine vlees' uit het lijf. Het witte spiervlees, uit poten, scharen en ook uit het lichaam, is vetarm en de concentraties dioxines en PCB’s zijn laag. Uit de voorgaande onderzoeken blijkt dat er een sterk geografische invloed is op de concentraties contaminanten in WHK; bijvoorbeeld WHK gevangen in de grote rivieren zijn sterker vervuild dan WHK uit het IJsselmeer. Over een eventuele tijdsafhankelijke variatie waardoor bijvoorbeeld gedurende het trekseizoen de concentraties contaminanten in de WHK sterk wisselen op een bepaalde locatie, is nog niets bekend. Er is wel reden om dat aan te nemen. WHK leggen in het trekseizoen grote afstanden af. De kans is daarom reëel dat de WHK die in een bepaald gebied worden gevangen van verschillende locaties afkomstig zijn, waardoor de concentraties contaminanten tussen individuele WHK kunnen verschillen.

To properly conserve, restore and manage riverine ecosystems and the services they provide, it is pertinent to understand their functional dynamics. However, there is still a major knowledge gap concerning the functioning of tropical rivers in terms of energy sources supporting riverine fisheries. I reviewed the anthropogenic influences on organic matter processes, energy sources and attributes of riverine food webs in the Lake Victoria basin, but also expanded the review to incorporate recent research findings from the tropics. Contrasting findings have been presented on the diversity of shredders and their role in organic matter processing in tropical streams. Recent tropical research has also highlighted the importance of autochthonous carbon, even in small forested streams. However similar studies are very limited in Afrotropical streams making it difficult to determine their place in emerging patterns of carbon flow in the tropics. This study was conducted in the Mara River, which is an important transboundary river with its headwaters in the Mau Forest Complex in Kenya and draining to Lake Victoria through Tanzania. In its headwaters, the basin is drained by two main tributaries, the Amala and Nyangores Rivers which merge in the middle reaches to form the Mara River mainstem. The overall objective of this dissertation was to better understand the functioning of the Mara River by assessing the spatio-temporal dynamics of organic matter sources and supply under different land-use and flow conditions and the influence of these dynamics on energy flow for consumers in the river. I collected benthic macroinvertebrates from open- and closed-canopy streams and classified them into functional feeding groups (FFGs) using gut content analysis. In total, I identified 43 predators, 26 collectors, 19 scrapers and 19 shredders. Species richness was higher in closed-canopy forested streams where shredders were also the dominant group in terms of biomass. Seven shredder taxa occurred only in closed-canopy forested streams highlighting the importance of maintaining water and habitat quality, including the input of leaf litter of the right quality, in the studied streams. The findings suggest that Kenyan highland streams harbor a diverse shredder assemblage contrary to earlier findings that had identified a limited number of shredder taxa. I subsequently used the composition of invertebrate functional feeding groups (FFGs) and the ecosystem process of leaf breakdown as structural and functional indicators, respectively, of ecosystem health in upland Kenyan streams. Coarse- and fine-mesh litterbags were used to compare microbial (fine-mesh) with shredder + microbial (coarse-mesh) breakdown rates, and by extension, determine the role of shredders in litter processing of leaves of different tree species (native Croton macrostachyus and Syzygium cordatum and the exotic Eucalyptus globulus). Breakdown rates were generally higher in coarse- compared with fine-mesh litterbags for the native leaf species and the relative differences in breakdown rates among leaf species remained unaltered in both agriculture and forest streams. Shredders were relatively more important in forest compared with agriculture streams where microbial breakdown was more important. Moreover, shredder mediated leaf litter breakdown was dependent on leaf species, and was highest for C. macrostachyus and lowest for E. globulus, suggesting that replacement of indigenous riparian vegetation with poorer quality Eucalyptus species along streams has the potential to reduce nutrient cycling in streams. To study organic matter dynamics is these streams, I assessed the influence of land use change on the composition and concentration of dissolved organic matter (DOM) and investigated its links with whole-stream ecosystem metabolism. Optical properties of DOM indicated notable shifts in composition along a land use gradient. Forest streams were associated with higher molecular weight and terrestrially derived DOM whereas agriculture streams were associated with autochthonously produced and low molecular weight DOM and photodegradation due to the open canopy. However, aromaticity was high at all sites irrespective of catchment land use. In agricultural areas high aromaticity likely originated from farmlands where soils are mobilized during tillage and carried into streams and rivers by runoff. Gross primary production (GPP) and ecosystem respiration (ER) were generally higher in agriculture streams, because of slightly open canopy and higher nutrient concentrations. The findings of this study are important because, in addition to reinforcing the role of tropical streams and rivers in the global carbon cycle, they highlight the consequences of land use change on ecosystem functioning in a region where land use activities are poised to intensify in response to human population growth. Lastly, I used natural abundances of stable carbon (δ13C) and nitrogen (δ15N) isotopes to quantify spatial and temporal patterns of carbon flow in food webs in the longitudinal gradient of the Mara River. River reaches were selected that were under different levels of human and mammalian herbivore (livestock and wildlife) influences. Potential primary producers (terrestrial C3 and C4 producers and periphyton) and consumers (invertebrates and fish) were collected during the dry and wet seasons to represent a range of contrasting flow conditions. I used Stable Isotope Analysis in R (SIAR) Bayesian mixing model to partition terrestrial and autochthonous sources of organic carbon supporting consumer trophic groups. Overall periphyton dominated contributions to consumers during the dry season. During the wet season, however, the importance of terrestrially-derived carbon for consumers was higher with the importance of C3 producers declining with distance from the forested upper reaches as the importance of C4 producers increased in river reaches receiving livestock and hippo inputs. This study highlights the importance of large mammalian herbivores on the functioning of riverine ecosystems and the implications of their loss from savanna landscapes that currently harbour remnant populations. The results of this dissertation contribute data to discussions on the effects of land use change on the functioning of upland streams and food webs in savanna rivers with regard to carbon flow and the vectoring role played by large mammalian herbivores as they transfer terrestrial organic matter and nutrients into streams and rivers. This study also provides information and recommendations that will guide future research and management actions for the sustainability of the Mara River and linked ecosystems in the Lake Victoria basin.

Thesis abstract The thesis explored how the global market for Nile Perch fish has reconfigured the social and the natural in dramatic ways. The demand for Nile Perch and Dagaa played, willingly or unwillingly, an important role in converting its products into regionally and globally desired commodity. It has also simultaneously restructured the organisation of fisheries into a complex and aggressively managed sector. In fishing and fish trade, one needs to externalize costs and risks to the lower actors in the production and business hierarchy. From an historical point of view, power has shifted from many points of coordination and decision making into a few hands, those that own fishing camps and export processing factory. Moreover, illegal fishing and trading are continuous and corruption is rife to safe guard individual interest in turn shaping the local practices (governance) of Lake Victoria. Finally the debate about fisheries policies and fisheries regulation in L. Victoria does not address local realities and are largely irrelevant and that the real focus of power and driver of change is the international and regional markets for Nile Perch and Dagaa and global players with a lot of capital.

Thesis abstract The thesis explored how the global market for Nile Perch fish has reconfigured the social and the natural in dramatic ways. The demand for Nile Perch and Dagaa played, willingly or unwillingly, an important role in converting its products into regionally and globally desired commodity. It has also simultaneously restructured the organisation of fisheries into a complex and aggressively managed sector. In fishing and fish trade, one needs to externalize costs and risks to the lower actors in the production and business hierarchy. From an historical point of view, power has shifted from many points of coordination and decision making into a few hands, those that own fishing camps and export processing factory. Moreover, illegal fishing and trading are continuous and corruption is rife to safe guard individual interest in turn shaping the local practices (governance) of Lake Victoria. Finally the debate about fisheries policies and fisheries regulation in L. Victoria does not address local realities and are largely irrelevant and that the real focus of power and driver of change is the international and regional markets for Nile Perch and Dagaa and global players with a lot of capital.

The aim of the present study was to analyse the causes for the absence and persistence of chironomids in sediments with a complex pollution history. Observations on the benthic community composition in floodplain lake sediments of the River Rhine, differing in contaminant level and food quality, were combined with laboratory and in situ bioassays, using the midge Chironomus riparius as a model species. The positive relation between food quality and the diversity of the communities indicated that food quality is an important factor regulating benthic community composition, overriding the potential effects of contaminants. Nevertheless, in sediments with high contaminant levels, high densities of species classified as 'pollution-tolerant', such as Chironomus sp., were observed. The laboratory bioassays verified that the contaminant concentrations in these sediments had no direct effect on survival and growth of C. riparius, although higher contaminant concentrations in the sediment resulted in a higher incidence of mentum deformities. Also, the high growth rate of C. riparius in the in situ enclosures in two of the most contaminated sediments indicated chemical stress and mentum deformities did not exclude rapid growth. This observation agrees with the abundance of the resident pollution-tolerant chironomids of the Chironomus plumosus group. In addition, the in situ bioassay pointed out that predation on chironomids added to the field enclosures was significant and is also an important factor in regulating the abundance of chironomids. In sediments with the highest food quality and low levels of contaminants the opportunistic chironomids may have been outcompeted by benthic invertebrate taxa that are not able to persist at more contaminated sites. It is concluded that tolerance of opportunistic chironomids, such as Chironomus sp., combined with their rapid rate of development, enables them to persist in contaminated sediments that are organically enriched avoiding competition with other invertebrates

In recent years, interest has grown in restoring floodplain function of regulated rivers. Successful rehabilitation of riparian systems requires knowledge of how regulation of river flow affects biodiversity and ecosystem function. The effects of changes in the river's low water-level regime on aquatic ecosystems in floodplains has received little attention so far. The aquatic vegetation of 215 floodplain lakes along the Lower Rhine (the Netherlands) was analysed in relation to lake characteristics and lake water-level fluctuations in 1999–2000. Vegetation composition was related to lake morphology and age, cattle access to the shoreline, the amount of time the river was in flood, and lake sediment area exposed at low water level (drawdown). Surprisingly, vegetation composition was correlated more strongly with lake age and occurrence of drawdown than the amount of time the river was in flood1. In recent years, interest has grown in restoring floodplain function of regulated rivers. Successful rehabilitation of riparian systems requires knowledge of how regulation of river flow affects biodiversity and ecosystem function. The effects of changes in the river's low water-level regime on aquatic ecosystems in floodplains has received little attention so far. 2. The aquatic vegetation of 215 floodplain lakes along the Lower Rhine (the Netherlands) was analysed in relation to lake characteristics and lake water-level fluctuations in 1999-2000. 3. Vegetation composition was related to lake morphology and age, cattle access to the shoreline, the amount of time the river was in flood, and lake sediment area exposed at low water level (drawdown). Surprisingly, vegetation composition was correlated more strongly with lake age and occurrence of drawdown than the amount of time the river was in flood. 4. In older lakes, water-level fluctuations are reduced due to an accumulation of clay and silt that 'seals' sediment, preventing drawdown during periods of low river levels. Our results suggest that this clay sealing process is a major driving force for aquatic vegetation succession in floodplain lakes along the Lower Rhine, as succession drives from desiccation-tolerant species (e.g. Chara spp.) in young lakes to desiccation-sensitive species (e.g. Nuphar lutea) in old lakes. 5. Water levels were stable in lakes along a river branch that was impounded below mean flow only. Here, the original low water-level regime has been replaced by an artificial regime with higher water levels than would be expected naturally. Consequently, in these lakes drawdown was rare or absent, and the aquatic macrophyte vegetation was characterized by low species richness and frequent dominance by the invasive species Elodea nuttallii. 6. Synthesis and applications. Our results show that stabilization of river water levels during low flow may negatively affect vegetation composition and succession in floodplain lakes adjacent to these rivers. A management scheme including incidental temporary lowering of the river water level, which results in drawdown of floodplain lakes, would enhance the ecological status of those rivers with stabilized water levels during low flow.

The characteristic ecology of floodplain lakes is in part due to their relatively strong water-level fluctuations. We analyzed the factors determining water-level fluctuations in 100 floodplain lakes (during non-flooded conditions) in the active floodplains of the Lower Rhine in the Netherlands. Furthermore, we explored the relationship between water-level fluctuations and macrophyte species richness, and analyzed the suitability of artificially created lakes for macrophyte vegetation. During non-flooded conditions along the Rhine, lake water-level fluctuations are largely driven by groundwater connection to the river. Hence, water-level fluctuations are largest in lakes close to the main channel in strongly fluctuating sectors of the river and smallest in isolated lakes. Additionally, water-level fluctuations are usually small in old lakes, mainly due to reduced groundwater hydraulic conductivity resulting from accumulated clay and silt on the bottom. Species richness of floating-leaved and emergent macrophytes was reduced at both small and large water-level fluctuations, whereas species richness of submerged macrophytes was reduced at small water-level fluctuations only. In addition, species richness of submerged macrophytes was higher in lakes that experienced drawdown, whereas no similar pattern was detected for floating-leaved and emergent macrophytes. The decline in amplitude of lake water-level with lake age implies that the number of hydrologically dynamic lakes will decrease over time. Therefore, we suggest that excavation of new lakes is essential to conserve the successional sequence of floodplain water bodies including conditions of high biodiversity. Shallow, moderately isolated, lakes with occasional bottom exposure have the highest potential for creating macrophyte-rich floodplain lakes along large lowland rivers. The water-level regime of such lakes can in part be designed, through choice of the location along the river, the distance away from the river and the depth profile of the lake. [KEYWORDS: connectivity ; river ecology ; river restoration ; species riches]

The fish community in the shallow Lake Veluwe (3400 ha) has recently changed in composition. The total biomass of fish decreased to ca. 35% of the original biomass. The change in fish biomass followed the introduction of a commercial fishery and was in its turn followed by a rapid expansion of macrophyte beds dominated by Chara aspera Deth. ex Willd. An individual-based fish model was used to evaluate the causation of these events. We estimated natural mortality of the bream population (Abramis brama L.) using data on growth, recruitment and size distribution. The model predicted that without the fishery the biomass of the bream population would hardly change and that the fishery could explain the change in the biomass and size compositionThe fish community in the shallow Lake Veluwe (3400 ha) has recently changed in composition. The total biomass of fish decreased to ca. 35% of the original biomass. The change in fish biomass followed the introduction of a commercial fishery and was in its turn followed by a rapid expansion of macrophyte beds dominated by Chara aspera Deth. ex Willd. An individual-based fish model was used to evaluate the causation of these events. We estimated natural mortality of the bream population (Abramis brama L.) using data on growth, recruitment and size distribution. The model predicted that without the fishery the biomass of the bream population would hardly change and that the fishery could explain the change in the biomass and size composition. The rapid expansion of C. aspera was unlikely without the fishery, as Chara needs transparency down to the bottom and undisturbed sediment. The reduction of the bream population created favourable conditions for zebra mussels Dreissena polymorpha (Pallas) to settle which in turn created the clear water, necessary for the development of the Chara beds. (C) 2003 Published by Elsevier B.V.

Water-level fluctuations may be used to promote the expansion of emergent vegetation along lakeshores. The authors present the case of the lake Volkerak-Zoommeer in the Netherlands, a freshwater lake created in 1987 after the enclosure of an estuary. Using an experimental area in which the water level could be manipulated, it was shown that partial summer drawdown of the shoreline created suitable conditions for germination and growth of tall emergent species (in particular Phragmites australis). Plant survival and growth depended on subsequent water-level fluctuations and grazing by waterbirds. Based on the experiment and empincai data, a model was developed to predict the effects of the water-level regime on potential reed bed development. The model was applied for four hydrological scenarios that have been considered for the water-level management of the lakeWater-level fluctuations may be used to promote the expansion of emergent vegetation along lakeshores. We present the case of the lake Volkerak-Zoommeer in the Netherlands, a freshwater lake created in 1987 after the enclosure of an estuary. Using an experimental area in which the water level could be manipulated, it was shown that partial summer drawdown of the shoreline created suitable conditions for germination and growth of tall emergent species (in particular Phragmites australis). Plant survival and growth depended on subsequent water-level fluctuations and grazing by waterbirds. Based on the experiment and empirical data, a model was developed to predict the effects of the water-level regime on potential reed bed development. The model was applied for four hydrological scenarios that have been considered for the water-level management of the lake.

It is unclear whether accumulation of sediment-bound chemicals in benthivorous fish depends on the degree of sequestration in the sediment like it does for invertebrates. Here, we report on the potential of slow and fast desorbing sediment-bound polychlorobiphenyl (PCB) fractions for accumulation in carp (Cyprinus carpio) in lake enclosures treated with different nutrient doses It is unclear whether accumulation of sediment-bound chemicals in benthivorous fish depends on the degree of sequestration in the sediment like it does for invertebrates. Here, we report on the potential of slow and fast desorbing sediment-bound polychlorobiphenyl (PCB) fractions for accumulation in carp (Cyprinus carpio) in lake enclosures treated with different nutrient doses. Routes of PCB uptake were quantitatively evaluated for 15 PCBs (log K-0W range 5.6-7.8) using model analysis. Fast-desorbing PCB fractions in the sediment were defined as the ratio of 6-h Tenax-extractable to (total) Soxhlet-extractable concentrations. These fractions varied between 4 and 22% and did not show a clear trend with log K-0W. However, bioaccumulation of PCBs in carp correlated much better with Tenax-extractable concentrations than with total-extractable concentrations. Nutrient additions in the enclosures had a positive effect on PCB accumulation. Model results show that PCB uptake in carp can be explained from (1) uptake through invertebrate food, (2) uptake from fast-desorbing fractions in ingested sediments, and (3) uptake from water, where PCBs are in partitioning equilibrium with fast-desorbing fractions. The main implication of this research is that fast-desorbing PCB fractions in sediments have great predictive potential for bioaccumulation in benthivorous fish.

Op verscheidene plaatsen in Nederland wordt getracht verdroging van vennen tegen te gaan door verwijdering van veel verdampende bomen. Het effect van deze maatregel op de levensgemeenschap in het ven is echter sterk afhankelijk van de ligging van slecht doorlatende lagen en de grootte van het schijnspiegelsysteem, die op hun beurt de reactie van het venpeil op neerslag beïnvloeden. Meer inzicht in de hydrologie van vennen is verkregen via het tijdreeksmodel PIRFICT, met als onderzoekslocatie Beegderheide

In many water quality models production of dissolved organic carbon (DOC) is modelled as mineralisation from particulate organic matter (POM). In this paper it is argued that the DOC production from dessicated sediments by water turbulence may be of similar importanceIn many water quality models production of dissolved organic carbon (DOC) is modelled as mineralisation from particulate organic matter (POM). In this paper it is argued that the DOC production from dessicated sediments by water turbulence may be of similar importance. The DOC production from sediments was measured under sterile conditions, as a function of shaking time, sediment to water ratio and OC content. Furthermore, the maximum fraction of particulate organic carbon (POC) that can be mobilised by agitation (excluding the contribution of mineralisation) was assessed. The experiments can be considered as laboratory simulations of resuspension of aquatic sediments due to turbulence caused by wind, benthivorous fish or dredging. It was found that aquatic resuspension of dessicated sediment may cause mobilisation of up to 100% of indigenous particulate carbon in a few weeks. An empirical equation was derived that expresses mobilised carbon (DOC) as a function of sediment concentration (POC), OC fraction (foc) and time. At a typical value of 25 mg/L suspended sediment, the halflife for mobilisation is 0.5 day, which is much faster than bacterial degradation of POM in aquatic systems. (C) 2002 Elsevier Science Ltd. All rights reserved.

This study describes the chydorid (Branchiopoda, Anomopoda) assemblages from 66 European shallow lakes, and presents data relating the assemblages to lake type and ecological qualityThis study describes the chydorid (Branchiopoda, Anomopoda) assemblages from 66 European shallow lakes, and presents data relating the assemblages to lake type and ecological quality. Forty species, out of a total recorded European fauna of 60 species, were found in the study sites. No significant differences were found between chydorid assemblages associated with rock and plant substrata. Patterns of distribution were best explained primarily by latitude and pH. Chlorophyll-a, total phosphorus, water temperature and Secchi depth were also correlated with assemblage descriptors. Alonopsis elongata, Alona rectangula, Alonella excisa and Pleuroxus uncinatus were shown to have higher prevalence in certain lake types. The dominance of Chydorus sphaericus in a third of the study sites was linked to eutrophication and high levels of chlorophyll-a. The relationship between chydorids and lake ecological quality was more apparent at species rather than community level. This study identifies important typological factors affecting chydorid distribution, and confirms that patterns of chydorid distribution previously reported from regional studies hold true across Europe.

1. We analysed the vegetation structure of 215 lakes in the flood plain of the river Lower Rhine in relation to environmental variables related to hydrological connectivity, lake morphometry, lake age and land use on adjacent land. 2. The frequency distribution of the cover of submerged macrophytes was not normal, implying that submerged macrophytes in any one lake were either scarce or abundant. 3. We observed clear water lakes with submerged macrophyte dominance over a wide range of total P concentration (0.020-0.40 mg total P L-1). 4. Multiple logistic regression indicated that the probability of dominance by submerged macrophytes decreased markedly with the surface area, depth and age of the lakes. The surface area effect occurred independently of the depth. Further, there was a negative relationship between submerged macrophyte dominance and the long-term annual duration of inundation by the river. 5. Nymphaeid cover showed a distinct optimum with respect to mean lake depth, being almost absent in lakes shallower than 0.5 m. In contrast to what was found for submerged plants, the probability of occurrence of nymphaeids increased with lake age. 6. The probability of helophyte occurrence increased with lake age, and decreased with the presence of trees, cattle grazing, surface area, use of manure and mean lake depth. 7. In all cases the critical level of one factor (e.g. mean lake depth) depended on other factors (e.g. surface area or age of lake). Thus, in the present study, small lakes tended to remain dominated by submerged macrophytes up to a greater depth than large lakes, and helophytes colonised smaller lakes in an earlier phase. 8. The effect of inundation by the river was modest. This could be because most of our lakes are rarely inundated during the growing season and experience only moderate current velocities while flooded. 9. The results have practical implications for future management of flood plains for conservation purposes. In new water bodies, macrophyte domination will be promoted if many small shallow lakes, rather than few large deep ones, are excavated.

Biomass size distributions (BSDs) can be useful tools to (1) summarize complex information about fish community structure in a condensed graphical form, facilitating the characterization of freshwater fish communities, (2) compare the position of fish communities along environmental gradients and (3) elucidate major trophic interactions in freshwater fish communities. Biomass size distributions are presented by taxonomic and trophic group, for a selection of fish communities from 35 Scandinavian and eight Dutch lakes. They were used for the analysis of taxonomic and trophic shifts in the fish communities along a large environmental gradient, with productivity (expressed as total phosphorus concentration, TP) as its most important component. Regression analysis of fish community variables (such as proportion of cyprinids, or biomass of benthivores) were consistent with the semiquantitative conclusions drawn from BSDs, regarding taxonomic and trophic shifts with changes in TP in both Scandinavian and Dutch lakes, especially an increase in the amount and size of benthivorous fishes with increasing TPlevels. In addition, differences in mortality and growth rates were shown to partly explain differences in BSDs. Biomass size distributions thus provide an integrative tool for qualitative and quantitative comparisons among fish communities

Submerged plant dominance and a turbid state with few submerged plants have been hypothesized to represent alternative stable states in eutrophic shallow lakes. Here, we analyze the conditions for occurrence of alternative stable states in shallow lakes further, using Charisma, a simulation model describing the growth of Chara aspera. The model includes seasonality and spatial structure, aspects which were absent in earlier models predicting alternative equilibria. The parameterization of the model is largely based on experimental results and field observations. Over a range of conditions, the model does indeed predict alternative stable states. The range of conditions over which alternative stable states exist, appeared most sensitive to the assumed reduction of local turbidity by plants and the maximum growth rate of the plants. Aboveground biomass disappears during winter in most lakes in temperate regions. Our analysis indicates that from an evolutionary perspective there is an optimum biomass allocation strategy with respect to investment in overwintering structures. Too little investment reduces chances to regain dominance in the subsequent year, whereas too much investment in dormant overwintering structures such as seeds and tubers reduces photosynthesis. Interestingly, the optimal investment is lower for obtaining maximal summer biomass than for realizing the maximum stability of the vegetated state. The model also suggests that a short clear-water phase enhances the probability of vegetation survival. The optimal timing for a clear-water phase is at the end of May or in June, as is indeed the case in many lakes. In line with earlier theory, shallow lakes with a ‘flat’ depth profile are predicted to have the strongest hysteresis. In lakes with a depth gradient, the response to changes in turbidity is predicted to depend strongly on horizontal mixing of the water between vegetation stands and the open water. Hysteresis disappears predominantly due to a strong horizontal mixing of water. In case of little mixing, on the other hand, local alternative stable states are predicted to occur.

1. The development of bream populations, water transparency, chlorophyll-a concentration, extent of submerged vegetation and densities of the zebra mussel, Dreissena polymorpha, were analysed in three shallow eutrophic lake systems subject to different fish management. 2. In Lake Veluwemeer, the bream population was reduced from c. 100 to 20 kg ha1 after 5 years of fishing. The mortality caused by the fishery was estimated at 38␘f bream >15 cm in addition to a 13␗atural mortality of bream >17 cm. The decline was followed by an expansion of the Chara beds present in the shallow parts, an increase in water transparency in the open-water zone, an increase in the density of zebra mussels and a decrease in chlorophyll-a concentrations. 3. The newly created Lake Volkerak showed trends opposite to those in Lake Veluwemeer. Bream colonised the lake in 1988 and reached a biomass of c. 140 kg ha1 in 1998. The water transparency decreased from a maximum of 3 m to c. 1 m and the chlorophyll-a concentration increased from 5 to 45 g L1. Submerged vegetation colonised up to 20␘f the total lake area in the first 5 years after creation of the lake in 1987 but decreased to 10s turbidity increased. 4. Seine fishery in the Frisian lake system did not appear to affect the bream population despite annual catches as high as 40-50 kg ha1. The estimated natural mortality of fish >15 cm was 15nd mortality by fishery was 26ÐThe high loss was apparently compensated by good recruitment and high growth rates resulting from a c. 1 °C higher water temperature during the years when bream were removed by fishing. There was only a slight decrease in chlorophyll-a concentrations and a slight increase in water transparency. 5. The results of this study suggest that the effects of bream exploitation in eutrophic lakes can vary depending on the efficiency of the fishery, recruitment success and temperature regime. In the absence of fishery, bream dominated the fish community in the study lakes and apparently prevented D. polymorpha and submerged vegetation from establishing because of physical disturbance, enhanced internal P-loading and resettling of resuspended sediments.

Het visbestand in het IJsselmeer wordt sterk bepaald door de intensieve visserijactiviteiten. Het IJsselmeer biedt goede potenties voor trekkende en (nu nog) zeldzame vissoorten, zeker wanneer de dynamische aspecten van een estuarium meer kunnen worden ontwikkeld

Mature bream (>30 cm LF) dominated the fish communities in floodplain lakes along the lower River Rhine in The Netherlands, in terms of biomass, in all lakes at all times. In some lakes bream made up 99 f the total fish biomass. While immature, medium-sized bream (10–30 cm) and white bream and roach were abundant in the main channel of the river, these species were almost absent from the floodplain lakes. Inundation of the floodplains did not lead to spatial homogenization of species and length distributions. Small bream (

Eighteen shallow lakes in The Netherlands were subjected to biomanipulation, i.e. drastic reduction of the fish stock, for the purpose of lake restoration. The morphology and the nutrient level of the lakes differed, as did the measures applied. In some lakes biomanipulation was accompanied by reduction of the phosphorus loading. In all but two lakes, the Secchi disk transparency increased after the fish removal. Eight lakes (no phosphorus loading reduction, except for one lake) showed a strong and quick response to the measures: the bottom of the lake became visible (`lake bottom view'') and there was a massive development of submerged macrophytes. In eight other lakes the water transparency increased, but lake bottom view was not obtained. In the biomanipulated lakes the decrease in total phosphorus and chlorophyll aand the increase in Secchi disk transparency were significantly stronger than the general trend occurring in Dutch lakes where no measures had been taken. The improvement in the Secchi depth and chlorophyll awas also stronger than in lakes where only the phosphorus load was reduced. The critical factor for obtaining clear water was the extent of the fish reduction in winter. Significant effects were observed only after >75% fish reduction. Success seems to require substantial fish manipulation. In such strongly biomanipulated lakes, wind resuspension of the sediment never prevented the water from becoming clear. No conclusion can be drawn with respect to the possible negative impact of cyanobacteria or Neomysison grazing by Daphniaand consequently on water clarity. In all lakes where there had been a high density of cyanobacteria or years with a high density of Neomysisother factors such as insufficient fishery may explain why lake bottom view was not obtained. In all lakes with additional phosphorus loading reduction the fish stock has been reduced less drastically (15–60%). In these lakes the effects on transparency were less pronounced than in the lakes with > 75% fish removal. Daphniagrazing seems responsible for spring clearing in all clear lakes but one. In three lakes the reduction of benthivorous fish also increased the transparency. The factors that determine water clarity in summer are less obvious. In most clear lakes a low algal biomass coincided with a macrophyte coverage of more than 25% of the lake surface area. However, it was not clear what mechanism caused the low algal biomass in summer, although inorganic nitrogen concentrations were regularly found to be very low. Daphniagrazing in open water seemed to be of little importance for suppressing the algal biomass in summer. Although in most lakes the total phosphorus concentration decreased after the biomanipulation, the dissolved phosphorus concentration remained too high to cause phosphorus limitation of the algal growth. In four out of six clear lakes for which there are long-term data the transparency decreased again after 4 years. In one lake with lower nutrient levels the Secchi disk transparency increased over the years. However, the number of lakes with low nutrient levels is too small for conclusions to be drawn regarding the impact of nutrient levels on the stability of the clear water state.

This study describes the 1987–1992 time variationof the bulk chemical composition, levels of heavymetals, arsenic, nitrogen and phosporous insuspended and settling solids in Lake Volkerak andLake Zoom (The Netherlands). Suspended and setlingsolids were collected with continuous flowcentrifuges and sediment traps, respectively. Therelations between solids characteristics and metalconcentrations in the particles were investigatedusing correlation and factor-analysis. Heavy metals,except manganese, were associated with clay contentof the particles and with organic carbon which wasmainly determined by phytoplankton growth. Manganesewas correlated with Org-C only. Clay and organicmatter with associated heavy metals showed negativetrends on all locations, and a seasonality due toalgal biomass dilution. It is concluded that thetrophic state significantly affects trace metalcycling. Sediment traps collect different types ofparticles than continuous flow centrifuges. Thetraps collect particles with higher clay content,lower Org-C concentrations and higher Org-C/Nelemental ratios than the centrifuges.

This study evaluates the effects of sediment erosion and primary production on the temporal and spatial variability of heavy metals concentration in settling solids in the lakes IJsselmeer and Ketelmeer. Settling solids were sampled fortnightly using sediment traps at two sites in the IJsselmeer and contents of clay, organic matter, calcium carbonate, six heavy metals and δ 13 C and δ 18 O values were measured. Wind speed and direction, suspended solids, organic matter, chlorophyll, phaeophytin and oxygen concentrations were measured in the water compartment, as was pH, temperature and Secchi depth. Regression and multivariate analyses were used to evaluate the relationships within the data set. Principal components analysis and stepwise multiple regression show that the variation in the heavy metals concentration in settling solids is related to the windspeed and clay content, both variables that are related to erosion of sediments; and to pH, chlorophyll and CaCO 3 , which are related to algal growth in the lake. This supports the hypothesis that the spatial gradient in the heavy metals concentrations in deposited solids in the IJsselmeer is the result of dilution of contaminated sediments due to erosion of older, less contaminated sediments and primary production related to algal growth.

In veel podzolgronden komt onder de humusinspoelingshorizont (BZh) plaatselijk een ijzerbandje (BZir) voor. De vorming daarvan hangt samen met een specifiek vochtfront dat optreedt als een slecht doorlatende bodemlaag op een goed doorlatende voorkomt. Op sommige kleine, relatief lage plekken, 'mini-vennetjes: is onder invloed van accumulatie van water en van veengroei het ijzerbandje verplaatst naar de diepere zandondergrond. Daardoor zijn hierin 'ijzerkuipen' gevormd. Van een aantal grotere vennen is bekend dat ze zijn gemarkeerd door verticale ijzerwanden. In dit artikel gaan we nader in op het ontstaan en de verbreiding van deze ijzerlaagjes en hun invloed op de waterbeweging in de grond

A dynamic, deterministic model is presented to simulate the phosphorus cycle and plankton growth in the shallow, hypertrophic Loosdrecht Lakes (The Netherlands) before and after restoration measures. Both the water and the upper sediment layer are modelled. The model comprises three algal groups, zooplankton, fish detritus, zoobenthos and upper sediment (all modelled both in carbon and in phosphorus) besides inorganic phosphorus (SRP) in both the surface water and the interstitial water. Within the model system, the phosphorus cycle is completely closed. Carbon and phosphorus are described independently, so that dynamics of P/C ratios can be modelled. Sediment processes are described in a simplified form. Simulated values are largely within the range of observed ones. De detrital fraction of the seston (= phytoplankton + detritus) varies from 50-60 % in summer to about 90% in winter. SRP in the surface water is very low during most of the year. The sensitivity for external phosphorus input is larger for algal and detrital P than for algal and detritial C and chlorophyll-a. So the P/C ratio of the seston descreases following restorations measures, as is observed in the lakes, while the P/C ratios of zooplankton and fish remain constant and much higher. Phosphorus mobilisation from the sediment decreases with decreasing external input. Adaption of the model system to the reduced loading takes place in about two years. This means that without additional measures no further changes in the system are to be expected. Further reduction of phosphorus loading will lead to an improvement of water quality. Sources of uncertainty in the model include the limited knowledge on selective grazing as well as on mortality and mineralisation processes.

Phoslock(®) is a lanthanum (La) modified bentonite clay that is being increasingly used as a geo-engineering tool for the control of legacy phosphorus (P) release from lake bed sediments to overlying waters. This study investigates the potential for negative ecological impacts from elevated La concentrations associated with the use of Phoslock(®) across 16 case study lakes. Impact-recovery trajectories associated with total lanthanum (TLa) and filterable La (FLa) concentrations in surface and bottom waters were quantified over a period of up to 60 months following Phoslock(®) application. Both surface and bottom water TLa and FLa concentrations were0.001 mg L(-1) in all lakes prior to the application of Phoslock(®). The effects of Phoslock(®) application were evident in the post-application maximum TLa and FLa concentrations reported for surface waters between 0.026 mg L(-1)-2.30 mg L(-1) and 0.002 mg L(-1) to 0.14 mg L(-1), respectively. Results of generalised additive modelling indicated that recovery trajectories for TLa and FLa in surface and bottom waters in lakes were represented by 2nd order decay relationships, with time, and that recovery reached an end-point between 3 and 12 months post-application. Recovery in bottom water was slower (11-12 months) than surface waters (3-8 months), most probably as a result of variation in physicochemical conditions of the receiving waters and associated effects on product settling rates and processes relating to the disturbance of bed sediments. CHEAQS PRO modelling was also undertaken on 11 of the treated lakes in order to predict concentrations of La(3+) ions and the potential for negative ecological impacts. This modelling indicated that the concentrations of La(3+) ions will be very low (0.0004 mg L(-1)) in lakes of moderately low to high alkalinity (0.8 mEq L(-1)), but higher (up to 0.12 mg L(-1)) in lakes characterised by very low alkalinity. The effects of elevated La(3+) concentrations following Phoslock(®) applications in lakes of very low alkalinity requires further evaluation. The implications for the use of Phoslock(®) in eutrophication management are discussed.