Reduction Of External Loading Research Articles

杭州西湖北里湖沉积物氮磷内源静态释放的季节变化及通量估算

通过采集北里湖不同季节的柱状芯样,在实验室静态模拟沉积物氨氮(NH₄⁺-N)和可溶解性磷酸盐(PO₄^3--P)的释放,同时研究了沉积物间隙水中NH₄⁺-N及PO₄^3--P的垂直分布特征。结果表明,沉积物间隙水NH₄⁺-N随深度的增加有上升的趋势,PO₄^3--P随深度的增加呈先升后降的趋势。氮、磷营养盐在沉积物-水界面均存在浓度梯度,表明存在自间隙水向上覆水扩散的趋势。沉积物NH₄⁺-N在春季、夏季、秋季、冬季的释放速率分别为0.074 mg·m^-2·d^-1、0.340 mg·m^-2·d^-1、0.087 mg·m^-2·d^-1、0.0004 mg·m^-2·d^-1,PO₄^3--P的释放速率则分别为0.340 mg·m^-2·d^-1、0.518 mg·m^-2·d^-1、0.094 mg·m^-2·d^-1、-0.037 mg·m^-2·d^-1。不同采样点表现出明显的季节和空间差异性,释放速率表现为夏季＞春季、秋季＞冬季。根据静态模拟出的不同季节下内源氮、磷释放速率计算,全湖内源氮、磷营养盐的贡献分别为0.0037、0.0057 t/a。该研究可为北里湖富营养化及内源污染的治理提供基础数据。;Eutrophication is one of the most widespread environmental problems of inland waters, either because external loading has not been reduced sufficiently or because internal lake mechanisms. High internal loading of nitrogen (N) and phosphorus (P) from lake sediments is frequently reported as an important mechanism delaying lake recovery after a reduction of external loading. In this paper, the seasonal changes of ammonia nitrogen and phosphorus releasing rate from sediment were evaluated using intact sediment core simulation methods in the Beili Lake of the Hangzhou West Lake. Besides, vertical distribution characteristics of ammonia nitrogen and phosphors in the pore water were analyzed. The results showed that the concentration of ammonia nitrogen was expressed as the increasing trend with depth, while an increasing trend followed by decreasing was found for phosphorus. The difference in ammonia nitrogen and phosphorus concentration between pore water and overlying water can cause inner nitrogen and phosphorus nutrients releasing. The seasonal releasing rates from sediments were 0.074 mg·m^-2·d^-1, 0.340 mg·m^-2·d^-1, 0.087 mg·m^-2·d^-1, 0.0004 mg·m^-2·d^-1 for ammonia nitrogen, and 0.340 mg·m^-2·d^-1, 0.518 mg·m^-2·d^-1, 0.094 mg·m^-2·d^-1, -0.037 mg·m^-2·d^-1 for phosphorus, respectively. Seasonal and spatial variations of releasing rate were found from this study. The ammonia nitrogen and phosphorus releasing rate was higher in summer than in spring and autumn, but lowest in winter. The release fluxes of nitrogen and phosphorus across the sediment-water interface in different points were estimated using results from intact sediment core simulation methods. This difference is mainly focus on nitrogen and phosphorus releasing from the sediments in different environmental conditions including temperature, pH, dissolved oxygen (DO) and transparency. In summer, the temperature was the highest, the activity of bacterium was stronger than in other seasons. At the same time, the DO value decreased. The transparency was the lowest in summer, indicating the disturbance strength was the highest than other seasons. The results demonstrated the internal loading were 0.0004 t, 0.0025 t, 0.0008 t, 0 t for ammonia nitrogen, and 0.0015 t, 0.0038 t, 0.0009 t, -0.0005 t for phosphorus, respectively. The conclusions indicated that the internal loading of N and P in the whole lake were estimated about 0.0037 and 0.0057 t/a, respectively. Therefore, nutrients from sediment made a contribution to lake eutrophication. The phosphorus was the limiting factor to the lake entrophication in Beili Lake, and the releasing of phosphorus was related to the forms. The active phosphorus including the dissolved phosphorus, Fe-bound P and Al-bound P, can release from sediment to overlying water easily. The proportion of active P was accounted 5.3% of TP in spring, and then decreased to 3.72% in summer. After that, the active P increased to 6.16% and 6.73% in autumn and winter. The seasonal variation characteristics of active P were consitent with those of the phosphorus releasing. However, the simulation method used in this study didn't analyze the effect of environmental factors such as waves' disturbance and turbulence on nitrogen and phosphorus releasing rate, the results from simulation may be lower than the dynamic simulation value. Afterwards, the results from this study are important for the eutrophication control and ecological restoration in Beli Lake.

Relative importance of expertise, lifting height and weight lifted on posture and lumbar external loading during a transfer task in manual material handling

The objective of this study was to measure the effect size of three important factors in manual material handling, namely expertise, lifting height and weight lifted. The effect of expertise was evaluated by contrasting 15 expert and 15 novice handlers, the effect of the weight lifted with a 15-kg box and a 23-kg box and the effect of lifting height with two different box heights: ground level and a 32 cm height. The task consisted of transferring a series of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables (moments) while expertise had low impact. On the other hand, expertise showed a significant effect of posture variables on the lumbar spine and knees. All three factors are important, but for a reduction of external back loading, the focus should be on the lifting height and weight lifted. Practitioner Summary The objective was to measure the effect size of three important factors in a transfer of boxes from a conveyor to a hand trolley. Lifting height and weight lifted had more effect size than expertise on external back loading variables but expertise was a major determinant in back posture.

Water quality of Loch Leven: responses to enrichment, restoration and climate change

It is usually assumed that climate change will have negative impacts on water quality and hinder restoration efforts. The long-term monitoring at Loch Leven shows, however, that seasonal changes in temperature and rainfall may have positive and negative impacts on water quality. In response to reductions in external nutrient loading, there have been significant reductions in in-lake phosphorus concentrations. Annual measures of chlorophyll a have, however, shown little response to these reductions. Warmer spring temperatures appear to be having a positive effect on Daphnia densities and this may be the cause of reduced chlorophyll a concentrations in spring and an associated improvement in water clarity in May and June. The clearest climate impact was the negative relationship between summer rainfall and chlorophyll a concentrations. This is highlighted in extreme weather years, with the three wettest summers having very low chlorophyll a concentrations and the driest summers having high concentrations. To predict water quality impacts of future climate change, there is a need for more seasonal predictions from climate models and a greater recognition that water quality is the outcome of seasonal responses in different functional groups of phytoplankton and zooplankton to a range of environmental drivers.

Dilemma of Non‐Steady State in Lakes – Development and Predictability of In‐Lake P Concentration in Dimictic Lake Scharmützelsee (Germany) after Abrupt Load Reduction

AbstractBased on measured and calculated long‐term data on external phosphorus (P) load (1920–2009), hypolimnetic P accumulation and trophic parameters for the dimictic Lake Scharmützelsee, we aimed to identify factors which cause variable P net sedimentation and the importance of internal P loading for different time periods especially focusing on non‐steady state after abrupt external load reduction in 1988. P retention (R) decreased from 0.85 during the high external P loading (0.37 g m–2 a–1) phase (1950–1988) to 0.71 during the following transient phase, and increased to 0.81 for the present recovery phase (0.17 g m–2 a–1) beginning in 2003. Mean net sedimentation coefficients for the same periods were 0.47, 0.22 and 0.30. Our results show that a) empirical models overestimate R during the high loading phase and underestimate R during the transient phase after load reduction, and b) the application of simple one‐box models which assume that a portion of in‐lake P stock is retained requires the consideration of the variability of the net sedimentation coefficient. We identified multiple reasons for variable gross sedimentation (e.g., particle trapping in the elongated lake; efficient accumulation of sewage P) as well as release of P (delayed release of mobile P from sediment; changes in plant colonization and food webs), so that their relation (net sedimentation) varies. Despite a new equilibrium reached in 2003, it is still unclear when the lake will reach mesotrophic reference conditions and a good ecological status. Historical data and elevated Cl– concentration (22 mg L–1) indicate that P import from sewage contaminated groundwater still continues, so that the mean in‐lake P concentration is still too high (53 µg L–1), and biological structures have not fully recovered yet. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Modelling the response of a highly eutrophic lake to reductions in external and internal nutrient loading

The reduction of macronutrients to levels that limit primary production is often a critical element of mitigating eutrophication and reducing the potential for algal blooms. Lake Okaro has remained highly eutrophic despite an intensive catchment and in-lake restoration programme, including implementation of a constructed wetland, riparian protection, an alum application and application of a modified zeolite mineral (Z2G1) to reduce internal nutrient loading. A one-dimensional process-based ecosystem model (DYRESM-CAEDYM) was used in this study to investigate the need for further nutrient loading reductions of both nitrogen (N) and phosphorus (P). The model was calibrated against field data for a 2-year period and validated over two separate 1-year periods. Model simulations suggest that the trophic status of the lake, measured quantitatively with the Trophic Level Index (TLI), could shift from highly eutrophic to mesotrophic with external and internal loads of both N and P reduced by 75–90%. The magnitude of the nutrient load reductions is indicative of a major challenge in being able to effect transitions across trophic state categories for eutrophic lakes.

Cladoceran community responses to biomanipulation and re‐oligotrophication in Lake Vesijärvi, Finland, as inferred from remains in annually laminated sediment

Summary1. We studied the role of zooplankton in biomanipulation and the subsequent recovery phase in the Enonselkä basin of Lake Vesijärvi, using subfossil cladocerans in annually laminated sediment. Measures to restore the Enonselkä basin included reduction in external nutrient loading and mass removal of plankti‐ and benthivorous fish. Water clarity increased and the lake changed from a eutrophic to a mesotrophic state. However, some signs of increased turbidity were observed after 5–10 years of successful recovery.2. Annual laminae in a freeze core sample were identified and sliced, based on the seasonal succession of diatoms. Cladoceran remains and rotifer eggs were counted, and Daphnia ephippia and Eubosmina and Bosmina ephippia and carapaces were measured. Annual changes in pelagic species composition were studied with principal component analysis. Individual species abundance, size measurements and various cladoceran‐based indices or ratios (commonly used to reconstruct changes in trophic state and fish predation) were tested for change between four distinct periods: I (1985–1988) dense fish stocks, poor water quality; II (1989–1992) fish removal; III (1993–1997) low fish density, improved water quality; IV (1998–2002) slightly increased fish density and poorer water quality.3. After the removal of fish, the mean size of Daphnia ephippia and Eubosmina crassicornis ephippia and carapaces increased significantly. In contrast, the percentage of Daphnia did not increase. When based on ephippia, the ratio Daphnia/(Daphnia + E. crassicornis) increased, but the interpretation was obscured by the tolerance of fish predation by small Daphnia and by the fact that bosminids were the preferred food of roach. Moreover, ephippial production by E. crassicornis decreased in recent years.4. The abundance of Diaphanosoma brachyurum and Limnosida frontosa increased significantly after the fish population was reduced, while that of Ceriodaphnia and rotifers decreased.5. The expanding littoral vegetation along with improved water clarity was clearly reflected in the concentration of littoral species in the deep sediment core. The species diversity index for the entire subfossil community also increased.6. The period of faltering recovery was characterised by greater interannual variability and an increased percentage of rotifers. Nevertheless, the mean sizes of Daphnia ephippia and E. crassicornis ephippia and carapaces indicated a low density of fish. The deteriorating water quality was apparently related to multiple stressors in the catchment after rehabilitation, such as intensified lakeshore building, as well as to exceptional weather conditions, challenging the management methods in use.

A long-term record of epilimnetic phosphorus patterns in recovering Onondaga Lake, New York

A retrospective analysis for a 23 year (1987-2009) record of spectrophotometically determined forms of phosphorus (P) is presented for the epilimnion of Onondaga Lake, New York, a period over which it was trans- formed from severely culturally eutrophic to mesotrophic through reductions in point source inputs. Patterns of fi ve forms of P were evaluated, total P (TPe), total dissolved P (TDPe), soluble reactive P (SRPe), particulate P (PPe = TPe - TDPe) and dissolved organic P (DOPe = TDPe - SRPe) based on weekly observations from multiple epilim- netic depths for the April (spring turnover) - October (fall turnover) interval of each year. A 10-fold reduction in the effl uent concentration of the dominant source (TPMetro), made in several steps, caused dramatic decreases in summer average epilimnetic P concentrations, 6.8-fold for TPe, 4.8-fold for PPe, 14.9-fold for TDPe, 8.4-fold for DOPe, and 70-fold for SRPe, as well as improvements in common metrics of trophic state. Long-term patterns of summer average concentrations of each of these forms tracked the progression of TPMetro at an annual time step. Both short- and long-term patterns of in-lake concentrations, particularly for SRPe, support the position that the lake remained often nutrient-saturated through much of the record, but shifted to distinctly P limited in response to the most recent (2005) reduction in TPMetro. The diagnostic value of monitoring the dynamics of these forms of P at multiple time steps to track lake metabolism and features of the P cycle is demonstrated, including uptake of dissolved forms, effects of Daphnia metabolism, dynamics of phytoplankton uptake, and long-term responses to reductions in external loading. Evidence is presented that a noteworthy fraction of the PPe pool is associated with non-phytoplankton particulates.

Effect of intensive catchment and in-lake restoration procedures on phosphorus concentrations in a eutrophic lake

Lake Okaro is a small, warm monomictic lake in central North Island, New Zealand, which progressed from oligotrophic to eutrophic through the 1960s. Trends in phosphorus (P) concentrations in the lake are linked to multiple restoration efforts over a 5-year period (2003–2008). The restoration procedures include a 2.3 ha constructed wetland established in February 2006 and riparian margin protection to reduce external loading, as well as an Alum application in December 2003 and sediment capping using modified zeolite in September 2008 to reduce internal loading. The annual average total phosphorus (TP) concentration in the lake decreased by 41% from 2004–2005 to 2007–2008. Two predictive models based on external P loading data generally underestimated the measured TP concentrations in the water column due to internal P loading. The relatively rapid response of TP concentrations after reduction of the internal loading using modified zeolite suggests that this technique can effect a rapid decrease in lake water TP concentrations though the trophic state of Lake Okaro showed high resilience to the reduced P loading. It is concluded that the combined effect of all restoration procedures resulted in a relatively rapid decrease in TP concentrations in Lake Okaro, which may be prolonged by continued external load reduction.

Влияние климатических изменений на уровень эвтрофирования Куршского залива

The article presents the results of many years research (1991-2007) on the chlorophyll and nutrient concentrations, biomass and primary production of phytoplankton in the Curonian Lagoon. The main factors affecting the level of biological production and the trophic status are determined by the comparison of hydrological and chemical indicators. Water temperature is a key environmental factor that regulates the phytoplankton production and abundance in the Curonian Lagoon. The greater seasonal rise in water temperatures in 1990-2000, presumably, a consequence of the climate change, alongside with other factors (freshness, slow water flow) creates conditions for cyanobacterial «hyperblooms». Probably, the 1990-2000 climate warming is the cause of the eutrophication of the Curonian Lagoon, which continues despite significant reduction in external nutrient load.

Comparative analysis of nutrients, chlorophyll and transparency in two large shallow lakes (Lake Taihu, P.R. China and Lake Okeechobee, USA)

This article compares limnological attributes of two of the world’s largest shallow lakes—Lake Okeechobee in Florida, USA and Lake Taihu in P.R. China. Both the systems support an array of ecological and societal values including fish and wildlife habitat, public water supply, flood protection, and recreation. Both have extensive research programs, largely because of concern regarding the lakes’ frequent cyanobacterial blooms. By evaluating these systems together, we compare and contrast properties that can generally advance the understanding and management of large shallow lowland lakes. Because of shallow depth, long fetch, and unconsolidated mud sediments, water chemistry, and transparency in both the lakes are strongly influenced by resuspended sediments that affect light and nutrient conditions. In the central region of both the lakes, where depth is the greatest, evaluation of limiting factors by a trophic state index approach indicates that light most often limits phytoplankton biomass. In contrast, the more sheltered shoreline areas of both the lakes display evidence of nitrogen (N) limitation, which also has been confirmed in nutrient assays conducted in earlier studies. This N limitation most likely is a result of excessive levels of phosphorus (P) that have developed in the lakes due to high external loads over recent decades and the currently high internal P recycling. Comparisons of these lakes show that Lake Taihu has higher N than, similar total phosphorus (TP) and similar light conditions to that of Lake Okeechobee, but less chlorophyll a (CHL). The latter may be as a result of lower winter temperatures in Lake Taihu (around 5°C) compared to Lake Okeechobee (around 15°C), which could reduce phytoplankton growth and abundance through the other seasons of the year. In these systems, the important role of light, temperature, and nutrients in algal bloom dynamics must be considered, especially due to possible adverse and unintended effects that might occur with projects such as sediment removal, and in the long term, in regard to buffering lake responses to external load reduction.

Rapid Recovery from Eutrophication of a Stratified Lake by Disruption of Internal Nutrient Load

Restoration of anthropogenically eutrophied lake ecosystems is difficult due to feedback mechanisms that stabilize the trophically degraded state. Here, we show rapid recovery of a eutrophic stratified lake in response to multiple restoration that targeted the feedback mechanisms of high external and internal nutrient loads, lack of a trophic cascade, and lack of structured littoral habitats. Lake Tiefwarensee (Germany) was exposed to aluminium and calcium treatment and fisheries management over 5 years. Within this period, in-lake phosphorus concentrations declined by more than 80%, and transparency, zooplankton biomass and fish assemblage structure and biomass responded immediately and almost linearly to the reduction in phosphorus concentrations. Phytoplankton biomass and chlorophyll a (chl a) concentrations likewise decreased in response to restoration, but the declining trend was interrupted by one recovery year with unusually high phytoplankton biomasses. The zooplankton:phytoplankton biomass ratio and the chl a:phosphorus ratio approached values observed in other stratified lakes during natural recovery from eutrophication. The slow response of Tiefwarensee to the reduction of external load, and the quick response to the chemical treatment suggest that the disruption of internal P recycling and loading was the decisive restoration measure in Tiefwarensee. The external load reduction was a necessary but not sufficient measure, at least in the short-term, whereas the low-effort fisheries management was of minor importance. A comparison with other case studies confirms that measures aiming to inactivate phosphorus are the most efficient approaches to restore stratified lakes in the short-term, but a shift to a permanent near-pristine state is possible only by additional P input control.

Management measures and long-term water quality changes in Lake Balaton (Hungary)

A reduction in the external and internal P loadings along with the removal of benthivorous cyprinids resulted in a significant improvement of the water quality in the hypertrophic western part of Lake Balaton by 1996, some ten years following first management measure. Enhanced internal P loading after the reduction in external nutrient load is clearly an important factor explaining the duration of recovery in shallow lakes. Phytoplankton responded quite rapidly to reductions in nutrient loading and fish stock, whereas the effect on both the structure and abundance of the crustacean plankton was less pronounced. The role of top-down control of phytoplankton by crustaceans following nutrient reduction is not clearly understood in shallow lakes. Grazing did not play a significant role in the overall reduction of algal biomass in Lake Balaton. Since high concentration of suspended mineral particles from both sediment resuspension and lime precipitation causes permanent food limitation of daphnids, the top-down effects were negligible in Lake Balaton following fish stock reduction. Fish community structure did not respond to the lower external and internal TP loads cyprinids being dominant in both the pre- and post-management periods.

Development of phosphorus load reduction goals for seven lakes in the Upper Ocklawaha River Basin, Florida

External phosphorus loading targets have been developed for seven lakes in the Upper Ocklawaha River basin (UORB), and for six of these lakes the loading targets have been used as a basis for total maximum daily loads (TMDLs). External phosphorus loads for the seven lakes were estimated from 1986–2005. Three of the lakes that have received partial reductions in external loads have shown proportional reductions in phosphorus concentrations. Natural background phosphorus concentrations for the lakes were determined through a weight-of-evidence approach combining existing concentrations in reference lakes and modeling of natural background conditions in the basin. Target phosphorus concentrations were established by allowing a 10% degradation from natural background water transparency, as specified in Florida water quality standards. Data collected from the UORB lakes were analyzed to determine the relationship between phosphorus concentrations and water transparency. Target phosphorus concentrations ranged from 14–32 μg/L, which range from 15–100% of existing concentrations in the lakes. Recommended external phosphorus load reductions for the lakes range from 0 to 85%. We estimated that meeting the target phosphorus concentrations in the lakes with poorest water quality would reduce mean chlorophyll a concentrations up to 80%, substantially reduce algal bloom frequencies, and more than double mean Secchi transparency.

Assessment of internal and external lake restoration measures for two Berlin lakes

Two previously highly eutrophic lakes, Lake Tegel and Schlachtensee in Berlin, Germany, were subjected to similar external but different internal restoration measures during the last 20–25 years. External phosphorus (P) load was reduced in both lakes by P-stripping their main inflows using P-elimination plants; internal P load was treated by aeration in Lake Tegel and by hypolimnetic withdrawal in Schlachtensee. Loads before and after treatment are compared with the targets using the Vollenweider model and a modified One-Box model. The results indicate that external load reduction was the main cause of the pronounced lake water quality improvements. The hypolimnetic withdrawal in Schlachtensee was effective only in the initial years. No significant positive effect can be identified for the aeration of Lake Tegel.

Lake restoration: successes, failures and long‐term effects

1. Eutrophication constitutes a serious threat to many European lakes and many approaches have been used during the past 20–30 years to improve lake water quality. Results from the various lake restoration initiatives are diverse and the long-term effects are not well described. 2. In this study we evaluated data from more than 70 restoration projects conducted mainly in shallow, eutrophic lakes in Denmark and the Netherlands. Special focus was given to the removal of zooplanktivorous and benthivorous fish, by far the most common internal lake measure. 3. In more than half of the biomanipulation projects, Secchi depth increased and chlorophyll a decreased to less than 50% within the first few years. In some of the shallow lakes, total phosphorus and total nitrogen levels decreased considerably, indicating an increased retention or loss by denitrification. The strongest effects seemed to be obtained 4–6 years after the start of fish removal. 4. The long-term effect of restoration initiatives can only be described for a few lakes, but data from biomanipulated lakes indicate a return to a turbid state within 10 years or less in most cases. One of reasons for the lack of long-term effects may be internal phosphorus loading from a mobile pool accumulated in the sediment. 5. Synthesis and applications. Lake restoration, and in particular fish removal in shallow eutrophic lakes, has been widely used in Denmark and the Netherlands, where it has had marked effects on lake water quality in many lakes. Long-term effects ( > 8–10 years) are less obvious and a return to turbid conditions is often seen unless fish removal is repeated. Insufficient external loading reduction, internal phosphorus loading and absence of stable submerged macrophyte communities to stabilize the clear-water state are the most probable causes for this relapse to earlier conditions Eutrophication constitutes a serious threat to many European lakes and many approaches have been used during the past 20-30 years to improve lake water quality. Results from the various lake restoration initiatives are diverse and the long-term effects are not well described. In this study we evaluated data from more than 70 restoration projects conducted mainly in shallow, eutrophic lakes in Denmark and the Netherlands. Special focus was given to the removal of zooplanktivorous and benthivorous fish, by far the most common internal lake measure. In more than half of the biomanipulation projects, Secchi depth increased and chlorophyll a decreased to less than 50% within the first few years. In some of the shallow lakes, total phosphorus and total nitrogen levels decreased considerably, indicating an increased retention or loss by denitrification. The strongest effects seemed to be obtained 4-6 years after the start of fish removal. The long-term effect of restoration initiatives can only be described for a few lakes, but data from biomanipulated lakes indicate a return to a turbid state within 10 years or less in most cases. One of reasons for the lack of long-term effects may be internal phosphorus loading from a mobile pool accumulated in the sediment. Synthesis and applications. Lake restoration, and in particular fish removal in shallow eutrophic lakes, has been widely used in Denmark and the Netherlands, where it has had marked effects on lake water quality in many lakes. Long-term effects (> 8-10 years) are less obvious and a return to turbid conditions is often seen unless fish removal is repeated. Insufficient external loading reduction, internal phosphorus loading and absence of stable submerged macrophyte communities to stabilize the clear-water state are the most probable causes for this relapse to earlier conditions.

Modeling phosphorus flux in the sediments of Onondaga Lake: Insights on the timing of lake response and recovery

External (i.e. point source and tributary) phosphorus loads are an appropriate target for lake managers concerned with eutrophication control. However, internal loads associated with phosphorus that has accumulated in lake sediments can retard the response to reductions in external loading. This paper describes the development, calibration, confirmation and application of a model for phosphorus exchange at the sediment–water interface of a eutrophic, urban system, Onondaga Lake, New York. The model is based on the principles of mass balance and accommodates the delivery of labile particulate phosphorus to the sediment surface, diagenesis of that phosphorus within the sediment (conversion to the soluble form), redox-mediated adsorption and desorption, and diffusion and release of soluble phosphorus to the water column. Values for most model inputs and coefficients were determined through a suite of field and laboratory measurements and experimentation, establishing the credibility of the tool for site-specific application. The model was calibrated to measured sediment profiles of soluble and labile particulate phosphorus, limiting coefficient adjustment to experimentally defined bounds. The confirmation process included comparison of model-predicted and measured rates of sediment phosphorus release with no further coefficient adjustment. The model was then applied in the prediction of the sediment response to changes in the phosphorus content of sedimenting particulate matter, changes which would accompany reductions in external phosphorus loads to the lake. It is demonstrated that steady state rates of sediment phosphorus release, i.e. those in equilibrium with newly established rates of labile particulate phosphorus deposition, are achieved over a period of 19–26 years. These findings have important implications with respect to the natural recovery of eutrophic systems and will assist water quality managers in developing appropriate public expectations for the timing of lake restoration.

Nitrate‐depleted conditions on the increase in shallow northern European lakes

We determined relative nitrate‐nitrogen (NO3‐N) loss rates in 100 north‐mid‐European lakes from late spring to summer by using the exponential function N2 = N1e−k(t2−t2), where N1 and N2 are NO3‐N concentrations at the beginning (t1) and the end (t2) of the time interval, respectively, and k is the specific NO3‐N loss rate. We found that k decreased with increasing lake depth. Adjusting k to the lake depth (kadj), we observed that kadj was positively related to spring NO3‐N concentrations, but this relationship became insignificant at mean lake depths exceeding 12.5 m. A relationship between kadj and spring NO3‐N concentrations in lakes shallower than 12.5 m implies that changes in spring NO3‐N concentrations influence the NO3‐N loss rate and thereby summer NO3‐N concentrations. Time series from one Estonian, one German, and 14 Swedish lakes shallower than 12.5 m since 1988 revealed that May to August NO3‐N concentrations have decreased over time everywhere, and the number of time periods exhibiting a NO3‐N depleted condition, i.e., NO3‐N levels below 10 mg L−1, in these lakes has tripled since 1988. We explained the decreasing NO3‐N concentrations by a reduction in external nitrogen loading including atmospheric deposition, and by changes in climate. The observed prolongation of NO3‐N depleted conditions might be one possible explanation for the increasing occurrence of nitrogen‐fixing cyanobacteria in a variety of lake ecosystems.

Long term changes in the eutrophication process in a shallow Mediterranean lake ecosystem of W. Greece: Response after the reduction of external load

Lake Pamvotis is a shallow Mediterranean lake located in Western Greece near the city of Ioannina. The lake has been recognized as an internationally important conservation site under European Community legislation due to its rich biodiversity. However, during the last three decades the trophic status of the lake has changed as a result of anthropogenic activity (among others irrigation and domestic sewage discharge), resulting in serious problems. Here we present data about the long-term development in eutrophication of Lake Pamvotis. Water samples were collected and analyzed (water temperature, pH, dissolved oxygen, nutrients, chlorophyll- a) during three monitoring periods: 1985–1989, 1998–1999, 2004–2005. The high nutrient concentrations in the lake water during the three monitoring periods, as well as its eutrophic to hypertrophic status reflect the degree of impact anthropogenic activity has had on the lake. Commencement of a restoration plan in 1995–1996, involving sewage diversion, led to a reduction in external nutrient load and consequently to lower in-lake nutrients and Chlorophyll- a concentrations. Orthophosphate concentration decreased by about 87%, nitrates fell below 1.20 mg/l, whilst the total reduction of inorganic N compounds showed a weaker downward trend, fluctuating between 0.39 and 1.24 mg N/l with an average value of 0.76 mg N/l. However, after a short-term recovery the eutrophic status of the lake remains eight years later (2004–2005), suggesting the importance of the internal loading process and the absence of the top-down effect of fish. This study provides evidence for the need of greater restoration efforts utilized in Mediterranean shallow lakes.

Internal Ecosystem Feedbacks Enhance Nitrogen-fixing Cyanobacteria Blooms and Complicate Management in the Baltic Sea

Eutrophication of the Baltic Sea has potentially increased the frequency and magnitude of cyanobacteria blooms. Eutrophication leads to increased sedimentation of organic material, increasing the extent of anoxic bottoms and subsequently increasing the internal phosphorus loading. In addition, the hypoxic water volume displays a negative relationship with the total dissolved inorganic nitrogen pool, suggesting greater overall nitrogen removal with increased hypoxia. Enhanced internal loading of phosphorus and the removal of dissolved inorganic nitrogen leads to lower nitrogen to phosphorus ratios, which are one of the main factors promoting nitrogenfixing cyanobacteria blooms. Because cyanobacteria blooms in the open waters of the Baltic Sea seem to be strongly regulated by internal processes, the effects of external nutrient reductions are scale-dependent. During longer time scales, reductions in external phosphorus load may reduce cyanobacteria blooms; however, on shorter time scales the internal phosphorus loading can counteract external phosphorus reductions. The coupled processes inducing internal loading, nitrogen removal, and the prevalence of nitrogen-fixing cyanobacteria can qualitatively be described as a potentially self-sustaining "vicious circle." To effectively reduce cyanobacteria blooms and overall signs of eutrophication, reductions in both nitrogen and phosphorus external loads appear essential.

Strategy and current status of combating eutrophication in two Berlin lakes for safeguarding drinking water resources

After reduction of the external phosphorus load by phosphorus elimination plants, Lake Tegel and Schlachtensee in Berlin underwent a significant trophic improvement. The phosphorus elimination plants work by precipitation/coagulation/flocculation--sedimentation--post precipitation--filtration. The external load was reduced by one to two orders of magnitude down to 10-20 mgg PL(-1). The inlake phosphorus concentration followed. The development of algae and cyanobacteria was reduced substantially below a threshold value of about 50 microg PL(-1) clearly due to phosphorus limitation. In Lake Tegel, the external load reduction of the main inflow was counteracted partially by the external load of the second main inflow by the River Havel and the internal load. This has to be managed further in future.