External Phosphorus Loading Research Articles

Stochastic modelling of nutrient loading and lake ecosystem response in relation to submerged macrophytes and benthivorous fish

SUMMARY 1. The strong stabilising effect of increased submerged macrophytes (charophytes) and benthivorous fish reduction on the clear water state was shown for shallow Lake Veluwe and Lake Wolderwijd.2. The first two links in the chain of relationships from external phosphorus (P) loading to in‐lake total‐P concentrations to chlorophyll a concentrations to water transparency, showed a significant correlation with the areal fraction of coverage with charophytes. Higher coverages lead to (i) lower ratios of the in‐lake total‐P concentration compared with the volume weighted average concentration in the inlet water, indicating a higher retention of P in the presence of charophytes (ii) lower chlorophyll a to total‐P ratios, indicating a positive effect of charophytes on top‐down control of algae, and (iii) higher water transparency because of lower algal turbidity. Transparency further improved as a result of benthivorous fish reduction and a significant positive correlation between non‐algal turbidity and benthivorous fish biomass.3. A model was developed taking into account the inherent variability in precipitation and uncertainties in the empirical relationships determining phosphorus export from stream catchments and other sources and eutrophication variables in the receiving lakes. The model was used to compute (i) probability distributions for in‐lake total‐P, chlorophyll a and Secchi Disc transparency in relation to the coverage with charophytes and benthivorous fish biomass, and (ii) exceedence probabilities with respect to critical values for in‐lake total‐P and water transparency for several management scenarios.4. The effects of an expected rise in external nutrient loading on the in‐lake total‐P and chlorophyll a concentrations and on water transparency can be compensated for by two proposed control measures: (i) extended treatment at a waste water treatment plant directly discharging into Lake Veluwe, and (ii) diverting the outlet of a stream draining a catchment with high fertilisation. The minimal internal charophyte coverage needed to sufficiently stabilise the clear water state and to meet with the objective of a summer mean water transparency of at least 1 m was estimated at well over 30% of the lake area, while the benthivorous fish stock should be maintained at the present level of c. 20 kg ha−1.

Why the phosphorus retention of lakes does not necessarily depend on the oxygen supply to their sediment surface

In order to improve the trophic state of Lake Sempach, a eutrophied lake in central Switzerland, its external phosphorus (P) load has been decreased and its hypolimnion has been artificially oxygenated to lower the lake‐internal P recycling. Based on more than 15 yr of experience, we conclude that the reduction of the external P load resulted in a corresponding decrease of the lake’s P concentration. However, contrary to initial expectations, increased hypolimnetic dissolved oxygen concentrations neither (1) reduced the P release from sediments during summer nor (2) resulted in anincreased permanent P retention. These observations warrant a reevaluation of the well‐accepted management strategy of decreasing the lake internal P cycling by maintaining an aerobic hypolimnion and sediment surface. We hypothesize that oxygenation only results in an increased permanent benthic P burial if, because of the depressed sulfide production, more ferrous phosphate (e.g., vivianite) and less FeS is deposited in the anoxic sediment. Hence, it is not the oxic sediment surface that directly affects the permanent redox‐dependent sediment P retention but the molar ratio of the available reactive Fe(II) : S2‐ : PO4 in the anoxic sediment. This ratio is driven by the settling rate and the nature of organic matter and particulate iron, as well as the supply of oxygen, nitrate, and sulfate to the sediment.

Development of a Total Phosphorus Concentration Goal in the TMDL Process for Lake Okeechobee, Florida (USA)

ABSTRACT This paper describes the approach used to establish an in-lake concentration goal for total phosphorus in the total maximum daily load (TMDL) process carried out by die Florida Department of Environmental Protection (FDEP) for Lake Okeechobee. In order to specify the in-lake phosphorus goal, the first consideration was to identify the most suitable indicator of “imbalance in flora or fauna” pursuant to Subsection 62–302.530(48)(b) of the Florida Administrative Code (FAC). Blooms of cyanobacteria (blue-green algae) were previously identified as one of the most serious symptoms of cultural eutrophication in this lake, and there existed a large data set relating total phosphorus to chlorophyll a, which can be used to index the occurrence of blooms. We evaluated the occurrence of samples with chlorophyll a in excess of 40 μg·L−1 (moderate bloom) and 60 μg·L−1 (severe bloom) as a function of total phosphorus concentrations in order to specify the lake phosphorus goal. A cross-tabulation procedure was used to identify a range of total phosphorus corresponding to a rapid increase in bloom frequency. Near-shore and pelagic data sets were sorted by total phosphorus and each was sub-divided into ten intervals of approximately equal sample size. When total phosphorus averaged below 30 μg·L−1, the probability of moderate blooms was below 3% and the probability of severe blooms was near 1% in the near-shore region. When total phosphorus averaged between 35 and 45 μg·L−1, frequencies were between 15 and 35% for moderate blooms and between 2 and 5% for severe blooms, respectively. Pelagic bloom frequencies also increased with increasing phosphorus, but the response was considerably muted relative to that observed in the near-shore area. To ensure an acceptable level of risk in terms of algal bloom occurrence, a total phosphorus goal of 40 μg·L−1 was selected by the FDEP. Mass-balance modeling results (Walker 2000) indicate that an average external phosphorus load of 140 metric tons y1 (compared to a 1973 to 1999 mean of 498 metric tons y1) would provide a long-term average phosphorus concentration in the lake's pelagic zone of 40 μg·L−1. Based on our empirical model relating bloom frequencies to total phosphorus, it is predicted that under these TMDL loading conditions, bloom frequencies in the near-shore region would be 2 to 9%, as compared to 5 to 33% under present conditions. Successful implementation of the TDML should significantly reduce near-shore bloom frequencies in Lake Okeechobee.

Environmental impact of two successive chemical treatments in a small shallow eutrophied lake: Part I. Case of aluminium sulphate

This paper deals with the efficiency and effects of addition of aluminium sulphate on soft water quality of a shallow eutrophic lake. Almost all the controlled variables improved with treatment, especially nutrient concentrations such as soluble reactive phosphorus (SRP) and transparency. However, aluminium sulphate was not added in sufficient quantity to reduce the total phosphorus content, SRP concentration was significantly reduced in the short term. Moreover, external loading of phosphorus was high and not taken into account by the in-lake treatments. Finally, resuspension of sediment (polymictic lake) removed the alum hydroxide layer on the sediment surface, which reduced treatment effectiveness. No significant pH decrease was noted following alum addition. According to bibliographical toxicological data, monomeric aluminium content does not show any toxic effect on aquatic fauna and flora. In spite of low SRP in the water column, the treatment did not prevent appearance of Microcystis sp. colony (>10 colony per ml) approximately 30 days after alum application.

Seasonal dynamics in the concentrations and retention of phosphorus in shallow Danish lakes after reduced loading

To investigate how phosphorus concentrations and retention were affected during recovery after reduced external phosphorus loading, phosphorus concentrations and loading were seasonally monitored for 11 years in 15 shallow eutrophic Danish lakes. Hydraulic loading, inlets, and lake water were sampled monthly during the winter (October–April) and bimonthly during the summer (May–September) to estimate monthly mass balances. Total phosphorus concentrations in the lakes were two to three times higher during summer than during winter; internal loading seems to be of great significance. The retention of phosphorus was negative from April until October, the most negative values generally occurring in May and July (–30% of external loading). In June, retention was less negative (−12%), probably influenced by clearwater phases which often appear in early summer. During the 11 years, the period with negative phosphorus retention decreased from five to four months and, as a median, the maximum phosphorus concentration declined from ca. 0.4 to 0.2 mg phosphorus l−l. In Lake Engelsholm, which was biomanipulated by a 66% reduction of the zooplanktivorous fish stock (mainly bream, Abramis brama), marked changes were observed in biological structure. Also affected were phosphorus concentrations and retention. Simultaneously, with an increase in Secchi depth from 0.7 to 2.0 m, summer concentrations of total phosphorus decreased to less than 50% of the pre-biomanipulation level. The period exhibiting negative phosphorus retention declined from six months before the intervention to four months afterwards. It is concluded that the recovery period after reduced phosphorus loading may last long in shallow eutrophic lakes, but that the recovery is significantly influenced by biological structure. During recovery, both the duration and net phosphorus release rates from the sediment declined progressively.

Partial recovery of a eutrophic reservoir through managed phosphorus limitation and unmanaged macrophyte growth

Recovery from an algal to a macrophyte-dominated state in a freshwater system can require both nutrient reduction and a switch to initiate the change. Despite ferric dosing of pumped inputs since 1983 to reduce external phosphorus loading and forced mixing of the main basin by helices since 1985, Alton Water reservoir, Suffolk, UK suffered from increasing planktonic chlorophyll aconcentrations in the main basin, from construction in 1981 until 1992. From 1987 blooms of cyanobacteria were increasingly frequent, with a major bloom of Woronichinia(originally Coelosphaerium) in 1992, resulting in water supply problems. Since 1993 a partial recovery from a eutrophic to mesotrophic state occurred. Studies of chlorophyll a, orthophosphate (OP), total phosphorus (TP), Alkaline Phosphatase Activity (APA) and response of phytoplankton to in situ phosphate addition experiments between 1995 and 1997, revealed gradients indicative of increasing phosphorus limitation of phytoplankton along the length of the reservoir, away from the pumped input. From 1993 to 1997 no algal blooms occurred in the main basin of the reservoir and extensive macrophyte growth was observed around the reservoir periphery, principally Elodea spp. This corresponded with an increase in weed-eating birds, such as coot (Fulica atra) and tufted duck (Aythya fuligula). The partial recovery of the main basin was attributed to bottom-up control through phosphorus limitation as a result of management in the form of ferric dosing, as well as competition for nutrients and light by the extensive macrophyte beds. Increased macrophyte growth was probably induced by changes in the reservoir ecology following the cyanobacterial bloom and associated fish kill in 1992.

The effect of an emergent macrophyte (Typha angustifolia) on sediment resuspension in a shallow north temperate lake

1. The effects of emergent macrophytes on water turbidity and sediment resuspension in the shallow Kirkkojärvi basin of Lake Hiidenvesi were studied with sediment traps, and concomitant sediment and water samples. The study was conducted during May–August in three different zones of a stand of the emergent Typha angustifolia.2. Within the stand (5 m from the edge), both the concentration of suspended solids and the rate of sediment resuspension were significantly lower than at the edge and outside the stand (5 m from the edge). The differences between the zones increased towards the end of summer together with the growing stem density. During the study period (82 days), 2210 g dw m−2 of sediment was resuspended in the outer zone. At the edge and in the inner zone, the corresponding numbers were 1414 and 858 g dw m−2, respectively.3. With the resuspended sediment, 39.4 mg P m−2 day−1 was brought to the water column outside the stand, 22.4 mg P m−2 day−1 at the edge and 13.4 mg P m−2 day−1 within the stand.4. In early summer, the concentration of suspended solids had a highly significant positive effect on soluble reactive phosphorus (SRP) concentration in the water, whereas in late summer no effect was found. During the study period, phosphorus retention by emergent macrophyte stands corresponded to 3–5% of the present annual external phosphorus loading of the Kirkkojärvi basin.

Exploring the dynamics and fate of total phosphorus in the Florida Everglades using a calibrated mass balance model

The Everglades protection area, which encompasses five Water Conservation Areas (WCA), Everglades National Park (ENP), and a network of canals, levees, structures, and pump stations, exhibits elevated nutrient concentrations in the water and sediments, primarily as a result of phosphorus loads in agricultural runoff. A mass balance model was developed to predict phosphorus fate and transport in the Everglades Protection Area that could result from proposed phosphorus reduction strategies. The modeled area is about a 7000 km 2 region that is divided into 642, 3.2×3.2 km cells, plus additional cell areas for canals. Phosphorus is transported between model cells and canals in accordance with output from a regional hydrology model. Simulated water column phosphorus dynamics within each cell and canal is further controlled by a simple, apparent net settling rate coefficient that integrates the effects of chemical, biological, and physical processes, and leads to net deposition of phosphorus in the sediments. After specification of external phosphorus loads (surface water and atmospheric wet and dry deposition) and system boundary conditions, the model was calibrated to available field data. The calibration procedure consisted of varying the apparent net settling rate coefficients in the WCA and the ENP. The goodness of fit of predicted water column total phosphorus concentrations varied temporally and spatially. Sediment phosphorus net deposition rates calculated by the model matched well with in situ observations where available. The model indicates that phosphorus in seasonal rainfall is a dominant influence on water column phosphorus dynamics in remote areas of the Everglades, whereas phosphorus dynamics in cells directly downstream of runoff inputs exhibit well-documented, nutrient gradients in receiving waters and sediments that could not be caused by rainfall alone. The model suggests that reductions of phosphorus concentrations leaving agricultural areas at the north end of the system will lead to lower concentrations entering ENP at the south end of the system.

The potential for phosphorus release across the sediment–water interface in an eutrophic reservoir dosed with ferric sulphate

Alton Water, Suffolk, UK is a pumped storage reservoir that has a history of cyanobacterial blooms. Dosing of the input water with ferric sulphate to control external phosphorus loading has occurred since 1983. A detailed study of the sediment chemistry of the site was carried out between May 1995 and July 1997. Sequential phosphorus fraction analysis indicated a decrease along the length of the reservoir in sediment labile phosphorus content from 0.62 to 0.08 mg P g −1 dw and iron-bound phosphorus content from 3.22 to 0.46 mg P g −1 dw. These gradients positively correlated with water column chlorophyll a concentrations reported in a parallel study. Labile and iron-bound sediment phosphorus contents were in a dynamic equilibrium due to diffusional release, contributing to internal loading to the water column. Equilibrium phosphorus concentrations (EPC) determined from phosphorus adsorption capacity (PAC) experiments were lower inside the bunded region (0.01–0.03 mg P-PO 4 l −1) where iron content was greatest compared to outside the bund (0.15–0.20 mg P-PO 4 l −1) suggesting greater potential for diffusional release of phosphorus at the latter site. PAC experiments indicated that anaerobic and pH-mediated loadings were of less importance than diffusional release, although the latter may have contributed to internal loading in the main reservoir. Sulphate concentrations may act to increase the potential for anaerobic internal loading near to the pumped input in microstratified sediment. Sediment iron content decreased from 250±13.1 to 51±4.0 mg Fe g −1 dw across the line of a constructed bund at the north-west end near to the pumped input, which indicated successful control of dispersal of the fine ferric floc. The management implications with regard to phosphorus loadings indicated by these results are discussed.

Wind-induced increases in trophic state characteristics of a large (27 km 2 ), shallow (1.5 m mean depth) Florida lake

Nutrient and chlorophyll concentrations in Lake Newnan (27 km2, mean depth 1.5 m), Florida showed dramatic increases from 1991 to 1998. Historical data showed Lake Newnan never had sufficient aquatic macrophyte abundance for a shift in alternate stable states to account for increases in trophic state characteristics. External phosphorus and nitrogen loads from incoming streams were monitored from August 1997 to July 1998 to determine if external supplies of nutrients were responsible for increases in lake nutrient and chlorophyll concentrations. During the study period, external nutrient loading rates were not correlated to lake nutrient concentrations. Phosphorus and nitrogen models based on the external loading estimates predicted the lake total phosphorus and total nitrogen concentrations to be 370% and 680% less, respectively, than the observed lake total phosphorus and total nitrogen mean concentrations. Consequently, phosphorus and nitrogen exports were 280% and 540% greater, respectively, than stream input loading. Data during the study period revealed strong inverse relations between lake stage and total phosphorus (r=−0.78), total nitrogen (r=−0.71), and chlorophyll (r=−0.90) concentrations. Long-term data (1965–1998) also revealed inverse correlations (r=−0.48 to −0.52) between lake stage and total phosphorus, total nitrogen, and chlorophyll concentrations. Applying fundamental wave theory and using a bathymetric map, it is probable that as much as 70% of the lake bottom sediment could be subjected to resuspension 50% of the time when the lake stage falls below 19.9 m mean sea level (msl). Above a lake stage of 19.90 m msl, less than 20% of the lake bottom sediment can potentially be resuspended 50% of the time. A percent frequency distribution from 1991 to 1998 showed that over 30% of the lake stages fell below 19.9 m msl. However, from 1967 to 1990, only 8% of the lake stage values fell below 19.9 m msl. Increases in total phosphorus, total nitrogen and chlorophyll concentrations in Lake Newnan were likely caused by an increased probability of internal loading due to decreased lake levels, and not to external loading of phosphorus and nitrogen.

Response of Lake Piediluco to the change of hydrodynamic conditions and nutrient load reductions

In order to verify and quantify the efficacy of management strategies utilised to improve the water quality of Lake Piediluco in the long and short term, a model was developed to represent the unsteady and three-dimensional form of eutrophication processes and water anoxia phenomena. Such a model, taking into account the hydrodynamic mean and turbulent characteristic of the lake, allows the simulation of the spatial and temporal evolution of the concentration fields of the following species: (a) in water, dissolved oxygen, algal organic carbon, particulate organic carbon, dissolved organic carbon, orthophosphate, hydrogen sulphide; and (b) in sediments, dissolved oxygen, particulate organic carbon, dissolved organic carbon, orthophosphate, adsorbed phosphorus, hydrogen sulphide. This paper presents the simulation results using the following configurations characterised by: different hydraulic regimes of the lake; different external phosphorus loads introduced into the lake from discharges; different initial adsorbed phosphorus concentrations in sediments (obtainable by sediment reclaim) in certain critical areas of the lake. The simulations have detailed: (1) the characterisation of the lake's eutrophication behaviour in its present state and the vulnerability of the different areas of the lake to summer water anoxia; (2) the role of external and internal phosphorus loads on the eutrophication processes; and (3) the management options to improve the environmental conditions of the lake.

Rehabilitation of Delavan Lake, Wisconsin

ABSTRACT A comprehensive rehabilitation plan was developed and implemented to shift Delavan Lake, Wisconsin, from a hypereutrophic to a mesotrophic condition. The plan was threefold: (1) reduce external phosphorus (P) loading by applying Best Management Practices in the watershed, enhance an existing wetland, and short-circuit the inflows through the lake, (2) reduce internal P loading by treating the sediments with alum and removing carp, and (3) rehabilitate the fishery by removing carp and bigmouth buffalo and adding piscivores (biomanipulation). The first and second parts of the plan met with only limited success. With only minor reductions in internal and external P loading, P concentrations in the lake returned to near pre-treatment concentrations. The intensive biomanipulation and resulting trophic cascade (increased piscivores, decreased planktivores, increased large zooplankton populations, and reduced phytoplankton populations) eliminated most of the original problems in the lake (blue-green algal blooms and limited water clarity). However, now there is extensive macrophyte growth and abundant filamentous algae. Without significantly reducing the sources of the problems (high P loading) in Delavan Lake, the increased water clarity may not last. With an improved understanding of the individual components of this rehabilitation program, better future management plans can be developed for Delavan Lake and other lakes and reservoirs with similar eutrophication problems.

Quantitative inferences of past hypolimnetic anoxia and nutrient levels from a Canadan Precambrian Shield lake

Paleolimnological analyses were used to infer limnological changes during the past ~ 300 yrs in the west basin of Peninsula Lake, a small (853 ha) Precambrian Shield lake in Ontario, Canada, that has been subjected to moderate cultural disturbances (forest clearance, cottage and resort development). This study represents a pioneering attempt to use sedimentary chironomid assemblages and weighted-averaging models to quantify past hypolimnetic anoxia (expressed as the anoxic factor, AF). Impacts of forest clearance and human land-use on deepwater oxygen availability and surface water quality were assessed by comparing chironomid-inferred AF and diatom-inferred total phosphorus concentration ([TP]) to changes in terrestrial pollen and historical data. This study also discusses the ability of chironomids to quantitatively infer changes in AF.Pre-disturbance chironomid assemblages were stable and dominated by taxa indicative of oxygen-rich hypolimnetic conditions (e.g., Protanypus, Heterotrissocladius, Micropsectra type), while diatoms indicated oligotrophic lake status (diatom inferred [TP] = 5-7 μg·l-1). Chironomids characteristic of lower oxygen availability (e.g., Chironomus, Procladius) increased following land-clearance, road construction, establishment of a grist mill and lakeshore development beginning ca. 1870. Increased abundances of Tanytarsus s. lat., a multigeneric group of mainly littoral chironomids, since 1900, indicated that littoral chironomids may have comprised a greater proportion of fossil assemblages during periods of eutrophication and prolonged anoxia. Abundances of meso-eutrophic diatom taxa (e.g., Fragilaria crotonensis, Asterionella formosa, Aulacoseira ambigua, A. subarctica) increased concurrent with European settlement (ca. 1870) and diatom-inferred [TP] doubled (~ 6-12 μg·l-1), further indicating that naturally-oligotrophic Precambrian Shield lakes were extremely sensitive to initial land-clearance activities.Recent increases in oligotrophic diatom taxa (e.g., Cyclotella stelligera) indicate a shift to more oligotrophic conditions since ca. mid-1960s, with greatest changes since ca. 1980. The chironomids Heterotrissocladius and Micropsectra type also increased at this time suggesting greater deepwater oxygen availability. These recent water-quality improvements, possibly in response to enhanced nutrient removal from detergents and sewage, climate-related reductions in external phosphorus loads, and catchment (but not lake) acidification and reforestation, suggest that habitat for commercially-valuable cold-water fishes has improved in recent decades despite greater recreational lake-use.Paleolimnological assessment of trophic status changes in Peninsula Lake using fossil diatom and chironomid assemblages were in good agreement. Diatom inferences of [TP] and chironomid inferences of AF both suggest that Peninsula Lake was historically oligotrophic, became oligo-mesotrophic after European settlement, and returned to oligotrophy in recent yrs. Chironomid inferences of [TP] consistently underestimated the trophic status of Peninsula Lake, possibly due to its relatively large hypolimnion. These results suggest that AF represents a useful tool for quantitatively reconstructing the past trophic status of deeper, stratified lakes.

External phosphorus load estimates and P-budget for the hydroelectric reservoir of Bort-Les-Orgues, France

This paper presents two methods used to estimate the externalphosphorus load for the hydroelectric reservoir ofBort-Les-Orgues (in France's `Massif Central' region). The Pload calculated from export coefficients (150 t yr-1) agreedwith field measurements taken during a one-year survey carriedout from March 1996 to February 1997 (121 t yr-1). Exportcoefficients represent a good option for estimating phosphorusloads of other reservoirs in the region of Bort-Les-Orgues,provided that they display similar characteristics. Theseexternal load estimates are greater than the maximum permissibleload for a mesotrophic lake calculated from the OECD equations(40 t yr-1); thus, a yearly reduction of about 60 t shouldideally be achieved. This target, however, is probably tooambitious, and measures to reduce both point and non-pointsources must be implemented for the reservoir to tbe mesotrophic.Moreover, Bort-Les-Orgues is a tourist area. As such, thepresence of algal blooms (a regular occurrence on the lake) dueto an excess of P in the lake's water inflow could actuallyexert a negative impact on regional economy.

Internal phosphorus loading in Lake Ringsjön

Lake Ringsjon did not respond with decreased algal production following a substantial reduction in external phosphorus loading. This is typical of many shallow lakes which for decades have received excessive amounts of nutrients. The inertia is due to large amounts of phosphorus (P) stored in sediments and biota, causing internal phosphorus loading. Much of this phosphorus is thought to be released from the organic-rich profundal sediments. In Lake Ringsjon, only one third of the total bottom area is covered by such sediments, the rest being dominated by sand and silt. In the profundal sediments bulk P content was not exceptionally high (approximately 2 mg P·g DW−1), while the pore water phosphate concentrations, especially in Satofta Basin, were very high, indicating large potential for phosphorus release to the water. This is also indicated by the large proportion of Fe- and Al-bound P in the sediments of Satofta Basin. Although there are no direct quantifications of phosphorus release from the sediments in Lake Ringsjon, measurements of phosphorus concentrations in the water mass as well as budget calculations for the three basins clearly show a high capacity for internal loading. Phosphorus concentrations generally increase during summer, when external additions are minimal. Until 1980, the annual external phosphorus addition to Lake Ringsjon greatly exceeded the output, showing that the lake was an efficient phosphorus trap. Since then, input and output have been balanced, but in recent years signs that the lake is once again retaining phosphorus on an annual basis are evident. There are marked differences between the three basins, with Western Basin generally retaining phosphorus, while the upstream Eastern Basin and Satofta Basin during the 1980s often exported phosphorus. It is not possible to evaluate the effects of the fish biomanipulation on the internal loading of phosphorus from the sediment.

Planning and execution of the fish reduction in Lake Ringsjön

Lake Ringsjon, a naturally productive lake in southern Sweden, was subjected to high external phosphorus loading during the 20th century and became highly productive. In spite of effective waste water treatment which reduced external phosphorus loading, the improvements in lake water quality were moderate and proceeded at low rate. Consequently cyprinid reduction, which was expected to affect water clarity via several intertrophic mechanisms, was implemented as a measure for restoring the lake. The fish removal was performed using a board trawler equipped with a herring trawl modified for bottom trawling. The bottom rope of the trawl was 11 m and the total length of the trawl was 20 m, the cod end mesh size was 11 mm (knot to knot). In the beginning the catch amounted to 1 ton of fish in less than 2 hours, logistic problems bringing the catch to shore and replacing all piscivorous fish into the lake reduced the initial daily catch to 2–3 tons. A grand total of 240 tons of fish were removed by trawling Lake Ringsjon over a 3 year period. The catch was either fermented with myric acid or deep-frozen before being used as mink and pelican food respectively. Some of the catch was also used for gas production. The cost for boats and equipment was approximately 60 000 US$. The three persons needed for fishing and sorting as well as the costs of taking care of the catch on land are not included in this sum.

Summer water clarity responses to phosphorus, Daphnia grazing, and internal mixing in Lake Mendota

Linear models were developed for predicting mean Secchi disk depth readings as a measure of water clarity for the summer months in Lake Mendota, Wisconsin. The 20‐yr (1976–1995) data set also included external phosphorus (P) loadings and in‐lake April P concentrations as indices of lake nutrient status, and monthly (28 d) water column stabilities and Daphnia biomasses as indices of lake mixing and algal grazing potentials, respectively. June Secchi depths were mostly controlled by food web dynamics, which dictated whether the larger‐bodied Daphnia pulicaria or the smaller‐bodied D. galeata mendotae dominated during the spring clear‐water phase. May Daphnia biomasses were significantly greater in D. pulicaria‐dominated years than in D. galeata‐dominated years; D. pulicaria‐year biomasses were also greater during the summer months. The model for the midsummer (July–August) months indicated that Secchi depths were inversely related to April P concentrations and positively related to midsummer Daphnia biomasses and lake stabilities. Scenarios for midsummer Secchi depths were tested using the observed minimum and maximum values for each predictor variate. While holding two variates constant, April P, Daphnia biomass, and lake stability each resulted in relatively similar Secchi ranges (0.81, 0.81, and 1.17 m, respectively). Our results suggest that summer water clarity in eutrophic Lake Mendota is controlled by interacting ecosystem processes linked to land use activities, lake food web dynamics, and climate.

Vertical movements by planktonic cyanobacteria and the translocation of phosphorus: implications for lake restoration

1. Vertical movements of cyanobacteria during summer stratification were studied in Cauldshiels Loch, a small dimictic temperate lake. 2. Filaments of the three species of cyanobacteria dominant in the summer phytoplankton were recorded moving vertically in both directions, by simple traps deployed just below the thermocline. 3. These phytoplankters could potentially exploit opposing ends of the vertical light/nutrient gradient that existed in the water column during stratification. 4. The hypolimnion and water overlying the sediments were enriched by phosphorus released from the sediments. This internal loading to the lake provides a source of phosphorus available to migrating cyanobacteria to sustain production and growth in the epilimnion. 5. In this study, cyanobacteria filaments moving from the hypolimnion were found to be translocating phosphorus to the epilimnion. 6. Following reduction of external phosphorus loading, utilization of such internal phosphorus sources may delay expected reductions of bloom-forming cyanobacterial communities, and consequent improvements in other aspects of water quality. Copyright © 1999 John Wiley & Sons, Ltd.

Synthesis of theoretical and empirical experiences from nutrient and cyprinid reductions in Lake Ringsjön

The reduction in external phosphorus load to Lake Ringsjon during the 1980s, did not result in improved water transparency during the following ten-year period. Furthermore, a fish-kill in the Eastern Basin of the lake, in addition to a cyprinid reduction programme (biomanipulation; 1988–1992), in contrast to theory, did not lead to any increase in zooplankton biomass or size. This absence of response in the pelagic food chain may have been attributed to the increase in abundance of YOY (0+) fish, following the fish reduction programme. Despite the lack of effect on zooplankton, there was a decrease in phytoplankton biomass, a change in species composition and an increase in water transparency following biomanipulation. In 1989, one year after the fish-kill in Eastern Basin, the Secchi depth (summer mean) increased from 60 cm to 110 cm. In the following years, water transparency increased further, despite an increase in phosphorus loading. An unexpected effect of the biomanipulation was an increase in benthic invertebrate and staging waterfowl abundances, which occurred 2–4 years after fish reduction. Hence, the response in the benthic community following biomanipulation was considerably stronger than in the pelagic community. A likely explanation is that reduction in abundance of the benthic feeding fish species bream (Abramis brama), strongly affected the benthic invertebrate fauna. In this paper, we present what we believe happened in Lake Ringsjon, and which processes are likely to have been important at various stages of the restoration process.

Multiple techniques for lake restoration

Lake Finjasjon is a shallow, eutrophic lake (area 1100 ha, mean depth 3 m, maximum depth 13 m) in southern Sweden. In the 1920s, the lake was clear, with a summer Secci depth of about 2 m. During the first half of the 20th century, untreated sewage from the town polluted the lake. In the 1930s, the lake began to show eutrophic characteristics, and in the 1940s, the cyanobacterium Gloetrichia echinulata dominated in summer. In 1949, the first municipal sewage treatment plant was built. The treatment was, however, insufficient, since the lake continued to be the recipient of the effluent with the result that the occurrence of cyanobacteria became more frequent. Species such as Microcystis and Anabaena caused skin rash and allergic symptoms among swimmers. The phosphorus load on Lake Finjasjon increased as the population of Hassleholm grew and reached a peak value of 65 tons annum−1 in 1965. In 1977, the sewage plant was rebuilt to include chemical flocculation, reducing the total external phosphorus load to about 5 tons annum−1. Despite this improvement the lake did not recover from its chronic and toxic cyanobacterial blooms. Phosphorus-leaking black sediments were identified as the cause of the lake’s failure to recover. Some 60% of lakebed area is covered with sediments on average 3 m thick. Dredging the sediments was started on a large scale in 1987. Five years later, 25% of the sediment area had been removed but the dredging was stopped since phosphorus continued to be released into the water from these areas. In 1992, a new restoration policy, a combination of further reduced external nutrient loading and food-web manipulation was initiated. A constructed wetland (30 ha) to reduce phosphorus and nitrogen was created in connection to the effluent from the sewage treatment plant. Protection zones along the feeder streams into Lake Finjasjon were also established. A cyprinid reduction programme by trawling was carried out between 1992 and 1994. When it started, the fish community was composed of 90–95% bream and roach. After two years of trawling, the ratio between piscivorous and planktivorous fish was 1:1. In 1994 and 1995, the transparency increased due to a considerably reduced biomass of phytoplankton and a radically altered phytoplankton community. The monoculture of Microcystis was replaced by a diverse phytoplankton community. The increased transparency made possible the development of submerged macrophytes such as Elodea, Myriophyllum and Potamogeton. The internal loading of phosphorus decreased dramatically in 1994 and 1995, possibly as a result of reduced sedimentation of phytoplankton.