The occurrence of internal phosphorus loading in two small,eutrophic, glacial lakes in Northeastern Ohio

Internal loading of phosphorus for the summers of 1972–1974 in the eutrophic Twin Lakes, Ohio, USA was calculated from nutrient budgets, and was found to account for 65–100% of the increase in phosphorus content of the lakes during this period. Recovery of lakes of this type after nutrient diversion may be delayed by internal loading and chemical inactivation of phosphorus may be needed. A discussion of sources of this internal loading is presented.     

Biomanipulation,new perspectives for restoration of shallow,eutrophic lakes in The Netherlands

Eutrophication of Dutch lakes has led to massive algal growth, disappearance of most of the macrophytes and disturbance of the food chain. The pike population has fallen sharply and bream developed very strongly, in the absence of this predator. Eutrophication problems are primarily being tackled by reducing nutrient loading. Restoration of water quality, however, seems to be impeded by the present structure of the food chain,i.e. the large bream stock. Biomanipulation, especially fish stock control with the aim of reducing the bream stock and increasing that of predatory fish, can accelerate the process of restoration. For the further development of biomanipulation it is very important that water authorities and managers of fish stocks agree on a common strategy.     

Experiences with reducing point sources of phosphorus to lakes

Experiences over the last 25 years have demonstrated that eutrophication can be reversed, and that phosphorus is most often the nutrient through which control should be exerted.The reduction of the external load of phosphorus to a eutrophic lake is a necessary condition for lake restoration, but may not in itself be sufficient. Three main response patterns to a reduction in external load are identified. These include reduction in lake phosphorus that leads to sufficient reduction in chlorophyll to change the trophic category, to make the lakes less eutrophic or have small or no effect. The factors that determine the actual response are discussed.It is clear that interventions to restore eutrophic lakes have not always given the results expected. Limnological studies are necessary for well-grounded predictions.     

Biomanipulation additional to nutrient control for restoration of shallow lakes in The Netherlands

Eutrophication control is one of the major issues in the environmental policy in The Netherlands. As a result of international action programmes the average phosphorus loading of freshwater systems should decrease by 50% between 1985 and 1995. However, in many cases the restoration of water quality requires additional measures. Recovery is hampered by the structure and functioning of the present food-chain.

The feeding behaviour of the dominant fish species in Dutch lakes, bream and roach, tend to impose a homeostasis on the system, resisting restoration of water quality. In shallow lakes, biomanipulation, including drastic reduction of fish-stocks, may induce a shift from a stable ‘turbid-water state’ to a stable ‘clear-water state’.

To assess the possibilities of biomanipulation for the restoration of a particular lake, three questions are relevant: (1) is a drastic reduction of fish-stocks feasible?, (2) will a shift occur from ‘turbid to clear’ after the fish reduction? and (3) will the new situation of clear water be stable? This paper focuses attention on the last two questions. The increase in water clarity, following fish reduction, largely depends on the increase in the density of the Daphnia-population and the contribution of benthivorous fish to the resuspension of sediments. A ‘turbid to clear’ shift may be expected if the total biomass of planktivorous and benthivorous fish is reduced to levels<50 kg ha^?1. The stability of the achieved clear-water state largely depends on the development of submerged macrophytes in the lake and on the level of nutrient loading. It is tentatively concluded that a stable clear-water state may be expected at initial total-P concentrations<0.10 mg l^?1.

Because the water managers in The Netherlands have no fishing rights, they have to.co-operate with anglers and commercial fishermen to apply biomanipulation as a tool for water management.

     

Persistent internal phosphorus loading during summer in shallow eutrophic lakes

Nutrient availability, in particular of phosphorus (P), is a key factor for the structure and functioning of shallow lakes, and not least the sediment plays an important role by acting as both a nutrient source and sink. We used 21 years of monthly mass balance and lake water data from six shallow (mean depth = 1.2–2.7 m) and fast flushed (mean hydraulic retention time = 0.6–2.6 months) eutrophic Danish lakes (mean summer P concentrations ranging from 0.09 to 0.61 mg/l) to investigate long-term trends in yearly and seasonal patterns of P retention. To one of the lakes, the external P input was reduced by 70% in the early 1990s, whereas none of the other lakes have experienced major changes in external P loading for more than 20 years. All lakes showed a distinct seasonal pattern with high P concentrations and typically negative P retention during summer (up to ?300% of the external loading from May to August). During winter, P retention was overall positive (up to 50% of the external loading from December to April). Internal P loading from the sediment delayed lake recovery by approximately 10 years in the lake with the most recently reduced external loading, but in all the lakes net release of P from the sediment occurred during summer. P release in the six lakes has not abated during the past decade, indicating that the sediment of eutrophic and turbid shallow lakes remains a net source of P during summer. The seasonal variations in P retention became more pronounced with increasing P levels, and retention decreased with increasing temperature, but increased if clear water conditions were established.     

Engineering and biological approaches to the restoration from eutrophication of shallow lakes in which aquatic plant communities are important components

Engineering approaches (nutrient removal, sediment pumping, hypolimnion oxygenation, alum treatments) may be most appropriate to deep lakes where the aim of restoration from eutrophication is simply to reduce the production and crop of one component, the phytoplankton. They do not always give the desired results because the nutrient loading may only be reduced to a limited extent. There are additional problems in shallow lakes where change of state between community dominance (aquatic plants versus plankton) is wanted. Each community has powerful buffering mechanisms and biomanipulation may be essential to switch one state to another even with considerable nutrient reduction. For the phytoplankton-dominated community the buffers include the advantages of early growth, lower diffusion pathways for CO₂, overhead shading, and an absence of large cladoceran grazers. This later is because open-water shallow environments provide no refuges against predation for the large Cladocera which are both the most efficient grazers and the most favoured prey for fish. Restoration of aquatic plants may then require provision of refuges for the grazers. Different sorts of refuge are discussed using case studies of Hoveton Great Broad and Cockshoot Broad in the Norfolk Broadland.     

Role of sediment and internal loading of phosphorus in shallow lakes

Current velocity is a pervasive feature of lotic systems, yet this defining environmental variable is rarely examined as a factor for regulating stream herbivory. To investigate how current modifies herbivory in the upper Colorado River, U.S.A., loops of electrified fencing wire were used to reduce in situ grazer densities on 30 × 30 cm tile substrates. After 45 d, electrified tiles had significantly fewer grazers (P = 0.03) and >2X more algal biomass than controls (P = 0.0002). Reduced grazing on electrified tiles yielded periphytic assemblages having more diatoms and chlorophytes, as well as greater algal species richness. Current velocity effects alone did not significantly regulate algal abundance; however, the interaction between current velocity and grazer exclusion resulted in more algae in slow vs. fast current (P = 0.02). Grazer abundances were similar between fast and slow current velocities, suggesting that grazers in the Colorado River differ in their ability to regulate algae across the current velocity gradient. Our results indicate that stream current-mediated herbivory in streams may be more important than is generally recognized.     

Submerged vegetation development in two shallow, eutrophic lakes

Submerged macrophyte vegetation in two shallow lakes in the Netherlands, Lake Veluwemeer and Lake Wolderwijd, has been affected by eutrophication in the late 1960's and 1970's. Recent changes in the vegetation occurred in the period following lake restoration measures. Between 1987 and 1993, the dominance of Potamogeton pectinatus decreased, while Charophyte meadows expanded over the same time interval. The pattern of change of the dominant macrophyte species might result from changes in the underwater light climate. Seasonally persistent clear water patches associated with the Chara meadows have been observed in the last few years. The interaction between submerged macrophyte vegetation succession and water transparency in the lakes is discussed.     

Prediction of phosphorus and nitrogen concentrations in lakes from both internal and external loading rates

A model predicting phosphorus and nitrogen concentrations from loading rates was tested using data collected from a shallow, nutrient-rich lake where both internal and external loading occurred. Predicted nutrient concentrations agreed closely with obaerved values and it is suggested that the model could be used to predict the reduction in loading rate required to effect lake restoration.     

Internal phosphorus loading to shallow Edinboro Lake in northwestern Pennsylvania

The summer stratification phosphorus budget for eutrophic Edinboro Lake in northwestern Pennsylvania was determined. Phosphorus loading from internal sources contributed 141 kg, (79%) and 55 kg, (68%) of the mass phosphorus increase in the lake in 1981 and 1982, respectively. Calculated anaerobic sediment release rates of total phosphorus were 9.9 and 3.7 mg m^–2 day^–1 for these two years. The observed summer maximum chlorophyll a concentration was 1.5–3 times greater than that predicted by existing models. Year-to-year variability in the internal phosphorus load for this lake and others is discussed. Without a data base that will permit the comparison of lakes and with and without a significant supply of internal phosphorus, prediction of the relative importance of internal loading in a particular lake will be difficult.     

Carbon burial by shallow lakes on the Yangtze floodplain and its relevance to regional carbon sequestration

Floodplain lakes may play an important role in the cycling of organic matter at the landscape scale. For those lakes on the middle and lower reaches of the Yangtze (MLY) floodplain which are subjected to intense anthropogenic disturbance, carbon burial rates should, theoretically, be substantial due to the high nutrient input, increased primary production and high sediment accumulation rates. There are more than 600 lakes >1 km² on the Yangtze floodplain including 18 lakes >100 km² and most are shallow and eutrophic. ²¹⁰Pb‐dated cores were combined with total organic carbon (TOC) analyses to determine annual C accumulation rates (C AR; g C m^?2 yr^?1) and the total C stock (since ~1850). The sediment TOC content is relatively low with an average <2% in most lakes. C AR ranged from ~5 to 373 g C m^?2 yr^?1, resulting in C standing stocks of 0.60–15.3 kg C m^?2 (mean: ~5 kg C m^?2) since ~1850. A multicore study of Chaohu lake (770 km²) indicated that spatial variability of C burial was not a significant problem for regional upscaling. The possible effect of changes in lake size and catchment land use on C burial was examined at Taibai lake and indicated that lake shrinkage and declining arable agriculture had limited effects on C AR. The organic C standing stock in individual lakes is, however, significantly dependent on lake size, allowing a simple linear scaling for all the MLY lakes. Total regional C sequestration was ~80 Tg C since ~1850, equivalent to ~11% of C sequestration by soils, but in ~3% of the land area. Shallow lakes from MLY are a substantial regional C sink, although strong mineralization occurs due to their shallow nature and their role as C sinks is threatened due to lake drainage.     

Eutrophication control strategies for three shallow Vecht lakes in the province of North Holland

Like many shallow surface waters in the Netherlands the North Holland Vecht lakes, formerly known for their rich variety of flora and fauna, now face a serious eutrophication problem. Nutrient enrichment has been mainly in the form of (treated) wastewater discharges, and the continuing ingress of nutrient-loaded water from the river Vecht. Yet, this water has to be supplied in order to compensate for water shortages resulting from (i) changes in the groundwater flow pattern due to reclamation of the deep polder Horstermeer, (ii) extensive groundwater extraction in the Gooi hills, and (iii) extensive drainage for agricultural purposes.The present policy of eutrophication abatement and restoration of the Ankeveen and Kortenhoef lakes ecosystems is focused on eliminating wastewater discharges and Vecht water supply. It also allows for additional dredging measures. Because of the un-suitable major ion composition of the Vecht, the aim is to compensate for this water supply by (i) partial restoration of the original groundwater flow from the Gooi hills and (ii) periphere additional supply with fresh seepage water from the skirts of the Horstermeer polder. However, uncertainty exists about the amounts of water needed.Water balances and phosphorus budgets have been established to ascertain the water demands of the lakes and to gain a detailed insight into the nutrient fluxes through the lakes. A groundwater flow model is used to assess the beneficial effects of the proposed measures.The results obtained, question the current unilateral restoration objectives. Calculations reveal that, both in the present situation and after (total) reduction of groundwater extractions in the future, the available quantity of fresh seepage water from the skirts of the polder Horstermeer is not sufficient to replace the inlet from the river Vecht into the Kortenhoef lakes. Additional supply options are available but the ones favoured from an ecological viewpoint are either the most expensive or less favoured from a social point of view. Although the sediments of the lakes appear to be a major source of eutrophication, the possibility of dredging the lakes will be considered only after reviewing results of a pilot-dredging project in the Hollands Ankeveen lakes in 1991.     

The response of periphyton and submerged macrophytes to nitrogen and phosphorus loading in shallow warm lakes: a mesocosm experiment

1. Recent experimental and field studies on temperate shallow lakes indicate that nitrogen may play a greater role in their functioning than previously thought. Several studies document that abundance and richness of submerged macrophytes, both central in shallow lake ecology, may decrease with increasing nitrogen loading, especially at high phosphorus levels. However, the role of nitrogen in warm lakes with fluctuating water regimes remains to be described in detail. 2. The effect of increasing nitrate and phosphate concentrations on submerged macrophyte growth was examined in a 3‐month mesocosm experiment conducted in summer in a shallow freshwater lake on the north western coast of Turkey with a Mediterranean climate. Twenty four field mesocosms, open to the sediment and atmosphere, were stocked with Myriophyllum spicatum shoots and small cyprinid fish. Three nitrate loadings in combination with two phosphate loadings were applied in a fourfold replicated design. 3. Mean ± SD nutrient concentrations maintained throughout the experiment were 0.55 ± 0.17, 2.2 ± 0.97, 9.2 ± 5.45 mg L^?1 total nitrogen and 55 ± 19.2, 73 ± 22.9 μg L^?1 total phosphorus. Mean periphyton biomass increased with increasing nutrient concentrations and peaked at the highest nitrogen and phosphorus loadings, while the mean phytoplankton biomass remained relatively low in all treatments. 4. Percent volume inhabited (% PVI) by macrophytes throughout the experiment and total macrophyte biomass at the end of the experiment did not differ among treatments. In addition to stocked M. spicatum, Ceratophyllum demersum and Potamogeton crispus appeared in the majority of the mesocosms. The plants grew continuously up to 50% PVI throughout the experiment and remained resilient to shading provided by periphyton and phytoplankton. 5. The mean summer air temperature in 2007 was 2.2 °C higher than the average of the last 32 years, which resulted in a water level decrease of 0.3 m in the mesocosms over three months. This might have counteracted the shading of submerged macrophytes provided by phytoplankton and periphyton. The results of the experiment are consistent with observations of higher macrophyte resilience to nutrient loading in Mediterranean lakes compared with northern temperate lakes.     

Influence of internal loading on phosphorus concentration in shallow lakes before and after reduction of the external loading

An analysis of data from 49 shallow lakes showed, that the parameters of empirical models between phosphorus loading and concentration in the lake (e.g. Vollenweider type of relations) differ significantly for lakes without or with a reduced external loading. For lakes without a reduction of the external loading the summer phosphorus concentration is determined by the external phosphorus loading and the hydraulic loading. For these lakes the classical models suffice; deviations between calculations and measurements are partly due to errors made in the determination of the loading.In contrast, for lakes where the external loading was reduced, the measured internal loading explains most of the variation in the summer lake concentration. The external loading is of minor importance and the classical models cannot be applied. The internal loading measured before reduction of the external loading is not useful in predicting the concentration afterwards. Instead of the internal loading, the sediment composition can be used. The advantage of using sediment composition is that these variables are easier to determine and vary less in time. The most promising variable is the ratio between total P and total Fe in the sediment.Abbreviations: Q_s hydraulic loading (m y^-1) - hydraulic retention time (y) - L_ext external phosphorus loading (gP m^-2 y^-1) - L_int internal phosphorus loading (gP m^-2 y^-1) - P_lake phosphorus concentration in the lake (gP m^-3) - P_inlet phosphorus concentration in the inlet water (gP m^-3) - P_sed phosphorus content on the sediment (gP kg^-1 d.w.) - Fe_sed iron content of the sediment (gFe kg^-1 d.w.) - Y dependent variable multiple regression calculations - X1, X2 independent variables multiple regressions calculations - a, a1 constants - a2, b constants     

Biomanipulation by introduction of herbivorous zooplankton. A helpful shock for eutrophic lakes?

In 1988 and 1989Daphnia magna were introduced into the Gewerbepark Pond and into the University Pond, and also into a limnocorral (10 m diameter, 5 m deep) in Postfelden Reservoir. Limnological parameters were regularly monitored in all the three water bodies over three months or longer. The objective of the experiments was to induce a clear-water period in turbid eutrophic waters by direct manipulation of the zooplankton community. Introduction of four million individuals ofD. magna (10 ind. 1⁻¹) caused a long lasting clear-water period in the limnocorral. While Secchi-disc transparency in the reservoir decreased from 1.3 m to<1 m during summer, transparency increased to a stable level of about 4 m in the corral. Despite very low phytoplankton biomass,D. magna was still abundant. The experiment was terminated after three months because of oxygen depletion in the corral. In the Gewerbepark Pond 3.3 million individuals ofD. magna (0.6 ind. 1⁻¹) were introduced at the beginning of August. The daphnids soon increased 60-fold in their density causing a persistently high water transparency and low phytoplankton biomass up to November. Ammonia concentration rose steeply, whereas that of nitrate and oxygen decreased. Subsequently in summer, concentrations of chlorophyll as well as of inorganic phosphorus and nitrogen were extremely low. Instead of the phytoplankton,Elodea canadensis grew enormously and covered 12% of the pond surface. The biomanipulation experiment in University Pond failed, probably due to predation by carp.     

Control of phosphorus loading and flushing as restoration methods for Lake Veluwe,The Netherlands

As a result of high nutrient loading Lake Veluwe suffered from an almost permanent bloom of the blue-green algaOscillatoria agardhii Gomont. In 1979, the phosphorus loading of the lake was reduced from approx. 3 to 1 g P.m^–2.a^–1. Moreover, since then the lake has been flushed during winter periods with water low in phosphorus. This measure aimed primarily at interrupting the continuous algal bloom. The results of these measures show a sharp decline of total-phosphorus values from 0.40–0.60 mg P.l^–1 (before 1980) to 0.10–0.20 mg P.l^–1 (after 1980). Summer values for chlorophylla dropped from 200–400 mg.m^–3 to 50–150 mg.m^–3.The increase in transparency of the lake water was relatively small, from summer values of 15–25 cm before the implementation of the measures to 25–45 cm afterwards. The disappointing transparency values may be explained by the decreasing chlorophylla and phosphorus content of the algae per unit biovolume. Blue-green algae are gradually loosing ground. In the summer of 1985 green algae and diatoms dominated the phytoplankton for the first time since almost 20 years. To achieve the ultimate water quality objectives (transparency values of more than 100 cm in summer), the phosphorus loading has to be reduced further.     

Restoration and recovery of shallow eutrophic lake ecosystems in The Netherlands: epilogue

During the symposium Restoration and recovery of shallow lake ecosystems in The Netherlands studies on restoration of eutrophic lakes were addressed and discussed. Many Dutch shallow lakes have received high external loadings of phosphorus through supply water that is influenced by the River Rhine and loadings in The Netherlands. Two important Action Plans (the Rhine Action Plan, the North Sea Action Plan) are now in operation to reduce nutrient emissions. The targets set are not likely to be fully reached, so that supplementary reduction of phosphorus supplied to inland fresh waters will be required. In several shallow lakes such a reduction has been achieved recently, but without leading to discernible recovery. The main causes of delay are phosphorus storage and its subsequent release from sediments and foodweb; however, the remaining extraneous phosphorus supply is often still too high. Supplementary actions are, therefore, called for. A further reduction of phosphorus inputs is suggested, besides supplementary measures proposed, viz. dredging, flushing, biomanipulation, chemomanipulation. Restoration to the past situation via upwelling groundwater appeares to be feasible in some cases. There is a common consensus that each lake behaves differently depending to its morphology, hydrology and history of eutrophication. Therefore each lake has to be studied before restoration measures can be applied. Besides, the ecosystem should not only be studied as a separate entity, but as a part of systems of a higher integration level.     

Seasonal response of nutrients to reduced phosphorus loading in 12 Danish lakes

1. Concentrations of phosphorus, nitrogen and silica and alkalinity were monitored in eight shallow and four deep Danish lakes for 13 years following a phosphorus loading reduction. The aim was to elucidate the seasonal changes in nutrient concentrations during recovery. Samples were taken biweekly during summer and monthly during winter. 2. Overall, the most substantive changes in lake water concentrations were seen in the early phase of recovery. However, phosphorus continued to decline during summer as long as 10 years after the loading reduction, indicating a significant, albeit slow, decline in internal loading. 3. Shallow and deep lakes responded differently to reduced loading. In shallow lakes the internal phosphorus release declined significantly in spring, early summer and autumn, and only non‐significantly so in July and August. In contrast, in deep lakes the largest reduction occurred from May to August. This difference may reflect the much stronger benthic pelagic‐coupling and the lack of stratification in shallow lakes. 4. Nitrogen only showed minor changes during the recovery period, while alkalinity increased in late summer, probably conditioned by the reduced primary production, as also indicated by the lower pH. Silica tended to decline in winter and spring during the study period, probably reflecting a reduced release of silica from the sediment because of enhanced uptake by benthic diatoms following the improved water transparency. 5. These results clearly indicate that internal loading of phosphorus can delay lake recovery for many years after phosphorus loading reduction, and that lake morphometry (i.e. deep versus shallow basins) influences the patterns of change in nutrient concentrations on both a seasonal and interannual basis.     

Shifting states,shifting services: Linking regime shifts to changes in ecosystem services of shallow lakes

1. Shallow lakes can shift between stable states as a result of anthropogenic or natural drivers. Four common stable states differ in dominant groups of primary producers: submerged, floating, or emergent macrophytes or phytoplankton. Shifts in primary producer dominance affect key supporting, provisioning, regulating, and cultural ecosystem services supplied by lakes. However, links between states and services are often neglected or unknown in lake management, resulting in conflicts and additional costs.
2. Here, we identify major shallow lake ecosystem services and their links to Sustainable Development Goals (SDGs), compare service provisioning among the four ecosystem states and discuss potential trade‐offs.
3. We identified 39 ecosystem services potentially provided by shallow lakes. Submerged macrophytes facilitate most of the supporting (86%) and cultural (63%) services, emergent macrophytes facilitate most regulating services (60%), and both emergent and floating macrophytes facilitate most provisioning services (63%). Phytoplankton dominance supports fewer ecosystem services, and contributes most to provisioning services (42%).
4. The shallow lake ecosystem services we identified could be linked to 10 different SDGs, notably zero hunger (SDG 2), clean water and sanitation (SDG 6), sustainable cities and communities (SDG 11), and climate action (SDG13).
5. We highlighted several trade‐offs (1) among ecosystem services, (2) within ecosystem services, and (3) between ecosystem services across ecosystems. These trade‐offs can have significant ecological and economic consequences that may be prevented by early identification in water quality management.
6. In conclusion, common stable states in shallow lakes provide a different and diverse set of ecosystem services with numerous links to the majority of SDGs. Conserving and restoring ecosystem states should account for potential trade‐offs between ecosystem services and preserving the natural value of shallow lakes.