Effect of summer weather on internal loading and chlorophyll a in a shallow lake: a modeling approach

The effect of weather on the eutrophication of a shallow lake was estimated by a hydrodynamic lake model coupled with a simple water quality module. The model was applied to Lake Villikkalanjärvi in southern Finland. This shallow, agriculturally loaded lake may stratify during warm and calm periods in summer and as a result oxygen is often consumed from the hypolimnion, causing high internal loading of phosphorus. Vertical mixing and temperature distribution in the lake were simulated by a one-dimensional, horizontally integrated hydrodynamic model. State variables included in the water quality model were dissolved reactive phosphorus, chlorophyll a and dissolved oxygen. The model was first calibrated against observations from 1989 and 1990. Thereafter, simulations were carried out using weather data from the years 1961 to 1988. The results indicated that warm summer periods may cause high chlorophyll a concentrations due to high internal loading. In four years with exceptionally warm summers the model predicted maximum chlorophyll a concentrations almost twice as high as in years without remarkable internal loading. The model simulates accurately temperature and mixing but the reliability of water quality predictions could be improved by adding more factors regulating algal biomass and sediment phosphorus release.     

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.     

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.     

Predicting concentration of total phosphorus and chlorophyll a in a lake with short hydraulic residence time

The relationship between total phosphorus and chlorophyll a concentration was determined for Skinner Lake, Indiana over an annual cycle in 1978–79. Total nitrogen:total phosphorus ratios in the epilimnion ranged from 19 to 220 suggesting a phosphorus-dependent algal yield in the epilimnion. Approximately 90% of annual TP loading reached the lake via streamflow, and 93% of this entered during snowmelt and spring-overturn periods. At that time incoming water flushed the lake 2.4 times. Atmospheric loading accounted for 1.4% of annual TP load. Internal hypolimnetic TP loading occurred during summer stratification. Mean [chl a] for the ice-free period was 15.15 mg m^–3, within the range expected for eutrophic lakes.The 1978–79 data were used in conjuction with the Vollenweider & Kerekes (1980) model to produce a model specific for the Skinner Lake system. The model predicted mean epilimnetic total phosphorus and chlorophyll a concentrations from mean total phosphorus concentration in inlet streams and from lake water residence time during the period of spring overturn and summer stratification. The Skinner-specific model was tested in 1982 and it closely predicted observed mean epilimnetic [TP] and [chl a] during the ice-free period. This study shows that variability in lake models which average data over an annual period can be reduced by considering lake-specific seasonal variation in hydrology and external TP loading.     

A model study on the role of wetland zones in lake eutrophication and restoration

Shallow lakes respond in different ways to changes in nutrient loading (nitrogen, phosphorus). These lakes may be in two different states: turbid, dominated by phytoplankton, and clear, dominated by submerged macrophytes. Both states are self-stabilizing; a shift from turbid to clear occurs at much lower nutrient loading than a shift in the opposite direction. These critical loading levels vary among lakes and are dependent on morphological, biological, and lake management factors. This paper focuses on the role of wetland zones. Several processes are important: transport and settling of suspended solids, denitrification, nutrient uptake by marsh vegetation (increasing nutrient retention), and improvement of habitat conditions for predatory fish. A conceptual model of a lake with surrounding reed marsh was made, including these relations. The lake-part of this model consists of an existing lake model named PCLake. The relative area of lake and marsh can be varied. Model calculations revealed that nutrient concentrations are lowered by the presence of a marsh area, and that the critical loading level for a shift to clear water is increased. This happens only if the mixing rate of the lake and marsh water is adequate. In general, the relative marsh area should be quite large in order to have a substantial effect. Export of nutrients can be enhanced by harvesting of reed vegetation. Optimal predatory fish stock contributes to water quality improvement, but only if combined with favourable loading and physical conditions. Within limits, the presence of a wetland zone around lakes may thus increase the ability of lakes to cope with nutrients and enhance restoration. Validation of the conclusions in real lakes is recommended, a task hampered by the fact that, in the Netherlands, many wetland zones have disappeared in the past.     

Internal and external loading as regulators of nutrient concentrations in the agriculturally loaded Lake Pyhäjärvi (southwest Finland)

This study assesses the effects of external and internalloading on the nutrient concentrations in an agriculturallyloaded shallow lake. Using 13 years of observations of thelake's input and outflow, we calculated the long-term balancesof Tot-P and Tot-N. A more detailed balance, which includeddissolved nutrients and suspended solids, was estimated for anice-free period of one year. The contribution of the externalload was assessed using a mass-balance model. The internalload was estimated from the nutrient balances and on the basisof sedimentation measurements and bioassays. The drainagebasin of the lake provided most of the external nutrientinput; the remaining load was derived from atmosphericdeposition to the lake. The proportions of river-transported Pand N in dissolved form were 25% and 77%, respectively. Thelake retained >80% of the external load. Particulatenutrients settled to the bottom and were probably resuspendedseveral times before permanent sedimentation. Dissolvednutrients were bound by primary producers and a highproportion of dissolved P was removed with the fish catch.Dissolved N was also lost via denitrification. Themass-balance model showed that external loading only partlyregulated the mean annual nutrient concentrations in the lake.The regulation was probably due to internal loading, which washigh despite the efficient net retention of nutrients. Duringthe ice-free period, the temporal variations in nutrientconcentrations were controlled almost solely by internalprocesses, such as resuspension of inorganic and organicbottom matter. Although the internal load of bioavailable Pmay, under favourable conditions, exceed the external load,the mechanism by which bioavailable P is translocated from thebottom sediments to the water could not be fully identified.Abbreviations used in this paper follow the editor'srules.     

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.     

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.     

Response of a eutrophic, shallow subtropical lake to reduced nutrient loading

1. Lake Apopka (FL, U.S.A.) was subjected to decades of high nutrient loading from farms developed in the 1940s on converted riparian wetlands. Consequences included perennially high densities of cyanobacteria, low water transparency, elimination of submerged vegetation, modified fish community, and deposition of nutrient‐rich, flocculent sediments. 2. Initial steps were taken to reduce phosphorus (P) loading. Through strengthened regulation and purchase of farms for restoration, external P loading was reduced on average from 0.56 to 0.25 g P m^?2 year^?1 (55%) starting in 1993. The P loading target for the lake is 0.13 g P m^?2 year^?1. 3. For the first 6 years of P loading reduction the annual sedimentation coefficient (σ) averaged 13% less than the prior long‐term value (0.97 versus 1.11 year^?1). The sedimentation coefficient, σ, was lower in the last 3 years of the study, but this period included extreme low‐water conditions and may not be representative. Annual σ was negative (net P flux to the water column) only 1 year. 4. Wind velocity explained 43% of the variation in σ during the period before reductions in total phosphorus (TP) concentration of lake water, but this proportion dropped to 6% after TP reductions. 5. Annual mean TP concentrations differed considerably from values predicted from external loading and hydraulic retention time using the Vollenweider–Organization for Economic Co‐operation and Development relationship. Reductions in lake water TP concentration fit model predictions better when multiyear (3‐year) mean values were used. 6. Evidence available to date indicates that this shallow, eutrophic lake responded to the decrease in external P loading. Neither recycling of sediment P nor wind‐driven resuspension of sediments prevented improvements in water quality. Reductions in TP concentration were evident about two TP‐resident times (2 × 0.9 year) after programmes began to reduce P loading. Improvements in concentrations of chlorophyll a and total suspended solids as well as in Secchi transparency lagged changes in lake‐water TP concentration but reached similar magnitudes during the study.     

The effects of reduced phosphorus and nitrogen loading on phytoplankton in Mondsee,Austria

Following restoration measures (ring canalization, treatment plant with phosphorus precipitation), phosphorus loading declined step-wise from 26.21 t year^–1 to 9.18 t year^–1 during the period 1979–1984 while P-retention increased from 48% to 78%. Phosphorus loading was poorely correlated with precipitation. Inorganic nitrogen load, largely NO₃– N, did not decline but was significantly correlated with precipitation (r = 0.95) throughout the investigation period (1978–1989).Total phosphorus loading reached acceptable levels in 1984 when compared to critical loading calculated according to Vollenweider (1976). Phosphorus input to the lake has remained at these levels in recent years.Average annual chlorophyll-a concentrations and biovolume of phytoplankton in the top 20 m layer of the lake decreased, in correspondence with the respective phosphorus concentrations, from eutrophic to mesotrophic levels. The decline was accompanied by a drastic reduction of blue-green algal populations, and especially of Oscillatoria rubescens D.C..     

Lake restoration by reducing external phosphorus loading: the influence of sediment phosphorus release

SUMMARY. 1. This review considers the factors which determine the recovery of eutrophic lakes following a reduction in the external phosphorus loading.
2 The mean phosphorus content of a lake should decrease roughly in proportion to the reduction in phosphorus input. Where the lake phosphorus concentration does not decrease as predicted, then the release of phosphorus from the sediment is implicated.
3. The current understanding of the processes by which sediment phosphorus is mobilized and transported into the photic zone is described. The extent to which phosphorus release can maintain lake phosphorus concentrations following the reduction in external loading is influenced by: lake morphometry, flushing rate, sediment type, trophic state and history of enrichment.
4. A reduction in the phytoplankton biomass of a lake is dependent upon the size of the decrease in lake phosphorus concentration and the degree to which phosphorus limits primary production. The importance of phosphorus in limiting phytoplankton production tends to decrease with increasing lake trophic status.
5. Improvements in the condition of highly eutrophic lakes require very large reductions in external phosphorus loading, whereas in mildly enriched lakes moderate changes in the supply of phosphorus have noticeable effects on phytoplankton biomass.     

Stable sulfur isotopic distributions and sulfate reduction in lake sediments of the Adirondack Mountains,New York

Cores from five lakes of the Adirondack Mountains, New York, were analyzed for sedimentary sulfur concentrations and stable sulfur isotopic compositions. Isotopic values of total sedimentary sulfur were as much as 6 to 8 lower than isotopic values of sulfur sources (soils, tree leaves and lake water sulfate) which showed little isotopic variation in the Adirondacks. The low isotopic values of recent sediments indicate increased sulfate reduction, probably in response to increased sulfate loading and acid deposition. Detailed historical reconstruction of sulfate loading histories from sedimentary sulfur profiles is premature, however, since model calculations indicate that sulfur can be added to deeper older sediments by ongoing, contemporary sulfate reduction.     

Arsenic transport between water and sediments

Arsenic discharged into the Moira River has accumulated in the sediments of Moira Lake during the past century. The chronology of arsenic concentrations in the sediments, established using Pb-210 dating, has a subsurface concentration maximum (> 1000 g g^–1) that reflects higher inputs to the lake 15 to 45 years ago. The distribution coefficient (K_d) of arsenic in the surficial sediments was low (4000–6000 L kg^–1) and decreased below the sediment water interface. Higher concentrations of exchangeable As also were extracted deeper in the sediments. As a result, arsenic is mobile in the sediment column and the flux of arsenic via diffusion and particle resuspension from the sediments into the water is greater than current external loading from the Moira River. Less than 20% of the external input of arsenic is buried in the lake sediments. Using these flux measurements and a one dimensional model of arsenic transport in the sediment column, we constructed the history of arsenic exchange between water and sediments throughout the past century. The simulations predict that arsenic input into the water from the sediments has been > 20 % of external loading for the past 25 years and will continue to be important in the future as diffusion and resuspension regenerate arsenic from the mixed layer of the sediments into the overlying water.     

Influence of Sediment Inflow on Phytoplankton Primary Productivity in Lake Tahoe (California-Nevada)

Light, nutrient concentrations and phytoplankton photosynthesis were studied in a Lake Tahoe sediment plume during maximum spring runoff. They were compared with conditions in clear lake waters not influenced by inlets. In the plume, nutrient concentrations increased in proportion to sediment density whereas light transmission of water was reduced with little effect on the spectral composition except for red light. Light inhibition of photosynthesis at the lake surface was less pronounced in the plume than in clear water and light limitation occurred more rapidly in deeper layers. Evidence from both lake experiments and laboratory bioassays suggests that iron had the greatest stimulatory effect on both photosynthetic activity and biomass growth at maximum sediment densities near the stream inlet. Because of less surface inhibition, photosynthetic light energy utilization efficiency was usually higher in the sediment plume which occurred in relatively shallow areas near the shore. In order to estimate overall effects of enhanced turbidity associated with nutrient loading on Lake Tahoe's primary productivity, profiles taken in shallow areas near the lakeshore were extrapolated to the maximum depth of photosynthesis. Light limitation would cause decreasing productivity, but nutrient stimulation would make this effect less pronounced. The overall effect would depend on the extent of sediment loading relative to nutrient loading.     

Water quality effects following establishment of the invasive <Emphasis Type="Italic">Dreissena polymorpha</Emphasis> (Pallas) in a shallow eutrophic lake: implications for pollution mitigation measures

This study investigates whether ecosystem alteration occurred in a shallow, lowland lake following establishment of the alien zebra mussel, Dreissena polymorpha (Pallas). Measurements of total phosphorus (TP) loads, lake TP concentrations, phytoplankton (chlorophyll a) and transparency levels for the period 1990–2008 were examined to determine the water quality effects of D. polymorpha. The period of time also included the implementation of catchment measures aimed at reducing phosphorus (P) loading to the lake. A range of loading-response models was tested to explore changes in the net sedimentation rate for P. Results show that while high TP loads from the catchment were reduced, TP concentrations in the lake remained high after D. polymorpha invasion. Decoupling of the previous chlorophyll a–TP relationship also occurred. Results from a TP loading-response model that closely simulated observed concentrations in the lake prior to establishment of D. polymorpha indicated that measured TP post establishment was statistically higher than predicted for the same conditions but without the presence of D. polymorpha. Data presented in the paper highlight the need to consider the potential impacts of invasive species in evaluations of the effectiveness of measures aimed at mitigating aquatic pollution.     

Monitoring lake recovery from point-source eutrophication: the use of diatom-inferred epilimnetic total phosphorus and sediment chemistry

1. Diatom and geochemical responses to reduced nutrient loading were followed in a small, monomictic eutrophic lake in Northern Ireland by use of short sediment cores taken c. 15 years after redirection of creamery waste away from the lake. 2. Epilimnetic total phosphorus (TP) concentrations (fig TP 1-^?1) were estimated for the period 1850–1990 using weighted averaging regression and calibration. Background TP levels, inferred using the diatom model, were c. 35μg TP 1-^?1 and increased to >140μg TP 1-^?1 in the late 1960s to early 1970s. Total P concentrations dropped to 80 μg TP 1-^?1 within 5 years of waste diversion (c. 1978–79), but varied between 1980 and 1990 (range 70–140 μg TP 1-^?1), perhaps due to internal loading, occasional continued disposal from the creamery and natural variations in stream P load. 3. Diatom-inferred TP concentrations were compared with monitored data where available, and the diatom model tended to overestimate TP concentrations by about 25 fig TP 1-^?1. Possible reasons for this are discussed (errors in the diatom model, stratigraphic variability, variability in the monitoring data). 4. Post-1980 geochemistry profiles (concentrations and accumulation rates) indicated some changes when compared with sediments deposited before 1980, perhaps reflecting the redirection of the creamery waste and reduced productivity of the lake (e.g. reduced calcium deposition). Phosphorus concentrations in the sediments changed very little over the last 150 years and, while sedimentary TP fluxes (g cm^?2 yr^?1) increase steadily up-core, they do not record the effluent redirection in the mid-1970s. There is, however, some indication of a slight lowering of P retention in the most recent sediments (1985–90). 5. The general implications of such an approach to monitoring (i.e. the use of short cores) are discussed and the value of diatom-inferred TP assessed. Diatom models offer the possibility of determining background TP concentrations and indicate that, despite the redirection of the creamery waste over 15 years ago, the pre-creamery epilimnetic TP concentrations have not yet been reached.     

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.     

A review and reassessment of lake phosphorus retention and the nutrient loading concept

1. We conducted a statistical reassessment of data previously reported in the lake total phosphorus (TP) input/output literature (n = 305) to determine which lake characteristics are most strongly associated with lake phosphorus concentration and retention. We tested five different hypotheses for predicting lake TP concentrations and phosphorus retention. 2. The Vollenweider phosphorus mass loading model can be expressed as: TP_out = TP_in/(1 + στ_w), where TP_in is the flow‐weighted input TP concentration, τ_w is the lake hydraulic retention time and σ is a first‐order rate constant for phosphorus loss. 3. The inflow‐weighted TP input concentration is a moderately strong predictor (r² = 0.71) of lake phosphorus concentrations when using log–log transformed data. Lake TP retention is negatively correlated with lake hydraulic retention time (r² = 0.35). 4. Of the approaches tested, the best fit to observed data was obtained by estimating σ as an inverse function of the lake's hydraulic retention time. Although this mass balance approach explained 84% of the variability in log–log transformed data, the prediction error for individual lakes was quite high. 5. Estimating σ as the ratio of a putative particle settling velocity to the mean lake depth yielded poorer predictions of lake TP (r² = 0.77) than the approach described above, and in fact did not improve model performance compared with simply assuming that σ is a constant for all lakes. 6. Our results also demonstrate that changing the flow‐weighted input concentration should always have a directly proportionate impact on lake phosphorus concentrations, provided the type of phosphorus loaded (e.g. dissolved or particulate) does not vary.     

Rehabilitation and management of a moderately deep-stratifying reservoir by the use of nutrient reduction and food-web management

As expected from its short renewal time, the hypertrophic Lingese Reservoir responded rapidly but incompletely to external nutrient reduction in 1993. Although there was a sudden decline in lake phosphorus and chlorophyll concentrations, the ‘turbid regime’ revealed resistance in this weakly stratified reservoir of intermediate depth. Draining the reservoir in autumn 1995 provided an unprecedented opportunity for sediment treatment—the phosphorus-rich surface layer was inverted and covered with deeper nutrient-poor layers—and removal of the dense cyprinid fish stock which had not responded to sewage diversion, at least in the short term. Commencing with refilling in 1999, a new fish stock was built up from 2000 by only stocking predators (fingerlings of pike, pike-perch and larger specimens of rainbow-trout in the first years) in combination with catch restrictions. Concomitantly, with the appearance of daphnids in 1999, a ‘clear water regime’ was established and lake water phosphorus concentrations decreased at unchanged external loading. Reduced zooplanktivory as well as reduced fish-mediated phosphorus release from the sediments were driving mechanisms behind successful reduction of internal loading and achievement of a ‘clear water regime’. Hence, phosphorus concentrations were revealed to be a response variable not only to input management but also to food-web management. As the development of cyprinid dominance was prevented in the long-term, there is ample evidence that the fish community responded to the applied management measures as expected featuring successful food-web management. Overall, the biological structure was revealed to be of major importance for lake phosphorus availability and turbidity as in shallow lakes, without, however, the establishment of macrophyte dominance.     

Drought-induced changes in nutrient concentrations and retention in two shallow Mediterranean lakes subjected to different degrees of management

While extensive knowledge exists on the relationship between nutrient loading and nutrient concentrations in lakes in the cold temperate region, few studies have been conducted in warm lakes, not least in warm arid lakes. This is unfortunate as a larger proportion of the world’s lakes will be situated in arid climates in the future due to climate change and a larger proportion will suffer from a higher frequency of intensive drought. We conducted a comprehensive 11–13 year mass balance study in two interconnected shallow Mediterranean lakes in Turkey, covering a period with substantial changes in climate conditions. The upstream lake was only affected by natural changes in nutrient loading, while the downstream lake was additionally influenced by sewage diversion and restoration by fish removal. Contrasting to experience from north temperate lakes we found an increase in in-lake concentrations of total phosphorus and inorganic nitrogen (ammonia as well as nitrate) in dry years despite lower external nutrient loading, and submerged macrophytes did not increase the nitrogen retention capacity of the lakes. In contrast, fish removal modulated the nitrogen concentration as in north temperate lakes, but the effect was not long-lasting. Our results suggest that climate warming reduces the nutrient retention capacity of shallow lakes in the Mediterranean and exacerbates eutrophication. Lower thresholds of nutrient loading for shifting turbid shallow lakes to a clear water state are therefore to be expected in arid zones in a future warmer climate, with important management implications.