Applicability of phosphorus budget models to southern African man-made lakes

An investigation of the phosphorus loading characteristics of 31 southern African man-made was lakes made. The lakes were characterized by low water retention times, with most of the lakes having retention times of less than one year. Catchment phosphorus export rates showed wide variation (1–162 mg P m^-2 y^-1) with those lakes experiencing excessive municipal wastewater inputs having export rates in excess of 53 mg m^-2 y^-1. The phosphorus data were tested against the Vollenweider (1976) and Dillon & Rigler (1974) phosphorus budget models which predict in-lake steady state concentrations of phosphorus. It was found that both models displayed good potential for the prediction of steady state concentrations of phosphorus, with better results being obtained from the Dillon & Rigler (1974) model. However, because phosphorus concentrations within these lakes may not necessarily be related to trophic status the use of these models as a predictive tool for eutrophication control still requires further development.     

水库磷收支及其调控措施研究进展

磷是主要的富营养化限制性因子,其收支途径和调控措施的研究是水库生态安全的重要保障.为推动水库磷的系统研究,进一步完善磷调控体系,本文归纳了水库磷的收支途径,分析了不同因素对磷收支的影响,并对常规调控措施进行了梳理.水库磷输入的主要来源包括上游河流、地表径流、生活生产污染、养殖投入、大气沉降和底泥释放.其中上游河流的输入是水库磷最大的来源.磷主要的输出途径为泄洪或放水,并受不同排水方式的影响.另外生物收获也会输出部分磷.水库磷收支平衡受水库功能、水文条件和水体理化性质等因素的影响,不同季节和地域的收支情况差异很大.在磷的调控治理上,人工湿地、前置库、底泥疏浚和生物操纵法是目前采取的主要措施.为有效去除磷,需根据水库不同生态特性和水质管理目标,选择或结合多种不同的调控方式.因此,未来的研究应突出水库生态特性,进一步对水库磷收支及其影响因素进行系统分析,并开展水库磷收支与其他营养物质之间影响关系的研究,在此基础上构建科学完善的管理体系,是有效削减水库磷的根本保障.     

Relations between trophic state indicators and plant biomass in Florida lakes

We collected quantitative data on macrophyte abundance and water quality in 319 mostly shallow, polymictic, Florida lakes to look for relationships between trophic state indicators and the biomasses of plankton algae, periphyton, and macrophytes. The lakes ranged from oligotrophic to hypereutrophic with total algal chlorophylls ranging from 1 to 241 mg m^–3. There were strong positive correlations between planktonic chlorophylls and total phosphorus and total nitrogen, but there were weak inverse relationships between the densities of periphyton and the trophic state indicators total phosphorus, total nitrogen and algal chlorophyll and a positive relationship with Secchi depth. There was no predictable relationship between the abundance of emergent, floating-leaved, and submersed aquatic vegetation and the trophic state indicators. It was only at the highest levels of nutrient concentrations that submersed macrophytes were predictably absent and the lakes were algal dominated. Below these levels, macrophyte abundance could be high or low. The phosphorus–chlorophyll and phosphorus–Secchi depth relationships were not influenced by the amounts of aquatic vegetation present indicating that the role of macrophytes in clearing lakes may be primarily to reduce nutrient concentrations for a given level of loading. Rather than nutrient concentrations controlling macrophyte abundance, it seems that macrophytes acted to modify nutrient concentrations.     

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.     

江西省余江县水体营养状况分析

用营养状态指数法(TSI)对江西省红壤区余江县东塘、五湖、仙塘三个水库的水质状况进行综合评价,结果表明三个水体的水质状况均呈富营养态。在此基础上对三个水库的10项主要水化学指标进行主成分分析,表明亚硝态氮和氨氮对第一主成分的贡献最大,COD为第二主成分的主要贡献者;而且三个水库的水质受水中有机物含量的影响最大,这一结果与当地的农业结构密切相关。     

Eutrophication of reservoirs in South Korea

Natural lakes in South Korea are limited in number and generally quite small. As a result, reservoirs and regulated rivers are the major sources of freshwater for society. About 18 000 reservoirs have been constructed in South Korea, and they are particularly important for domestic water supply. Thirteen of the major reservoirs were surveyed in this general assessment of the trophic state of South Korean reservoirs. Ten reservoirs were from the upper or middle reaches of rivers (including eight of the ten largest reservoirs in Korea), and three were estuarine reservoirs. Reservoirs in the mountainous district of South Korea were usually mesotrophic, whereas the estuarine reservoirs were highly eutrophic. Because total nitrogen to total phosphorus ratios were always between 18 and 163, phosphorus was probably more limiting than nitrogen for algal growth. However, hydraulic residence time and light penetration may be limiting in the nutrient-enriched downstream reservoirs. In winter, algal density was lowest in deep reservoirs, perhaps due to deep mixing. During the same season, algal density was high in shallow reservoirs, perhaps due to a favorable hydraulic residence time. Factors contributing to the observed eutrophication patterns, including nutrient runoff from agriculture, animal farms, fish aquaculture, and urban areas, are discussed. According to the national budget of phosphorus, fertilizer and livestock manure are major source of phosphorus, contributing 133 400 and 73 700 tons of phosphorus per year, respectively, while human excretion discharges 30 000 tons P year⁻¹. Reduction of the application of fertilizer, proper treatment of manure, and conservation of topsoil may be the most practical and effective measures to prevent further lake eutrophication. Received: February 15, 2001 / Accepted: July 2, 2001     

Effects of invasive zebra mussels on phytoplankton,turbidity, and dissolved nutrients in reservoirs

Few experiments have quantified the effects of invasive zebra mussels (Dreissena polymorpha) on man-made reservoirs relative to other aquatic habitats. Reservoirs, however, are the dominate water body type in many of the states that are at the current front of the zebra mussel invasion into the western United States. The objective of this research, therefore, was to determine how zebra mussels affected phytoplankton, turbidity, and dissolved nutrients in water that was collected from three Kansas reservoirs that varied in trophic state (mesotrophic to hypereutrophic), but all experienced frequent cyanobacterial blooms. Laboratory mesocosm experiments were conducted to document the effects of zebra mussels on cyanobacteria and general water quality characteristics in the reservoir water. Zebra mussels significantly reduced algal biomass, and the total biovolume of cyanobacteria (communities were dominated by Anabaena) in each reservoir experiment. The effects of zebra mussels on other major algal groups (diatoms, flagellates, and green algae) and algal diversity were less consistent and varied between the three reservoir experiments. Similarly, the effects of zebra mussels on nutrient concentrations varied between experiments. Zebra mussels increased dissolved phosphorus concentrations in two of the reservoir experiments, but there was no effect of zebra mussels on dissolved phosphorus in the mesotrophic reservoir experiment. Combined, our results strongly suggest that zebra mussels have the potential to significantly impact reservoirs as they continue to expand throughout the western United States. Moreover, the magnitude of these effects may be context dependent and vary depending on the trophic state and/or resident phytoplankton communities of individual reservoirs as has similarly been reported for natural lakes.     

Testing predictions of the lake food web theory on pelagic communities of Australian reservoirs

Several predictions of the theory developed for pelagic food webs of the Northern Hemisphere were tested on water bodies of Eastern Australia. Eleven reservoirs, representing trophic and latitudinal gradients were sampled for nutrients, phytoplankton, zooplankton and pelagic fish. Two models of regression analysis, which analysed possible interactions between trophic levels were based on different sets of data. In one, each reservoir was represented by only one pair of observations – annual mean or single observation (“regional model”). In the other, seasonal means of four frequently sampled reservoirs similar in productivity were used (“temporal model”). Significant variation in total phytoplankton biovolume (TPB) was predicted by total phosphorus concentration (TP), total nitrogen concentration (TN), mean crustacean length and acoustic biomass of planktivorous fish in both models. This suggested that nutrient limitation, zooplankton grazing and positive effects of fish were probably important in controlling the biomass of primary producers at both regional and temporal scales. In the regional model, the biomass of fish was also negatively correlated with Daphnia biomass and mean crustacean length, suggesting that the trophic cascade hypothesis may be applicable to Eastern Australia for the considered range of reservoir productivities. The biovolume of cyanobacteria was not correlated to any variables tested in the regional model. In contrast, nutrient and food web variables had significant effects on cyanobacterial biovolume in the temporal model. This suggested that factors governing seasonal succession were probably more important for cyanobacteria than variation in reservoir productivity or location. Contrary to previous views, no negative relationship between total biomass of zooplankton and TPB was found in both models, suggesting that the community structure of zooplankton rather than its total biomass mediates top‐down effects. Many predictions of the food web theory remained robust in spite of substantial differences in animal taxonomy and physical environment of Australian ecosystems.     

Distinct age and landscape influence on two reservoirs under the same climate

With the purpose of finding out whether different landscape occupation could affect water quality in two reservoirs of distinct age and subjected to the same climatic influence, several factors were investigated in a study lasting from January 2000 till December 2001. Total phosphorus, orthophosphate, chlorophyll a concentrations and water colour were determined monthly in winter and biweekly in summer, in two reservoirs located in the Portuguese part of River Douro catchment. Complementarily, variables such as nitrate, ammonium ion and ammonia gas, as well as water temperature, dissolved oxygen, conductivity, and transparency were measured. Trophic state of both reservoirs was assessed by computation of Carlson's Trophic State Index. The potential allochthonous sources of phosphorus and nitrogen to both reservoirs were identified and estimated. Differences between reservoirs were found for conductivity, water temperature, transparency and water colour. According to Carlson's Trophic State Index both reservoirs were classified as meso-eutrophic from winter to the beginning of summer, and as eutrophic from this period onwards. Intrinsic factors such as age of reservoir, organic matter inputs from decomposition of flooded terrestrial vegetation and exposure of littoral sediments to cycles of drying and wetting, as well as extrinsic factors such as grazing and frequent land fires, can explain the trophic state of S. Serrada Reservoir. Allochthonous sources of nutrients originated from agriculture and grazing in the catchment area, and recreational activities in the reservoir probably are the factors with greatest influence on Azibo Reservoir trophic state. Based upon the obtained data, management measures are suggested to prevent further eutrophication and water quality degradation in both reservoirs.     

Assimilation and turnover time of phosphorus by size fractions of microplankton in lakes of different types

In a direction from oligotrophic lakes to more productive ones concentration of biologically available phosphorus (BAP) increased from 1 up to 35 ??g/L, turnover time of BAP ?? from 19 up to 1300 min. Relative participation of phytoplankton and bacterioplankton fractions of microplankton in phosphorus assimilation depend on productivity of lakes. In oligotrophic lakes the phosphorus assimilation by bacteria reached 80?C90% of total assimilation. As the trophic status of lakes increased the participation of bacteria in phosphorus assimilation decreased to 10?C20%, accordingly assimilation activity of phytoplankton increased. It testifies to domination of phosphorus flow through microbial organisms (0.23?C1.5 ??m) in oligotrophic lakes.     

The phosphorus status of sediments in a hypertrophic impoundment (Hartbeespoort Dam): implications for eutrophication management

The phosphorus status and distribution of sediments in a hypertrophic water supply reservoir (Hartbeespoort Dam) were investigated, with a view to assessing the role of sediments in counteracting the effects of reduced external phosphorus loading as a restoration measure. In comparison with similar water bodies in South Africa, the sediments in Hartbeespoort Dam contained high levels of both total and potentially mobile phosphorus. The potentially mobile fraction constituted about 60% of the total phosphorus content of the sediments, compared with about 11% in other reservoirs. The excessive eutrophication of Hartbeespoort Dam is clearly reflected in the phosphorus status of the sediments. Sediment distribution in the impoundment was found to be extremely heterogeneous, due to the combined influences of morphometry, hydrology and an imbalance in the nutrient loads entering via rivers at remote points in the water body. It is concluded that sufficient mobile phosphorus has accumulated in the sediments to prolong the response time of the impoundment to phosphorus load reductions. Since phosphorus release from sediments is dependent on dynamic processes not addressed in this study, the extent of the delays in trophic response to load reduction cannot be estimated.     

Nutrient additions by waterfowl to lakes and reservoirs: predicting their effects on productivity and water quality

Lakes and reservoirs provide water for human needs and habitat for aquatic birds. Managers of such waters may ask whether nutrients added by waterfowl degrade water quality. For lakes and reservoirs where primary productivity is limited by phosphorus (P), we developed a procedure that integrates annual P loads from waterfowl and other external sources, applies a nutrient load-response model, and determines whether waterfowl that used the lake or reservoir degraded water quality. Annual P loading by waterfowl can be derived from a figure in this report, using the days per year that each kind spent on any lake or reservoir. In our example, over 6500 Canada geese (Branta canadensis) and 4200 ducks (mostly mallards, Anas platyrhynchos) added 4462 kg of carbon (C), 280 kg of nitrogen (N), and 88 kg of P y^–1 to Wintergreen Lake in southwestern Michigan, mostly during their migration. These amounts were 69% of all C, 27% of all N, and 70% of all P that entered the lake from external sources. Loads from all external sources totaled 840 mg P m^–2 y^–1. Application of a nutrient load-response model to this concentration, the hydraulic load (0.25 m y^–1), and the water residence time (9.7 y) of Wintergreen Lake yielded an average annual concentration of total P in the lake of 818 mg m^–3 that classified the lake as hypertrophic. This trophic classification agreed with independent measures of primary productivity, chlorophyll-a, total P, total N, and Secchi disk transparency made in Wintergreen Lake. Our procedure showed that waterfowl caused low water quality in Wintergreen Lake.Contribution 824 of the National Fisheries Research Center-Great Lakes, 1451 Green Road, Ann Arbor, Michigan 48105, U.S.A. and 722 of the Kellogg Biological Station, Michigan State University.     

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     

Restoration of shallow lakes by nutrient control and biomanipulation—the successful strategy varies with lake size and climate

Major efforts have been made world-wide to improve the ecological quality of shallow lakes by reducing external nutrient loading. These have often resulted in lower in-lake total phosphorus (TP) and decreased chlorophyll a levels in surface water, reduced phytoplankton biomass and higher Secchi depth. Internal loading delays recovery, but in north temperate lakes a new equilibrium with respect to TP often is reached after <10–15 years. In comparison, the response time to reduced nitrogen (N) loading is typically <5 years. Also increased top-down control may be important. Fish biomass often declines, and the percentage of piscivores, the zooplankton:phytoplankton biomass ratio, the contribution of Daphnia to zooplankton biomass and the cladoceran size all tend to increase. This holds for both small and relatively large lakes, for example, the largest lake in Denmark (40 km²), shallow Lake Arresø, has responded relatively rapidly to a ca. 76% loading reduction arising from nutrient reduction and top-down control. Some lakes, however, have proven resistant to loading reductions. To accelerate recovery several physico-chemical and biological restoration methods have been developed for north temperate lakes and used with varying degrees of success. Biological measures, such as selective removal of planktivorous fish, stocking of piscivorous fish and implantation or protection of submerged plants, often are cheap versus traditional physico-chemical methods and are therefore attractive. However, their long-term effectiveness is uncertain. It is argued that additional measures beyond loading reduction are less cost-efficient and often not needed in very large lakes. Although fewer data are available on tropical lakes these seem to respond to external loading reductions, an example being Lake Paranoá, Brazil (38 km²). However, differences in biological interactions between cold temperate versus warm temperate-subtropical-tropical lakes make transfer of existing biological restoration methods to warm lakes difficult. Warm lakes often have prolonged growth seasons with a higher risk of long-lasting algal blooms and dense floating plant communities, smaller fish, higher aggregation of fish in vegetation (leading to loss of zooplankton refuge), more annual fish cohorts, more omnivorous feeding by fish and less specialist piscivory. The trophic structures of warm lakes vary markedly, depending on precipitation, continental or coastal regions locations, lake age and temperature. Unfortunately, little is known about trophic dynamics and the role of fish in warm lakes. Since many warm lakes suffer from eutrophication, new insights are needed into trophic interactions and potential lake restoration methods, especially since eutrophication is expected to increase in the future owing to economic development and global warming.     

Factors governing nutrient status of mountain lakes in the Tatra Mountains

1. Nutrient and chlorophyll a levels, and bacterial numbers of 84 glacial lakes in the Tatra Mountains (Slovakia and Poland, Central Europe) were determined to assess the impact of catchment vegetation and water acidity on lake trophic status. 2. Catchment vegetation was the crucial factor governing nutrient content of lakes. 3. Concentrations of organic carbon, organic nitrogen, and chlorophyll a, and bacterial numbers were tightly correlated with total phosphorus (TP) content. Their levels were the highest in forest lakes, then decreased in alpine lakes with decreasing amount of catchment vegetation and soil cover, and were the lowest in lakes situated in bare rocks. 4. The above pattern was further modified by lake water acidity. Concentrations of TP, organic carbon, and chlorophyll a were lower in alpine lakes with pH between 5 and 6 than in more or less acid alpine lakes. Zooplankton was absent in all alpine lakes with pH between 5 and 6. 5. Nitrate concentrations followed an inverse trend to TP; lowest values were in forest lakes, then increased with decreasing amount of catchment soils and vegetation. Within the lakes of the same type of catchment vegetation, nitrate concentrations were negatively correlated to TP. N‐saturation of catchment areas and lake primary production were dominant processes controlling nitrate levels in lakes and nitrate contribution to lake acidification.     

Fish manipulation as a lake restoration tool in shallow,eutrophic temperate lakes 1: cross-analysis of three Danish case-studies

The use of fish manipulation as a tool for lake restoration in eutrophic lakes has been investigated since 1986 in three shallow, eutrophic Danish lakes. The lakes differ with respect to nutrient loading and nutrient levels (130–1000 μg P l⁻¹, 1–6 mg N l⁻¹). A 50% removal of planktivorous fish in the less eutrophic cyanobacteria-diatom dominated Lake V?ng caused marked changes in lower trophic levels, phosphorus concentration and transparency. Only minor changes occurred after a 78% removal of planktivorous fish in eutrophic cyanobacteria dominated Frederiksborg Castle Lake. In the hypertrophic, green algae dominated Lake S?byg?rd a low recruitment of all fish species and a 16% removal of fish biomass created substantial changes in trophic structure, but no decrease in phosphorus concentration. The different response pattern is interpreted as (1) a difference in density and persistence of bloomforming cyanobacteria caused by between-lake variations in nutrient levels and probably also mixing- and flushing rates, (2) a difference in specific loss rates through sedimentation of the algal community prevaling after the fish manipulation, (3) a decreased impact of planktivorous fish with increasing mean depth and (4) a lake specific difference in ability to create a self-increasing reduction in the phosphorus level in the lake water. This in turn seems related to the phosphorus loading.     

Sediment resuspension: rescue or downfall of a thermally stratified eutrophic lake?

The aim of this study was to estimate the effect of sediment resuspension, a common phenomenon in many lakes, on the phosphorus budget of a eutrophicated lake. We used two different approaches, mass balance calculations and spatially comprehensive resuspension measurements, to determine the level of phosphorus loading from which rehabilitation action is started in a dimictic north temperate lake. The effect of resuspension was assumed to be significant, since it often is a governing process for cycling of material in lakes. Internal loading was multifold to that of external loading as determined by the budget calculation. Spatially comprehensive sedimentation and resuspension measurements were necessary, since deep site versus spatially comprehensive measurements had a marked difference in their results. Resuspension of P slightly exceeded the internal loading assessed by budget calculations and thereby proved its significance as a governing in-lake process that influences P cycling strongly. The shallow areas were of importance, since most of the total P load originated from there. The fate of P after resuspension depends on the retention capacity of resuspended particles in addition to prevailing biological and physico-chemical conditions. Therefore, sediment resuspension can either strengthen or diminish internal nutrient load and the processes of the shallow zones are of importance.     

El Niño Southern Oscillation and climate trends impact reservoir water quality

Low dissolved oxygen concentration in bottom layers of lakes and reservoirs usually indicates low water quality. In lakes, empirical models predicting anoxia are almost entirely based on the decay of plankton biomass, while in reservoirs recent findings suggest a prominent role of streamflow and load of organic carbon. This suggests a potential link between water quality in reservoirs and climate processes affecting streamflow. Here we support this hypothesis presenting evidence that both interannual climate variability and recent climate change, mainly consisting in a significant increase in potential evapotranspiration in the upstream basin, affected the oxygen content in a Mediterranean reservoir (Sau Reservoir, Spain). Using a 44‐year monthly record, we found strong and consistent signatures of El Niño Southern Oscillation in the inflow and reservoir oxygen content. Spectral and wavelet techniques showed that the El Niño, streamflow, and reservoir oxygen content series oscillated in common periods, which coincided with the main El Niño variability modes. An empirical model explaining the annual oxygen content in the reservoir suggested that a decreasing streamflow trend reduced the oxygen content of the reservoir by about 20%, counteracting remediation measures implemented at the basin upstream the reservoir. Our results provide the first quantitative evidence of climate change effects on reservoir water quality using long‐term instrumental data, and indicate that streamflow should be considered as a key variable in assessing climate change impact on reservoir water quality. These results are especially relevant in regions of the world where reservoirs are abundant and most climate models predict a decrease in runoff during the next decades. Both the expected trends and the sensitivity of reservoir water quality to global interannual climate variability should be considered for a correct management of water resources in the present and to design adaptation policies in the future.     

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.     

Soil and vegetation as the determinants of lake nitrogen concentrations in forested watersheds in British Columbia,Canada

This study used soil type, drainage class, parent material, and vegetation type to quantify the concentrations of lake water nitrogen for nine coastal lakes in forested watersheds in British Columbia, Canada. The results showed that forest soil type, drainage class, parent material, and vegetation type can predict more than 80% of the variance of nitrogen concentrations in these coastal lakes. The lakes in the watersheds with Orthic Humic Gleysol soils, moraine soil parent material, poor drainage, and coastal western hemlock and Douglas-fir vegetation type had higher nitrogen concentrations than the lakes in the watersheds with the other soil and vegetation types and properties. The impact factor of forest soil and vegetation on lake nitrogen concentration was defined according to the ability of each watershed soil and vegetation to contribute lake nitrogen input. This paper also demonstrates how to construct a model of lake nitrogen concentrations in response to the change of forest vegetation types and soil properties. The developed models can be used as watershed management tools for establishing and maintaining high quality water through managing soil and vegetation in forested watersheds.