The coupled dynamics of human socio-economic choice and lake water system: the interaction of two sources of nonlinearity

We study a mathematical model for the coupled dynamics of human socio-economic choice and lake water system. In the model, many players choose one of the two options: a cooperative and costly option with low phosphorus discharge, and an economical option with high phosphorus discharge. The choice is affected by an economic cost, a social concern about water pollution, and a conformist tendency. The pollution level in the lake is determined by total phosphorus discharge by the players, the sedimentation and the outflow of phosphorus, and the recycling of phosphorus from the sediment. The model has two sources of nonlinearity: the cooperation level tends to be bistable due to conformist tendency of people (social hysteresis) and pollution level tends to be bistable because phosphorus recycling occurs faster in more eutrophic lakes (ecological hysteresis). The combination of these two sources may cause multiple stable equilibria or oscillations with a long periodicity. Small economic cost and strong social concern about pollution level can decrease the pollution level, but may not be very effective in enhancing the cooperation level. In contrast, strong conformist tendency produces a stable state with a high cooperation level and a low pollution level. We discuss implications of these results to the water quality management.     

河湖生态系统生态用水优化研究——以滇池流域为例

生态需水是河流与湖泊生态系统健康的重要基础。湖泊流域的河流与湖泊生态系统之间存在密切的水量联系,目前对流域内生态用水的研究多为单一生态系统生态需水简单相加,忽略了河流和湖泊之间复杂的水量联系。基于河湖复合生态系统之间的水量联系构建了河湖生态系统生态用水优化模型,并以滇池为例分析了河湖生态系统生态用水规律。结果表明：湖泊流域中单一河流或湖泊生态需水计算结果不能满足复合生态系统的生态用水要求,需要综合考虑河流和湖泊之间的水量联系;在当前水质状况下,牛栏江每年的调水量不能满足滇池流域的生态用水要求;滇池流域水体污染对流域内生态用水影响较大,随着水体污染程度的下降,流域生态用水量和调水量呈指数下降,河流生态用水呈线性下降。     

基于环境流体动力学模型的浅水草藻型湖泊水质数值模拟

针对内蒙古乌梁素海面临的污染现状,基于三维环境流体动力学模型(EFDC)的计算模式,将其与CE-QUAL-ICM模型耦合,模拟了不同情景下的乌梁素海藻类、总氮、总磷、化学需氧量年际和季节变化规律,模型中不仅考虑了风速和蒸发对模型的影响,也加入了挺水植物密度、高度、直径等形态指标,以此反映水生植物存在的区域中植物吸收降解污染物质、风速对底部应力、流场变化等因素对模型模拟结果的影响。研究结果表明:考虑挺水植物分布的耦合模型能够很好的模拟藻类和污染物质在乌梁素海内的年际、季节变化过程,模拟结果更接近于实际,模拟值与实测值间的相对误差基本控制在30%以内,大部分相对误差已控制在20%以内;降低入湖污染物质负荷直接影响着湖区内水质浓度的变化。另外,对于浅水或挺水植物密集而无法进入取得信息的区域,利用所建耦合模型能够为这些区域湖泊规划、管理、修复提供依据。     

Combined effects of social pressure from the inside and outside of fishing communities promote cooperation for stock management

We analyze a coupled socioeconomic and bioeconomic dynamics model for fisheries management in a local community. In the model, fishermen choose to cooperate with the fisheries management rule or not at each time step, depending on the utilities of each option. The social dynamics is coupled with the dynamics of the stock biomass. Fishermen’s utility basically depends on their catch, and the utility function of the noncooperative option is penalized by a psychological punishment, “social pressure”, which is affected by both the fraction of cooperative players and stock abundance. We classify social pressure into two categories: pressure originating from the inside and from the outside of the community. We develop a utility function with these social pressures and analyze the coupled dynamics. Our results show that all fishermen can end up cooperating with the management rule if both sources of internal and external social pressure have sufficient effect. The results imply that psychological aspects on fishermen can have an important role for successful management of local fisheries.     

Nitrogen dynamics and microbial food web structure during a summer cyanobacterial bloom in a subtropical,shallow, well-mixed,eutrophic lake (Lake Taihu,China)

Nitrogen dynamics and microbial food web structure were characterized in subtropical, eutrophic, large (2,338 km²), shallow (1.9 m mean depth), and polymictic Lake Taihu (China) in Sept–Oct 2002 during a cyanobacterial bloom. Population growth and industrialization are factors in trophic status deterioration in Lake Taihu. Sites for investigation were selected along a transect from the Liangxihe River discharge into Meiliang Bay to the main lake. Water column nitrogen and microbial food web measurements were combined with sediment–water interface incubations to characterize and identify important processes related to system nitrogen dynamics. Results indicate a gradient from strong phosphorus limitation at the river discharge to nitrogen limitation or co-limitation in the main lake. Denitrification in Meiliang Bay may drive main lake nitrogen limitation by removing excess nitrogen before physical transport to the main lake. Five times higher nutrient mineralization rates in the water column versus sediments indicate that sediment nutrient transformations were not as important as water column processes for fueling primary production. However, sediments provide a site for denitrification, which, along with nitrogen fixation and other processes, can determine available nutrient ratios. Dissimilatory nitrate reduction to ammonium (DNRA) was important, relative to denitrification, only at the river discharge site, and nitrogen fixation was observed only in the main lake. Reflecting nitrogen cycling patterns, microbial food web structure shifted from autotrophic (phytoplankton dominated) at the river discharge to heterotrophic (bacteria dominated) in and near the main lake.     

污染湖泊生态系统服务净价值评估——以滇池为例

水污染是我国湖泊生态系统面临的主要环境问题,评估污染条件下湖泊生态系统服务价值对认识和恢复污染湖泊生态功能具有重要意义。针对当前湖泊生态系统服务价值评估方法中未体现水污染负面影响的问题,在传统生态系统服务功能价值评估框架的基础上,将水污染造成的经济损失内化到生态系统服务功能价值中,提出了针对污染湖泊的生态系统服务功能价值评估方法。应用该方法核算了滇池生态系统服务净价值。结果表明:2006—2015年滇池年均生态系统服务功能价值为386.55亿元/a,水污染改变了滇池的价值结构,供水价值丧失,气候调节、旅游和蓄水等成为滇池生态系统服务功能价值的主体;滇池水污染对流域内外经济社会发展造成的损失达到15.23亿元/a,主要经济损失体现在生态补水费用和蓝藻去除费用;综合湖泊生态系统服务的正向价值和水污染的负向损失,滇池的净生态系统服务价值为371.33亿元/a,占流域GDP的11.7%。将水污染损失纳入湖泊生态系统服务功能价值评估中增强了评价结果的合理性,能够更加真实地反映湖泊生态系统对人类经济社会发展的综合影响。     

Dynamics of phytoplankton pigment characteristics in the Biya head water

Concentrations of phytoplankton photosynthetic pigments were measured at the Biya head water (in the immediate vicinity of Lake Teletskoe) every 10 days from July 1998 through December 2001. Comparison of phytoplankton pigment characteristics at the river’s head waters and at the surface of the lake’s pelagic zone for the ice-free period shows that the monitoring data give a clear picture of the formation and functioning of the lake phytoplankton. Analysis of seasonal fluctuations of pigment characteristics and their proportions revealed characteristic stages in the development of lake algal cenoses. We have demonstrated similarities and differences in the seasonal dynamics of the phytoplankton between Lake Teletskoe, a deep oligotrophic lake in the south of West Siberia, as compared with the general pattern observed in temperate lakes. According to the concentration of chlorophyll a, the trophic status of the lake’s pelagic zone and the Biya head water is ultraoligotrophic-oligotrophic. Yellow:green pigment ratio ranks Lake Teletskoe among “carotenoid lakes“. Low coefficients of linear correlation between chlorophyll a, water level and biogenic matter concentration shows that the relationships analyzed are far more complex than has been inferred from the combined effect of abiotic and biotic factors.     

Invasion biology of Ponto-Caspian onychopod cladocerans (Crustacea: Cladocera: Onychopoda)

Lake Taihu is characterized by its shallowness (mean depth = 1.9 m) and large surface area (2,338 km²). Runoff sources are mostly from the mountainous west and southwest, and outflows are located throughout East Taihu. This causes shorter retention times in the south. In contrast, urban pollutants discharge into northern Taihu and result in poor water quality. Non-point pollution from rural areas and sewage wastewater is the primary pollution source. Water current velocity ranges from 10–30 cm s⁻¹, and surface currents normally follow wind direction. Bottom currents appear to be a compensation flow. Most wave heights are less than 40 cm, and underwater irradiance correlates to seston in the water column. Lacustrine sediment is distributed in littoral zones, mostly along the western shoreline, with almost no accumulation in the lake center. Intensive aquaculture in East Taihu caused eutrophication and hampered water supply in surrounding areas. In addition, development of marshiness in the eastern littoral zones and East Taihu has occurred. The function of flood discharging of East Taihu has been limited by flourishing macrophytes. The problems facing in Lake Taihu will be alleviated by improving the management of nutrient sources into the lake. Guest editors: B. Qin, Z. Liu & K. Havens Eutrophication of shallow lakes with special reference to Lake Taihu, China     

Chemical and biological response of two small lakes in the Khumbu Valley,Himalayas (Nepal) to short-term variability and climatic change as detected by long-term monitoring and paleolimnological methods

The most remote regions of the globe are home of the least disturbed ecosystems, yet they are threatened by air pollution and by climatic change. The Himalayas are one of the most isolated and least explored wilderness areas in the world outside the Polar Regions and it is for this reason that the Tibetan Plateau is often referred to as the ‘Third Pole’. Since 1990, an annual limnological survey (including chemistry and biology) has been carried out at two lakes located in the Kumbhu Valley, Nepal, at 5200 and 5400 m a.s.l., respectively. Lake water chemistry surveys reveal a persistent increase in the ionic content of the lake water, a trend which appears to be closely linked to increasing temperature. In this study, we also analysed lake sediment cores for historical changes in algal abundance and community composition to evaluate how long-term variations in primary producer communities corresponded to known regional variations in climate systems during the past 3500 years. Paleolimnological results support the evidence that the strong variability observed in the chemical data drives the variability in lake production and in the composition of algal assemblages. These variabilities can be related to known features of local climate and the values recorded in the recent years compare well with those recorded during warm periods, such as around 2000 BP, and thus support the idea that this area of the Himalayan Range, influenced by the South Asia monsoon, is closely linked to Northern Hemisphere climate dynamics.     

Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake

Bacterial growth and grazing mortality were estimated from Mayto October in two south Swedish oligotrophic lakes, one beinga clearwater lake (water colour 5–10 mg Pt l^–1 DOC2.9–3.4 mg l^–1, Secchi disk depth 5.0–9.4m) and the other a humic, brownwater lake (water colour 105–165mg Pt l^–1, DOC 13.7–22.7mg l^–1, Secchi diskdepth 1.3–2.1 m). Specific rates of growth and grazingmortality were generally similar for both lakes. However, theabundance of bacteria was consistently 2–3 times higherin the water of the humic lake, suggesting that the total productionand consumption of bacterial cells were also higher than inthe dearwater lake. The ratio of bacterial secondary productionto primary production was higher in the humic lake than in theclearwater lake, indicating that the bacterioplankton of thehumic lake utilize allochthonous substrates, in addition tosubstrates originating from autochthonous primary production.Most of the bacterial loss in both lakes could be attributedto small protozoan grazers. This implies that allochthonousand autochthonous organic carbon fixed by bacterioplankton isless important in terms of carbon flow to higher trophic levelsthan would be expected if macrozooplankton were the dominantbacterivores, providing a more direct and efficient transferof carbon to larger organisms.     

Influence of the Hydrological Cycle on the Bacterioplankton of an Impacted Clear Water Amazonian Lake

Abstract Free-living and attached bacterial population sizes were determined fortnightly from December 1991 to December 1992 in natural and disturbed areas of an Amazonian clear water lake (Batata Lake, Pará, Brazil) impacted by bauxite tailings. The bacterioplankton showed distinct patterns during different phases of the hydrological cycle. Total bacterial population size and rates of thymidine incorporation (measured during high and low water phases) were high during low water, with values ranging from 3.3 × 10⁵ to 1.1 × 10⁶ cells ml⁻¹, and from 0.28 to 4.01 μg C l⁻¹ h⁻¹, respectively. The population size of free-living bacteria was larger at the natural station, while no differences were observed between attached bacterial populations at both stations. However, production and turnover rate of attached bacteria were high at the disturbed area. During low water, bacterial growth appeared to be driven mainly by the input of dissolved organic carbon (DOC) from phytoplankton origin. During high water, bacterial abundance was reduced, probably as the result of dilution and the input of less labile DOC from floodplains. The presence of bauxite tailings seems to influence bacterial dynamics in an indirect way, probably due to shading of phytoplankton cells and, hence, reducing the DOC supply for bacterial growth. This study, the first on the microbial ecology of an Amazonian clear water lake, demonstrated that water level variations exert a strong influence on the bacterioplankton dynamics. Received: 9 January 1996; Accepted 6 November 1996     

Food selection in <Emphasis Type="Italic">Eucypris virens</Emphasis> (Crustacea: Ostracoda) under experimental conditions

Identification of sources of nutrient pollution is a first step towards remediation of eutrophication in aquatic ecosystems. The stable isotope nitrogen-15 (¹⁵N) is a natural indicator of nitrogen (N) source and biogeochemistry. We sampled Lake Taihu, a hyper-eutrophic lake in eastern China, and major inflow rivers during winter and spring of 2004 to determine concentration and δ¹⁵N of nitrate (NO ₃ ⁻ ). Nitrate concentrations in rivers and the lake were higher, in most cases, in spring than in winter. δ¹⁵N of NO ₃ ⁻ was not correlated with NO ₃ ⁻ concentration, indicating that concentrations alone are insufficient to describe N sources. Results show that riverine N inputs in winter are influenced by discharge of human sewage into rivers and the lake. In spring, however, wastewater inputs to the lake appear to be balanced by fertilizers, atmospheric, and/or N₂ fixation sources. Rain NO ₃ ⁻ concentrations were seasonally high and isotopically enriched compared to potential sources, indicating that rain may be a significant or even dominant source of N to the lake during the rainy season. δ¹⁵N values show that urbanized areas of the lake have more sewage-derived N than those areas dominated by agriculture, aquaculture, or industry. This observation has important implications for human health, since Lake Taihu is a source of drinking and irrigation water as well as fish for human consumption.     

On polynuclear aromatic hydrocarbons contamination levels in the ecosystem of Lake Peipsi in the 1970s–1980s

The data on benzo(a)pyrene (BaP) and other polynuclear aromatic hydrocarbons (PAH) pollution of Lake Peipsi and its tributaries in the 1970s–1980s have been summarized. Systematic data on the pollution of Lake Peipsi by BaP are available. The BaP content in lake water varied from <0.1 to 237 ng 1^–1. The content and qualitative composition of PAH varied in different observations. PAH were accumulated mainly in the bottom sediments; in their surface layer BaP concentrations were 10³–10⁶ times higher than in water. The concentration of the PAH in bottom sediments varied from 16 to 580 µg kg^–1 in the lake and from 40 to 2800 µg kg^–1 in its tributaries. The content of BaP in fish and biota was not high. Among the pollution sources the atmospheric pollution played quite a significant role. The lake was also polluted by oil spillage and fuel exhausts from boats and ships. The PAH concentration in the water of the tributaries was somewhat higher than in the area of Lake Peipsi.     

Phosphorus dynamics at multiple time scales in the pelagic zone of a large shallow lake in Florida,USA

Phosphorus (P) dynamics in large shallow lakes are greatly influenced by physical processes such as wind-driven sediment resuspension, at times scales from hours to years. Results from long-term (30 year) research on Lake Okeechobee, Florida (area 1,730 km², mean depth 2.7 m) illustrate key features of these P dynamics. Variations in wind velocity result in changes in water column transparency, suspended solids, and total P (TP). In summer there are diurnal changes in TP associated with afternoon winds, and in winter, when strong winds occur for multiple days, monthly average TP remains high compared to summer. The magnitude of daily and seasonal TP changes can exceed 100 μg l⁻¹. Hurricanes and tropical storms also cause extreme changes in TP that are superimposed on seasonal dynamics. When a hurricane passed 80 km south of the lake in October 1999, mean pelagic TP increased from 88 to 222 μg l⁻¹. During large resuspension events, light attenuation is substantially increased, and this influences the biomass and spatial extent of submerged plants, as well as water column TP. In Lake Okeechobee, TP concentrations typically are ∼20 μg l⁻¹ when submerged plants are dense, and soluble reactive P concentrations are reduced below detection, perhaps by the periphyton and plant uptake and by precipitation with calcium at high pH. In contrast, TP exceeds 50 μg l⁻¹ when submerged plants and periphyton are absent due to prolonged deep water, and phytoplankton biomass and algal bloom frequency both are increased. In Lake Okeechobee and other large shallow lakes, complex models that explicitly consider wind-wave energy, hydrodynamics, and sediment resuspension, transport, and key biological processes are needed to accurately predict how lake water TP will respond to different management options.     

Changes in Internal Phosphorus Loading and Fish Population as Possible Causes of Water Quality Decline in a Shallow,Biomanipulated Lake

Impacts of internal nutrient loading and the role of biota in phosphorus (P) dynamics were studied in a shallow, hypertrophic, biomanipulated lake. Reduced fish stock resulted in clearing water and the development of a dense submerged vegetation by 2005. However, an abrupt shift occurred in 2007, seven years after the fish manipulation. Simultaneously, water quality deteriorated which became obvious in elevated chlorophyll‐a concentration in lake water, associated with increased biomass of fish and decay of a previously extended macrophyte cover. There were no significant differences in lake water P concentrations between the two periods (2005–2006 and 2007); however, peaks of different P forms were markedly higher in 2007 than in 2005–2006. At the same time, P content of sediment pore water declined considerably in 2007. Our mesocosm experiment, carried out in the manipulated lake, emphasize the positive role of the dominant fish species (roach) in P regeneration. We suggest that fish manipulation should be carried out every 5 year to maintain clear water conditions permanent, until the total removal of redundant nutrients accumulated in the lake ecosystem. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Evaporation and the hydrologic budget of Crater Lake, Oregon

The hydrologic budget of Crater Lake, Oregon is investigated by taking advantage of its relatively simple geometry, climatic circumstances, and the concurrent availability of many years of traditional data. Buoy data are here utilized for the first time for this purpose. The lake gains water through precipitation and delayed runoff from the caldera sides and Wizard Island. The lake loses water through evaporation and seepage. Seepage can be estimated quite well from ice-covered precipitation-free intervals in 1985, and is 127 cm/year. Evaporation has previously been determined as a residual, but is here estimated directly from the floating buoy, with an approximate value of 76 cm/year, a downward revision from previous estimates. These losses are balanced by precipitation input, nearly all in the form of snow or snowmelt runoff. Factors contributing to the uncertainty in each of the water budget components are discussed in some detail. The buoy data corroborate previous findings based on studies of stage that evaporation is greatest on the coldest days. Seasonally, the greatest evaporation occurs in the autumn and the least in spring. Proxy records are used to extend the effective length of the buoy record. Monthly estimates of evaporation are calculated for 1950–1996 and used to deduce temporal characteristics. The standard deviation of water year precipitation is 4.6 times larger than that of evaporation. Thus the water budget is controlled more by variability of precipitation than evaporation. An additional 15 years of data since earlier studies confirm that the annual lake level variations from one September 30 to the next are highly correlated (r = 0.96) with Park Headquarters water year precipitation for the 42 years from 1961–2003. The lake rises 1.4 cm for every cm of measured precipitation over equilibrium value (168.6 cm) at Park Headquarters. Sources of this “magnification” are discussed.     

Analyzing the mutual feedbacks between lake pollution and human behaviour in a mathematical social-ecological model

Does the adoption of environment-oriented actions by individuals necessarily improve the state of an ecosystem in the most effective way? We address this question with the example of eutrophication in shallow lakes. When exposed to fertilizers, such lakes can undergo a critical transition called eutrophication, resulting in a loss of biodiversity and ecosystem services. We couple a generic model of eutrophication with a best-response model of human behaviour, where agents can choose to pollute the lake at a high level (defection) or at a low level (cooperation). It is known that feedbacks between the interacting lake pollution and human behaviour can give rise to complex dynamics with multiple stable states and oscillations. Here, we analyze the impact of all model parameters on the shape of the nullclines. S-shaped nullclines are a condition for complex dynamics to occur. Moreover, we find that agents decreasing their pollution discharge into the lake is not necessarily the most effective way to reduce the pollution level in the lake. This is due to coexisting counterintuitive stable equilibria where the lake is in a clear state despite a high level of pollution discharge. We analyze the complex dynamics of the system and describe in detail Hopf, saddle-node, homoclinic and Bogdanov-Takens bifurcations. The complex dynamics with potential multistability and counterintuitive equilibria suggests that generic management recommendations holding for every level of pollution and of cooperation are impossible. Apart from the direct perturbation of an ecological variable, we identify three ways a management strategy can influence the social-ecological system: it can change the location, the resilience and the existence of stable equilibria.     

中国湖泊水量调节能力及其动态变化

湖泊水量调节是指湖泊生态系统通过洪水蓄积和径流补给实现水资源的再分配,进而减轻洪旱灾害。在围湖造田、退田还湖和气候变化影响下,科学评估我国湖泊水量调节能力现状及变化情况,是实现湖泊洪水调蓄功能和水资源调节功能评价的重要基础。基于全国湖泊调查数据,将全国划分为5个评价区,探讨了面向全国尺度的湖泊水量调节能力评价方法,在此基础上对全国湖泊的水量调节能力及其动态变化进行了分析评价。结果表明:(1)我国湖泊(面积1 km2)水量调节总量为1500.02亿m3,其中东部平原区和青藏高原区的调节量最高,分别占全国总量的44.46%和43.63%;(2)湖泊调节水量的效能以东部平原区最高(310.19万m3/km2),其次是东北平原与山区(191.19万m3/km2),围湖造田/退田还湖将导致该区湖泊水量调节能力明显削弱/增强;(3)近几十年来,我国湖泊水量调节能力呈小幅增长(增长量9.76亿m3,增幅0.65%),5个评价区仅蒙新高原区湖泊水量调节能力明显削弱,其余区域均呈不同程度增强,且以东部平原区增加最多,东北平原与山区增幅最大。研究可以为评估我国湖泊生态系统水量调节能力、分析土地利用变化对流域洪水调蓄和水资源调节功能的影响提供参考。     

Bioluminescence characteristics of Lake Shira water

Bioluminescence bioassays based on luminous bacteria (Photobacterium phosphopreum) and coupled enzyme system NADH-FMN-oxidoreductase-luciferase were adapted for monitoring the saline-water conditions of Lake Shira (Khakasia, Siberia). The differences in bioluminescence responses have been found to be related to the salt composition and the oxidation-reduction properties of water. Bioluminescent kinetics parameters, which are mostly sensitive to pollution under conditions of saline water, have been observed. The enzymatic system in the presence of 1,4-benzoquinone are shown to be more sensitive to redox characteristics of the salt water than this in the absence of 1,4-benzoquinone. 1,4-benzoquinone should be applied for the preparation of a model solution for the monitoring of redox properties of the salt water. Using this technique, the results of bioluminescence analysis are used to construct a heterogeneity map that characterizes the spatial and temporal water quality of lake Shira. A partial map was based on the bioluminescence characteristics of water samples taken along the shoreline, sampling stations in the different places and in different depths of the lake. It has been demonstrated that the bioluminescence assay measurements must be done within two hours after the sampling time.     

Parameters affecting polymerase chain reaction detection of waterborne Cryptosporidium parvum oocysts

Cryptosporidium parvum is an enteric protozoan parasite of medical and veterinary importance. Dissemination of environmentally resistant oocysts in surface water plays an important role in the epidemiology of cryptospridiosis. Although the polymerase chain reaction (PCR) is a well-established technique and is widely used for detecting microorganisms, it is not routinely applied for monitoring waterborne C. parvum. In order to facilitate the application of PCR to the detection of waterborne C. parvum oocysts, a comparison of published PCR protocols was undertaken and different sample-preparation methods tested. The sensitivity of a one-step PCR method, consisting of 40 temperature cycles, was 10 purified oocysts or fewer than 100 oocysts spiked in raw lake water. The detection limit of two primer pairs, one targeting the ribosomal small subunit and another specific for a C. parvum sequence of unknown function, was approximately ten-fold lower than achieved with a primer pair targeting an oocyst shell protein gene. Three cycles of freezing/thawing were sufficient to expose oocyst DNA and resulted in higher sensitivity than proteinase K digestion, sonication or electroporation. Inhibition of PCR by surface water from different local sources was entirely associated with the soluble fraction of lake water. Membrane filtration was evaluated in bench-scale experiments as a means of removing lake water inhibitors and improving the detection limit of PCR. Using gel and membrane filtration, the molecular size of inhibitory solutes from lake water was estimated to less than 27 kDa. Received: 14 November 1996 / Received revision: 18 March 1997 / Accepted: 27 March 1997