Evaluation and application of a three-dimensional water quality model in a shallow lake with complex morphometry

Fundamental hydrodynamic and ecological processes of a lake or reservoir could be adequately depicted by one-dimensional (1D) numerical simulation models. Whereas, lakes with significant horizontal water quality and hydrodynamic gradients due to their complex morphometry, inflow or water level fluctuations require a three-dimensional (3D) hydrodynamics and ecological analyses to accurately simulate their temporal and spatial dynamics. In this study, we applied a 3D hydrodynamic model (ELCOM) coupled with an ecological model (CAEDYM) to simulate water quality parameters in three bays of the morphologically complex Lake Minnetonka. A considerable effort was made in setting up the model and a systematic parameterization approach was adopted to estimate the value of parameters based on their published values. Model calibration covered the entire length of the simulation periods from March 29 to October 20, 2000. Sensitivity analysis identified the top parameters with the largest contributions to the sensitivity of model results. The model was next verified with the same setup and parameter values for the period of April 25 to October 10, 2005 against field data. Spatial and temporal dynamics were well simulated and model output results of water temperature (T), dissolved oxygen (DO), total phosphorus (TP) and one group of algae (Cyanobacteria) represented as chlorophyll a (Chla) compared well with an extensive field data in the bays. The results show that the use of the model along with an accurate bathymetry, a systematic calibration and corroboration (verification) process will help to analyze the hydrodynamics and geochemical processes of the morphologically complex Lake Minnetonka. An example of an ecological application of the model for Lake Minnetonka is presented by examining the effect of spatial heterogeneity on coolwater fish habitat analysis in 3D and under a scenario where horizontal spatial heterogeneity was eliminated (1D). Both analyses captured seasonal fish habitat changes and the total seasonal averages differed moderately. However, the 1D analysis did not capture local and short duration variabilities and missed suitable fish habitat variations of as much as 20%. The experiment highlighted the need for a 3D analysis in depicting ecological hot spots such as unsuitable fish habitats in Lake Minnetonka.     

Structural changes in lake functioning induced from nutrient loading and climate variability

Climate variability is increasingly recognized as an important regulatory factor, capable of influencing the structural properties of aquatic ecosystems. Lakes appear to be particularly sensitive to the ecological impacts of climate variability, and several long time series have shown a close coupling between climate, lake thermal properties and individual organism physiology, population abundance, community structure, and food web dynamics. Thus, understanding the complex interplay among meteorological forcing, hydrological variability, and ecosystem functioning is essential for improving the credibility of model-based water resources/fisheries management. Our objective herein is to examine the relative importance of the ecological mechanisms underlying plankton seasonal variability in Lake Washington, Washington State (USA), over a 35-year period (1964–1998). Our analysis is founded upon an intermediate complexity plankton model that is used to reproduce the limiting nutrient (phosphate)–phytoplankton–zooplankton–detritus (particulate phosphorus) dynamics in the lake. Model parameterization is based on a Bayesian calibration scheme that offers insights into the degree of information the data contain about model inputs and allows obtaining predictions along with uncertainty bounds for modeled output variables. The model accurately reproduces the key seasonal planktonic patterns in Lake Washington and provides realistic estimates of predictive uncertainty for water quality variables of environmental management interest. A principal component analysis of the annual estimates of the underlying ecological processes highlighted the significant role of the phosphorus recycling stemming from the zooplankton excretion on the planktonic food web variability. We also identified a moderately significant signature of the local climatic conditions (air temperature) on phytoplankton growth (r = 0.41), herbivorous grazing (r = 0.38), and detritus mineralization (r = 0.39). Our study seeks linkages with the conceptual food web model proposed by Hampton et al. [Hampton, S.E., Scheuerell, M.D., Schindler, D.E., 2006b. Coalescence in the Lake Washington story: interaction strengths in a planktonic food web. Limnol. Oceanogr. 51, 2042–2051.] to emphasize the “bottom-up” control of the Lake Washington plankton phenology. The posterior predictive distributions of the plankton model are also used to assess the exceedance frequency and confidence of compliance with total phosphorus (15 μg L⁻¹) and chlorophyll a (4 μg L⁻¹) threshold levels during the summer-stratified period in Lake Washington. Finally, we conclude by underscoring the importance of explicitly acknowledging the uncertainty in ecological forecasts to the management of freshwater ecosystems under a changing global environment.     

Exploring the influence of lake water chemistry on chlorophyll a: A multivariate statistical model analysis

A multivariate statistical approach integrating the absolute principal components score (APCS) and multivariate linear regression (APCS-MLR), along with structural equation modeling (SEM), was used to model the influence of water chemistry variables on chlorophyll a (Chl a) in Lake Qilu, a severely polluted lake in southwestern China. Water quality was surveyed monthly from 2000 to 2005. APCS-MLR was used to identify key water chemistry variables, mine data for SEM, and predict Chl a. Seven principal components (PCs) were determined as eigenvalues >1, which explained 68.67% of the original variance. Four PCs were selected to predict Chl a using APCS-MLR. The results showed a good fit between the observed data and modeled values, with R² = 0.80. For SEM, Chl a and eight variables were used: NH₄-N (ammonia-nitrogen), total phosphorus (TP), Secchi disc depth (SD), cyanide (CN), arsenic (As), cadmium (Cd), fluoride (F), and temperature (T). A conceptual model was established to describe the relationships among the water chemistry variables and Chl a. Four latent variables were also introduced: physical factors, nutrients, toxic substances, and phytoplankton. In general, the SEM demonstrated good agreement between the sample covariance matrix of observed variables and the model-implied covariance matrix. Among the water chemistry factors, T and TP had the greatest positive influence on Chl a, whereas SD had the largest negative influence. These results will help researchers and decision-makers to better understand the influence of water chemistry on phytoplankton and to manage eutrophication adaptively in Lake Qilu.     

湖泊营养物参照状态建立方法研究

建立生态分区内各类型湖泊营养物的参照状态是营养物基准制定过程中最为核心的内容之一。在系统分析和评价国外确定湖泊营养物参照状态的若干种方法,包括参照湖泊法、湖泊群体分布法、三分法、回归分析等几种统计学方法以及模型推断和古湖沼学重建方法后,文章根据总磷（total phosphorus,TP）、总氮（total nitrogen,TN）、叶绿素a（chlorophyll a,Chl-a）和塞氏透明度（Secci depth）等四项指标的历史监测数据,应用若干统计学方法建立了巢湖的营养物基准参照状态。通过互相之间的比较分析以及长江中下游湖区古湖沼学重建数据的验证,推荐采用湖泊群体分布法5％点位对应的值作为巢湖营养物基准的参照状态。因此,巢湖营养物参照状态阈值范围为总磷0．023～0．27mg·L^-1,总氮O．62～0．63mg·L^-1,叶绿素aO．65～0．67mg·m^-1,塞氏透明度0．65-0．72m。     

Spatial and temporal hydrodynamic and water quality modeling analysis of a large reservoir on the South Carolina (USA) coastal plain

Two-dimensional, 31-segment, 61-channel hydrodynamic and water quality models of Lake Marion (surface area 330.7 km²; volume 1548.3×10⁶ m³) were developed using the WASP5 modeling system. Field data from 1985 to 1990 were used to parameterize the models. Phytoplankton kinetic rates and constants were obtained from a related in situ study; others from modeling literature. The hydrodynamic model was calibrated to estimates of daily lake volume; the water quality model was calibrated for ammonia, nitrate, ortho-phosphate, dissolved oxygen, chlorophyll-a, biochemical oxygen demand, organic nitrogen, and organic phosphorus. Water quality calibration suggested the model characterized phytoplankton and nutrient dynamics quite well. The model was validated (Kolmogorov–Smirnov two-sample goodness-of-fit test at P<0.05) by reparameterizing the nutrient loading functions using an independent set of field data. The models identified several factors that may contribute to the spatial variability previously reported from other research in the reservoir, despite the superficial absence of complex structure. Sensitivity analysis of the phytoplankton kinetic rates suggest that study site-specific estimates were important for obtaining model fit to field data. Sediment sources of ammonia (10–60 mg m⁻² day⁻¹) and phosphate (1–6 mg m⁻² day⁻¹) were important to achieve model calibration, especially during periods of high temperatures and low dissolved oxygen. This sediment flux accounted for 78% (nitrogen) and 50% (phosphorus) of the annual load. Spatial and temporal variability in the lake, reflected in the calibrated and validated models, suggest that ecological factors that influence phytoplankton productivity and nutrient dynamics are different in various parts of the lake. The WASP5 model as implemented here does not fully accommodate the ecological variability in Lake Marion due to model constraints on the specification of rate constants. This level of spatial detail may not be appropriate for an operational reservoir model, but as a research tool the models are both versatile and useful.     

Using a 1-D mixing model to simulate the vertical flux of heat and oxygen in a lake subject to episodic mixing

A one-dimensional, two-layer lake model is used to simulate the daily temperature and oxygen profile of an English Lake in response to changes in wind, air temperature and radiation. The thermal model component derives from the TEMIX lake model originally developed by the Institute of Limnology in St. Petersburg [Mironov, D.V., Golosov, S.D., Zilitinkevitch, S.S., Kreiman, K.D., Terzhevik, A.Y., 1991. Seasonal changes of temperature and mixing conditions in a lake. In: Zilitinkevitch, S.S. (Ed.), Modelling Air–Lake Interactions. Springer-Verlag, pp. 74–90]. This paper describes the model's adaptation and its extension to incorporate dissolved oxygen to simulate periods of anoxia of short duration during summer stratification. The new oxygen model component, which is based on mass-balance principles, divides the lake into two layers in a similar way to the thermal model. Its primary purpose is to model periods of anoxia for use in studies of fish survival. The model has been tested over a 10-year period from 1991 to 1999 using daily weather data and fortnightly observations of chlorophyll a and secchi depth. Ten years of fortnightly oxygen measurements, together with 2 years of more detailed (hourly) oxygen data, indicate that simulated and observed oxygen levels are in reasonable agreement considering the sparseness of the chlorophyll observations. The balance between the relative effects of temperature and BOD on oxygen depletion is of particular importance to model accuracy.     

Lake phosphorus dynamics and climate warming: A mechanistic model approach

Model results in this work indicate that lakes may respond very differently to climate change depending on their physical character. A physical lake model and a mechanistic phosphorus model are combined with two temperature scenarios generated by a regional climate model (RCM) in three sites in central Sweden—Lake Erken and two basins of Lake Mälaren (Galten and Ekoln). In the phosphorus model water mixing, mineralization, diffusion and biouptake are temperature dependent. In the simulations, Lake Erken is much more sensitive to climate warming than the two basins of Lake Mälaren, and the reason is shown to be the much longer water residence time in Lake Erken (7 years), stressing the importance of internal lake processes. In Galten and Ekoln the water residence times are less than 1 year, and the effects of water temperature changes are small. In Lake Erken the concentration of epilimnetic-dissolved phosphorus is almost doubled in spring and autumn in the warmest climate scenario. Since the lake is mostly phosphorus limited, this means that the potential for phytoplankton production is almost doubled. The implication would be that in Lake Erken, and in other eutrophic lakes with long water residence times, eutrophication problems may become serious in the future, and that managers may need to take action today in order to maintain good water quality in these lakes.     

Modelling the fate of mirex and lindane in Lake Ontario,off the Niagara river mouth

A seasonally averaged (June–November) fate model (toxfate) of organic contaminants was used to assess the relative importance of transport, degradation and volatization processes in the Niagara River and in Lake Ontario near the river mouth. The model predicts concentrations in several compartments, including suspended sediments, water, plankton, and fish. Two contaminants were modelled, Mirex and α-hexachlorocyclohexane (Lindane). Simulations show that the largest amount of contaminants is usually in the water fraction. About 70% of Mirex is in the water at the mouth of the Niagara River, compared with 99% for Lindane. For these two contaminants, the processes of degradation and volatilization are two to three orders of magnitude slower than the rate of transport through the river and the plume. In the water, the most important process is a change of phase, from the dissolved to the adsorbed state. This change of state is mostly relevant for strongly hydrophobic compounds like Mirex, and less so for Lindane. Simulations were performed for a 40-day period during which fish concentrations did not reach equilibrium with Mirex, but they did with Lindane. Spatial distributions of these contaminants were also predicted and compared with available data.     

Watershed integrated management: A decision-support model on Sete Cidades Lake (Azores)

The Sete Cidades Lake (437 ha) is located in a volcanic crater of São Miguel Island (Azores). The watershed is 1923 ha, and 25% of it is covered with water. Extreme logging and the use of fertilizers in the cattle grazing areas have caused sedimentation and eutrophication of the main water body (Carlson Trophic State Index TSI = 40–50 mesoeutrophy). To deal with the issue, a linear programming model was created to generate efficient land-use scenarios for different amounts of phosphorus release. The model was calibrated for the present situation and then simulated to obtain 29 scenarios. It allowed us to derive parameters for each scenario. Then a multiobjective program was used to derive an acceptable weight for each indicator. The dominant solutions were discovered and discussed with different stakeholders.     

Microplastic contamination in Great Lakes fish

Freshwater ecosystems, generally adjacent to human population and more contaminated relative to adjacent marine ecosystems, are vulnerable to microplastic contamination. We sampled 7 species of fish from Lake Ontario and Lake Superior and assessed their gastrointestinal (GI) tracts to quantify ingested microplastics and other anthropogenic particles. A subset of the microparticles were chemically analyzed to confirm polymer types and anthropogenic origins. We documented the highest concentration of microplastics and other anthropogenic microparticles ever reported in bony fish. We found 12,442 anthropogenic microparticles across 212 fish (8 species) from nearshore Lake Ontario, 943 across 50 fish (1 species) from Humber River, and 3094 across 119 fish (7 species) from Lake Superior. Fish from Lake Ontario had the greatest mean abundance of anthropogenic microparticles in their GI tracts (59 particles/fish [SD 104]), with up to 915 microparticles in a single fish. Fish from Lake Superior contained a mean [SD] of 26 [74] particles/fish, and fish from Humber River contained 19 [14] particles/fish. Most particles were microfibers. Overall, ≥90% of particles were anthropogenic, of which 35-59% were microplastics. Polyethylene (24%), polyethylene terephthalate (20%), and polypropylene (18%) were the most common microplastics. Ingestion of anthropogenic particles was significantly different among species within Lake Ontario (p < 0.05), and the abundance of anthropogenic particles increased as fish length increased in Lake Ontario (ρ = 0.62). Although we cannot extrapolate the concentration of microplastics in the water and sediments of these fish, the relatively high abundance of microplastics in the GI tracts of fish suggests environmental exposure may be above threshold concentrations for risk.     

A thermodynamic model of freshwater Antarctic lake ice

Antarctic lakes with simple plankton ecosystems are believed to be sensitive biological indicators of climate change. Models of the physical environment, in particular the ice layer, support understanding of how the ecosystems respond to meteorological variables. This paper describes how data from a previously reported automatic measuring probe and meteorological data from Davis station were used to develop a detailed thermodynamic model of the ice layer on Crooked Lake, one of the largest and deepest freshwater lakes in Antarctica. The general model structure is similar to a previously reported model of sea ice but with modifications specific to the Antarctic freshwater lake case informed by the data. The model inputs are atmospheric variables as well as water temperature, ice albedo and the radiation extinction coefficient for the ice. Heat and radiation fluxes at the ice–air and ice–water boundaries are calculated using equations chosen for their suitability for the Antarctic. In the case of shortwave radiation, equations were fitted to data from the automatic probe. Using the heat fluxes to establish boundary conditions, and incorporating the known thermodynamic properties of ice, the temperature profile within the ice and the resulting growth and melt of the ice can be calculated. The model uses a largely mechanistic approach, with most equations taken from established thermodynamic theories or empirical studies and only one adjustable parameter related to the sensible heat flux from the water, which is not easily calculated from the available data. It was found to accurately reproduce ice temperature and ice thickness data for the year 2003, with r² = 0.89, n = 2005. Finally, the model was simplified to run with air temperature as the only input variable and was shown to perform well—this suggests that freshwater lake ice is affected more by air temperature than any other variable, and is therefore a useful indicator of climate change in its own right.     

Evaluating the trophic status of a shallow polluted lake,lake Ioannina,Greece

Lake Ioannina in western Greece is a shallow lake, charged by springs as well as untreated discharges of the nearby urban area. Physicochemical and biological parameters have been evaluated verifying the deterioration of the lake aquatic environment. The water quality and the trophic status of the lake are greatly altered during the last ten years while the total phosphorus critical loading is already at a dangerous state.     

A test of two models of different levels of complexity for predicting changes of phosphorus concentration in a lake's outflow

The objective of this investigation is to compare the ability of a one-box model and a two-box model to predict the temporal variations of total phosphorus in the outflow of several lakes. The one-box model considers only total phosphorus and treats the lake as well-mixed, all year round. The two-box model considers two phosphorus fractions and the epilimnion and hypolimnion. For five shallow Canadian Shield lakes, some of the temporal variability in lake phosphorus concentrations can be explained on the basis of changes in hydraulic inflows and phosphorus loadings. Both models give roughly similar predictions for shallow lakes with high hydraulic loadings (1–200 m/year) and small variations in total phosphorus concentrations. In comparing the settling and hydraulic outflow mechanisms, a sensitivity analysis shows that both models are more sensitive to the settling rate if the lake's hydraulic loadings are less than 1 m/year, and more sensitive to the outflow rate if the lake's hydraulic loadings are greater than 100 m/year. This analysis suggests a priority for concentrating economic and other resources in conducting field measurement programs for testing phosphorus models.     

Catastrophic-like shifts in shallow Turkish lakes: a modeling approach

A generalized logistic model (GLM) was developed to determine occurrence of submerged macrophytes in shallow Lake Eymir, and the model was tested independently on the upstream shallow Lake Mogan using the data collected fortnightly from both lakes during 1997–2002. The independent variables included concentrations of chlorophyll-a (chl-a), suspended solids (SS) and total phosphorus (TP), Secchi disc transparency and z-scores of water levels. The dependent variable was the binary index of submerged plant occurrence. We used bootstrapping to determine the maximum number of epochs to train the model and to execute training when the corrected average cross entropy (c-index) leveled off. The model predicted that SS concentration, z-scores of water levels and TP concentration were the most important variables for determining occurrence of submerged plants. Sensitivity analyses showed that the probability of submerged plant occurrence followed a strong hysterisis response to varying water levels and the concentrations of SS and TP, with the break points being ±50 cm, 12–17 mg l⁻¹ and 200–300 μg l⁻¹, respectively. This observed sensitivity was in accordance with the alternative stable states hypothesis of shallow lakes. For occurrence of submerged plants, chlorophyll-a concentration and Secchi disc transparency had low significance. This was in concert with both relevances of input variables and the field results. The model gave a good definition of the system since the c-index and corrected c-index on the training data were high (0.970 and 0.963, respectively). Testing the model on Lake Mogan produced a c-index of 0.815 with around 80% of the cases being correctly classified. This showed that the model had a high ability to generalize over a spatially independent test set; therefore, it had a great reliability as well. In addition, the predictive power of the model was indeed very high. Consequently, the model captured the relationships between the input and output variables successfully and consistently with alternative stable states hypothesis.     

Structure and function of a warm monomictic lake

Construction and simulation of a model of Lake Conway, Florida, U.S.A., provided a framework for defining major characteristics of this ecosystem. The relationships that are formalized in this model comprise a set of hypotheses about the nature of a warm monomictic lake. The data that were used to parameterize the model came primarily from literature estimates, although approximations of biomass levels were available from associated research conducted on the lake.Submersed macrophytes are a major biomass component in Lake Conway; simulation suggested that their role in nutrient recycling overshadows their importance in the grazing food chain. Phytoplankton biomass and degree of fluctuation are considerably lower than are observed in most cool temperate lakes, although simulated respiration and herbivory rates are closer to temperate values than tropical values. Simulated epipelic algae biomass varies an order of magnitude during the year, and this group appears to be a significant part of the food chain. Simulated zooplankton consumption and turnover rates are very high, in part because of the relatively small biomass per individual. Simulation of the model suggests that slightly more carbon is processed through the grazing food chain in Lake Conway than through the detritus food chain.     

Food web structure and trophic interactions of the tropical highland lake Hayq, Ethiopia

We generated a mass-balance model to figure out the food web structure and trophic interactions of the major functional groups of the Ethiopian highland Lake Hayq. Moreover, the study lay down a baseline data for future ecosystem-based investigations and management activities. Extensive data collection has been taken place between October 2007 and May 2009. Ecotrophic efficiency (EE) of several functional groups including phytoplankton (0.8) and detritus (0.85) was high indicating the utilization of the groups within the system. However, the EE of Mesocyclops (0.03) and Thermocyclops (0.30) was very low implying these resources were rather a ‘sink’ in the trophic hierarchy. Flows based on aggregated trophic level sensu Lindeman revealed the importance of both phytoplankton and detritus to higher trophic levels. The computed average transfer efficiency of 11.5% for the first four trophic levels was within the range for highly efficient African lakes. The primary production to respiration (P/R) ratio (1.05) of Lake Hayq indicates the maturity of the ecosystem. We also modeled the food-web by excluding Tilapia and reduced phytoplankton biomass to get insight into the mass balance before Tilapia was introduced. The analysis resulted in a lower system omnivory index (SOI = 0.016) and a reduced P/R ratio (0.13) that described the lake as immature ecosystem, suggesting the introduction of Tilapia might have contributed to the maturity of the lake. Tilapia in Lake Hayq filled an ecological empty niche of pelagic planktivores, and contributed for the better transfer efficiency observed from primary production to fish yield.     

Calibration of the forest vegetation simulator (FVS) model for the main forest species of Ontario,Canada

The forest vegetation simulator (FVS) model was calibrated for use in Ontario, Canada, to predict the growth of forest stands. Using data from permanent sample plots originating from different regions of Ontario, new models were derived for dbh growth rate, survival rate, stem height and species group density index for large trees and height and dbh growth rate for small trees. The dataset included black spruce (Picea mariana (Mill.) B.S.P.) and jack pine (Pinus banksiana Lamb.) for the boreal region, sugar maple (Acer saccharum Marsh.), white pine (Pinus strobus L.), red pine (Pinus resinosa Ait.) and yellow birch (Betula alleghaniensis Britton) for the Great Lakes-St. Lawrence region, and balsam fir (Abies balsamea (L.) Mill.) and trembling aspen (Populus tremuloides Michx.) for both regions. These new models were validated against an independent dataset that consisted of permanent sample plots located in Quebec. The new models predicted biologically consistent growth patterns whereas some of the original models from the Lake States version of FVS occasionally did not. The new models also fitted the calibration (Ontario) data better than the original FVS models. The validation against independent data from Quebec showed that the new models generally had a lower prediction error than the original FVS models.     

Adsorption of sulfonamides on lake sediments

Sulfonamides (SAs) are one class of the most widely used antibiotics around the world. Their fate and transport in the aquatic environment is of great concern. In this study, adsorption of four SAs—sulfadiazine (SD), sulfamethoxazole (SMZ), sulfadimethoxine (SDM) and sulfamethazine (SM2)—in single-solute and multi-solute systems on sediments of Dianchi (DC) Lake and Taihu (TH) Lake, China was investigated with batch experiments. In the single-solute adsorption system, the Langmuir model and the dual-mode model described the adsorption process better than the Freundlich model. Model fitness was better on DC sediment than on TH sediment. The order of adsorption capacity approximately followed a decreasing order of SDM>SD>SM2>SMZ on both sediments, which was likely attributed to the distinctly different water solubility of the four SAs. In the multi-solute system, the order of adsorption capacity was SM2>SDM>SD>SMZ, which was probably related to the compound speciation caused by the pH values of the experimental solution. In the multi-solute system, both competitive and cooperative adsorption played important roles in the adsorption of sulfonamides on sediments.     

Elucidation of ecosystem attributes of an oligotrophic lake in Hokkaido,Japan, using Ecopath with Ecosim (EwE)

The fishing practices in the oligotrophic Lake Toya, Hokkaido, Japan, have profound implications in the ecosystem sustainability. The status of the sockeye salmon (Oncorhynchus nerka) population has become a serious concern among the lake managers and policy makers during the last decades. While the decline of the sockeye salmon population has been well documented in Lake Toya, there is considerable uncertainty with regards to the impact on the broader system dynamics. In this study, our objective is to address this knowledge gap by undertaking a synthesis of the Lake Toya food web using the mass-balance modeling software Ecopath with Ecosim (EwE). Our primary research question is to examine the repercussions of the declining sockeye salmon population on the trophic dynamics of the lake. Namely, we assess if there are any competing species that might have benefited from the decrease of sockeye salmon standing biomass and to what extent do these changes propagate through the Lake Toya food web? Our analysis pinpoints the critical role of the Japanese smelt (Hypomesus transpacificus nipponensis) in the system, which demonstrates a wide range of effects on several functional groups at both higher and lower trophic levels in Lake Toya. In particular, being a substantial portion of the masu salmon (Oncorhynchus masou) and adult sockeye salmon diets, the Japanese smelt has a positive impact on the top predators of the system. Amphipods, insects, and shrimp strongly benefit from the autochthonous and allochthonous organic matter in the system, while the tight coupling between phytoplankton and zooplankton seems to be particularly critical for the integrity of the Lake Toya food web. Whereas the values of the different ecosystem attributes (e.g., primary production/biomass, biomass/total throughput, system omnivory index, amount of recycled throughput, Finn's cycling index) provide evidence that Lake Toya is an immature system, we note that the internal redundancy and the system overhead estimates suggest that the lake possesses substantial reserves to overcome external perturbations. We also examined the effects of a variety of fishing policies on the biomass of masu salmon and adult sockeye salmon, which verify the belief that the adult sockeye population is quite fragile with high likelihood to collapse. Our analysis also predicts that sockeye will not rebound unless the fishing pressure exerted is substantially reduced (>50% of the reference levels used). Masu salmon seems to benefit under all the scenarios examined indicating that the intensity of the current fishing activities is significantly lower than its biomass accumulation rate in the system.     

浮叶植物重建对富营养化湖泊氮磷营养水平的影响

利用太湖五里湖污染底泥，在不破坏表层沉积物形态和结构的情况下，利用大口径的采样器采集沉积物柱状样，研究浮叶植物荇菜（Limnanthemun nymphoides）在此底泥上适应性生长情况及其对水体以及沉积物中氮磷的影响。结果表明，在培养实验过程中，荇菜生长使上覆水体中的氮、磷营养水平逐渐降低，藻类的生长明显受到克制，水体透明度增加，水质逐渐改善。通过植物根系对沉积物和问隙水中营养盐的直接吸收，使表层（0～5cm）沉积物与问隙水中氮磷营养盐的水平在实验结束后有明皿下降，对于控制沉积物内源营养盐释放有重要的作用。因此，恢复以水生植物为丰的水生生态系统足重建富营养湖泊生态系统和控制湖泊内源负荷的重要措施。