Lake Total Phosphorus Research Articles

Green Bay, Lake Michigan: A proving ground for Great Lakes restoration

Abstract Green Bay has sometimes been referred to as the largest freshwater “estuary” in the world. Its watershed, much of it in intensive agriculture, comprises one-third of the Lake Michigan basin and delivers one-third of the lake's total phosphorus load. At one time, the major tributary, the Fox River, was considered the most heavily industrialized river in North America, primarily from paper manufacturing. Deterioration in water quality and the loss of beneficial and ecological uses have been extensive and began well back into the last century. More recently, the bay has also become a test case for our resolve to remediate and restore ecosystems throughout the Great Lakes and elsewhere. Green Bay has stimulated a significant amount of widely relevant research on the fate and behavior of toxics, biogeochemistry, habitat, biodiversity, and ecological processes. The bay represents a true “proving ground” for adaptive restoration. Key findings of the recent summit on the Ecological and Socio-Economic Tradeoffs of Restoration in the Green Bay Ecosystem are summarized here. Foremost among recommendations of the workshop was the creation of a “Green Bay Ecosystem Simulation and Data Consortium” serving as a data clearing house, building upon the significant progress to date, and developing a modeling framework and visualization tools, furthering public outreach efforts, and ensuring a sustained growth in scientific expertise. Funding was estimated to be on the order of ~$15–20M over the next ~5 years – a modest investment relative to the value of the ecosystem and the long-term cost of inaction.

Asynchronous changes in trophic status of a lake and its watershed inferred from sedimentary diatoms of different habitats

Paleolimnology is one important approach for setting realistic goals for restoration and management decisions for lakes. Two important assumptions for reconstructing total phosphorus in lakes using diatoms are: (1) phosphorus is the key limiting factor for diatom growth; (2) other factors (chemical, biologic, or physical activity) had not significantly affected the natural way the remains of diatoms were buried. Both other lines of evidence, which is evaluated by a form of causal criteria analysis–Eco Evidence, and evidence from Muskegon Lake itself, were analyzed to evaluate whether total phosphorus (TP) is the causal factor for diatom species composition change in Muskegon Lake. Both evidence supported the cause-effect linkage between TP and diatom species composition change. Interestingly, our data analysis on a long sediment core from Muskegon Lake showed benthic and planktonic diatoms responded to changes in phosphorus conditions at two spatial scales. When inferring TP based on species composition of all diatoms, relationships to geochemical proxies and land use were poor. However, when inferring TP using benthic and planktonic diatoms, benthic diatom inferred TP was related most to in-lake conditions and planktonic diatom inferred TP was related most to changes in the Muskegon River, upstream from the lake. Only benthic diatom inferred TP correlated well with geochemical proxies in the lake. With a 23 day residence time of water in Muskegon Lake, species composition of planktonic diatoms in the core was likely more regulated by exogenous (allochthonous) environmental factors and processes, Muskegon River, than benthic diatoms. Therefore, freshwater landscape and other landscape variables, such as water residence time, are as important as biological feature for understanding long term trophic status change for some lakes. Caution should be used when diatom assemblages are more influenced by other factors than in-lake condition.

Assessing ecological effects of glacial meltwater on lakes fed by the Greenland Ice Sheet: The role of nutrient subsidies and turbidity

ABSTRACT Meltwater discharge from the Greenland Ice Sheet (GrIS) exports sediment, solutes, total phosphorus (TP), dissolved inorganic nitrogen (DIN), and other macro- and micronutrients to associated aquatic ecosystems. It remains unclear how this meltwater affects the ecology of glacially fed (GF) lakes. We assessed a suite of physical, chemical, and biological features of four GF lakes, and compared them to those of four nearby snow- and groundwater-fed (SF) lakes. We found that TP concentrations were six times higher in GF compared to SF lakes, but microbial extracellular enzyme activities and aluminum, iron, and phosphorus sediment fractions suggested that much of this TP in GF lakes is likely not biologically available. Turbidity was fifteen times higher in GF lakes, and DIN was twice as high than in SF lakes, but these nitrogen differences were not significant. While diatom species richness did not significantly differ between lake types, GF lakes had higher water column chlorophyll a (Chl a). Diatom species distributions across all lakes were strongly associated with turbidity, TP, and dissolved organic carbon (DOC). While cosmopolitan diatom taxa such as Discostella stelligera were found in both lake types, diatom communities differed across lake types. For instance, Fragilaria and Psammothidium species dominated GF lakes, while Achnanthes species and Lindavia ocellata were dominant in SF lakes. In addition to turbidity, the moderate amounts of DIN in GF lakes may play an important role in shaping diatom communities. This is supported by the high abundance in GF lakes of taxa such as Fragilaria tenera and D. stelligera, which reflect nitrogen enrichment in some lakes. Our results demonstrate how GrIS meltwaters alter the ecology of Arctic lakes, and contribute to the growing body of literature that reveals spatial variability in the effects of glacial meltwaters on lake ecosystems.

The identification of nutrient limitations on eutrophication in Dianchi Lake, China

AbstractDue to rapid urbanisation and economic development, Dianchi Lake has been eutrophic since the 1980s. Control of nutrient loading is regarded as the primary restoration method. Two independent approaches: analysis of 13 years of Dianchi Lake monitoring data using Support Vector Machines (SVM) and a nutrient‐enrichment experiment, identified phosphorus and nitrogen as simultaneously limiting. Total phosphorus (TP) has a clear, stable correlation with chlorophyll‐a (Chl‐a) concentrations in Dianchi Lake. Compared to total nitrogen (TN), TP shows a stronger correlation when Chl‐a concentration is below 0.051 mg/L and an almost equal correlation when it is above this level. An excessive phosphorus to nitrogen ratio may stimulate compensatory fixation of atmospheric N2 by cyanobacteria; therefore, only reducing the external nitrogen input may not necessarily result in lower total nitrogen and chlorophyll concentrations. We recommend that reducing the lake TP load to the lowest economically‐feasible level is the most cost‐effective restoration measure.

Influence of Lake Surface Area and Total Phosphorus on Annual Bluegill Growth in Small Impoundments of Central Georgia

The relationships between environmental variables and the growth rates of fishes are important and rapidly expanding topics in fisheries ecology. We used an informationtheoretic approach to evaluate the influence of lake surface area and total phosphorus on the age-specific growth rates of Lepomis macrochirus (Bluegill) in 6 small impoundments in central Georgia. We used model averaging to create composite models and determine the relative importance of the variables within each model. Results indicated that surface area was the most important factor in the models predicting growth of Bluegills aged 1–4 years; total phosphorus was also an important predictor for the same age-classes. These results suggest that managers can use water quality and lake morphometry variables to create predictive models specific to their waterbody or region to help develop lake-specific management plans that select for and optimize local-level habitat factors for enhancing Bluegill growth.

Human development modifies the functional composition of lake littoral invertebrate communities

Residential shoreline and watershed development by humans are leading agents of environmental change in lake ecosystems that reduce abundances and diversity of littoral invertebrates. Invertebrate functional and life history traits are robust indicators of environmental quality and ecosystem functioning, yet surprisingly few studies have utilized trait-based approaches to assess impacts of human development to lake littoral communities. We assessed environmental characteristics of human development that impact functional diversity and structure the trait composition of invertebrate communities in lakes of northwestern United States. Multiple linear regressions revealed that functional diversity declined with increasing watershed development, lake total phosphorus, and littoral macrophyte cover. Results from multivariate constrained ordination and fourth corner analysis indicated that high phosphorus concentrations and abundant macrophyte cover removed taxa with semivoltine life histories and filter feeders from lake communities, and that both regional ecosystem and local habitat characteristics of human development were important determinants of invertebrate community structure. Human development had particularly pronounced effects on invertebrate communities in the sublittoral zone, for which overall community abundances declined. Our study indicates that human development favors lake communities dominated by multivoltine taxa and herbivores, which may have important implications for energy flow among terrestrial, littoral, and pelagic food webs.

Empirical Estimation of Total Nitrogen and Total Phosphorus Concentration of Urban Water Bodies in China Using High Resolution IKONOS Multispectral Imagery

Measuring total nitrogen (TN) and total phosphorus (TP) is important in managing heavy polluted urban waters in China. This study uses high spatial resolution IKONOS imagery with four multispectral bands, which roughly correspond to Landsat/TM bands 1–4, to determine TN and TP in small urban rivers and lakes in China. By using Lake Cihu and the lower reaches of Wen-Rui Tang (WRT) River as examples, this paper develops both multiple linear regressions (MLR) and artificial neural network (ANN) models to estimate TN and TP concentrations from high spatial resolution remote sensing imagery and in situ water samples collected concurrently with overpassing satellite. The measured and estimated values of both MLR and ANN models are in good agreement (R2 > 0.85 and RMSE < 2.50). The empirical equations selected by MLR are more straightforward, whereas the estimated accuracy using ANN model is better (R2 > 0.86 and RMSE < 0.89). Results validate the potential of using high resolution IKONOS multispectral imagery to study the chemical states of small-sized urban water bodies. The spatial distribution maps of TN and TP concentrations generated by the ANN model can inform the decision makers of variations in water quality in Lake Cihu and lower reaches of WRT River. The approaches and equations developed in this study could be applied to other urban water bodies for water quality monitoring.

Integrating hierarchical Bayes with phosphorus loading modelling

The causal linkage between lake productivity and phosphorus loading has provided the basis for a family of models that predict lake total phosphorus concentrations as a function of lake morphometric/hydraulic characteristics, such as the areal phosphorus loading rate, mean lake depth, fractional phosphorus retention and areal hydraulic loading. Most of these empirical models have been derived from “cross-sectional” datasets, comprising multiple point measurements or single averages from a number of lakes, and are typically used to predict changes within a single system at different points in time. This practice implicitly postulates that the large scale (cross-sectional) patterns described in the model are also representative of the dynamics of individual systems. In this study, we relax this assumption using a Bayesian hierarchical strategy that aims to accommodate the role of significant sources of variability (morphology, hydraulic regime). We first examine several hierarchical structures representing different characterizations of model error, parameter covariance, and prior distribution followed by the hyperparameters. Our analysis primarily highlights the robustness of the posterior group-level patterns to the hierarchical formulation developed. We also show that the delineation of homogeneous subsets of lakes with respect to their morphological/hydraulic characteristics and the subsequent integration with hierarchical frameworks may give empirical phosphorus retention/loading models with better predictive ability. We then present a complementary exercise that aims to accommodate the spatial and seasonal total phosphorus variability within individual systems, using a spatially-explicit simple mass-balance model forced with idealized sinusoidal loading. Our study concludes by advocating that the hierarchical Bayes provides a conceptually appealing framework to gradually accommodate different sources of variability and more prudently increase the complexity of simple empirical models.

Historical trophic evolution resulting from changes in climate and ecosystem in Lake Taihu and seven other lakes, China

Under natural conditions, the evolution of lake trophic status is not only influenced by climatic and hydrological factors but also by the biological interactions and feedbacks occurring within the lake ecosystem. To envision long-term changes of lake trophic status in several shallow lakes from the Yangtze River basin, we focused on variations in nutrient level and attempted to use both hydrologic- and ecodynamic-based physical models. One was an equivalent-hydrology model with external driving forces from catchment climate and hydrology changes (Model 1), the other was a dynamic model with internal driving forces from the biological interactions within the lake ecosystem (Model 2). Two models were employed in order to distinguish the effects of climate change (by Model 1) from ecosystem feedbacks (by Model 2) on the long-term lake nutrients changes and to further determine which driver was the primary cause for the changes. Variations in lake producer biomass and total phosphorus (TP) were simulated for a historic 200-yr period (1640–1840 AD) in Lake Taihu and seven other similar lakes from the Yangtze River basin after calibration and validation of the control runs (1950–2000 AD). Results showed that (1) TP simulations by Model 1 and Model 2 were mostly consistent with the reconstructed nutrient records from lake sediment cores; (2) the long-term variations of lake nutrients were primarily affected by precipitation under the influence of climate change and the decreases in lake nutrients were more likely to respond to the dry climate; and (3) approximately 49% of the variability in nutrient concentrations were contributed by the feedback of biological interactions in the lake ecosystem. The study results provide a scientific basis for long-term control of nutrients in lakes along the middle–lower reaches of the Yangtze River.

Should a water colour parameter be included in lake total phosphorus prediction models used for the Water Framework Directive?

Under the Water Framework Directive (WFD) lakes are classified according to a variety of criteria. This classification facilitates state of the environment assessments and helps identify work needed to achieve the objectives of the WFD, which are broadly to maintain and/or restore water quality and ecological status at a level recognised as good or high. To achieve high or good status, lakes must meet a criterion for total phosphorus (TP) that is linked to a predicted reference condition value that is derived by various models. Lakes which fail to meet good status may require expensive remedial actions to be undertaken, thus accurate identification of the reference condition TP concentration is vital for effective environmental management. However, the models currently employed could be improved for some regions, particularly those with carbon rich soils. By examining 19 reference condition lakes (i.e. lakes essentially non-impacted by humans) in peaty areas of Scotland, we found that a simple parameter linked to water colour and humic substances was a better predictor of TP than the currently employed models (R2 0.585 vs R2 < 0.01). Therefore, for Scotland and elsewhere, in regions with carbon rich soils and lakes with humic waters the TP predictive models could be improved by development and incorporation of a parameter related to water colour and humic components.

Water quality effects following establishment of the invasive Dreissena polymorpha (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.

Evidence for iron‐regulated cyanobacterial predominance in oligotrophic lakes

Summary Cyanobacterial harmful algal blooms (cyanoHABs) are most often associated with large eutrophic lakes. In the past decade, an increase in the incidence of cyanoHABs in small oligotrophic lakes within the Laurentian Great Lakes‐St. Lawrence River Basin has been documented. We explored macro‐ and micronutrient conditions during peak phytoplankton biomass that result in high cyanobacterial densities. We hypothesised that the availability of biologically available iron (Fe) both limits the assimilation of phosphorus by competing taxa and selects for the predominance of cyanobacteria. We tested this hypothesis with a comparative study of 25 oligotrophic lakes in central Ontario, Canada. These lakes were sampled during peak phytoplankton biomass for 3 years (from 2009 to 2011) for concentrations of total phosphorus (TP), total nitrogen (TN), ammonium (), nitrate (), total dissolved Fe (TDFe) and pH for comparison with phytoplankton community biomass and composition. Correlation analyses indicated that predominance of cyanobacteria was assured at all TP levels when the availability of Fe was low. Proportionally high cyanobacterial density occurred at relatively low TP in lakes with a molar ratio of dissolved inorganic N (DIN):TP of 8:1 and a modelled ferric Fe concentration &lt;1.0 × 10−19 m at which cyanobacteria have been shown to be competitive for Fe in laboratory experiments. Regression tree analysis confirmed the minimal influence of P and N, implying that cyanobacteria thrive in low Fe environments. Our findings suggest that an increased susceptibility of oligotrophic lakes to cyanobacterial bloom formation is based on the presence of cyanobacteria that have adapted sufficiently to low Fe environments and are able to multiply rapidly when pulses of P are made available to the ecosystem. Physiologically prepared transitionary cyanobacteria do not have to compete with the eukaryotic species but rather exploit the new P and replicate faster.

Effects of agricultural conservation practices on oxbow lake watersheds in the Mississippi River alluvial plain

Agricultural lands are considered to be a major source of nonpoint source pollutants such as sediment, pesticides and nutrients in the United States. Conservation practices have been tested for their effectiveness in reducing agricultural related pollutants on smaller scales, but farm watershed scale assessments of these practices are limited and few of these have evaluated the impacts on downstream ecology. Several projects focused on oxbow lake watersheds in the Mississippi River alluvial plain were designed to utilize working farms to evaluate primary pollutants in water resources and to identify conservation practices that are most effective in reducing the transport of those pollutants in surface and ground water on a watershed scale. Major findings of these studies include: (1) conservation practices reduced sediment in oxbow lakes, resulting in improved water clarity, plankton growth, and fish stocks; (2) total phosphorus in lakes decreased between 39&amp;ndash;50% following application of conservation practices; (3) conservation tillage and cover crops reduced NO&lt;sub&gt;3&lt;/sub&gt;-N losses by 73%, sediment losses by 70; and (4) riparian areas mitigated the transport of sediment in runoff and enhanced the degradation of pesticides.

Research on Algal Bloom Control for Environment Engineering

Aquatic plants inhibit algae through nutrient competition, sludge sedimentation and the release of allelochemicals in three ways. Investigating Pistia stratiotes L in East Lake, Shao (2001) observed removal rate of the BOD5 achieved more than 70%; the total nitrogen removal efficiency was 60%, the total phosphorus removal efficiency was approximately 70% or more, and this biochemical inhibitory effect may promote algal settlement. Ho Pool (1999) found that the Rhizoma AcoriGraminei could cause a water total nitrogen (TN) removal rate of 87.4%, a total phosphorus (TP) removal rate of 43.9%, and a dissolved oxygen (DO) increase of 26.6%. These studies suggest that through the promotion of the lake TN and TP, aquatic plants influence bio-deposition into sediments, in addition to their role in the nutrient cycling of lakes. Furthermore, many studies have shown that aquatic macrophytes can produce allelochemicals that could inhibit the growth of algae (Donk &amp; Bund, 2002; A. Gross &amp; Boyd, 1998; Elisabeth M. Gross, 2003; E. M. Gross &amp; Sütfeld, 1994; Mulderij, Smolders, &amp; Van Donk, 2006; Mulderij, Van Nes, &amp; Van Donk, 2007).

Landscape factors influencing lake phosphorus concentrations across Minnesota

Total phosphorus (TP) concentrations are known to be a significant factor influencing fish populations in Minnesota lakes. Consequently, a primary focus of the Minnesota Department of Natural Resources to address fish habitat in lakes across the state has been to determine relationships between TP concentrations and watershed conditions in Minnesota lakes. Because phosphorus concentrations in Minnesota lakes vary widely corresponding to differences in geomorphology, nutrient criteria were established by the Minnesota Pollution Control Agency for specific ecoregions. To refine these relationships in Minnesota lakes managed for fishing, we gathered mean summer epilimnetic TP concentrations on 1330 natural lakes to identify where agricultural and urban development have elevated phosphorus levels. Random forest, regression tree, and generalized additive models were used to model spatial variation in lake phosphorus concentrations across Minnesota. Key landscape variables known to influence TP concentrations in lakes, including lake depth and watershed size, were used as explanatory variables in these models, along with agricultural and urban development quantified for lake watersheds from the National Land Cover Dataset. These models explained up to 60% of the variation in TP in lakes across the state and showed a critical benchmark of anthropogenic land use disturbance at 40%, that once exceeded could significantly alter TP levels and consequently fish populations. This information should be useful for fish managers to prioritize conservation efforts and to set appropriate fish population goals.

Total phosphorus changes in New York and New Jersey lakes (USA) inferred from sediment cores

We used sediment diatoms to infer historical changes in total phosphorus (TP) concentration in 26 New York and New Jersey (USA) lakes using a top–bottom approach (i.e., the top of the core represents present conditions and the bottom represents past conditions). Detailed stratigraphic analyses were performed on cores from Greenwood (New Jersey and New York) and Cossayuna lakes (New York). TP reconstructions were performed using 2 weighted-averaging partial-least-squares (WA-PLS) transfer functions: (1) an interregional 278-lake calibration set from northeastern United States (NE-US model: r2 boot = 0.69, root mean square error of prediction RMSEP) = 1.8 μg/L); and (2) a regional 33-lake (callibration set from New Jersey and New York (NJ-NY model: r2 boot = 0.54, RMSEP = 1.5 μg/L). The NJ-NY model provided better estimates for modern TP but failed to provide reliable estimates for low TP values and reliable modern analogs for half of the bottom samples. Low TP concentrations were better inferred by the NE-US model, which included a higher number of oligotrophic lakes. Average change for all lakes was an increase of 2 to 7 μg/L TP. Greenwood and Cossayuna lakes inferred TP concentrations have increased up to 21 μg/L, presumably as a result of post-settlement anthropogenic activities. The inferred TP temporal changes provide important insight on the magnitude of cultural eutrophication. The use of 2 different inference models demonstrates the advantage of using a regional versus a larger-scale inference model in estimating the degree of change in historical lake TP. Careful interpretation of TP reconstructions can be used to provide reliable estimates of cost-effective targets for lake restoration programs.

Multiyear destratification study of an urban, temperate climate, eutrophic lake

Hanson D, Austin D. 2012. Multiyear destratification study of an urban, temperate climate, eutrophic lake. Lake Reserv Manage. 28: 107-119. An aeration system for destratification has been in operation since 1973 at Sweeney Lake, an urban, eutrophic lake near Minneapolis, Minnesota, USA. Water quality data are available for 19 of the 39 years, 1972–2010. Growing season (Apr–Oct) aeration was turned off in 2007–2008 to evaluate the effect of destratification on water quality. Aeration never achieved the Lorenzen and Fast (LF) complete destratification airflow criterion of 9.2 m3/min/km2. When aeration was at 51% LF, hypoxia (DO < 2.0 mg/L) occurred June–August at the lake bottom. When airflow increased to 65% LF, bottom water hypoxia was restricted to 4 weeks in late July–August. There were no systematically significant differences (p > 0.05) in median Secchi disk depths, chlorophyll a concentrations, or surface total phosphorus (TP) concentrations between destratified and stratified lake conditions, but interannual variability linked to other influences, including partial stratification and stormwater inputs, limits statistical power. The median April–October lake TP mass of 67 kg in 2007–2008 (stratified conditions) was significantly higher (p < 0.0001) than the 44 kg median mass in1998–2005 and 2009 (destratified conditions). Destratification can suppress internal phosphorus loading when properly applied.

Multiscale landscape and wetland drivers of lake total phosphorus and water color

We quantified relationships between local wetland cover in the riparian lake buffer and lake total phosphorus (TP) and water color (color) using multilevel mixed‐effects models that also incorporate landscape features such as hydrogeomorphology and land use at broad regional scales to determine the following: (1) Within regions, are local wetland relationships with TP and color affected by interactions with local land use or hydrogeomorphic variables? (2) Across regions, are local wetland relationships with TP and color different? And if so, (3) Are differences in local wetland relationships with TP and color a result of cross‐scale interactions? We answered these questions by analyzing TP, color, and multiscaled landscape data for 1790 north temperate lakes. Local wetland‐TP and wetland‐color relationships were not affected by local‐scale interactions. However, these same relationships were different when compared across regions, and these differences were related to cross‐scale interactions with regional landscape characteristics. For example, regional human land use affected local wetland‐TP relationships such that in regions with high amounts of agriculture, local wetlands were associated with decreased lake TP. However, in regions with low amounts of agriculture, local wetlands were associated with increased lake TP. In contrast, regional hydrogeomorphic characteristics influenced local wetland‐color relationships such that in regions with high groundwater contribution, the strength of local wetland relationships were weak. Regional landscape setting influences local wetland relationships with TP and color through cross‐scale interactions, and lake TP and color are controlled by both local‐scale wetland extent and regional‐scale landscape variables.

A re-evaluation of the empirical relationships connecting dissolved oxygen and phosphorus loading after dreissenid mussel invasion in Lake Simcoe

Lake Simcoe has had many stressors in the past decades. High total phosphorus (TP) loads into the lake led to end-of-summer volume-weighted hypolimnetic dissolved oxygen (MVWHDO) levels that were lethal to coldwater fish species. Additionally, dreissenid mussels became established in the lake around 1996, and may have affected nutrient cycling and phytoplankton abundance. Using data averaged over two time periods following dreissenid mussel establishment, we re-evaluated empirical relationships connecting TP loading rate and MVWHDO concentration that were developed with pre-dreissenid data. Oxygen depletion rate predicted chlorophyll (Chla) well within the range of other Ontario lakes. Measured Chla corresponded well with Chla predicted from lake TP concentration except at the shallowest station, indicating that the lake may becoming more spatially heterogeneous. Interestingly though, the Chla:TP temporal trend increased post-dreissenids when it is often observed to decrease (e.g., Great Lakes). Measured lake TP concentration was better predicted by TP loads when averaged over the ice-free season compared with the spring. The final model predicted MVWHDO directly from TP load very closely except for the 2002–2006 time period when the end-of-summer date was September 15 rather than September 30, suggesting that the date when MVWHDO is determined should be continuously re-evaluated. Additionally, trend analyses on each variable from 1980 to 2008 did not identify any changes that coincided with dreissenid establishment. Given that this model appears to be sound for the post-dreissenid period, an estimated TP load of 44 tonnes/yr was predicted that would yield the current MVWHDO target level of 7 mg/L.

Environmental status of a tropical lake system

Eutrophication has become a serious threat to the lake systems all over the world. This is mainly due to the pollution caused by anthropogenic activities. Carlson trophic state index (CTSI) is commonly used for the classification of trophic conditions of surface waters. The study is conducted to assess the trophic status of a tropical lake (Akkulam-Veli lake, Kerala, India) using CTSI based on Secchi disc depth (SD), total phosphorus (TP) and chlorophyll-a. The TSI values based on SD and TP are high (>70), indicating the hypereutrophic state which needs urgent action for the restoration of the fragile ecosystem. The higher TP in both lakes, and the lower value of chlorophyll-a in the Akkulam part, warrant explanation, are discussed here. The influence of other biochemical parameters in both the Akkulam and the Veli part of the lake has been assessed. Correlation analysis is conducted to study the effect of various water quality parameters. The variation in the water quality before and after the opening of sand bar is studied using paired t test. As almost all the lakes in the world are experiencing similar situation of extinction, this study is helpful to have an insight in the hydrochemistry of the lake as well as to identify the worst affected areas of the lakes.