Trophic State Index Research Articles

Carbon, nitrogen, and phosphorus dynamics in China's lakes: climatic and geographic influences.

Given the differences in geomorphology, climate, hydrology, and human activities in various regions, lake chemometrics may also vary. However, the spatial distribution of lake chemistry and the factors affecting such pattern are still unclear. Here, we collected data for carbon, nitrogen, and phosphorus from published literature and databases in 224 lakes and calculated the trophic status index to represent the nutrient classification state of lakes. We found that lakes with high carbon concentrations were located in the Tibet-Qinghai Limnetic Region of western China, whereas lakes with high nitrogen and phosphorus concentrations were located in the Inner Mongolia-Xinjiang Limnetic Region and Northeast Limnetic Region of northern China. Areas with larger cropland and urban residential land (such as the junction of the three lake regions, i.e., the Northeast Limnetic Region, East Limnetic Region, and Inner Mongolia-Xinjiang Limnetic Region) tended to have lakes with high nitrogen and phosphorus concentrations. Our analysis suggested that spatial distribution of carbon, nitrogen, and phosphorus concentrations reflect the effect of climate, geomorphology, and land use in each lake region and nationwide.

Spatial and temporal variations of trophic state conditions of Lake Bunyonyi, south-western Uganda

The study was conducted to assess the spatial and temporal variations of the trophic state condition of Lake Bunyonyi between October 2019 and September 2020. Secchi depth (SD), total phosphorus (TP) and chlorophyll-a (Chl-a) concentration were measured to aid the quantification of trophic state condition of the lake using Carlson trophic state index (CTSI). The trophic state index (TSI) values based on SD, TP and Chl-a values were 60.82 ± 5.35, 68.99 ± 11.04 and 61.74 ± 7.51, respectively. March 2020 recorded the highest mean CSTI value (70.14 ± 4.04), while in September 2020, the lowest mean value (58.03 ± 6.22) was obtained. Based on CTSI values, the lake was eutrophic in both dry and wet seasons albeit with differences in the values. The eutrophic state of the lake is possibly attributed to nutrients inflow into the lake via runoff and drainage from farmlands. The negative value of TSI-Chl-a–TSI-TP deviation pointed out that the lake is not phosphorus limited, while the positive TSI-Chl-a–TSI-SD mean deviation value revealed the effect of Chl-a and TP on the lake’s transparency. The study recommends the necessity to develop guidelines for the trophic status monitoring of Lake Bunyonyi since results suggest the deteriorating lake condition.

Water quality assessment of Lake Burullus, Egypt, utilizing statistical and GIS modeling as environmental hydrology applications.

GIS is a very powerful tool for analyzing huge amount of data and connecting them with the geography; moreover, recently, there is great advancement in the field. The main objective of this study is to assess the water quality (WQ) and trophic status (TS) conditions of Lake Burullus, Egypt, using statistical modeling (PCA/FA and CA), WQ index (L-WQI), and trophic status index (Carlson TSI and TRIX) approaches, in addition to using GIS tools for building models able to automatically calculate the various indices and producing color coded maps for the lake. The results indicated that PCA/FA grouped the twenty-four WQ parameters into nine principal components explaining 72.6% of the total variance, domestic, and agriculture pollution were dominant. CA divided the twelve sampling stations into most and least polluted groups. The lake WQ was classified as a "Very Poor," according to L-WQI. Moreover, the results of the Carlson TSI and TRIX indices were coincided and classified the eutrophication levels in the lake as "Hyper-Eutrophic" and "Elevated Trophic," respectively. Based on the results of this study, Lake Burullus needs urgent plans for recovering its WQ. Pre-treatment for its drains' effluents and implementing of a periodical WQ monitoring program are highly recommended.

Trophic status analysis and estimation of fish production in Aneuk Laot Lake aquatic resources management, Sabang City

Aneuk Laot Lake is located in Sabang City, Weh Island, Aceh Province. Changes in water quality can affect the life of aquatic organisms found in Aneuk Laot Lake. The purpose of this research estimates fish production and trophic status for the aquatic resource management of Aneuk Laot Lake, Sabang. This research was conducted in September 2019 in Lake Aneuk Laot using the TRIX, TSI (Trophic State Index) and Nygaard index methods to determine fish production and trophic status. Observation stations were determined using stratified random sampling method and analysis of chlorophyll-a was done using the Trichomatic method. The results showed that the waters of Aneuk Laot Lake can be categorized as eutrophic to hypereutrophic. The estimated value of fish production in Aneuk Laot Lake based on the chlorophyll-a approach is 109.84 kg/ha/year, with a lake area of 61.6 ha. Water quality parameters are still in a state that can be tolerated by aquatic organisms in Aneuk Laot Lake.Keywords:Fish production potential Water qualityTRIX indexBiomass productionWater productivity

Using Multiple Indices for the Water Resource Management of a Monomictic Man-Made Dam in Southern Africa

This study employed different indices, namely the weighted arithmetic water quality index (WQI), Carlson Trophic State Index (TSI), van Ginkel TSI, and Trophic Level Index (TLI) to determine the water quality status of a man-made dam for the needs of sustainable water resource management in Southern Africa. The selection of indices for the study was based on the impacts of anthropogenic activities on the dam. The Roodeplaat Dam exhibited the spatial variation of physicochemical characteristics, indicative of influence by point-source pollution. Although the dam was classified as being eutro-hypertrophic, it was evident that water clarity was not a limiting factor but was P-limited, which was an indication of limiting conditions on primary production. Moreover, the WQI calculated for the dam with an average of 93.94 demonstrated very poor water quality that could be used for crop irrigation purposes only. As such, continued nutrient enrichment must be mitigated to sustain fitness for irrigation, at least. However, strategic goals should involve widening fitness for use. The selected indices were found to be effective for water resource management and could be applied to dams impacted by point-source pollution in Southern Africa. Thus, this study recommends the implementation of an integrated management approach, which needs to prioritize nutrient management to retain societal resource value.

Assessing temporal variability of lake turbidity and trophic state of European lakes using open data repositories

Water turbidity is one of the more important water quality parameters that is strictly linked with the productivity of the lake and is commonly used as an indicator of the trophic state. However, limited field data availability across wide geographic gradients may hinder the conduction of large scale longitudinal studies. In this study, time series of lake turbidity and trophic state index (TSI) between 2002 and 2012 were obtained from the Copernicus Lake Water products to create a large longitudinal dataset of lake variables for 22 European lakes. The dataset was combined with estimates of nutrient concentrations and surface water temperature obtained from the Hydrological Predictions for the Environment (HYPE) and ERA5-Land data repositories, that were used as environmental predictors. Hence, the validity of the lake water quality parameters was tested by a) exploring their spatial and temporal variability and b) identifying associations with the environmental predictors. For this purpose, seasonal Mann-Kendall tests were applied to find significant inter-annual trends of turbidity and TSI for each lake, and generalized additive models (GAMs) were employed to identify the main parameters that shape their temporal dynamics. Although we did not find significant inter-annual changes, our findings highlighted the strong influence of seasonality and surface water temperature in defining the temporal variability patterns in most of the lakes. In addition, the importance of nutrients varied among lakes as several lakes exhibited narrow nutrient gradients reflecting relatively stable nutrient conditions during the examined period. Other lake intrinsic factors, such as local climate and biotic interactions, are important drivers of shaping turbidity and nutrient dynamics. This study highlighted the usefulness of combining lake data from large repositories in conducting large scale spatial studies as a valuable asset for future lake research and management purposes.

Analysis of the eutrophication in a wetland using a data-driven model.

Eutrophication is a major problem in the international Anzali wetland (northern Iran). The present research initially aimed to determine the trophic state index (TSI) in ten sampling sites in the main parts of the Anzali wetland (western, eastern, central, and Siahkeshim parts). After determining the TSI in the wetland, a data-driven method (classification tree model with a J48 algorithm) was implemented to predict the trophic condition in the wetland based on a set of water quality and physical-structural variables. One hundred twenty samples related to chlorophyll-a (the model's output) and environmental variables (the model's inputs) were measured monthly during 1-year study period (2017-2018). Based on the TSI calculation, the western, Siahkeshim, eastern, and central parts of the wetland are classified as eutrophic, super-eutrophic, hyper-eutrophic, and hyper-eutrophic, respectively. When all environmental variables were introduced to the model (with five-time randomization effort, pruning confidence factor = 0.01, and seven-fold cross-validation), eight variables (bicarbonate, pH, water temperature, electric conductivity, dissolved oxygen, total phosphorus, water depth, and water turbidity) were predicted by the model. The model predicted that an increase in total phosphate, water turbidity, and electric conductivity concentration may contribute to the hyper-eutrophic state of the wetland. In contrast, the hyper-eutrophic of the wetland is associated with a decrease in water depth, dissolved oxygen, and pH concentration. According to ANOVA test, the trophic condition in the wetland can be affected by spatial and temporal patterns. Anthropogenic pressures such as the influx of chemicals particularly the nutrients (phosphorus and nitrogen) are the main cause of water enrichment (eutrophication problem) in main parts of the Anzali wetland ecosystem.

ASSESSING OF TROPHIC STATUS FOR SHATT AL-ARAB ESTUARY AND IRAQI MARINE WATERS

During the months of January to December 2019, the nutritional status of the waters of the Shattal-Arab estuary and Iraqi marine waters was investigated using the trophic state index (TSI) andan average assessment mesotrophic of the two above-mentioned environments using three factors:total phosphorous, water transparency and chlorophyll-a, were chosen three stations, The thirdstation had the highest value of the directory, 49.9 in February, and the third station had the lowestvalue, 17.8 in December. with transparency, was the Maximal value in April in the third stationreached 212 cm, as well as the minimum value, reached 13 cm in November my first station.

Spatiotemporal optical properties of dissolved organic matter in a sluice-controlled coastal plain river with both salinity and trophic gradients

Due to the combined effect of sluices and sea tide, the sluice-controlled coastal plain river would be characterized by both trophic state and salinity gradients, affecting the spatiotemporal optical properties of dissolved organic matter (DOM). In this study, we investigated the spatiotemporal variation of water quality parameters and optical properties of DOM in the Haihe River, a representative sluice-controlled coastal plain river in Tianjin, China. A significant salinity gradient and four trophic states were observed in the water body of the Haihe River. Two humic- and one protein-like substances were identified from the DOM by the three-dimensional fluorescence spectra combined with the parallel factor (PARAFAC) analysis. Pearson's correlation analysis and redundancy analysis (RDA) showed that the salinity significantly affected the abundance of chromophoric DOM (CDOM) but did not cause significant changes in the fluorescence optical characteristics. In addition, the effect of Trophic state index (TSI) on the CDOM abundance was greater than that on the fluorescence intensity of fluorescent dissolved organic matter (FDOM). In the water body with both salinity and trophic state gradients, TSI posed a greater influence than salinity on the CDOM abundance. Our results fill the research gap in spatiotemporal DOM characteristics and water quality variation in water bodies with both salinity and trophic state gradients. These results are beneficial for clarifying the joint influence of saline intrusion and sluices on the DOM characteristics and water quality in sluice-controlled coastal plain rivers.

Response of eutrophication and water quality drivers on greenhouse gas emissions in lakes of China: A critical analysis

AbstractLakes, especially shallow lakes, contribute disproportionately to greenhouse gas (GHG; particularly CO2 and CH4) emissions and have received global attention due to their high potential to contribute to global warming and future climate change. Recent studies have identified eutrophication as a critical factor in GHG emissions. However, the role of lake trophic state index (TSI) and the impact of important water quality parameters (WQP) such as pH, Chl‐a, total nitrogen, total phosphorus and organic carbon on GHG emissions are still a subject of debate and an area of intense research. To further understand the relationship between GHG and lake eutrophication, datasets (GHG and WQP) from the scientific literature have been compiled, and statistical analyses of these secondary data were performed to determine the influence of eutrophication on GHG emissions. In this review, GHG emissions from Chinese lakes are quantified, and the important factors affecting these emissions are analysed systematically. The statistical analysis reveals that chlorophyll a and carbon (as TOC) are the key factors of lake eutrophication and have a significant effect on the GHG potential (mainly CH4 fluxes). In addition, the proposed mitigation measures could serve as a guide for scientists and young researchers to reduce future climatic risks.

The response of benthic diatom community to anthropogenic eutrophication of a river basin under agricultural influence in NE China

Anthropogenic eutrophication has universally threatened river ecological health and has been a key issue in river conservation. Agricultural sewage, which leads to increased nutrient levels and results in the loss of ecological function and biodiversity, results from the growth of agricultural activity and human populations. Consequently, attention has increasingly been focused on developing a relevant tool to describe the relationship between the biotic communities and anthropogenic eutrophication. The benthic diatom community has been widely used as an effective tool for indicating the ecological status of river systems especially in the context of recent international Water Framework Directive policies (WFD). In this study, a statistical method consisting of trophic status index (TSI), Specific Polluosensitivity Index (IPS), Descy Index (DES), redundancy analysis (RDA), and the geographic information system (GIS) technique was conducted to explore the response of benthic diatom community to anthropogenic eutrophication. Initially, the TSI, IPS and DES were determined to differentiate the trophic gradient along a gradient in agricultural intensity. Subsequently, RDA was used to identify spatial distribution patterns of environmental parameters and benthic diatom communities. Finally, the RDA scores as calculated and spatially interpolated were applied to GIS technology to reveal the response of benthic diatom communities to anthropogenic impacts along the trophic gradient. Our results revealed that the studied basin mostly exceeded the eutrophic level. Two different patterns of diatom communities response to ecological gradient were identified based on RDA, one representing the agriculture eutrophication discharges in electrical conductivity (EC) and the other representing organic pollution as chemical oxygen demand (COD). The developed method in this study highlight that the EC was more effective than the nutrient indices in determining diatom distribution in a eutrophic agriculturally-influenced river system.

Linking river flow modification with wetland hydrological instability, habitat condition, and ecological responses.

Flow modification pursuing dams is widely found. Some works also focused on its impact on floodplain wetland hydrology. However, how this change can pose an impact on habitat conditions, ecological conditions, and trophic state is also a matter of investigation. The very least attention has been paid to this so far. Therefore, the present study focused on these, taking the dam-induced Lower Tangon river basin of India and Bangladesh as a case. The degree of flow alteration in the river was presented in a heat map. Multi-parametric machine learning (ML) approaches were applied to model hydrological instability and habitat condition. The ecological consequences like evaluating eco-deficit using flow duration curve (FDC) approach, trophic state using trophic state index (TSI), fish habitat zone using image-based hydrological parameters, etc. were measured. The study exhibited that after damming, the degree of river flow modification was about 41%. Consequently, the wetland hydrological instability and habitat conditions were degraded. In the post-dam period, > 50% of wetland area was lost, and hydrological instability was enhanced considerably over wider parts of the wetland. Habitat conditions of the existing wetland also witnessed fragility (poor and very poor areas increased by about 22.23 and 9.34%). As a result of this, adverse ecological responses were found. For instance, the eco-deficit area was increased by 36.19%, a good proportion (100%) of wetlands was witnessed the transformation of TSI from oligotrophic to mesotrophic state, and optimum fish habitat area was declined. The ecological strength map integrating all the cause-effect model parameters showed that good ecological strength was reduced from 49 to 2% in the post-dam. The result of the study would be very useful for wetland restoration for ecological and human well-being.

Micron-Scale Biogeography of Seawater Biofilm Colonies at Submersed Solid Substrata Affected by Organic Matter and Microbiome Transformation in the Baltic Sea.

The aim of this research was to determine temporal and spatial evolution of biofilm architecture formed at model solid substrata submersed in Baltic sea coastal waters in relation to organic matter transformation along a one-year period. Several materials (metals, glass, plastics) were deployed for a certain time, and the collected biofilm-covered samples were studied with a confocal microscopy technique using the advanced programs of image analysis. The geometric and structural biofilm characteristics: biovolume, coverage fraction, mean thickness, spatial heterogeneity, roughness, aggregation coefficient, etc., turned out to evolve in relation to organic matter transformation trends, trophic water status, microbiome evolution, and biofilm micro-colony transition from the heterotrophic community (mostly bacteria) to autotrophic (diatom-dominated) systems. The biofilm morphology parameters allowed the substratum roughness, surface wettability, chromatic organisms colony adaptation to substrata, and quorum sensing or cell to cell signaling effects to be quantitatively evaluated. In addition to the previous work, the structural biofilm parameters could become further novel trophic state indicators.

Composition of dissolved organic matter (DOM) in lakes responds to the trophic state and phytoplankton community succession

Dissolved organic matter (DOM), a heterogeneous mixture of diverse compounds with different molecular weights, is crucial for the lake carbon cycle. The properties and concentration of DOM in lakes are closely related to anthropogenic activities, terrigenous input, and phytoplankton growth. Thus, the lake's trophic state, along with the above factors, has an important effect on DOM. We determined the DOM sources and molecular composition in six lakes along a trophic gradient during and after phytoplankton bloom by combining optical techniques and the Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). CDOM pools in eutrophic lakes may be more biologically refractory than in oligotrophic and mesotrophic lakes. Molecular formulas of DOM were positively correlated with the TSI (trophic state index) value (R2 = 0.73), with the nitrogen-containing compounds (CHON) being the most abundant formulas in all studied lakes. Eutrophication modified the molecular formulas of DOM to have less CHO% and more heteroatom S-containing compounds (CHOS% and CHNOS%), and this was the synactic result of the anthropogenic perturbation and phytoplankton proliferation. In eutrophic lakes, summer DOM showed higher molecular lability than in autumn, which was related to the seasonal phytoplankton community succession. Although the phytoplankton-derived DOM is highly bioavailable, we detected a simpler and more fragile phytoplankton community ecosystem in autumn, which may be accompanied by a lower phytoplankton production and metabolic activity. Therefore, we concluded that the lake eutrophication increased the allochthonous DOM accumulation along with sewage and nutrient input, and subsequently increased its release with phytoplankton bloom. Eutrophication and phytoplankton growth are accompanied by more highly unsaturated compounds, O3S+O5S compounds, and carboxylic-rich alicyclic compounds (CRAMs), which are the biotransformation product of phytoplankton-derived DOM. Eutrophication may be a potential source of refractory DOM compounds for biodegradation and photodegradation. Our results can clarify the potential role of water organic matter in the future global carbon cycle processes, considering the increasing worldwide eutrophication of inland waters.

Assessment of eutrophication through ecological indicators at the entrance of a tropical urbanized estuary

Dissolved nutrients, phytoplankton biomass and other ancillary variables, were obtained at two sub-embayments at the entrance of Guanabara Bay (Rio de Janeiro, Brazil) and a composite trophic status index (TRIX) was used to assess the water quality of the study area. The role of bottom sediments in nutrient dynamics was also investigated through the evaluation nutrients, phytoplankton biomass and other sedimentary variables, since this compartment acts as a geological record of anthropogenic input. Jurujuba Sound, at the east margin, and Flamengo-Botafogo Sounds, at the western margin, were sampled during neap and spring tides in the dry season. Signs of eutrophication were detected through the extreme variations of dissolved oxygen concentrations at both margins, being more accentuated at Jurujuba Sound (2.20-14.07 mg.l-1). Dissolved inorganic nitrogen was elevated at both margins and mostly composed of ammonium, surpassing 87% at Flamengo Botafogo Sounds, which suggests a continued input of raw sewage at the western margin of the bay. TRIX revealed poor water quality for most stations at the study area, varying from 4.53 to 7.29 at Jurujuba Sound and from 5.67 to 7.87 at Flamengo-Botafogo Sounds. The increase of TRIX from neap to spring tide was registered at both margins, revealing decrease of water quality. The differences in grain size between both margins played a key role in nutrient dynamics, with predominance of fine sediments at Jurujuba Sound and coarser particles at the opposite margin. Accumulation of high concentrations of TOC (0.87-6.57%) and inorganic phosphorus (154.34-1516.82 µg.g-1) were favored by the predominance of fine sediments at Jurujuba Sound. The assessment of eutrophication in water column and bottom sediments revealed the maintenance of this process at the entrance of Guanabara Bay sustained by the recurrent anthropogenic input, what demands urgent action from public policies to mitigate this situation.

Seasonal Variations in Water Quality and Algal Blooming in Hypereutrophic Lake Qilu of Southwestern China

Understanding the spatiotemporal distributions and variation characteristics of water quality parameters is crucial for ecosystem restoration and management of lakes, in particular, Lake Qilu (QL), a typical plateau shallow lake on the Yunnan-Guizhou Plateau, southwestern China. To identify the main causes of harmful algal blooming and continuous water quality decline, the total phosphorus (TP), total nitrogen (TN), water temperature (WT), dissolved oxygen (DO), chlorophyll-a (Chl-a), pH, and turbidity in hypereutrophic Lake Qilu from January 2017 to December 2021 were analyzed. The results showed a complex pattern in spatiotemporal distribution and variation. WT showed no significant change in the vertical profile. DO and pH value variations were caused by both physical and biochemical processes, especially at the bottom of Lake QL with an anaerobic environment. The Trophic State Index (TSI) assessment results showed that Lake QL is a eutrophic (70.14% of all samples, 50 < TSI < 70) to a hypereutrophic lake (29.86%, 70 < TSI) with poor water quality (WQI < 25). TP and WT were the main factors controlling harmful algal blooms (HABs) based on the statistical analysis of Principal Component Analysis (PCA), Random Forest Model (RFM), and Correlation Analysis (CA). In lake QL, TP loading reduction and water level increase might be the key strategies for treating HABs in the future. Based on our results, reducing TP loading may be more effective than reducing TN to prevent HABs in the highly eutrophicated Lake Qilu.

Bacterial community composition and indicators of water quality in Caizi Lake, a typical Yangtze-connected freshwater lake.

Caizi Lake is an important lake connected to the Yangtze River in Anhui Province and a crucial connection for the Yangtze-to-Huaihe Water Diversion Project. There were marked differences in trophic status of the six sampling sites based on the physicochemical characterization. The Bacterial Eutrophic Index (BEI), used to quantify water quality, was well related to Carlson's trophic state index (TSI) (Spearman's ρ = 0.829, P<0.05). Mean TSI and BEI were 54 and 0.58, respectively, indicating that Caizi Lake was slightly eutrophic. Actinobacteriota were the predominant bacterial phylum in the water and Acidobacteriota in sediments. The diversity and composition of the bacterial community was markedly different between sites for water but not sediment samples. Unlike other Yangtze-connected freshwater lakes, the distance-based redundancy analysis revealed that dissolved oxygen affected the composition of the planktonic bacterial community (P<0.001), while total phosphorus was the major factor in the sediments (P<0.05). The water quality of Caizi Lake has significantly improved since a few years ago. These results contribute to the long-term monitoring of the ecological quality of the water environment along the Yangtze River to the Huaihe River.

Regime shifts, trends, and variability of lake productivity at a global scale

Lakes are often described as sentinels of global change. Phenomena like lake eutrophication, algal blooms, or reorganization in community composition belong to the most studied ecosystem regime shifts. However, although regime shifts have been well documented in several lakes, a global assessment of the prevalence of regime shifts is still missing, and, more in general, of the factors altering stability in lake status, is missing. Here, we provide a first global assessment of regime shifts and stability in the productivity of 1,015 lakes worldwide using trophic state index (TSI) time series derived from satellite imagery. We find that 12.8% of the lakes studied show regime shifts whose signatures are compatible with tipping points, while the number of detected regime shifts from low to high TSI has increased over time. Although our results suggest an overall stable picture for global lake dynamics, the limited instability signatures do not mean that lakes are insensitive to global change. Modeling the interaction between lake climatic, geophysical, and socioeconomic features and their stability properties, we find that the probability of a lake experiencing a tipping point increases with human population density in its catchment, while it decreases as the gross domestic product of that population increases. Our results show how quantifying lake productivity dynamics at a global scale highlights socioeconomic inequalities in conserving natural environments.

Clustering of bivariate satellite time series: A quantile approach

AbstractClustering has received much attention in statistics and machine learning with the aim of developing statistical models and autonomous algorithms which are capable of acquiring information from raw data in order to perform exploratory analysis. Several techniques have been developed to cluster sampled univariate vectors only considering the average value over the whole period and as such they have not been able to explore fully the underlying distribution as well as other features of the data, especially in presence of structured time series. We propose a model‐based clustering technique that is based on quantile regression permitting us to cluster bivariate time series at different quantile levels. We model the within cluster density using asymmetric Laplace distribution allowing us to take into account asymmetry in the distribution of the data. We evaluate the performance of the proposed technique through a simulation study. The method is then applied to cluster time series observed from Glob‐color satellite data related to trophic status indices with aim of evaluating their temporal dynamics in order to identify homogeneous areas, in terms of trophic status, in the Gulf of Gabes.

Application of trophic state indicators and nutrient ratios for the characterization of trophic state and primary productivity of the Volta Lake, Ghana

Application of trophic state indicators and nutrient ratios for the characterization of trophic state and primary productivity of the Volta Lake, Ghana