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INTRODUCTION Currently, the pollution from agricultural activities and sources of domestic waste in the Lam Vien highland has caused eutrophication to some water bodies, altering structure and function of aquatic ecosystems, in that phytoplankton communities are affected directly and indirectly The common phenomenon is the excessive growth of phytoplankton groups, usually cyanobacteria, damaging to other organisms in the water bodies To control this situation, we need identifying sources of impacts on basins and aquatic organisms In basins, phytoplankton is an important link in food web and is also the subject that affected by environmental factors Phytoplankton groups have different reactions when environmental conditions impact on them, through changes in composition, distribution, and growth Therefore, integrated analysis of phytoplankton responses to environmental factors can elucidate furthermore the effects of environmental conditions on phytoplankton From there, identifying which environmental factors that impact on the entire of aquatic ecosystem Worldwide, the study factors that impact on phytoplankton was done quite soon and mostly in temperate areas These researches are less and later in the tropics There are a lot of lakes and reservoirs in Vietnam but are not yet much researches about them In particular, the studies on relationship between environmental factors and phytoplankton as well as between phytoplankton and other organismal groups have not been mentioned adequately Most researches are focusing on taxonomy Until now, the researches on the impact of environmental factors on phytoplankton community structure in Vietnam as well as in Lam Vien highland based on year database have not been implemented Therefore, the study "Structure of phytoplankton communities in reservoirs at Lam Vien highland, Lam Dong Province" in addition to contribution on taxonomy for the flora of freshwater microalgae in Tay Nguyen area of Vietnam, it also help understanding clearly the responses of phytoplankton to environmental conditions as well as identifying dominant environmental factors that impact on aquatic ecosystems At the practical level, identifying which main impact on aquatic organisms, including phytoplankton community, will be the basis of science, support to management, protection of biodiversity of water resources in Lam Vien highland  Aims of study - Identifying characteristics of phytoplankton community structure in reservoirs at the Lam Vien highland - Determineing factors that impact on community structure of phytoplankton in the reservoirs  Thesis contents - Species composition, density and distribution of phytoplankton in Xuan Huong reservoir, Tuyen Lam reservoir and Dan Kia reservoir - The status of the waters of Xuan Huong reservoir, Tuyen Lam reservoir and Dan Kia reservoir - The relationship between phytoplankton and environmental characteristics of each reservoir - The impact of nutrition and grazing on growth of phytoplankton - Simulation and forecasting trends of change in ecosystem of reservoirs by modeling  Scientific contributions and applied aspects - Complementing database for tropical phytoplankton flora in general, tropical highland in particular Providing some information for phytoplankton researches and applications - Determining impacts of environmental factors on water bodies as well as on phytoplankton, the basis for building management solutions, effective use and proper exploitation of local water sources - Contributing to find causes of excessive growth of phytoplankton in lakes and resaervoirs, the basis for restricting outbreak of phytoplankton, especially cyanobacteria blooms in reservoirs at Lam Vien highland CHAPTER I OVERVIEW 1.1 The characteristics of tropical lakes and reservoirs 1.1.1 Overview 1.1.2 The temperature, stratification and mixing in water column 1.1.3 Radiation and clarity 1.1.4 Nutrition and solutes 1.1.5 Food webs, top-down and bottom-up control in lakes and reservoirs 1.2 Morphological characteristics and classification of freshwater phytoplankton 1.2.1 Morphological characteristics and classification of freshwater phytoplankton 1.2.2 Groups of freshwater phytoplankton 1.3 Ecology of phytoplankton 1.3.1 Light and photosynthesis of phytoplankton 1.3.2 Influence of temperature, stratification and mixing on phytoplankton 1.3.3 Metabolism and nutritional absorption of phytoplankton 1.3.4 Survival strategies of phytoplankton 1.4 Variation in phytoplankton community over time 1.4.1 The short-term changes (changes in molecules and cells) 1.4.2 Changes in medium term (phytoplankton community succession) 1.4.3 The long-term fluctuations (fluctuates by year) 1.5 Spatial distribution of phytoplankton 1.5.1 Voluntary movement of phytoplankton in water column 1.5.2 Passive movement of phytoplankton in water column 1.5.3 Loss phytoplankton in basins 1.6 AQUATOX model 1.6.1 AQUATOX model overview 1.6.2 Application of AQUATOX model 1.7 Some characteristics of studied area 1.7.1 Natural conditions of Lam Vien highland 1.7.2 Characteristics of reservoirs in Lam Vien highland 1.8 Situation of phytoplankton study in the world and Vietnam CHAPTER MATERIALS AND METHODS 2.1 Studied subjects Phytoplankton and characteristics of water in reservoirs, Xuan Huong, Tuyen Lam and Dan Kia 2.2 Materials and methods 2.2.1 At the field At each reservoir, sampling at stations At each station, sampling and measuring water parameters at layers Frequency of survey is once a month, from 11/2013 to 10/2014 Flow rate, water temperature, depth of reservoir, Secchi depth, pH, DO, turbidity, light intensity were measured by handheld electronic devices Collecting water samples for analyzing chemical and biological parameters 2.2.2 Experiments 2.2.2.1 Fixing cells, qualitative and quantitative phytoplankton Samples were fixed with 1% Lugol solution and formaldehyde solution of acetic acid (FAA) 2% Phytoplankton identification was based on classification keys of freshwater phytoplankton Quantitative forms, one liter of water was fixed by Lugol 1% and 2% FAA, within about 48 hours, siphon off above portion of water to remain 100 ml of water Let standing within 24 hours and then siphon off 20 ml remain water Taking ml of this one for counting by using Sedgewich – Rafter chamber 2.2.2.2 Fixed cells, qualitative and quantitative zooplankton Samples of quantitative zooplankton were fixed by a solution to final formadehyde concentration is 4% Analysizing zooplankton at Marine Plankton Department, Institute of Oceanography Zooplankton taxa were photographed, the quantitative zooplankton results are managed on MS-Excel 2.2.2.3 Analysis of chemical parameters Nutrition parameters are defined according to APHA (1995, 2005) Determination of chlorophyll a by UV-Vis 1020 – H, according to APHA (1995) Fecal coliform is determined by culturing multiple tubes method, most probable number, 9221 APHA (1999) Pesticide residues were analyzed at Environmental Research Institute, Dalat University 2.2.2.4 Experiments to determine plankton biological rates Bottom-up experiments arranged by Severiano (2012) Top-down experiments arranged by Landry & Hasset (1982) and Evans et al., (2003) 2.2.2.5 Calculation of phytoplankton biomass Phytoplankton biomass is calculated according to Wetzel et al., (2001) 2.2.2.5 Running AQUATOX model AQUATOX model was applied to assess impact of flows into Dan Kia reservoir 2.2.2.6 Application of statistical softwares and models Analyzing significant differences between two properties by analysising variances (ANOVA) base on MS-excel Data normalization, correlation analysis, regression by Statgraphic 5.0 statistical software Canonical-correlation analysis (CCA) between phytoplankton and environmental factors base on CANOCO 4.5 software Correlation analysis - RDA (Redundancy Analysis) for estimating biomass variability of morphological groups of phytoplankton and environmental factors was performed base on CANOCO 4.5 software Analysis Shannon index (H'), Simpson dominance index (D) with Primer 6.0 software CHAPTER RESULTS AND DISCUSSION 3.1 Species composition, density and distribution of phytoplankton in the reservoirs 3.1.1 Species composition of phytoplankton in Xuan Huong, Tuyen Lam and Dan Kia 3.1.1.1 Species composition of phytoplankton in Xuan Huong reservoir Total taxa of phytoplankton in Xuan Huong reservoir are 112, belonging to phyla, including Chlorophyta (60 taxa, 53%), Cyanophyta (18 taxa, 16%), Euglenophyta (16 taxa, 14%), Bacillariophyta (8 taxa accounting for 7%) Dinophyta (4 taxa, accounting for 4%) Both Cryptophyta and Chrysophyta has taxa, accounting for 3% of total taxa of phytoplankton (Figure 3.1) Species composition of phytoplankton in Xuan Huong reservoir is a typical one of phytoplankton in lentic waters with dominant species belong to green algae, and also has characteristics of eutrophic water-bodies with advantage of Cyanophyta density Cyanobacteria density accounting for 80% of phytoplankton in the reservoir throughout the year (Figure 3.1B) Of the seven phyla in Xuan Huong reservoir, Chrysophyta density is the lowest Although Euglenophyta not contribute significantly in density, they contributed significantly in biomass due to their cell size Of three reservoirs, Xuan Huong had the highest number of taxa, as well as the density phytoplankton concentrated in a few taxa Xuan Huong reservoir receives water from upstream sides So, phytoplankton species composition perhaps related to addition of these taxa from its basins Overall, the number of phytoplankton species in Xuan Huong reservoir is high but there are many species in low frequency, this maybe related to additional species temporarily from basins The density and biomass of phytoplankton are concentrated in some taxa of Cyanophyta phylum, the group dominant throughout the year 3.1.1.2 Species composition of phytoplankton in Tuyen Lam reservoir There are phyla in Tuyen Lam reservoir, with 43 taxa Among them, Chlorophyta contained 25 taxa, accounting for 58%; Bacillariophyta and Cyanophyta contained taxa, accounting for 14%; Dinophyta and Chrysophyta contained taxa, accounting for 7% and taxa, accounting for 5% respectively Euglenophyta phylum only has taxon, representing 2% (Figure 3.2) Phytoplankton in Tuyen Lam reservoir also featured lentic waters with advantage belonging to Chlorophyta phylum (Figure 3.2A) According to density, Cyanophyta has the highest density (Figure 3.2b) but base on biomass, Dinophyta was the most dominant group (Figure 3.2C) Some Chlorophyta genera are common, they are Desmids group (Desmidium, Coelastrum, Elakatothrix, Pleurotaenium, ), these genera usually present in less dirty waters (Reynolds, 2006) There are two dominant phyla in Tuyen Lam reservoir, Cyanophyta and Dinophyta However, while filamentous cyanobacteria was dominant in Xuan Huong reservoir, colony cyanobacteria was dominant in Tuyen Lam reservoir Two Dinophyta dominant genera are Ceratium and Peridinium 3.1.1.3 Phytoplankton species composition of Dan Kia reservoir Dan Kia reservoir has 44 taxa, distributed in phyla, including Chlorophyta, 17 taxa, accounting for 39%; Bacillariophyta, 11 taxa, accounting for 25%; Cyanophyta, taxa, accounting for 14%; Chrysophyta, taxa, accounting for 11%; Dinophyta, taxa, accounting for 4.5%; Euglenophyta also taxa, 4.5% and Cryptophyta taxon, representing 2% of total number of phytoplankton species in Dan Kia reservoir Though number of Chlorophyta taxa was lower than the others, this phylum was still dominant in number of species (Figure 3.3a) Bacillariophyta was dominant in Dan Kia reservoir both in species composition, density and biomass Cryptophyta accounting for only 2% of total taxa but they contributed not less in density and biomass of phytoplankton (Figure 3.3a, 3.3b and 3.3C) In short, species composition of phytoplankton was not the same in three reservoirs Th number of phytoplankton species was the highest in Ho Xuan Huong, 112 taxa Two remaining reservoirs had similar numbers, 43 and 44 taxa respectively for Tuyen Lam and Dan Kia Especially, comparing with similar reservoirs in the region (Le Thuong, 2010) showed that while Chrysophyta and Cryptophyta were found in studied reservoirs but they are completely absent in Eanhai reservoir, Easoup reservoir and Dak Ming reservoir Maybe, these groups distribute typically in some high mountains, low heat background around year Percentage of phyla in studied reservoirs is similar to reservoirs in the region, especially there is abundance and diversity of Chlorophyta in all 3.1.1.4 Biological diversity of phytoplankton in the reservoirs No seasonal differences in species diversity index of phytoplankton in Xuan Huong reservoir (T-test, p = 0.106) and Dan Kia reservoir (T-test, p = 0.285), but this difference occurred in Tuyen Lam reservoir (T-test, p = 0.016) Species diversity index of phytoplankton was highest in December in Tuyen Lam reservoir (2.24) and the lowest in October in Xuan Huong reservoir (0.51) H’ index of Tuyen Lam reservoir, Dan Kia reservoir was higher of Xuan Huong reservoir (T-test, p = 0.048; p = 0.004) Meanwhile, there was no difference between Dan Kia reservoir and Tuyen Lam reservoir (T-test, p = 0.382) on this index Phytoplankton species diversity of Xuan Huong reservoir was the lowest among them 3.1.1.5 Characteristics of phytoplankton community structure base on morphological - function groups The biomass dominant spiecies of phytoplankton in Xuan Huong reservoir, Tuyen Lam reservoir and Dan Kia reservoir were sorted by morphological - function groups of Reynolds et al., 2002, Salmaso & Padisak, 2007 and Kruk et al., 2010 Table 3.6, morphological - function groups presence in the studied reservoirs base on three systems Table 3.6 Morphological - function groups presence in the studied reservoirs According to Reynolds et al., (2002), D and Y group were in Xuan Huong reservoir and Dan Kia reservoir but completely absent in Tuyen Lam reservoir D and Y are the groups that characterized distribution in the shallow, turbid reservoirs, and susceptible to change or are affected by outside activities In this case, environmental and biological data in Dan Kia, Xuan Huong are consistent with above statements Among groups according to Reynolds et al., (2002), only LM present in three reservoirs, this group is known commonly present in high level of nutrition Thus, LM is not a good indicator for nutritional status but it is an evidence that functional morphological groups reflects nature of ecosystem The groups only present in Xuan Huong reservoir is H1 (typically in eutrophic, shallow, non-stratified reservoir), W1 (rich organic) and W2 (shallow, medium - eutrophic nutrient water bodies) all fit physic, chemical conditions that has been investigated for the reservoirs Similarly, N group (distributed at mixing reservoirs, 2-3 m thickness), was present only in Tuyen Lam reservoir WS group (in rich organic water bodies from decomposition process of plant; neutral pH), was present only in Dan Kia reservoir Figure 3.5 RDA chart of environmental factors and functional - morphological groups according to Reynolds et al., (2002) Trans = Transparency Secchi, L = light intensity, T = temperature, Cond = conductivity, DO = dissolved oxygen, TP = total phosphorus, phosphate PO4 =; TN = total nitrogen, NH4 = ammonium nitrate NO3 = Overall, the survey results on hydraulic, physic, chemical conditions in studied reservoirs fitted with ecological characteristics that morphological - function groups indicated However, there were some groups that featured in each water body where they present no resemblance to the survey results For example, A group (at Xuan Huong and Dan Kia) distribute in clean, deep, poor nutrient water bodies (Reynolds et al., 2002) while these reservoirs are not clean, shallow and eutrophic So, we can not completely rely on this system for monitoring quality, which should be combined with multivariate analysis between morphological-function groups and environmental factors Multivariate analysis techniques are often applied is RDA (Legendre, 1998) RDA analysis results between environmental factors and functional-morphological groups according to Reynolds et al., (2002) at Xuan Huong reservoir (Figure 3.5A) shows that in the first axis, morphological-function groups (including A group) correlate mainly with light intensity, concentration of nitrate and TN Meanwhile, the second axis correlates mainly with TP RDA chart (Figure 3.5C) also shows that A group correlate with nitrogen and phosphorus Thus, the presence of A group maybe related to nutritional status of water bodies Overall, three functional - morphological systems can be applied to evaluate characteristics of aquatic ecosystems 3.1.2 Variation of phytoplankton density in Xuan Huong, Tuyen Lam and Dan Kia 3.1.2.1 Variation of phytoplankton density in Xuan Huong reservoir Except Cyanophyta and Chlorophyta, all remaining phyla in Xuan Huong reservoir did not differ according to layer (Table 3.8) Table 3.8 Density of phytoplankton in Xuan Huong reservoir The dominance of Cyanophyta density is one of the characteristics of phytoplankton in Xuan Huong reservoir At time of algae blooms, density of Cyanophyta at the surface increased up to hundreds of millions of cells/liter The studied results showed that shallow, eutrophic, turbidity and no stratification reservoir could be characteristics enabling algae blooms in Xuan Huong reservoir These conditions are close to ecological characteristics for excessive development of Cyanophyta (Reynolds, 2006) Density of phytoplankton groups in Xuan Huong reservoir varied according to season (Table 3.8) Density of most groups did not differ according to layer, except Cyanophyta (ANOVA, p = 0.039) and Chlorophyta (ANOVA, p = 0.001) 3.1.2.2 Variation of phytoplankton density in Tuyen Lam reservoir Among phyla of phytoplankton in Tuyen Lam reservoir, Cyanophyta and Chrysophyta did not differ according to layer (Table 3.9) On other hand, only Cyanophyta (ANOVA, p = 0.022) and Chlorophyta (ANOVA, p = 0.046) are different according to season at the surface layer and cell density higher than in dry season Table 3.9 Density of phytoplankton Tuyen Lam reservoir Density of Chlorophyta, Bacillariophyta, Dinophyta fluctuated according to layer, at the surface layer was higher than at the bottom layer Density of diatoms in bottom layer varied according to season (ANOVA, p = 0027), in dry season is higher in rainy season Phytoplankton density of Tuyen Lam reservoir was much lower than in Xuan Huong reservoir Cyanophyta is also dominant group in this one However, while filamentous cyanobacteria is dominant in Xuan Huong reservoir, colony cyanobacteria is dominant in Tuyen Lam reservoir Perimidium and Ceratium are dominant genera in density and biomass, due to size of their cells These genera is typical for organisms that has K strategy, advantage in environments that density of higher organisms and concentration of nutrients is low relatively (Sigee, 2004) 3.1.2.3 Variation of phytoplankton density in Dan Kia reservoir Density of Bacillariophyta and Dinophyta are different according to season and layer, at the surface layer is higher than at the bottom layer In particular, density of 10 Bacillariophyta was higher in dry season than in rainy season Density of algal cells in Dan Kia reservoir was very low Most of algal density was higher in rainy season than in dry season, cell density in the surface layer was higher than in the bottom layer (Table 3.10) Density and biomass of Chrysophyta was quite high in Dan Kia reservoir Chrysophyta are known as adapting to changes of nutrient concentrations in water Especially, Dinobryon is a mixotrophic genus (Kristiansen, 2005), dominant throughout year While concentration of inorganic nutrients (N, P) in Dan Kia reservoir unlimited growth of phytoplankton, sources of remaining energy, such as light, is very noticeable Dan Kia reservoir is also very high turbidity, this is the factor limiting penetration of light into water In this case, the mixotrophic lifestyle will advance over nursing Diatom is dominant in Dan Kia reservoir both density and biomass Some diatom species are suitable for high turbidity conditions and rich nutrient (Bellinger & Sigee, 2010) Obviously conditions, environmental water of Dan Kia reservoir is quite suitable for the growth of diatoms Table 3.10 Density of phytoplankton in Dan Kia reservoir In short, most of phytoplankton groups in the reservoirs have seasonal fluctuations While filamentous cyanobacteria was dominant in Xuan Huong reservoir, colony cyanobacteria was dominant in Tuyen Lam reservoir Besides, in Tuyen Lam reservoir also appeared more a dominant one, that is Dinophyta While, Diatoms and Chrysophyta are dominant in Dan Kia reservoir 3.2 Assessment water status of Xuan Huong, Tuyen Lam and Dan Kia reservoir 3.2.1 Water status of Xuan Huong reservoir Hydraulic, physical, chemical and biological parameters of water in Xuan Huong reservoir were surveyed from 11/2013 to 10/2014, are shown in Table 3.11 11 Table 3.11 Hydraulic, physical, chemical and biological parameters of water in Xuan Huong reservoir Hydraulic, physical, chemical and biological parameters Min Max Average ±SD 1.0 4.9 2.86±1.46 Significant differences p≤0.05 Seas Layer on * 0.616 0.25 0.6 0.45±0.11 0.001 * 1466 31033 8147±7898 0.006 * 19.69±2.60 19.65±2.58 15.57 15.53 22.13 22.1 18.86±2.32 18.16±2.27 0.321 0.075 0.465 9.53 9.50 8.97±0.45 8.91±0.48 6.28 6.30 8.80 9.57 7.94±0.83 7.83±0.96 0.001 0.001 0.675 4.60 4.77 6.33 6.24 5.36±0.56 5.37±0.44 3.89 4.12 6.54 6.86 5.07±0.65 5.18±0.72 0.188 0.389 0.314 231.67 216.33 254.00 252.67 234.33±13.6 235.00±11.9 174.33 174.67 238.17 240.00 200.17±20.4 201.13±20.1 0.001 0.001 0.880 33.57 33.83 89.73 104.67 57.55±19.29 59.35±21.26 20.86 23.10 60.97 104.33 39.76±14.62 49.27±25.85 0.003 0.224 0.221 1.91 1.78 10.98 11.94 6.10±3.16 6.32±3.24 3.07 3.93 18.95 23.74 12.30±4.42 12.51±5.52 0.001 0.001 0.862 0.23 0.18 5.12 5.80 2.07±1.55 2.31±1.68 0.74 0.79 3.17 3.24 1.95±0.74 2.01±0.68 0.749 0.472 0.624 3.97 3.01 18.73 18.89 10.56±4.71 11.07±4.53 7.76 8.38 24.43 30.94 17.85±4.62 18.19±6.15 0.001 0.001 0.781 0.33 0.56 2.08 1.99 1.10±0.58 1.24±0.47 0.17 0.17 2.53 3.39 1.10±0.60 1.42±0.99 0.975 0.522 0.14 3.35 4.06 8.33 8.08 5.38±1.70 5.76±1.31 0.69 0.51 3.68 13.47 2.26±0.92 3.75±3.63 0.001 0.048 0.096 1.13/1 1.08/1 2.87/1 2.46/1 1.98±0.66 1.91±0.54 3.55/1 1.59/1 22.77/1 20.52/1 10.32±6.24 9.05±7.32 0.014 0.057 0.771 * * * * * * 0.00 * 22.68 * 10.23±7.68 * * * 12 12 2400 2400 1198±1067 1076±1124 313 300 2400 2400 1949±577 1794±710 0.010 0.024 0.519 49.51 19.84 247.93 173.34 165.96±55.5 114.56±43.2 32.79 15.61 161.19 131.71 103.37±39.65 55.27±30.73 0.001 0.001 0.001 0.00 0.08 3.14 8.50 1.15±1.19 3.83±2.77 0.83 1.25 13.75 12.92 7.34±3.68 6.99±3.50 0.001 0.006 0.805 0.00 1.08 19.92 18.20 3.60±4.87 6.98±5.56 0.00 1.33 9.80 13.44 5.20±2.81 5.70±3.06 0.221 0.381 0.079 0.00 0.00 17.50 9.32 3.33±4.54 3.55±2.95 1.67 0.33 28.22 27.13 10.92±7.16 10.21±6.94 0.001 0.001 0.842 0.00 35.33 5.95±10.56 0.89 10.33 5.29±2.68 0.788 0.999 Dry season (From November to March) Rainy season (From November to March) Min Max Water depth (m) 0.9 4.6 Average ±SD 2.61±1.44 Secchi depth (m) 0.25 0.4 0.34±0.06 Light intensity (lux) 353 4213 2215±940 15.07 15.20 22.80 22.87 7.81 7.57 Water temperature (°C) Surface layer Bottom layer pH Surface layer Bottom layer DO (mg/l) Surface layer Bottom layer Conductivity (µS/cm) Surface layer Bottom layer Turbidity (NTU) Surface layer Bottom layer NO-3-N (mg/l ) Surface layer Bottom layer NH+4-N (mg/l ) Surface layer Bottom layer TN (mg/l ) Surface layer Bottom layer PO3-4-P (mg/l ) Surface layer Bottom layer TP (mg/l ) Surface layer Bottom layer N:P Surface layer Bottom layer Pesticides (µg/l) Surface layer Bottom layer FC (MPN/100ml) Surface layer Bottom layer Chlorophyll a (µg/l) Surface layer Bottom layer Cladocera(indi./l) Surface layer Bottom layer Copepoda (indi./l) Surface layer Bottom layer Rotatoria (indi./l) Surface layer Bottom layer Larvae (indi./l) Surface layer 12 * Bottom layer 0.16 27.83 6.37±6.78 0.98 9.12 4.99±2.41 0.395 (*): No data 3.2.1.1 Hydraulic, physical, chemical and biological characteristics of water in Xuan Huong reservoir Xuan Huong is a shallow reservoir, average depth of 2.86 m Typically, shallow tropical reservoir none thermal stratification High pH values is a notable phenomenon in Xuan Huong reservoir, pH values in the surface layer is difference according to season (ANOVA, p = 0.001) and layer (ANOVA, p = 0.001), average value of pH in dry season was higher than in rainy season The highest pH values at time of algal blooms At the time, biomass of phytoplankton belonged to mainly several taxa of Cyanophyta Three compounds that containing nitrogen did not differ according to layer (ANOVA, p = 0.862), (ANOVA, p = 0.624) and (ANOVA, p = 0781) respectively for ammonium, nitrate and TN The ratio N/P of the surface layer has seasonal differences (ANOVA, p = 0.014), the average value in dry season (1.9/1) was lower than in the wet season (10.32/1) This N/P ratio is suitable for the growth of cyanobacteria (Paerl, 1996), and cyanobacteria is also the dominant group in Xuan Huong reservoir In summary, Xuan Huong is a shallow, turbid, non-stratified reservoir, high nutrient concentrations, quality water is very low, nutrient concentrations is suitable for the growth of Cyanophyta According to OECD (1982), Xuan Huong is a super eutrophic reservoir in terms of chlorophyll a, TP concentration and Secchi depth 3.2.1.2 Biological characteristics in Xuan Huong reservoir Concentration of chlorophyll a in Xuan Huong reservoir is very high and different according to layer and season The average chlorophyll a was higher in dry season than in rainy season in the surface layer There were presence of four groups of zooplankton in Xuan Huong reservoir, including Cladocera, Copepoda, Rotatoria and Larvae These groups did not differ spatial distribution in water column, p> 0.05 (Table 3.11) In there, density of Cladocera and Rotatoria varied according to season, higher in rainy season 3.2.2 Water status of Tuyen Lam reservoir The results of physical, chemical and biological parameters of water in Tuyen Lam reservoir from 11/2013 to 10/2014 are shown in Table 3.12 Table 3.12 Hydraulic physical, chemical and biological parameters of water in Tuyen Lam Hydraulic, physical, chemical and biological parameters Dry season (From November to March) Rainy season (From November to March) Min Max Average ±SD Min Max Water depth (m) 7.80 9.50 8.65±0.63 8.00 13.00 Average ±SD 9.88±1.28 Secchi depth (m) 0.80 2.40 1.55±0.34 1.10 3.50 1.75±0.6 13 Significant differences p≤0.05 Season Layer 0.004 * 0.015 * Light intensity (lux) Water temperature (°C) Surface layer Bottom layer pH Surface layer Bottom layer DO (mg/l) Surface layer Bottom layer Conductivity (µS/cm) Surface layer Bottom layer NO-3-N (mg/l ) Surface layer Bottom layer NH+4-N (mg/l ) Surface layer Bottom layer TN (mg/l ) Surface layer Bottom layer PO3-4-P (mg/l ) Surface layer Bottom layer TP (mg/l ) Surface layer Bottom layer N:P Surface layer Bottom layer Pesticides (µg/l) Surface layer Bottom layer Chlorophyll a (µg/l) Surface layer Bottom layer Cladocera(indi./l) Surface layer Bottom layer Copepoda (indi./l) Surface layer Bottom layer Larvae (indi./l) Surface layer Bottom layer 776 2286 1636±531 1343 50966 14180±1765 0.001 * 16.43 16.4 20.53 20.4 18.41±1.30 18.26±1.19 16.27 16.00 22.67 21.60 19.11±1.95 18.96±1.28 0.060 0.067 2.940 6.96 6.46 7.97 8.10 7.49±0.3 7.32±0.49 7.10 6.12 7.87 8.25 7.30±0.33 7.18±0.61 0.049 0.189 0.460 5.55 5.61 7.10 7.20 6.32±0.39 6.30±0.43 5.16 5.06 7.73 7.17 6.55±0.70 6.50±0.66 0.013 0.047 0.390 59.00 59.00 64.33 63.67 61.51±1.42 61.38±1.23 58.67 59.33 64.67 64.33 62.70±1.72 62.65±1.15 0.236 0.026 0.100 1.19 0.91 2.22 2.06 1.64±0.34 1.54±0.34 0.06 - 1.85 2.08 1.35±0.56 1.36±0.63 0.019 0.003 0.430 0.07 0.17 0.46 0.46 0.22±0.1 0.25±0.07 0.1 0.07 0.64 0.89 0.3±0.19 0.29±0.22 0.007 5.480 0.370 1.87 1.89 3.03 3.09 2.30±0.46 2.29±0.51 1.38 0.74 3.37 3.67 2.30±0.63 2.26±0.94 0.998 0.957 0.19 0.13 0.92 1.14 0.55±0.26 0.56±0.32 - 1.23 1.15 0.46±0.45 0.51±0.41 0.015 0.152 0.360 0.60 0.67 1.05 1.53 0.88±0.18 1.04±0.32 0.39 0.41 2.17 2.30 1.02±0.59 1.09±0.63 0.629 0.887 0.580 2.09/1 1.89/1 4.05/1 3.18/1 2.85/1±0.76/1 2.29/1±0.51/1 0.98/1 0.72/1 5.12/1 4.65/1 2.90/1±1.27/1 2.91/1±1.59/1 0.932 0.429 0.640 * * * * * * 4.1E-4 * 5.7E-4 * 5.1E-4±8.5E-5 * * * 11.21 3.74 37.88 16.02 21.21±7.43 7.82±3.59 11.21 3.73 52.33 22.43 23.71±11.21 8.81±5.26 0.102 0.045 0.000 0.17 0.00 1.25 1.17 0.64±0.31 0.64±0.29 0.83 1.00 3.08 3.33 1.82±0.65 1.96±0.67 0.001 0.001 0.903 0.75 0.75 3.50 2.83 2.07±0.66 2.07±0.56 1.67 2.42 12.79 8.25 5.02±2.44 4.65±1.72 0.001 0.001 0.915 1.25 1.33 3.17 3.33 2.37±0.53 2.28±0.61 1.42 1.25 3.71 3.33 2.62±0.57 2.49±0.52 0.191 0.250 1.000 0.915 * (*) No data 3.2.2.1 Hydraulic, physical, chemical characteristics of water in Tuyen Lam reservoir There was no difference in temperature between the layers (ANOVA, p = 2.940), known that Tuyen Lam reservoir is deeper than Xuan Huong reservoir about 3.5 times Except chlorophyll a, all parameters were not different according to layer, but different according to season Ammonium and phosphate concentrations also differed according to season but only at the surface layer Concentration of phosphates was higher in bottom layer in both seasons In summary, Tuyen Lam is not turbid, not stratified reservoir, and irregular disturbance; its depth is medium, nutrition ranged from moderate to eutrophic conditions 3.2.2.2 Biological characteristics of water in Tuyen Lam reservoir 14 Concentration of chlorophyll-a was different in water column (ANOVA, p = 0.001; Table 3.12), higher than in the surface layer Not seasonal differences of chlorophyll a in the surface layer (ANOVA, p = 0.102), while at the bottom layer, has the seasonal differences This phenomenon maybe relate to characteristics of phytoplankton community structure in Tuyen Lam reservoir In this reservoir, Dinophyta (Peridinium, Ceratium) and Cyanophyta (Microcystis) are dominant species in density Both groups have ability to move in the water column Microcystis can move at m/h speed, and favourite low light intensity (Walsby, 1994) Thus, chlorophyll a fluctuations in season at the bottom layer maybe relate to the distribution of two above groups For zooplankton, comparing to Xuan Huong reservoir, it is the absence of Rotatoria Cladocera and Copepoda fluctuated according to season in the layers (ANOVA, p = 0.001) Larvae not fluctuate over time 3.2.3 Water status of Dan Kia reservoir The results of hydraulic physical, chemical and biological parameters of water in Dan Kia reservoir from 11/2013 to 10/2014 are shown in Table 3.13 3.2.3.1 Hydraulic, physical, chemical characteristics of water in Dan Kia reservoir Water levels varied considerably according to season (ANOVA, p = 0.044), Secchi depths are low (0.38 and 0.34 m) and not different (ANOVA, p = 0.294) Table 3.13 The results of hydraulic physical, chemical and biological parameters of water in Dan Kia reservoir Min Max Average ±SD Min Max Average ±SD Water depth (m) 1.0 8.0 4.38±2.53 1.5 10 6.35±3.03 Significant differences p≤0.05 Season Layer * 0.044 Secchi depth (m) 0.25 0.6 0.38±0.12 0.2 0.5 0.34±0.11 0.294 * Light intensity (lux) 993 21333 4370±5793 367 31866 11372±11924 0.043 * 15.07 15.47 22.27 22.00 19.84±2.48 19.40±2.17 15.57 15.47 22.13 21.93 18.86±2.32 18.13±1.95 0.027 0.049 0.602 6.45 6.38 7.97 7.77 7.25±0.44 7.12±0.44 6.06 6.60 7.07 7.14 6.44±0.30 6.53±0.35 0.001 0.001 0.969 5.59 5.50 7.37 6.92 6.22±0.44 6.13±0.34 5.75 5.79 6.77 6.57 6.07±0.29 6.11±0.26 0.331 0.865 0.289 19.67 21.67 4.67 42.33 30.67±6.88 30.98±6.50 37.00 37.33 53.67 54.67 42.06±4.34 42.11±4.44 0.001 0.001 0.932 20.04 19.01 42.69 77.83 42.68±22.54 44.29±22.69 20.83 21.07 70.13 185.67 70.13±50.98 73.52±47.58 0.059 0.035 0.792 0.21 0.19 2.22 2.34 0.85±0.71 0.88±0.71 0.11 0.16 2.15 1.97 0.78±0.57 0.87±0.57 0.739 0.972 0.645 0.81 0.78 2.81 2.89 1.63±0.67 1.64±0.65 0.01 0.00 7.51 7.54 3.32±2.39 4.23±2.49 0.007 0.000 0.379 Hydraulic, physical, chemical and biological parameters Water temperature (°C) Surface layer Bottom layer pH Surface layer Bottom layer DO (mg/l) Surface layer Bottom layer Conductivity (µS/cm) Surface layer Bottom layer Turbidity (NTU) Surface layer Bottom layer NH+4-N (mg/l ) Surface layer Bottom layer NO-3-N (mg/l ) Surface layer Bottom layer Dry season (From November to March) Rainy season (From November to March) 15 TN (mg/l ) Surface layer Bottom layer PO3-4-P (mg/l ) Surface layer Bottom layer TP (mg/l ) Surface layer Bottom layer N/P Surface layer Bottom layer Pesticide (µg/l) Surface layer Bottom layer FC (MPN/100ml) Surface layer Bottom layer Chlorophylla(µg/l) Surface layer Bottom layer Cladocera(indi./l) Surface layer Bottom layer Copepoda(indi./l) Surface layer Bottom layer Rotatoria (indi./l) Surface layer Bottom layer Larvae (indi./l) Surface layer Bottom layer 1.45 1.39 4.78 4.88 2.97±1.02 3.02±1.06 0.54 0.87 11.22 11.20 5.07±3.48 6.12±3.43 0.031 0.002 0.373 0.71 1.13 3.74 3.61 1.85±0.96 1.98±0.87 0.89 0.72 9.19 9.47 2.87±2.53 2.85±2.61 0.146 0.223 0.932 1.50 1.47 4.05 4.10 2.31±0.84 2.33±0.86 1.27 1.34 9.87 10.51 3.46±2.72 3.58±2.73 0.126 0.097 0.879 0.74/1 0.69/1 2.33/1 2.17/1 1.42/1 ±0.71/1 1.28/1±0.67/1 0.42/1 0.44/1 2.37/1 3.06/1 1.45/1 ±0.71/1 2.05/1±0.92/1 0.951 0.353 0.383 * * * 17.92 * * 0.018 33.15 6.25 13.11±16.13 1.25±2.79 * * 150 93 39.27±38.91 34.07±26.91 43 23 1100 1100 342.57±352.95 325.38±307.38 0.002 0.000 0.859 3.74 24.92 26.70 11.82±6.64 9.25±8.55 7.47 59.81 74.76 17.80±10.70 11.21±15.59 0.064 0.662 0.072 0.00 0.42 3.14 2.92 1.15±1.19 1.57±0.78 1.33 1.41 4.83 4.94 2.76±0.98 2.76±0.92 0.000 0.000 0.650 0.08 0.33 2.67 9.33 1.27±0.65 2.83±2.57 1.42 1.17 52.36 34.58 12.47±12.86 11.13±10.02 0.002 0.004 0.955 0.00 0.00 0.50 9.32 0.08±0.16 3.55±2.95 1.67 0.33 28.22 27.13 10.92±7.16 10.21±6.94 0.222 0.243 0.334 0.00 0.00 1.08 14.08 0.26±0.39 1.17±3.59 0 2.58 4.16 0.98±0.75 1.27±1.11 0.002 0.898 0.193 * (*): no data Dan Kia reservoir is turbid frequently while chlorophyll a concentration are very low Thus, turbidity of Dan Kia reservoir can be caused by suspended solids pH values ranged from slight acid to slight alkaline This condition is suitable for the growth of Chrysophyta Surveying composition and density of phytoplankton showed regular presence of Synura genus in Dan Kia reservoir and Tuyen Lam reservoir while not seeing this taxon in Xuan Huong reservoir Ammonium and phosphate concentrations were high within the dry season, but did not differ according to layer and season (Table 3.13) Some parameters of quality water exceeded Vietnamese Standard for drinking water (QCVN08:2008/BTNMT) In short, Dan Kia is a muddy, medium depth, none stratified reservoir, some parameters of nutrition were in medium level, others were in eutrophic level Maybe agricultural activities around the basins causing a nutritional imbalance of nitrogen and phosphorus 3.2.3.2 Biological characteristics of water in Dan Kia reservoir Adequate presence of four zooplankton groups are Cladocera, Copepoda, Rotatoria and Larvae in Dan Kia reservoir While Rotatoria is dominant in Xuan Huong reservoir, they have low density in Dan Kia reservoir and absolutely no presence in Tuyen Lam reservoir 16 3.3 Characteristics of phytoplankton community and the impact factors 3.3.1 Chlorophyll a and environmental factors The results of multivariate regression analysis for reservoir, including chlorophyll a is the dependent variable, environmental factors are the independent variables, are shown in table 3.14 Table 3.14 Correlation between chlorophyll a and environmental factors Reservoirs/layers Xuan Huong Surface layer Bottom layer Tuyen Lam Surface layer an Kia Surface layer Correlation equations R2 (%) P Chla = -345.043 + 0.005* Conductivity*Turbidity + 144.3*log(Temperature) (1) Chla = -345.043 + 0.005* Conductivity*Turbidity + 144.3*log(Temperature) (2) 40.46 0.0002 65.83 0.0001 Log(Chla) = 1.5924 + 0.277*pH - 0.273*TN (3) 23.81 0.0112 Log(Chla) = 2.333 + 0.029*Crypt (4) 20.94 0.0050 At Xuan Huong reservoir, chlorophyll a concentrations in the surface layer and the bottom layer correlated with conductivity, turbidity and temperature, the largest R2 value at the bottom layer is 65.83% (p = 0.0001) Correlation equations (1) and (2) show that chlorophyll a in Xuan Huong reservoir correlated with temperature Meanwhile, concentration of chlorophyll a in Tuyen Lam reservoir correlated with pH and TN Thus, nitrogen is the main factor that dominated growth of phytoplankton in Tuyen Lam reservoir At Dan Kia reservoir, chlorophyll a concentration in the surface layer has a relationship with Cryptophyta (Equation 4, Table 3.14) 3.3.2 Correlation between density of phytoplankton (by phylum) and environmental factors Database of phytoplankton density in the layers of the reservoirs were standardized for multivariate regression analysis The results are shown in Table 3:15 Table 3:15 Results of multivariate regression analysis between phytoplankton groups and environmental factors Groups of phytoplankton Xuan Huong Cyanophyta Surface layer Bottom layer Chlorophyta Surface layer Cryptomophyta Surface layer Tuyen Lam Cyanophyta Surface layer Bottom layer Correlation equations between phytoplankton density & environmental factors R2 P 53.74 0.0001 57.46 0.0001 Log(Chlorophyta) = 0.878 + 0.588*DO (7) 20.33 0.0141 Log(Cryptophyta) = 3.943 - 1.087*log(TP) (8) 42.69 0.0004 Log(Cyanophyta) = 7.967 - 6.106*log(NO3) (9) Log(Cyanophyta) = 4.675 + 2.771*log(Turbidity) - 1.630*TN (10) 31.92 51.30 0.0006 0.0001 Cyanophyta = -3259.990 - 1429.510*PO43- + 491.351*Temperature - 1212.670*log(TP) (5) Sqrt(Cyanophyta) = 51.2801 - 9.266*DO + 2.774*Temperature - 0.315*log(NO3-)*TN (6) 17 Chrysophyta Surface layer Dan Kia Bacillariophyta Surface layer Bottom layer Dinophyta Surface layer Chrysophyta Surface layer Bottom layer Cryptomophyta Surface layer Bottom layer Log(Chrysophyta) = 1.193 + 1.871*PO43- (11) 26.43 0.0348 Log(Bacillariophyta) = -0.587 + 0.554*pH - 0.413*log(TN) (12) Bacillariophyta = 11.762 - 6.007*NO3- + 3.245*TN (13) 45.60 47.61 0.0001 0.0001 Log(Dinophyta) = -1.009 + 0.097*SQRT(Conductivity)*log(Turbidity) (14) 76.66 0.0001 Log(Chrysophyta) = 1.041 + 0.414*Cladocera (15) Log(Chrysophyta) = 5.568 + 0.629*Cladocera - 0.916*DO (16) 45.58 70.11 0.0001 0.0001 Cryptophyta = 99.494 - 1.293*T - 9.818*pH (17) Log(Cryptophyta) = 5.506 - 0.005* Temperature *pH*DO (18) 60.36 36.36 0.0001 0.0023 In Xuan Huong reservoir, density of Euglenophyta, Dinophyta and Bacillariophyta did not correlate with environmental factors in both layers Chlorophyta and Cryptophyta density in the surface layer have negative correlation with DO and TP (7, equation) Cyanophyta correlated with environmental factors clearly In particular, Cyanophyta density of surface layer correlated with phosphorus inversely (Equation 5), Cyanophyta density of the bottom layer inversely correlated with nitrogen (Equation 6) In addition, Cyanophyta in two layers was dependent on temperature Thus, both chlorophyll a and Cyanophyta density depended on temperature and nutrition in Xuan Huong reservoir In Tuyen Lam reservoir, Chrysophyta density correlated with concentration of phosphate and some environmental factors, especially they inversely correlated with nitrogen concentration Bacillariophyta depended on pH and nitrogen in Dan Kia reservoir (equation 12 and 13) Dinophyta correlated with turbidity and conductivity of Dan Kia water (Equation 14) while Cryptophyta can not resist to pH variations (equation 17 and 18) 3.3.3 Correlation between phytoplankton density and environmental factors The results of CCA analysis between phytoplankton species composition and environmental factors in the reservoirs are shown in Figure 3.18, 3.19, 3.20 In the surface layer of Xuan Huong reservoir (Figure 3.18A), most of the environmental factors were dominant species composition in months of dry season and are located in the first axis The advantage in these months belong to Euglenophyta and Cyanophyta as Euglena sp., Euglena caudata, Trachelomonas sp., and Oscillatoria boryana, Pseudanabaena catenata At rainy season, light intensity was maximum, nitrate and TN varied in opposite direction with ammonium, dominant species belonged to Microcystis aeruginosa, Pseudanabaena limnetica along with appearance of Cyclotella sp In months of rainy season and the first months of dry season, with abundant presence of Cyanophyta taxa as Pseudanabaena sp., Anabaena sp Anabaena genus known can fix N in case of ratio N/P of water is low The ratio 18 N/P in Xuan Huong reservoir at that time (June and July) was lower than in other months Figure 3.18 The CCA graph of phytoplankton species composition and environmental factors in Xuan Huong reservoir (A) surface layer, (B) bottom layer Phytoplankton species composition in the surface layer of Tuyen Lam reservoir (Figure 3.19A) was more diverse than in the bottom layer (Figure 3.19B) In the surface layer, Pinnularia sp and Cymbella sp positioned near the light intensity vector This species occurred in April and May, when light intensity increases Meanwhile, they were not present at the bottom layer, proved to that they are photophilic taxa Phytoplankton at the surface layer of Tuyen Lamreservoir was dominated by most of environmental factors in the final months of rainy season and the beginning of dry season (Figure 3.19) Coelastrum cambrium, Cosmarium pseudoconnatum, Pandorina charkowiensis and Ceratium hirundinella were found at the 2nd axis (Figure 3.19) related to concentration of low nutrients in July Microcystis aeruginosa, Microcystis wesenbergii, Oscillatoria sp on the graph (Figure 3.19A) showed their superiority density related to temperature and pH of water When these elements were added at the beginning of rainy season (April), accompanied by an increase in density of above genera Seasonal variation of species composition in Dan Kia reservoir is shown quite clearly in the CCA axis pH value at the first axis and temperature at the second axis increase in rainy season with dominant species is Rhizosolenia sp In the last months of rainy season, turbidity increased, this appropriate to Dinophyta as Peridinium sp., Ceratium sp and Bacillariophyta as Cyclotella sp Thus, turbidity is the dominant factor on phytoplankton species composition in Dan Kia reservoir 19 Figure 3.19 The CCA graph of phytoplankton species composition and environmental factors in Tuyen Lam reservoir (A) surface layer, (B) bottom layer 3.4 Impacts of nutrition and grazing on phytoplankton 3.4.1 Nutrition and growth of phytoplankton Figure 3.21 shows no has difference among supplement and complement experimental treatments of zooplankton in rainy season (dry season experiments were also similar) The pair of treatments added only N or P were also no different from no supplementation However, additional treatments simultaneously N and P had significant differences from non-supplement, in both experiments of rainy season (ANOVA, p = 0.002) and dry season experiments (ANOVA, p = 0.012) Figure 3.21 Variability of phytoplankton biomass at 6/2014 experiment for Tuyen Lam reservoir N=nitrogen (N-NO3-), P=phosphorus (P-PO43-), Z = zooplankton, C=control The results showed that in Tuyen Lam reservoir, grazing of zooplankton did not adjust biomass of phytoplankton, that is the role of nutrients Thus, the results obtained from experimental treatments and fields showed that bottom up control is a key driver in Tuyen Lam reservoir 20 3.4.2 Grazing of zooplankton on growth of phytoplankton Apparent growth rate of organisms groups in series of experimental dilutions (rainy and dry season) with average death rate is shown in Table 3.18 Table 3.18 The equations describing the regression line of apparent growth of plankton groups (preys) in different dilutions in two periods of experiment Time of experiments Preys filamentous cyanobacteria Dry season (24/2/2014) cyanobacteria Bacteria Algae filamentous cyanobacteria Rainy season (30/8/2014) cyanobacteria Bacteria Algae Serie of dilutions The regression equation between apparent growth rate (k) with dilutions R2 p 0.2µm 0.01µm 0.2µm 0.01µm 0.2µm 0.01µm 0.2µm 0.01µm 0.2µm 0.01µm 0.2µm 0.01µm 0.2µm 0.01µm 0.2µm 0.01µm y = 1.932 – 2.376x (19) y = 1.463 – 7.905x (20) y = 0.107 – 0.177x (21) y = 0.3  1.165x (22) y = 1.992  0.821x (23) y = 1.905 – 8.855x (24) y = 2.437  0.769x (25) y = 0.738 – 0.873x (26) y = 0.182 + 0.205x (27) y = 1.737  2.664x (28) y = 0.894 – 4.702x (29) y = 1.087 – 2.339x (30) y = 1.745 – 7.976x (31) y = 2.256 – 3.363x (32) y = 0.6433  2.531x (33) y = 0.8317  0.304x (34) 0.7405 0.9068 0.1536 0.6321 0.5667 0.9818 0.8708 0.2722 0.0334 0.9974 0.9694 0.7198 0.9559 0.9698 0.9672 0.1380 0.8267 0.0491 0.9937 0.1632 0.4671 0.0498 0.1020 0.8747 0.8235 0.9501 0.0499 0.6896 0.0487 0.9013 0.0496 0.2791 Average mortality (d-1) by Virus Zooplankton 0.713 - - - 0.258 - - - - - - 0.394 - 0.167 - 0.591 During dry season None increase in apparent growth rate of filamentous Cyanobacteria in the 0.2 micron dilution Conversely, the apparent growth of this group in the 0.01 micron dilution increased The regression line of 0.01 micron series of dilution (Equation 20, 24) shows, viruses effected on filamentous Cyanophyta and bacteria Indirect mortality of filamentous Cyanophyta and bacteria by viruses were estimated respectively 0.713 d-1 and 0.258 d-1 In contrast, zooplankton have no significant impact on filamentous Cyanophyta, bacteria, Cyanophyta and other phytoplankton during this experiment During rainy season Grazing of zooplankton was identified as lethal for bacteria (Equation 31), other Cyanophyta (Equation 29) and eukaryotic phytoplankton (Equation 33) Zooplankton indirectly lethal for bacteria, unicellular Cyanophyta and eukaryotic algae was estimated 0.167; 0.394 and 0.591 d -1respectively In summary, lysis of virus was the main cause of death for populations of filamentous Cyanophyta, bacteria in dry season Meanwhile, the grazing was the lethal source to single-cell cyanobacteria, microalgae and bacteria in rainy season 3.5 Simulation trends of ecosystem of Dan Kia reservoir by AQUATOX model 21 The first scenario, the current situation, the load of nutrients of all streams was introduced into the reservoir The second scenario, the quality water was controlled to reduce to 1/3 of nutrient load in S4 and S5 without affecting flows a) All sources were introduced into Dan Kia reservoir (status) a) Reducing to 1/3 of nutrient load in S4 and S5 streams Figure 3.23 Quality water of the surface layer of Dan Kia reservoir according to scenarios The first scenario Most concentration of NH4+, NO3- and PO43from the model and the survey (Figure 3.23a) were suitable The model results reflected quite well concentration of ammonium, nitrate and phosphate Accordingly, the model can forecast quality water of reservoir in the 22 future The results forecast that NH4+, NO3- and PO43- will increase in the future even if the scale of impacts are similar to the current situation (Figure 3.23a) Phytoplankton were simulated including Bacillariophyta, Chrysophyta and Dinophyta (Figure 3.24) Phytoplankton biomass increases in January and higher in April There was relatively consistent between chlorophyll a concentration from the model and monitoring On other hand, chlorophyll a concentration from the model varied simultaneously with Bacillariophyta, Dinophyta The simulation resuts of zooplankton groups showed that Copepod and Cladocera biomass (Figure 3.25) was the highest in March, respectively 5.5 mg/l and 2.25 mg/l These values fell to the lowest in August and September Most zooplankton biomass from the model were higher than actual results from 0.1 to 30% The second scenario When controlling to reduce the load of nutrients to 1/3 of S4 and S5 streams, concentration of ammonium, nitrate and phosphate decreased significantly Nitrate concentrations were lower than the allowed threshold of A1 level at all months Phosphate concentration still exceed the allowed standard of B2 level from 1.2 to 1.8 times When nutrient concentrations of water decreased, the model results showed that phytoplankton and zooplankton biomass also decreased Decreasing the load of nutrients of S4 stream and S5 stream is only one of many possible scenarios to simulate proposed quality water as expected from AQUATOX model CONCLUSION Recorded phyla of phytoplankton in the studied reservoirs, including Cyanophyta, Chlorophyta, Bacillariophyta, Euglenophyta, Dinophyta, Chrysophyta 23 and Cryptophyta Of these, 112 taxa of Xuan Huong reservoir, 43 and 44 taxa respectively in Tuyen Lam reservoir and Dan Kia reservoir Each reservoir has its own characteristics of phytoplankton community structure The structure of phytoplankton community in each reservoir is dominated by a few certain factors While density of filamentous Cyanophyta and Euglenophyta were dominant at Xuan Huong reservoir, colony Cyanophyta and Dinophyta dominated in Tuyen Lam reservoir Diatoms and Chrysophyta were dominant at Dan Kia reservoir Species diversity of phytoplankton in Xuan Huong reservoir is the lowest, higher and similar each other in Tuyen Lam reservoir and Dan Kia reservoir The phytoplankton groups according to morphological - function groups reflected nature and characteristic of aquatic ecology at certain levels When considering evaluate the relationship between phytoplankton community structure and environmental factors, the system of Reynolds et al., (2002) and Kruk et al., (2010) should be used All studied reservoirs are not stratified Most hydraulic, physical, chemical parameters of water did not differ according to layer but differing according to season Each reservoir owns ecological characteristics, including: - Xuan Huong is a shallow, turbid, high pH reservoir The main factor caused turbidity is phytoplankton biomass The ratio N/P of water is suitable for growth of Cyanophyta -Tuyen Lam reservoir has an average depth, few turbidity and low nutrient concentration among the reservoirs -Dan Kia reservoir frequently is turbid, this is not related to phytoplankton biomass Nutrient concentrations are high in months of dry season Composition, density and biomass of phytoplankton in the reservoirs respect to environmental factors Each reservoir was dominated by a few certain factors Turbidity dominated on phytoplankton community structure in three reservoirs Temperature effects on phytoplankton community structure of Dan Kia reservoir and Xuan Huong reservoir Conductivity dominated on phytoplankton community of Xuan Huong reservoir, Dan Kia reservoir and pH value dominated on phytoplankton of Dan Kia reservoir The concentration of compounds containing nitrogen and studied reservoirs Unclear roles of adjustment of zooplankton on phytoplankton in the reservoirs This control was selective in Xuan Huong reservoir There was no impact of zooplankton on filamentous Cyanophyta, but grazing were identified as the main lethal source for bacteria, single-celled cyanobacteria and others algae, with an estimated 0.167d-1; 0.394d-1and 0.591d-1 respectively Meanwhile, virus is a major cause of death for populations of filamentous Cyanobacteria and bacteria Indirect mortality of filamentous Cyanophyta and bacteria by virus estimated are 0.713 d-1 and 0.258 d-1 respectively 24 AQUATOX model was used to simulate quality water of Dan Kia reservoir including the load of nutrients, content of chlorophyll a, and biomass of zooplankton and phytoplankton The results showed that, when controlling to reduce the loads, concentration of ammonium, nitrate and phosphate was lower than the allowed thresholds of surface water standards NEW FINDINGS OF THE THESIS - This is the first study in Vietnam on impact of environmental conditions on natural phytoplankton community structure by analyzing aggregated responses of phytoplankton and environmental factors in the reservoirs at Lam Vien highland - Indicating structural characteristics of phytoplankton community in the reservoirs at Lam Vien highland by approaching and applying softwares, tools, experiments and new methods - This is the first study in Vietnam assessing the biological rates in reservoirs -The first time, the forecasting model of freshwater food web was applied in Vietnam LIST OF WORKS HAS BEEN PUBLISHED Tran Thi Tinh, Doan Nhu Hai, Le Ba Dung, 2015 Mortality impact of viral and microzooplankton upon bacteria and phytoplankton in a eutrophic in central highland Journal of Biology, Vol 37, No 2, 2015 (200-206) Tran Thi Tinh, Doan Nhu Hai, Le Ba Dung, 2015 Seasonal variation of phytoplankton in Tuyen Lam reservoir in Da Lat, Vietnam Journal of Biology, Vol 37, No 4, 2015 (414-424) Tran Thi Tinh, 2014 Assess the state of eutrophication of some reservoirs in Dalat by TSI and AQ index Journal No 13, May 12-2014 (36-43), Tay Nguyen University, ISSN 1859-4611 Tran Thi Tinh, Doan Nhu Hai, Bui Nguyen Lam Ha, Nguyen Thi Thanh Thuan, 2016 Impact assessment of Dan Kia inflows and application AQUATOX model in managing water quality Journal of Biology, Vol 38 No 1, 2016 (61-69) 25 ... was introduced into the reservoir The second scenario, the quality water was controlled to reduce to 1/3 of nutrient load in S4 and S5 without affecting flows a) All sources were introduced into... significant impact on filamentous Cyanophyta, bacteria, Cyanophyta and other phytoplankton during this experiment During rainy season Grazing of zooplankton was identified as lethal for bacteria (Equation... Correlation equations R2 (%) P Chla = -345.043 + 0.005* Conductivity*Turbidity + 144.3*log(Temperature) (1) Chla = -345.043 + 0.005* Conductivity*Turbidity + 144.3*log(Temperature) (2) 40.46 0.0002