Structure and dynamics of phytoplankton in an Amazon lake, Brazil

Natural lake systems represent important reservoirs for residential water supply, fish production, recreational activities and enjoyment of their natural beauty. Nevertheless, human impacts may affect their health status resulting in degradation and loss of biodiversity. The aim of the present study was to obtain data on the health status of a natural lake located in an indigenous reservation in the Brazilian Amazon, using the phytoplankton community changes along the rainy (June) and dry (November) seasons of 2006. We collected water (temperature, pH, Secchi depth and conductivity) and phytoplankton samples from the subsurface, middle of the water column, and approximately 30 cm above the bottom, over 24-hour sampling periods, from a central station in the lake. Samples taken from biotic and abiotic variables were correlated using canonical correspondence analysis (CCA). Results showed that the lake exhibited high temperatures in both seasons, and showed thermal stratification only during the rainy season. Dissolved oxygen exhibited a clinograde pattern in the rainy season and high oxygen in the hypolimnion in the dry season. In the rainy season, the water near the bottom was acidic, turbid and had a greater concentration of phosphorus. Dissolved oxygen, conductivity, pH, nitrite, total phosphorus and total dissolved phosphorus exhibited diel variations in the rainy season, whereas water temperature, dissolved oxygen, total nitrogen and total dissolved phosphorus exhibited significant differences between hours of the day in the dry season. The phytoplankton was represented by 39 taxa, and Chlorophyta showed the greatest species richness, totaling 25 taxa. Among Chlorophyta, desmids were the most diverse, accounting 52%. Bacillariophyta (nine species) was the second most diverse group. Cyanophyta was represented by three species, including Merismopedia tenuissima, the most abundant taxon. Despite the occurrence of taxa that indicate organic pollution, their biomass and frequency indicate that the system is not currently threatened. Lake Caracaran? is an oligotrophic system, with low algal density and isolated blooming episodes due to its shallow depth.     

Temporal and spatial variations of chemical oxygen demand in Lake Taihu,China, from 2005 to 2009

We undertook a study in Lake Taihu, China, from 2005 to 2009 including a total of 639 samples to determine: (i) the seasonal dynamics and spatial distribution of the chemical oxygen demand (COD) and (ii) the relationships between the COD concentration and the biochemical oxygen demand (BOD), phytoplankton pigment, total dissolved nitrogen (TDN), and total dissolved phosphorus (TDP) concentrations, as well as the chromophoric dissolved organic matter (CDOM) absorption coefficient. There were significant spatial differences in the COD concentration, which gradually decreased from Zhushan Bay in the northwest, to the north, the lake center, and the southeast of the lake. The COD concentration was significantly higher at near-shore sites than that at open water sites. The mean COD concentrations were significantly higher in the spring and summer than in the winter and autumn. The lowest annual mean COD concentration appeared in 2009, which could be attributed to improvements in water quality management and high rainfall. The COD concentrations in all four seasons were strongly correlated with phytoplankton pigment, suggesting that extracellular release of COD from phytoplankton was an important COD source. The correlation coefficients between the COD and phytoplankton pigment concentrations were higher in the spring, summer, and autumn than in the winter, showing a more important contribution of phytoplankton degradation to COD in the algal bloom season than in the non-algal bloom season. These new data on the temporal and spatial characteristics of the COD in Lake Taihu will be crucial for developing future strategies for water quality management.     

The Potential for Nitrogen Fixation in a Lake Receiving Sewage Effluent (Grasmere, English Lake District)

The potential for nitrogen fixation in the water column and sediment of Grasmere (English Lake District) was investigated using the acetylene reduction technique, with reference to seasonal changes and concentrations of dissolved oxygen and inorganic nitrogen in the epilimnion and hypolimnion. Potential rates of nitrogen fixation and MPN counts of nitrogen-fixing bacteria correlated with each other and with decreases in dissolved oxygen concentration. The results suggested that selective changes in the nitrogen-fixing microflora took place during the period of thermal stratification. Neither light-dependent fixation, nor cyanobacteria could be detected in water column samples, and acetylene reduction was detected, for most of the season, only in samples of water and sediment to which a source of carbon and phosphorus had been added. The possible role of the sewage effluent in suppressing nitrogen fixation is discussed.     

Seasonal changes in the chemistry and biology of a meromictic lake (Big Soda Lake,Nevada, U.S.A.)

Big Soda Lake is an alkaline, saline lake with a permanent chemocline at 34.5 m and a mixolimnion that undergoes seasonal changes in temperature structure. During the period of thermal stratification, from summer through fall, the epilimnion has low concentrations of dissolved inorganic nutrients (N, Si) and CH₄, and low biomass of phytoplankton (chlorophyll a ca. 1 mgm ^-3). Dissolved oxygen disappears near the compensation depth for algal photosynthesis (ca. 20 m). Surface water is transparent so that light is present in the anoxic hypolimnion, and a dense plate of purple sulfur photosynthetic bacteria (Ectothiorhodospira vacuolata) is present just below 20 m (Bchl a ca. 200 mgm^-3). Concentrations of N H₄ ⁺, Si, and CH₄ are higher in the hypolimnion than in the epilimnion. As the mixolimnion becomes isothermal in winter, oxygen is mixed down to 28 m. Nutrients (NH₄ ⁺, Si) and CH₄ are released from the hypolimnion and mix to the surface, and a diatom bloom develops in the upper 20 m (chlorophyll a > 40 mgm^-3). The deeper mixing of oxygen and enhanced light attenuation by phytoplankton uncouple the anoxic zone and photic zone, and the plate of photosynthetic bacteria disappears (Bchl a ca.10mgm^-3). Hence, seasonal changes in temperature distribution and mixing create conditions such that the primary producer community is alternately dominated by phytoplankton and photosynthetic bacteria: the phytoplankton may be nutrient-limited during periods of stratification and the photosynthetic bacteria are light-limited during periods of mixing.     

The Role of Allochthonous Inputs of Dissolved Organic Carbon on the Hypolimnetic Oxygen Content of Reservoirs

Hypolimnetic oxygen content in lentic ecosystems has traditionally been modeled as a function of variables measured at the epilimnion, or that are supposed to drive epilimnetic processes, like total phosphorus load. However, in man-made reservoirs the river inflow can plunge into deep layers, directly linking the hypolimnion with the surrounding watershed. In these circumstances, organic matter carried by the river can influence the hypolimnetic oxygen content without important intervention of epilimnetic processes. Taking long-term data from two reservoirs in Spain, we applied an empirical regression approach to show that the dissolved organic matter carried by the river is the main driver shaping the hypolimnetic oxygen content. By contrast, typical variables commonly included in the modeling of the oxygen content in the hypolimnion (nutrient concentrations, chlorophyll a, and dissolved organic carbon measured in the water column) did not show any significant correlation. Interpretations from this regression approach were supported by a comparison between the monthly oxygen consumption in the hypolimnion and the monthly dissolved organic carbon load from the river inflow. We also revisited the prediction of the year-to-year variability of the Nürnberg’s anoxic factor in four reservoirs from Spain and the USA, explicitly including the allochthonous sources in the equations. These sources were significant predictors of the anoxic factor, especially in those systems subject to relatively high human impact. Thus, effects of allochthonous dissolved organic carbon should always be considered in empirical modeling and management of reservoir hypolimnetic processes related to oxygen content (for example, anoxia, nutrient internal loading, or phosphorus cycle resilience).     

Some factors affecting the heterotrophic activity of bacteria in lake.

The heterotrophic activity of aquatic bacteria was measured by incorporation of 3H-thymidine, uptake of 14C-sodium acetate and mineralization of 14C-acetate. The temperature of water, oxygen content and concentration of chlorophyll a were also measured in the studied lakes. The obtained results of the studied processes in the surface layer and epilimnion were distinguished by negative correlation coefficients. A distinctive feature is the relation between chlorophyll a concentration in the water bodies and heterotrophic activity of bacteria. This indicates a negative dependence between these processes and phytoplankton activity in the photic zone. In contrast a positive relationship between phytoplankton activity and heterotrophic activity of bacteria was found in the metalimnion and hypolimnion of the studied lakes.     

Characteristics of Dissolved Organic Carbon in Three Layers of a Deep Reservoir,Lake Soyang,Korea

Vertical and seasonal distributions of dissolved organic carbon (DOC) were investigated in a deep reservoir by considering the biodegradability and optical properties of DOC from three different layers during the stratified season. DOC in the epilimnion was characterized by relatively labile compounds that may have originated from phytoplankton. DOC in the metalimnion was variable in its composi<\h>tion and was possibly affected by turbid water inputs to the reservoir during the summer monsoon season. DOC in the hypolimnion always showed refractory characteristics, with low decomposition rates and high ultraviolet (UV) absorption.     

The effect of river runoff and phytoplankton production on the seasonal variation of the chemical composition of coastal waters of the Amursky Bay, Sea of Japan

The factors that control the seasonal variability of the concentration of suspended matter, nutrients, dissolved forms of metals and phytoplankton biomass in coastal waters of the eastern part of the Amurskii Bay were determined. It was shown that the runoff of the Razdolnaya River determined the seasonal variability in suspended matter content, values of the chemical oxygen demand (COD), silicates, Fe, Mn, Cu, and Ni significantly affected the concentration of nitrates. The advection of sea water is the main source of phosphates and dissolved forms of Zn and Cd. In the autumn and winter seasons, the gradual decrease in the concentration of silicates and phosphates was related to phytoplankton activity. In the spring and summer seasons, the growth of phytoplankton determined the variability of the phosphate and nitrate contents and compensated for the effect of the Razdolnaya River runoff. It is concluded that the phytoplankton biomass, rather than variations in riverine flow, caused the long-term variability of the nitrate and phosphate concentrations in the coastal waters of the Amurskii Bay.     

Predicting the potential long‐term influence of climate change on vendace (Coregonus albula) habitat in Bassenthwaite Lake,U.K.

1. The long‐term suitability of Bassenthwaite Lake as a habitat for vendace (Coregonus albula) was assessed using two models. The first was the phytoplankton model (PROTECH) that provided temperature and phytoplankton biomass outputs that were used to drive a second model of lake oxygen (LOX). 2. Both temperature and oxygen concentrations were used to define the available habitat for the adult vendace, using 18 °C as an upper and 2 mg L^?1 as a lower threshold, respectively. The outputs of both models were compared with 4 years of observed data for the purposes of validation and produced good simulations of water temperature, total chlorophyll a and oxygen concentrations in the epilimnion, hypolimnion and at the lake bottom. 3. Using the outputs of a regional climate model (RCM) simulating 20 years of both present and future climate conditions for this part of the United Kingdom, both models were re‐run. These data suggest the future climate will cause a mean increase of >2 °C in water temperature, little change in overall phytoplankton biomass and a 10% decline in oxygen concentration. 4. Using the thresholds defined above, the habitat volume will decline greatly under the future climate scenarios, with all of the 20 years simulated having periods of zero habitat volume for >7 consecutive days, primarily caused by high temperature. These results suggest that the long‐term viability of the lake as a habitat for this rare fish is extremely low.     

Size-fractionated phytoplankton biomass and its implications for the dynamics of an oligotrophic tropical lake

1. Size-fractionated phytoplankton biomass was examined in relation to the hydrodynamics of tropical Lake Alchichica from 1999 to 2002.
2. Alchichica is a warm monomictic lake, in which mixing takes place from late December to early March. The lake is oligotrophic (mean total chlorophyll- a concentration 4.2 ± 4.2  μ g L⁻¹) and its phytoplankton biomass is dominated (72.3 ± 16.4%) by large individuals (>2  μ m). The degree of dominance of the large size class (nano- and microplankton) over the small size class (picoplankton) throughout the year is mainly determined by the availability of silicate and the Si/N ratio in the hypolimnion prior to the mixing period.
3. This is the first record of an oligotrophic tropical lake dominated by large size fractions of phytoplankton. Because of this dominance, the fate of most primary productivity is rapid sedimentation to the bottom followed by decomposition that promotes an anoxic hypolimnion.
4. Our findings in tropical Lake Alchichica challenge the idea that oligotrophic waters are dominated by small phytoplankton, as has been well established for the oligotrophic ocean and temperate lakes.     

The relationship between planktonic algae and bacteria in a small lake

The seasonal changes and vertical distribution of the aerobic and anaerobic bacteria in a small edaphically eutrophic lake which exhibited thermal and chemical stratification are described. There was some correspondence between the phytoplankton and particularly the aerobic bacteria but this was not consistent. Increases in the numbers of anaerobic bacteria coincided with the low dissolved oxygen concentrations in the hypolimnion when algal populations were first senescent and then increasing actively in size.     

Potential capacity of coprecipitation of dissolved organic carbon (DOC) with iron(III) precipitates

Hypolimnetic anoxic water of Lake Onogawa was subjected to aeration experiments. When the samples were agitated by magnetic stirrers for 24 h, dissolved oxygen increased from 0 to more than 7.6 mg l⁻¹, dissolved iron decreased from 98% to about 5% of the initial total iron, and from 32% to 48% of the dissolved organic carbon (DOC) disappeared. On the other hand, when the anoxic waters were left unstirred, dissolved oxygen increased from 0 to 2.2 mg l⁻¹, dissolved iron decreased from 98% to 31%, and 20% of the DOC disappeared within 48 h. Further 24-h incubation had little effect on the DOC loss, although dissolved oxygen increased to 3.9 mg l⁻¹ and dissolved iron decreased to 5%. These rates of DOC disappearance are too large to be explained by bacterial decomposition. It is quite conceivable that a part of the DOC is coprecipitated with iron(III) precipitates. When Fe(II) in the anoxic hypolimnion is oxidized by autumnal water mixing, probably anoxic water is mixed with aerobic water. The anoxic water must receive oxygen from the aerobic water during this mixing and be simultaneously diluted with the aerobic water. Because the present experimental conditions, especially the stirred one, significantly differ from in situ conditions, the present results are thought to be a potential capacity of DOC coprecipitation.     

Silicate availability, vertical mixing and grazing control of phytoplankton blooms in mesocosms

Lake Sempach, located in the central part of Switzerland, has a surface area of 14 km², a maximum depth of 87 m and a water residence time of 15 years. Restoration measures to correct historic eutrophication, including artificial mixing and oxygenation of the hypolimnion, were implemented in 1984. By means of the combination of external and internal load reductions, total phosphorus concentrations decreased in the period 1984–2000 from 160 to 42 mg P m^–3. Starting from 1997, hypolimnion oxygenation with pure oxygen was replaced by aeration with fine air bubbles. The reaction of the plankton has been investigated as part of a long-term monitoring program. Taxa numbers, evenness and biodiversity of phytoplankton increased significantly during the last 15 years, concomitant with a marked decline of phosphorus concentration in the lake. Seasonal development of phytoplankton seems to be strongly influenced by the artificial mixing during winter and spring and by changes of the trophic state. Dominance of nitrogen fixing cyanobacteria (Aphanizomenon sp.), causing a severe fish kill in 1984, has been correlated with lower N/P-ratio in the epilimnion. Buoyant algae such as Planktothrix rubescens (syn. Oscillatoria rubescens) increased in abundance due to enlargement of the trophogenic layer and extended mixing depth during winter. The interactions between zoo- and phytoplankton seemed to be depressed as a result of restoration measures. Zooplankton composition changed to more carnivorous and less herbivorous species. Oxygenation of the hypolimnion induced bioturbation of sediments, mainly by oligochaetae worms, and stimulated germination of spores and cysts and hatching of resting eggs.     

Bacterioplankton and Organic Carbon Dynamics in the Lower Mesohaline Chesapeake Bay

The mesohaline portion of the Chesapeake Bay is subject to annual summertime hypoxia and anoxia in waters beneath the pycnocline. This dissolved oxygen deficit is directly related to salinity-based stratification of the water column in combination with high levels of autochthonously produced organic matter and a very high abundance of metabolically active bacteria. Throughout the water column in the lower, mesohaline part of the bay, between the Potomac and Rappahannock rivers, near the southern limit of the mainstem anoxia, bacterial abundance often exceeded 10 × 10⁶ cells per ml and bacterial production exceeded 7 × 10⁹ cells per liter per day during summer. Bacterial biomass averaged 34% (range, 16 to 126%) of the phytoplankton biomass in summer. These values are equal to or greater than those found farther north in the bay, where the oxygen deficit is more severe. Seasonal variations in bacterial abundance and production were correlated with phytoplankton biomass (lag time, 7 to 14 days), particulate organic carbon and nitrogen, and particulate biochemical oxygen demand in spring; but during summer, they were significantly correlated only with dissolved biochemical oxygen demand. During summer, dissolved biochemical oxygen demand can account for 50 to 60% of the total biochemical oxygen demand throughout the water column and 80% in the bottom waters. There is a clear spring-summer seasonal shift in the production of organic matter and in the coupling of bacteria and autochthonous organic matter. The measurement of dissolved, microbially labile organic matter concentrations is crucial in understanding the trophic dynamics of the lower mesohaline part of the bay. The absolute levels of organic matter in the water column and the bacterial-organic carbon relationships suggest that a lower bay source of organic matter fuels the upper mesohaline bay oxygen deficits.     

Temporal distribution of phytoplankton in Lake Nyamusingiri in the Albertine Rift Valley,Uganda

A study aimed at investigating the temporal variation of phytoplankton assemblages in Lake Nyamusingiri was carried out during the period of December 1997–May 1998. Uganda’s freshwaters are ecologically diverse but a few are intensively studied. Research on phytoplankton has been restricted to large water bodies. There is little information on phytoplankton of the western Uganda crater lakes, which are important water and biodiversity resources. This study provided baseline data on phytoplankton, which will serve as a basis for monitoring the effects of human activities on the lake that might result in ecological transformations like loss of biodiversity because of overexploitation. A laboratory thermometer and Winker’s method were used to determine temperature and dissolved oxygen concentration, respectively. Lake transparency was measured by using the Secchi disc. A Van Dorn sampler was used to collect water samples. Nutrient and chlorophyll a concentrations were determined by using facilities at the Fisheries Resources Research Institute (FIRRI), Jinja. The Sedgwick‐Rafter counting chamber was used to analyse phytoplankton. Variation in temperature was small (25.4–26.2°C). Stable thermal stratification was not evident. The Secchi disc transparency was less than unity. The chlorophyll a value was high. Biomass was found to be light‐limited by nonalgal materials. Dissolved oxygen concentration was more than 100% in the surface waters but declined to <20% at the bottom, which reflected the eutrophic nature of the lake. Diversity indices were low. Eighteen species and five classes of phytoplankton were revealed by this study. The phytoplankton flora was dominated by chlorococcal green algae characteristic of the large eutrophic East African lakes.     

Design and calibration of a simplified river-water quality model including heterotrophic bacteria and chemical parameters

Abstract The results obtained in the design and calibration of a deterministic water quality model from the Llobregat River (Spain) are presented. The water quality indicators studied are dissolved oxygen, biochemical oxygen demand, ammonia, nitrite, nitrate, phosphate, and heterotrophic bacteria. The factors which describe the rates of the transformation processes are of two types: first-order and Michaelis-Menten kinetics. A decomposition process is applied to the model for calibration. The inclusion in the model of heterotrophic bacteria shows good predictive capacity to describe the behaviour of the biological processes in the river. The proposed model gives a correct evaluation of the indicators studied, and may be used in water quality management.     

Seasonal hydrochemical variation in a tropical coastal lagoon (A?u Lagoon, Brazil).

Hydrochemical conditions in the A?u Lagoon are described using spatial and temporal variations of various limnological variables (water temperature, dissolved oxygen, electric conductivity, total alkalinity, carbon dioxide, dissolved and total nutrients (N, P and Si), and chlorophyll a). Collected data was used in order to understand the structure and functioning of an enclosed coastal lagoon strongly influenced by climatic conditions. Water samples were collected monthly (November 1999-December 2000) in five sampling stations established along the lagoon. A decreasing spatial gradient of electrical conductivity was observed beginning from a sand bar region between the lagoon and the sea in the direction of the sweet-water input area. The positive correlation observed between the pH and dissolved oxygen (DO) values, and the negative one observed between pH values and those of carbon dioxide (CO2), evidenced coupled biological processes, e.g., primary production and decomposition. Both spatial and temporal variation of dissolved nutrients showed fast increase and decrease in the beginning of summer, suggesting that nutrient input resulting from rainfall stimulates phytoplankton production, as reflected by chlorophyll a concentration increase.     

Analysis of Dissolved Humic Substances in Eutrophic Waters Using the Fluorescence of Natural Samples: Calculations and Experiments

Theoretical analyses and experiments have been performed for rapid detection of dissolved humic substances in surface waters. The fluorescence has been generated by a nitrogen laser in a laboratory. An optical model has been developed, which has been applied to eutrophic waters around Berlin. The model is based on the assumption, that humic substances and phytoplankton dominate the fluorescence behaviour of the investigated waters. To verify the results of the model, regression analyses between fluorescence data of natural samples and the chemical oxygen demand of filtrates have been conducted. When green and red fluorescence values and the green fluorescence decay time were considered, a maximum squared correlation coefficient of typically 0.85 occurred.     

Bacterioplankton in a small polyhumic lake with an anoxic hypolimnion

Bacterioplankton biomass and dark fixation of inorganic carbon were measured in the highly humic (water colour up to 550 mg Pt l^?1) and acidic lake, Mekkojärvi. Strong thermal and chemical stratification developed in the water column early in spring and led rapidly to anoxia in the hypolimnion, which extended to less than 1.0 m from the surface. In the epilimnion only small bacteria were abundant. In the anoxic zone both the abundance and the mean size of bacteria were considerably higher than in the epilimnion. These differences are thought to be the result of different grazing pressure from zooplankton in the two zones. In late summer a high concentration of bacteriochlorophyll d in the upper hypolimnion indicated a high density of photosynthetic bacteria. Bacterial biomass was similar to that of phytoplankton in the epilimnion, but 23 times higher in the whole water column. In August, dark fixation of inorganic radiocarbon in the anaerobic zone was 51% of the total ¹⁴C-incorporation and the contribution of light fixation was only 5.4%. In the polyhumic Mekkojarvi, bacterioplankton was evidently a potentially significant carbon source for higher trophic levels, but bacterioplankton production could not be supported by phytoplankton alone. Allochthonous inputs of dissolved organic matter probably support most of the bacterial production.