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.     

The Morphometry of Lake Palmas,a Deep Natural Lake in Brazil

Lake Palmas (A = 10.3km²) is located in the Lower Doce River Valley (LDRV), on the southeastern coast of Brazil. The Lake District of the LDRV includes 90 lakes, whose basic geomorphology is associated with the alluvial valleys of the Barreiras Formation (Cenozoic, Neogene) and with the Holocene coastal plain. This study aimed to investigate the relationship of morphometry and thermal pattern of a LDRV deep lake, Lake Palmas. A bathymetric survey carried out in 2011 and the analysis of hydrographic and wind data with a geographic information system allowed the calculation of several metrics of lake morphometry. The vertical profiling of physical and chemical variables in the water column during the wet/warm and dry/mild cold seasons of 2011 to 2013 has furnished a better understanding of the influence of the lake morphometry on its structure and function. The overdeepened basin has a subrectangular elongated shape and is aligned in a NW-SE direction in an alluvial valley with a maximum depth (Z_max) of 50.7m, a volume of 2.2×10⁸ m³ (0.22km³) and a mean depth (Z_mv) of 21.4m. These metrics suggest Lake Palmas as the deepest natural lake in Brazil. Water column profiling has indicated strong physical and chemical stratification during the wet/warm season, with a hypoxic/anoxic layer occupying one-half of the lake volume. The warm monomictic pattern of Lake Palmas, which is in an accordance to deep tropical lakes, is determined by water column mixing during the dry and mild cold season, especially under the influence of a high effective fetch associated with the incidence of cold fronts. Lake Palmas has a very long theoretical retention time, with a mean of 19.4 years. The changes observed in the hydrological flows of the tributary rivers may disturb the ecological resilience of Lake Palmas.     

赣江流域土地利用方式对河流水质的影响

赣江是鄱阳湖的最大支流,是鄱阳湖水污染物的主要来源,查明流域土地利用方式对赣江水质的影响和鄱阳湖的水环境保护具有重要意义。基于2012年对赣江7个主要支流NH+4-N、TP、CODMn和DO浓度的每月测定结果,通过不同空间尺度和土地类型等级划分,利用相关分析和冗余分析研究土地利用方式对赣江流域河流水质的影响。研究结果表明,子流域的土地利用方式对TP的影响大于缓冲区;对CODMn的影响在丰水期大于缓冲区,在枯水期小于缓冲区;对NH+4-N的影响在丰水期与缓冲区接近,在枯水期小于缓冲区;DO受土地利用方式的影响较小。水田中的丘陵水田是赣江水体TP和丰水期CODMn的主要来源;平原水田是枯水期CODMn的主要来源。居民建设用地中的城镇用地是赣江水体TP、NH+4-N和丰水期CODMn的主要来源,农村用地是CODMn的主要来源。水域中的水库坑塘是赣江水体TP和丰水期NH+4-N、CODMn的主要来源。     

Studies on the biology of Chaoborus flavicans (Meigen) (Diptera: Chaoboridae) in a fish-free eutrophic pond,Japan

Population dynamics of Chaoborus flavicans larvae of various instars was studied from November 1986 to December 1987 in a eutrophic, fish-free pond, Japan. First and 2nd instar larvae were observed from late April to late October, indicating a reproductive period of about half a year. C. flavicans overwintered in the 4th instar larvae. In water column samples, total density of all instars was 680–23680 m^-2, and pupal density 0–2600 m^-2; larvae of the 1st, 2nd, and 3rd instars showed 5–6 density peaks in 1987, suggesting that 5–6 generations occur during a year (peaks of the 4th instar larvae were not clear, probably due to their longer development than those of younger instars). In sediment samples, no 1st and 2nd instar larvae were found, 3rd instar larvae were found occasionally but density of the 4th instar larvae was 280–18600 m^-2, and pupal density varied between 0–502 m^-2. Fouth instar larvae accumulated in sediment in the cold season and in the water column in the warm season; high temperature and low oxygen concentration were the most important factors limiting the distribution of larvae in the sediment in summer in the NIES pond. The dry weight of total C. flavicans larvae was 0.08–4.2 g m^-2 in sediment samples and 24–599 μg l^-1 (0.10–2.40 g m^-2) in water column samples. Comparisons of maximum densities in the NIES pond in different years and in waters of different trophic status show that density is generally higher in eutrophic than in oligotrophic habitats. This revised version was published online in July 2006 with corrections to the Cover Date.     

Effects of periodic flooding on the water chemistry and primary production of the Mapire systems (Venezuela)

The Mapire river mouth forms a complex floodplain system, where the river behaves as a river during the dry season, but changes to a transient lake which partially covers the inundation forest during the rainy season. Thus, we expected changes in water chemistry and a gradual increase of primary production during high waters. The system was sampled monthly for one year; two floodplain lakes were also studied for comparative purposes. Variations in the concentration of macro- and micronutrients occurred in a pulse-like manner and seemed to relate to mechanisms at work in the transient lake. Dissolved oxygen showed a stratification with low values at the bottom, but never reached anoxia. Net and gross primary production and respiration did not show any clear spatial pattern, reflecting a mosaic of different biochemical states within the transient lake. Heterotrophy tended to prevail in the transient lake, while autotrophy dominated floodplain lakes.     

Composition of phytobenthos in “lab-lab”, a periphyton-based extensive aquaculture technology for milkfish in brackishwater ponds during dry and wet seasons

The study compared the species composition of phytoperiphyton (“lab-lab”) present in ponds when gradually filled with water weekly to depths of 5, 10, 15 and 30 cm between the wet and dry seasons, for one month before the stocking of fish was studied. This was done during the dry season (March–April, 2003) and wet season (June–July, 2002). Periphyton was allowed to grow on 24 artificial substrates set at equal distances in a 1000 m² pond. “Lab-lab” that colonized the artificial substrates and that on the pond surrounding the substrates were scraped off from a measured surface area. Simultaneously, water was collected for the analysis of physical, chemical and biological parameters. Sampling was done bi-weekly coinciding with 2 and 7 days submergence at a desired depth before adjusting the water level. The major algae consisted of the diatoms (Bacilliarophyta), the blue green algae (Cyanobacteria), and the green algae (Chlorophyta). The diatoms were dominant during the dry season while the cyanobacteria dominated during the wet season. Twenty eight genera were observed during the dry season and 25 genera were noted in the wet season. Variation in genera and density that were observed every sampling period, was influenced by environmental conditions and the incoming water. The total algal density ranged from 100.7 × 10⁸ – 855.1 × 10⁸ and to 24.7 × 10⁸ – 83.9 × 10⁸ organisms.m⁻² during the dry and wet seasons, respectively. Presented at the 6th Meeting of the Asian Pacific Society of Applied Phycology, Manila, Philippines.     

Hydraulic Phases, Persistent Stratification, and Phytoplankton in a Tropical Floodplain Lake (Mary River, Northern Australia)

The Mary River, in the Australian wet/dry tropics, flows seasonally. When the river ceases flowing in the dry season, a series of isolated lakes remain along the river’s main floodplain channel. The limnology of a channel lake, which is 14 km long and 6-9 m deep in the dry season, was examined between April and December 2000. Four hydraulic phases were identified, these being (1) riverine (April), (2) riverine to lake transition (May), (3) lake (June–late-November), and (4) lake to riverine transition (late-November–December). These phases differ with respect to their duration and flow direction from lakes located on tropical floodplains of perennially flowing rivers. Despite the variable hydraulic conditions, the main channel remained thermally stratified, with only infrequent and short-lived deep mixing events, and sufficient light for photosynthesis in the diurnal mixed layer. During the period of isolation and in contrast to floodplain lakes in tropical South America, the depth of the Mary River channel lake always exceeded, by at least 2-fold, the depth of the diurnal mixed layer. The water quality (conductivity, dissolved oxygen, pH, Si and water clarity) and phytoplankton assemblage of the channel lake was primarily driven by its hydraulics, though this was not evident for the channel’s nutrient concentrations. Dissolved oxygen concentrations during lentic conditions were double values during the riverine and transition phases. This was attributed to the cessation of inflowing waters with a high biological oxygen demand, and enhanced photosynthetic activity of higher concentrations of phytoplankton retained under lentic conditions. The channel’s phytoplankton assemblage reflected the channel’s hydraulics, with the most common phytoplankton throughout the study period belonging to functional groups L_o(Peridinium inconspicuum), W1 (euglenoids), W2 (Trachelmonas) and Y (Cryptopmonas, Rhodomonas), with groups A (Acanthoceras) and D (Nitzschia agnita, Synedra alna) prominent during the lentic phase. Despite persistent stratification under lentic conditions, there was no clear evidence of autogenic succession or domination by any single phytoplankton functional group.     

Polymixis of a shallow lake (Groβer Müggelsee,Berlin) and its influence on seasonal phytoplankton dynamics

The polymictic properties of Lake Müggelsee, a eutrophic shallow lake in Berlin, are described by the water column stability (N ²) and gradients in saturation of oxygen at the deepest site of the lake (7.5 m). Mixing and stratification changed irregularly up to 7 times during the vegetation season (April to September), as was indicated by all of the stratification parameters. Thermally stable conditions generally lasted 1–2 weeks. A maximum of 5 weeks stratification was observed in 1982.In order to investigate the response of algal development, the internal rates of change of the dominant algal species in the lake during the vegetation period were estimated from weekly measurements of phytoplankton biomass from 1980 to 1990. The necessity taking a mixed sample in a shallow lake is discussed. The polymictic properties favoured the development of specific blue-green algal species; there dominance was also favoured by the trophic conditions. Among the dominant blue-greens the growth of Limnothrix redekei was independent of polymixis whereas stratification supported the starting conditions for the summer blue-greens Aphanizomenon flos-aquae and Planktothrix agardhii. After these algae reached a distinct level of biomass, they grew under mixing as well as under stratified conditions.For the development of solitary centric diatoms during summer regulation by growth restriction through nutrient limitation, esp. dissolved silicon was more important. However, Melosira sp. developed well under stratified conditions but collapsed due to increased sinking losses when the water column became too stable.An attempt is made to apply Reynolds' possibility matrix of the most likely phytoplankton assemblages as a function of nutrients and mixing in the shallow Lake Müggelsee.     

Comparison of water level and eutrophication indicators during the wet and dry period in a eutrophic urban lake

Eutrophication modifies lakes' ecological balances and threatens its viability. To date, eutrophication management strategies have been related to nutrient reduction in the lakes' water column. However, nutrient reduction strategies are complicated by the variations of the lake's water level, nutrient concentration, and eutrophication symptom, which are primarily known to be influenced by the local rainfall patterns. Therefore, this study aimed to compare the variability of water level, total phosphorus, and total chlorophyll-a concentrations in Slim River Lake during wet and dry seasons. In this study, water sampling and depth measurements were carried out from six sampling points for 1 year. Water samples were used to quantify total phosphorus and total chlorophyll-a. Our results showed that mean water levels in the studied lake ranged from 1.36 m to 5.46 m in the wet season and from 1.31 m to 5.41 m in the dry season, which implicated no significant difference (p > .05) between seasons in most sampling points. Total phosphorus present at concentrations exceeding 10 mg/L and showed small variations between wet and dry seasons. Mean total phosphorus concentrations varied from 10.55 mg/L to 26.66 mg/L in the wet season and 10.77 mg/L to 21.76 mg/L in the dry season and showed no significant difference between seasons. In addition, mean chlorophyll-a concentrations ranged from 14.35 mg/m³ to 180.13 mg/m³ and from 14.15 mg/m³ to 39.27 mg/m³ in wet and dry seasons, respectively. Chlorophyll-a concentrations showed significant differences (p < .05) between seasons in the deepest sampling points in the lake. The observed seasonal variations in total chlorophyll-a suggest the importance of algae monitoring during the wet season even when no apparent surge of phosphorus concentration is detected.     

A Guide to Tropical Freshwater Zooplankton

The levels of Cu, Fe and Zn in water, sediment and Cynothrissa mento, a commercially important fish species, were investigated from April 2007 to March 2008 at three sites in Ologe Lagoon. Metal levels in the water column and C. mento were lower than the FAO/WHO drinking water and food safety limits. In most cases, metal concentrations were highest during the dry season and lowest in the wet season. Metal concentrations in sediment and C. mento varied significantly among the sampling stations, but in the water column only the levels of Fe and Zn differed significantly between stations. Concentrations of metals in C. mento were significantly affected by wet and dry seasons. The metal concentrations in Ologe Lagoon were lower than the values reported in some Nigerian water bodies. While the Ologe Lagoon system contains the heavy metals studied, their levels in the water, sediment and fish are still within acceptable limits.     

Effects of thermal stratification and mixing on reservoir water quality

In this study, the effect of thermal stratification on water quality in a reservoir has been investigated by field observations and statistical analysis. During the summer period, when stratification is evident, field observations indicate that the observed dissolved oxygen concentrations drop well below the standard limit of 5 mg l⁻¹ at the thermocline, leading to the development of anoxia. The reasons for variations in the dissolved oxygen concentrations were investigated. Variations of air temperature and other meteorological factors and lateral flows from side arms of the lake were found to be responsible for the increase of dissolved oxygen concentrations. It was also observed that turbidity peaked mostly in the thermocline region, closely related to the location of the maximum density gradient and thus low turbulence stabilizing the sediments in the vertical water column. Relatively cold sediment-laden water flowing into the lake after rain events also resulted in increased turbidity at the bottom of the lake. Nondimensional analysis widely used in the literature was used to identify the strength of the stratification, but this analysis alone was found insufficient to describe the evolution of dissolved oxygen and turbidity in the water column. Thus correlation of these parameters was investigated by multivariate analysis. Fall (partial mixing), summer (no mixing), and winter (well mixed) models describe the correlation structures between the independent variables (meteorological parameters) and the dependent variables (water-quality parameters). Statistical analysis results indicate that air temperature, one day lagged wind speed, and low humidity affected variation of water-quality parameters.     

Spatiotemporal shift in sulphide concentration in hypolimnic water column in Lake Hiruga,a saline lake in Japan

A 2-year study was designed to investigate the spatiotemporal distribution of sulphide in the water column of a saline lake, Lake Hiruga in Fukui, Japan. Water samples were collected in Lake Hiruga periodically between April 2014 and March 2016 at appropriate depths with simultaneous in situ measurements of physicochemical parameters such as temperature, salinity and dissolved oxygen (DO) concentration. Sulphide started accumulating in the hypolimnion in September 2014, whilst that did in June 2015. The sulphide concentrations increased toward the end of December in both 2014 and 2015. Then the sulphide disappeared from the hypolimnion in April 2015 and February 2016. The difference in starting season of the sulphide accumulation was closely connected with DO concentrations in the hypolimnion in circulation periods of winter season. The DO concentrations were regulated by water column stability before thermal stratification periods. Our analysis of the sulphide distribution for the 2 years of the study suggested that accumulation of sulphide in the hypolimnion during stratified periods appears to be associated with strength of water column stability in the winter-spring seasons before stagnant periods in Lake Hiruga.     

Wildlife–water quality interactions in the Serengeti National Park, Tanzania

Surveys of water quality in the surface waters in the Serengeti National Park were undertaken in both the wet and dry seasons of 1996. Most rivers were ponded, with ponds having a flushing time of 1 month in the wet season and zero flushing in the dry season. pH values varied spatially from extremely alkaline conditions (pH>10) in the southern plains to acidic conditions in the northern region (pH=5·9). In the southern plains at the end of the dry season the salinity of surface waters was high (5–17‰) while there was abundant surface water and the zebras and wildebeest had started to migrate away, a finding suggesting excessive salinity may be the trigger initiating the annual migration. Most surface waters were heavily eutrophicated as a result of animal dung. As a result, the dissolved oxygen concentration near the surface fluctuated widely between 1 and 200% of saturation, smaller values occurring deeper in the water column. Stirring and mechanical aeration by hippos, crocodiles and mammals crossing the rivers prevented the formation of anoxic conditions. The oxygen stress was measurably lessened in wetland‐fringed water bodies as a result of filtering. Light penetration was high (≥10 cm) in saline waters as a result of flocculation enhanced by bacteria and vegetation detritus, elsewhere the euphotic zone was less than 1 cm thick and the waters generally inhospitable to aquatic life. The discontinuity between wooded savanna and grassland coincides with the occurrence of alkaline, high salinity waters in the dry season.     

Bacterial community composition in Lake Tanganyika: vertical and horizontal heterogeneity

Vertical and latitudinal differences in bacterial community composition (BCC) in Lake Tanganyika were studied during the dry season of 2002 by means of denaturing gradient gel electrophoresis analysis of PCR-amplified 16S RNA fragments. Dominant bands were sequenced and identified as members of the Cyanobacteria, Actinobacteria, Nitrospirae, green nonsulfur bacteria, and Firmicutes divisions and the Gamma- and Deltaproteobacteria subdivisions. The BCC in the lake displayed both vertical and latitudinal variation. Vertical changes in BCC were related to the thermal water column stratification, which influences oxygen and nutrient concentrations. Latitudinal variation was related to upwelling of deep water and increased primary production in the south of the lake. The number of bands per sample increased with bacterial production in the epilimnion of the lake, suggesting a positive diversity-productivity relationship.     

Longitudinal changes in protistan bacterivory and bacterial production in two canyon-shaped reservoirs of different trophic status

The drastic interactions of weather as El Niño events with catchment and hydrological processes can cause unexpected changes in physical, chemical and biological properties of freshwater aquatic ecosystems. The severe drought during 1998–1999 in the northeastern region of Brazil induced ecological changes in numerous reservoirs as in Tapacurá reservoir, one of the biggest drinking-water suppliers in Pernambuco state. Investigations were based on monthly sampling over 2 years (May 1998–May 2000) conducted at 3 representative stations with 3 sampled depths through the water column (0.5 m, middle and 0.5 m above the bottom). Temporal changes in ecological processes, especially stratification, were driven by two major precipitation patterns, with an initial marked dry period (period 1) followed by a rainy season (period 2). Dissolved oxygen and pH variations, higher conductivity and alkalinity values, higher concentrations of particulate organic material (carbon, nitrogen and phosphorus) and higher levels of algal biomass (chlorophyll a) characterized the dry period (May 1998–May 1999). During this phase of low water level when the reservoir storage capacity reached a minimum of 3.9%, the concentrations of chlorophyll a gradually increased with a cyanobacterial bloom (Cylindrospermopsis raciborskii) noted in April 1999. The decline in chlorophyll a and particulate organic matter were observed as a result of the first rains in May–June 1999, with the drastic changes of quality of matter (higher particulate C/N ratio). After a phase characterized by the entire water column turning anoxic, a second phase in the stratification process could be identified from June 1999 with the pronounced rainfalls accompanied by an overturn event. Annual rainfall deficit and lack of reservoir water renewal in 1998–1999 linked to the 1997 El Niño consequences were important determinants of high eutrophication levels and drastic ecological modifications in Tapacurá reservoir.     

Fingerprinting environmental conditions and related stress using stable isotopic composition of rice (Oryza sativa L.) grain organic matter

Sea level rise (SLR) is a primary factor responsible for inundation of low-lying coastal regions across the world, which in turn governs the agricultural productivity. In this study, rice (Oryza sativa L.) cultivated seasonally in the Kuttanad Wetland, a SLR prone region on the southwest coast of India, were analysed for oxygen, hydrogen and carbon isotopic ratios (δ¹⁸O, δ²H and δ¹³C) to distinguish the seasonal environmental conditions prevalent during rice cultivation. The region receives high rainfall during the wet season which promotes large supply of fresh water to the local water bodies via the rivers. In contrast, during the dry season reduced river discharge favours sea water incursion which adversely affects the rice cultivation. The water for rice cultivation is derived from regional water bodies that are characterised by seasonal salinity variation which co-varies with the δ¹⁸O and δ²H values. Rice cultivated during the wet and the dry season bears the isotopic imprints of this water. We explored the utility of a mechanistic model to quantify the contribution of two prominent factors, namely relative humidity and source water composition in governing the seasonal variation in oxygen isotopic composition of rice grain OM. δ¹³C values of rice grain OM were used to deduce the stress level by estimating the intrinsic water use efficiency (WUE_i) of the crop during the two seasons. 1.3 times higher WUE_i was exhibited by the same genotype during the dry season. The approach can be extended to other low lying coastal agro-ecosystems to infer the growth conditions of cultivated crops and can further be utilised for retrieving paleo-environmental information from well preserved archaeological plant remains.     

贵州草海岩溶湿地水体不同形态氮的时空分布特征

为揭示贵州草海岩溶湿地水体中不同形态氮时空分布特征及其变化规律,通过网格布点法采集了该湿地丰、枯水期表层水,并对其不同形态溶解性氮含量进行了测定,运用ArcGIS统计模块分析了草海不同形态氮的时空分布特征,并分析了各形态氮与环境因子之间的相关性.结果表明:草海水体丰、枯水期TN的平均含量分别为(0.96±0.52)、(...     

Bacterial Community Composition in Lake Tanganyika: Vertical and Horizontal Heterogeneity

Vertical and latitudinal differences in bacterial community composition (BCC) in Lake Tanganyika were studied during the dry season of 2002 by means of denaturing gradient gel electrophoresis analysis of PCR-amplified 16S RNA fragments. Dominant bands were sequenced and identified as members of the Cyanobacteria, Actinobacteria, Nitrospirae, green nonsulfur bacteria, and Firmicutes divisions and the Gamma- and Deltaproteobacteria subdivisions. The BCC in the lake displayed both vertical and latitudinal variation. Vertical changes in BCC were related to the thermal water column stratification, which influences oxygen and nutrient concentrations. Latitudinal variation was related to upwelling of deep water and increased primary production in the south of the lake. The number of bands per sample increased with bacterial production in the epilimnion of the lake, suggesting a positive diversity-productivity relationship.     

Oxygen depletion induced by adding whey to an enclosure in an acidic mine pit lake

Neutralization of acidic mine pit lakes by biotechnological means results in the production of labile metal-sulfides. These reaction products can theoretically be stored sustainably in the lake, provided reducing conditions are maintained at the lake bottom. In a field mesocosm experiment, we tested, if reducing conditions can be maintained in an acidic mine pit lake by the addition of a complex organic substrate.An enclosure of 30 m diameter was covered by a floating foil, and whey was repeatedly added to the water column to stimulate microbial respiration. A suspension of whey was successfully mixed into the enclosure by means of a boat motor. Whey was completely dissolved and subsequently consumed by microbial respiration in the water column. This resulted in oxygen consumption leading to anoxic conditions. About 10 mmol m⁻² d⁻¹ oxygen permanently entered the enclosure from the atmosphere, while a minor amount of oxygen was produced by primary production. By careful monitoring and repeated additions, it was possible to keep the bottom of the enclosure permanently anoxic, even during mixing periods in autumn and spring. Fe³⁺, however, was not reduced significantly. A laboratory experiment revealed that microbial iron reduction was inhibited by both low concentrations of organic substrates and low temperature. Since Fe^III is a potential oxidizing agent, it is questionable, if the stability of metal-sulfides in acidic mine pit lakes can be increased by the addition of complex organic substrates.     

Seasonal variations in the density of a small West African lake

The seasonal variation in the vertical density profile of Lake Asejire waters was investigated from February 1974 to February 1976.The range of density was of the magnitude of 2.0 mg/cc, at the temperatures of Lake Asejire (21°C–32°C). Water densities were generally lower in the dry season when aquatic temperatures were high and generally higher in the rainy season when aquatic temperatures were low.During the dry season, the pycnocline was observed at 3m–7m in 1974 and 6 m–9 m in 1975. During periods of density stratification, the water body was clearly layered.Seasonal longitudinal differences in water density were such that during the floods of 1975 the inflow station waters had a higher density than the dam-site waters which however had a concomitantly higher conductivity. This suggests the insignificance of conductivity in water density changes in a lake with rather low conductivity values.