Eutrophication of lakes in urbanized areas: The case of Yaounde Municipal Lake in Cameroon, Central Africa

A 1 year qualitative and quantitative evaluation of phytoplankton and chlorophyll‐a, as well as some physicochemical parameters, was recorded in a shallow tropical lake in Cameroon: the Yaounde Municipal Lake. Physicochemical measurements also were regularly made in its main tributary (Mingoa Stream). These analyses aimed to assess the lake's trophic status and to propose measures for controlling its degradation process. The Secchi disk transparency was low and rarely exceeded 100 cm. Conductivity was higher near the lake bottom. The oxygen deficiency, and sometimes anoxia, recorded from a 2.5 m depth leads to high quantities of ammonium‐nitrogen. The total phosphorus concentrations varied from 80–2290 µg P L^?1 and the total Kjeldhal nitrogen concentrations fluctuated between 3 and 15 mg  L^?1. Upstream to the lake, in the Mingoa Stream, total phosphorus concentrations ranged from 0.6–3.8 mg P L^?1 and total Kjeldhal nitrogen concentrations ranged from 10–22 mg  L^?1. There are up to 102 phytoplankton‐specific taxa, with Euglenophyta and Chlorophyta particularly more diversified. The phytoplankton biomass and chlorophyll‐a concentrations reached 225 µg mL^?1 and 566 mg m^?3, respectively. The analyses pointed out the allogenic nature of the functioning of this ecosystem as a result of bad waste management in the surrounding landscape. Urgent actions need to be undertaken in order to rehabilitate this lake, which rapidly shifted to a hypertrophic status.     

Phytoplankton composition and biomass in tropical soda Lake Shala: seasonal changes in response to environmental drivers

Although soda lakes are valuable, sensitive aquatic resources where phytoplankton play a decisive role for the entire ecological functions, they are among the least‐studied ecosystems. Seasonal variations in phytoplankton composition, abundance and biomass in relation to some environmental parameters of the little known, deep, large, volcanic and saline–alkaline Lake Shala were investigated over an annual cycle. The lake phytoplankton community consisted of relatively diverse taxa (23) belonging to Bacillariophyceae, Cryptophyta, Cyanoprokaryota and Dinophyta. Bacillariophyceae and Cryptophyta were the dominant groups throughout the annual cycle, accounting for about 57% and 22% of the total number of species, and 28% and 69% of the total abundance of the phytoplankton community, respectively. Cryptomonas spp. were most abundant throughout nearly all months, contributing about 59%–95% of total phytoplankton abundance, followed by Thalassiosira sp. (1%–35%). The chlorophyll‐a concentration, as a proxy for algal biomass, was generally low (mean 17 μg L^?1), exhibiting only small seasonal variation. The strong, inverse relation of chlorophyll‐a with water transparency (r = ?0.69; n = 11) and the persistent dominance of species adapted to low‐light conditions and mixing suggest the overriding importance of these factors in controlling the lake's phytoplankton. The results of the present study generally suggest the phytoplankton composition and biomass in Lake Shala exhibited muted seasonal changes, despite the environmental perturbations, probably because of the lake's high buffering capacity against allochthonous impacts because of its voluminous nature.     

Long‐term irreversible changes in a lake ecosystem affected by the Indian Ocean Tsunami

This study focused on ecosystem responses to the environmental perturbations caused by the 2004 Indian Ocean Tsunami in a small lake that was a freshwater body in 1996, prior to the tsunami. The physicochemical and biological characteristics of Kokilamedu Lake (KKM) revealed drastic changes, compared with pretsunami conditions. Monthly average observations on water quality indicated the electrical conductivity of the water increased steeply to 17.41 mS cm^?1 in 2009, from the lowest pretsunami value of 1.83 mS cm^?1 (range of 1.83–5.25 mS cm^?1). Simultaneously, the nitrate + nitrite (NO₃ + NO₂) values increased significantly from 0.49 μmol L^?1 in 1996 to 74.47 μmol L^?1 in 2006. Silicate (SiO₄‐Si) exhibited a dramatic increase in concentration, from an average of 64.87 μmol L^?1 in the pretsunami period to 309.71 μmol L^?1 the post‐tsunami period (2009–2010). Inorganic phosphate had increased to a maximum of 9.59 μmol L^?1 from a pretsunami maximum of 1.09 μmol L^?1. The chlorophyll‐α concentrations did not respond to the increased nutrient stoichiometry of the lake. There was a decreased chlorophyll‐α concentration under post‐tsunami conditions. The recent infilling of the lake with sediment during the tsunami, associated with wind‐driven resuspension reduced the light penetration. There was a significant improvement in dissolved oxygen concentrations (2006–2010 average of 8.27 mg L^?1) in the lake, however, compared with the pretsunami values (1994–1995 average of 5.94 mg L^?1). The algal component is now dominated by blue‐green algae, while green algae had dominated in the pretsunami period. Pre‐ and post‐tsunami observations from a control site did not exhibit such dramatic shifts from the 1995 and 1996 conditions, whereas a shift was apparent in the case of KKM. Certain marine fishes have adapted to this altered ecosystem. These marine species encountered (Elops machnata, Cociella punctata, Sphyraena jello, Platycephalus indicus, Glossogobius giuris) might have been recruited during the intrusion of the tsunami waves.     

洪泽湖浮游藻类变化动态及影响因素

针对2011年7月至2013年10月在洪泽湖11个采样点20次采样获得的湖水表层浮游藻类和水质理化指标数据,采用Shannon-Wiener指数H和优势度指数Y进行数据统计处理,分析洪泽湖浮游藻类时空分布动态及其影响要素。结果表明:洪泽湖浮游藻类共有7门60属144种,主要包括绿藻门66种、硅藻门36种、蓝藻门23种、裸藻门13种。浮游藻类群落具有明显的时空异质性,物种丰富度夏季最高,秋季中等,冬春季最低;西北部(成子湖区)和河流入湖口(溧河洼)高,湖心和河流出湖口(蒋坝)低;浮游藻类细胞密度全湖平均值变幅为157万~604万个/L,夏秋高,冬春--低;成子湖区等静水区高,入湖和出湖口低。浮游藻类组成和细胞密度的时间动态与温度、水位及营养盐(TN、TP等)的季节差异有关,而其空间动态与水动力因素和营养盐(NH3-N)的空间差异有关。建议限制洪泽湖营养盐陆地输入,合理调控洪泽湖水动力,以遏制洪泽湖蓝藻水华的发生。     

Changes in pollution indicators in Lake Victoria,Kenya and their implications for lake and catchment management

Monitoring of aquatic pollution is important for ascertaining the relationship between fisheries and the general ecosystem health of a lake. This study evaluated the use of changes in pollution indicators in Lake Victoria, Kenya, as a decision support tool for fisheries management and productivity. Principal component analysis (PCA; R² ≥ 0.5, P < 0.05) of physical and chemical parameters delineated sampling sites into ecological cluster zones consisting of the inner gulf (C1), mid‐gulf (C2) and open lake (C3). Test results for lead (Pb) and mercury (Hg) levels in the Nile perch tissues were found to be compliant with EU standards. The inner and mid‐gulfs of the Winam Gulf had high levels of total (1818.8 ± 102–1937.78 ± 94 cfu 100 mL^?1) and faecal (390 ± 21 cfu 100 mL^?1) coliforms attributable to urban sewage and industrial effluents exceeded WHO standards. Similarly, Winam Gulf was more polluted than the open lake, with higher total phosphorus and nitrogen concentrations, turbidity levels and electrical conductivity. Low phytoplankton biovolume and a low number of macroinvertebrates genera, and high zooplankton densities and pollution‐tolerant catfishes (e.g., Schilbe victoriae; Clarias gariepinus) were observed in Winam Gulf. Faecal coliforms and dissolved oxygen influenced the abundance of tolerant fish species (e.g., S. victoriae) in the lake. This study indicated a declining trend of ecological integrity in the Winam Gulf, compared with the open waters of Lake Victoria. An integrated management approach directed to minimizing pollution levels, especially in the Winam Gulf, is recommended to enhance fishery production.     

Recruitment of Heliozoa, rhizopods and rotifers from the sediments of an extremely acidic lake during spring and early summer

The goal of this study was to investigate the recruitment of zooplankton from the littoral sediment of Lake 111, an acidic lake in north‐east Germany, in April (spring) and June (early summer), and its role in coupling the benthos and the pelagic. Maximum heliozoan and rhizopod recruitment occurred in early summer from sediment cores incubated at ambient water temperatures (20°C). Conversely, recruitment of the rotifer Cephalodella sp. was highest in spring at ambient spring temperatures of 12°C. A combination of passive and active recruitment processes is likely responsible. The seasonal abiotic and biotic sediment characteristics were relatively constant and therefore not likely responsible for the observed temporal recruitment pattern. The sediment water and carbon content ranged from 20 to 50% (mean = 29 ± 6% standard deviation) and 2–12% (mean = 5 ± 2% standard deviation), respectively. Similarly, there was little variation in the chlorophyll‐a (mean = 0.2 ± 0.2 µg Chl‐a g^?1 dry weight ≡ 6.1 ± 3.9 mg Chl‐a m^?2). The in situ sediment bacterial density (0.82 × 10⁹ ± 0.26 × 10⁹ g^?1 dry weight ≡ 1.01 × 10⁹ ± 0.34 × 10⁹ cells cm^?3) was high. In contrast, the abundance of zoobenthos and their resting stages was low (< 25 individuals cm^?3, and mean of 90 ± 75 cysts cm^?3, respectively), with no temporal pattern being observed. Temperature was the only abiotic factor influencing recruitment. This study suggests that, even in relatively young, chemically extreme lakes, the benthos can play an important role in whole lake microbial processes and zooplankton community composition. Such benthic repositories of resting stages potentially provide protection against adverse environmental changes.     

Restoration of small lakes through cooperative management: A suitable strategy for poverty‐laden areas in developing countries?

Lake Rupa is a small, subtropical, shallow lake with a surface area of 100 ha situated 600 m a.s.l. in Central Himalaya, Nepal. This degraded lake was studied between 2000 and 2006, with the goal of determining whether or not it could be restored by a community‐based cooperative of local people living in its catchment. Main threats to the lake, its aquatic life and its very existence include encroachment by excessive aquatic vegetation, sedimentation, and low in‐lake concentrations of dissolved oxygen (DO). Small lakes (≤ 500 ha) are relatively prone to the process of ‘succession and climax’, although they also can be of social, environmental and ecological importance. Thus, their disappearance could spark social chaos and disorder in areas already having to cope with other rapid environmental changes. This means that credible mechanisms for revitalizing or protecting small lakes are an important goal. Accordingly, a lake cooperative of 329 households living in close vicinity to Lake Rupa was formed in 2002, by prioritizing traditional fishers, women and other deprived community members with the goals of respecting the citizenry and equity. Following the cooperative's campaigns directed at weed removal and fish stocking, encroachment of vegetation on the lake margins was halted, its aquatic weeds became under control, and its fisheries improved. In addition to the cooperative's restoration activities, several water quality parameters, including water transparency and DO and chlorophyll‐a concentrations, were monitored on a monthly basis. The measured water transparency was inconsistent, exhibiting large variations between 2000 and 2003. Relatively low, but consistent, values, however, were measured after 2004. The results imply that the removal of weeds, and sufficient nutrients and solar radiation are subsequently available to the lake's phytoplankton communities. Supporting this notion is that the chlorophyll‐a concentration spiked to 205 µg L⁻¹ in November 2006, the water transparency became consistent, and the DO concentration increased to >3.8 mg L⁻¹ during the critical months (March–May) after 2004. These water quality indicators indicated improvement in the degraded Lake Rupa, suggesting that the establishment of cooperatives such as that highlighted in this study could be a powerful and sustainable mechanism for restoring degraded lakes in similar socioeconomic settings by maintaining equity, by connecting communities with their resources, and by facilitating integrity, equity, citizenry and social justice.     

Environmental characteristics and phytoplankton productivity of a shallow Ramsar‐site floodplain in the western Himalaya

Hokersar wetland (altitude of 1584 masl), a shallow (0.5 m) floodplain waterfowl habitat in Kashmir, India, gained international importance in 2005 with its declaration as a Ramsar site. Although isolated in the western Himalayan mountains, it is being impacted by a silt‐laden river (Doodhganga). Human activities, not the least of which is affected by the closeness of the wetland to suburban areas, have gradually altered its trophic state. Its alkaline‐ and calcium‐rich waters contain many planktonic diatoms, green algae and cyanobacteria. Nitrate–nitrogen and total phosphorus concentrations are high (370–4750 and 101–968 μg L^?1, respectively). A pronounced seasonal cycle in phytoplankton production (g C m^?2 day^?1) was evident in a minimum value of 0.38 (Jan–Feb, 2002) and a maximum value of 4.02 (July, 2002), closely paralleling the seasonal cycles of temperature and light. The photosynthetic efficiency of the wetland was highest (1.97%) during the summer. The annual phytoplankton production of this wetland ecosystem was 210 × 10² KJ m^?2, suggestive of a meso‐eutrophic status.     

Microbial quality of water in Barekese reservoir and feeder streams in Ghana

Concern is growing in Ghana about the high incidence of water‐related diseases and deaths resulting from use of water contaminated with microorganisms. This water often remains unsafe for human consumption because of inadequate decontamination of microbially infected water. Data are presented in this study on the microbial quality of Barekese reservoir, which is the main drinking water reservoir for the city of Kumasi and its environs (population = 2, 5 million), and nine feeder streams in its catchment. Water samples were collected from 13 sampling sites and analysed for Escherichia coli, total and faecal coliforms. Furthermore, 18 faecal coliform isolates were randomly selected from all sampling sites and confirmed as E. coli using Analytical Profile Index 20E system. Escherichia coli, total and faecal coliforms were enumerated using the most probable number method. Mean bacterial indicator numbers from all sampling sites ranged from 1.45 × 10⁴ to 9.50 × 10⁷ 100 mL^?1 for total coliforms, 1.60 × 10³ to 9.00 × 10⁵ 100 mL^?1 for faecal coliforms and 1.50 × 10¹ to 9.50 × 10³ 100 mL^?1 for E. coli. Indicator numbers exceeded the World Health Organization (0.100 mL^?1) for E. coli and Ghana Water Resources Commission‐Target Water Quality Range of 5–100 100 mL^?1 for total coliforms and 0.100 mL^?1 for faecal coliforms in water used for domestic purposes. The identified isolates in Barekese Reservoir and its feeder streams belonged to Serratia, Enterobacter, Citrobacter, Salmonella and Klebsiella genera. Bacterial numbers were significantly (P < 0.05) higher in the feeder streams, compared with Barekese Reservoir water. This finding indicates the feeder streams pose health risks to local communities that withdraw water from them. The results of this study highlight the urgent need to raise public awareness on the adverse effects of water‐quality degradation through improper waste disposal methods in order to decrease the cost of treating the reservoir water.     

Estimation of bacterial production through the coupling of (methyl 3H) Thymidine incorporation and several parameters over a Microcystis aeruginosa bloom in a hypereutrophic reservoir

The coupling of the changes in bacterial quantitative and metabolic aspects during Microcystis aeruginosa bloom conditions together with several environmental parameters was studied in the hypereutrophic Villerest reservoir, France. Bacterial abundance varied from 5.20 to 21.28 × 10⁶ bacteria-mL^?1, while bacterial biomass ranged between 75 and 507 μg C mL^?1. These results confirmed the highly eutrophic status of the Villerest reservoir. The relative quantitative importance of attached bacteria increased as Microcystis proliferated. Methyl-³H incorporation and D-(U-¹⁴C) glucose uptake varied from 8.99 to 60.57 × 10⁶ cells-mL^?1, and 0.016 to 1.587 μg CL^?1 h^?1, respectively. Correlations between several abiotic and biotic parameters showed that phytoplankton regulated bacterial growth by releasing organic carbon which is directly uptaken by heterotrophic bacteria. In the hypolimnion, the sedimentation of decaying algae represented a substantial substrate for bacterial growth.     

Spatial and seasonal variations in phytoplankton community structure in alkaline–saline Lake Nakuru,Kenya

Spatial and seasonal variations of phytoplankton, expressed in terms of species composition, cell density, biovolumes and biomass, collected at 10 sampling sites in alkaline–saline Lake Nakuru, Kenya, were investigated monthly from March 2004 to February 2005, in relation to selected physical and chemical water quality parameters. A total of eight species, belonging to five genera and three classes, were recorded. The Cyanophyceae comprised the bulk of the phytoplankton, followed by Euglenophyceae, while Bacillariophyceae were the least abundant. Arthrospira fusiformis was the most dominant species, in terms of density, among the Cyanophyceae, while Euglena spp. and Navicula elkabs dominated the Euglenophyceae and Bacillariophyceae density, respectively. Seasonal variations in phytoplankton species composition, density and biomass were significant (P < 0.05) with minimum cell density and biomass occurring during the dry season, and being maximum following the end of the two rainy seasons from May to November 2004, suggesting the possible influence of various environmental factors on the lake. Sampling sites located close to the inlets of inflowing rivers exhibited significantly (P < 0.05) higher phytoplankton cell density and biomass than those by the inshore sites. Soluble reactive phosphorus, ammonia‐nitrogen, conductivity and total alkalinity accounted for most of the variations in the cell densities of phytoplankton species.     

The occurrence of microcystin-LR in Lake Chivero, Zimbabwe

Lake Chivero is a eutrophic reservoir, initially constructed to supply drinking water to the City of Harare, Zimbabwe. Blooms of blue‐green algae have been a problem in the lake for many years and concern has been expressed about the toxins produced by Microcystis spp. The concentrations of the toxin, microcystin‐LR, produced in cultures of Microcystis aeruginosa from Lake Chivero, were investigated from March–April 2003. Microcystin‐LR was found in algal cells cultured from the lake water in concentrations ranging between 18.02 and 22.48 µg L^?1, with a mean concentration of 19.86 µg L^?1. These concentrations are the highest recorded to date for the lake, raising concerns about the possible effects of the toxin on the health of people who are drinking the water. Based on these study results, there is a need to control eutrophication, reducing algal blooms in order to prevent their potentially detrimental effects from blue‐green algal toxins produced under such conditions.     

Assessment of the impacts of a new artificial lake mouth on the hydrobiology and fisheries of Chilika Lake, India

The degraded state of the fragile ecosystem of Chilika Lake, located on the east coast of India, was undergoing restoration through an effective hydrological intervention during the year 2000. Studies on the lake’s hydrobiology and fisheries for the period of 7 years before and 7 years after the hydrological intervention indicated a rapid recovery of the lake fishery immediately after opening of the new lake mouth, with a sixfold increase in the average annual fish landings. During 2000–2001 to 2006–2007, the average fisheries output (11 051.3 t), catch per unit effort (6.2 kg boat^?1 day^?1), the economic valuation of the average annual catch (637 million rupees) and productivity (11.97 t km^?2) exhibited dramatic increases of 498%, 464%, 1177% and 498.5%, respectively, compared with the 7‐year pre‐intervention data. Multivariate statistical analysis inferred that most of the lake’s environmental variables are strongly associated with salinity factor, which seems to have governed the lake ecology. The salinity dynamics of the lake are governed by both freshwater inflows and seawater ingress through the new artificial lake mouth. Correlation analysis indicated that salinity was positively correlated with prawn catch (R² = 0.542; d.f. = 25; P < 0.01), crab catch (R² = 0.628; d.f. = 25; P < 0.001) and fish catch (R² = 0.476; d.f. = 25, P < 0.05). The average increase in the salinity regime (43.8%) for the lake during the post‐hydrological intervention period, compared with the pre‐hydrological intervention period, appears to have positively impacted the fish, prawn and mud crab catches. A gradual decrease in total fisheries output since 2005–2006, however, was attributed mainly to a continuing increase in destructive fishing practices in the absence of any conservation and regulatory measures for fishing, and large‐scale collection of shrimp juveniles from the outer channel for shrimp aquaculture. Thus, carefully planned conservation and regulation measures must be ensured, with active participation of local communities during this early phase of lake restoration. In the absence of such measures, the present scenario of fisheries enhancement might not be sustained over the long term.     

Life under ice in the perennial ice‐covered Lake Glubokoe in Summer (East Antarctica)

This study focuses on hydrological and biotic variables in Lake Glubokoe, which is located in Thala Hills of Enderby Land (East Antarctica). Water and sediment samples and physical measurements were collected once a week in the austral summer (19 December 2010 – 6 February 2011). This lake exhibits perennial ice cover that reached a thickness of 2.5–2.7 m during the study period. A very low concentration of planktonic chlorophyll‐a (0.06–0.45 μg L^?1) was measured in the lake, indicating its ultra‐oligotrophic status. The water was poorly populated by algae and metazoans, especially in upper waters below ice cover to a depth of 2 m. Small planktonic organisms (2–5 μm) were observed throughout the study period, but larger organisms (>8 μm) such as the cyanobacteria Planktolyngbya limnetica occurred only during the warmest period (January). Only few individuals of metazoans (rotifers) were found in planktonic samples. Due to deep light penetration (10–15% of incoming active solar radiation reached the depth of 30 m), thick cyanobacterial mats (30 cm) cover all the bottom surface (grey silts) in the lake. Abundant benthic biota associated with these mats was found (up to 1000 ind. m^?2). Among the benthic metazoans, bdelloid rotifers and tardigrades were the dominating taxa. The results of this study suggest a typical ecological feature of most subglacial lakes in East Antarctica is that metazoans are very poor in the pelagic zone, preferring instead to occupy an area near the lake bottom because of a favourable constant temperature of 4 °C, good level of dissolved oxygen and available food resources as the bacterial detritus.     

The dying Dead Sea: The microbiology of an increasingly extreme environment

The Dead Sea, on the border between Israel and Jordan, currently contains around 348 g salt L^?1. Divalent cations (Mg²⁺, Ca²⁺) dominate over monovalent cations (Na⁺, K⁺), while Cl^? and Br^? are the main anions. The pH of the Sea is about 6. The water balance of the lake is negative, having dropped over 1 m year^?1 over the past decade. The water is supersaturated with Na⁺, with massive quantities of halite precipitating to the bottom of the lake. Biological monitoring since 1980 has indicated that blooms of the unicellular green alga Dunaliella and halophilic Archaea of the family Halobacteriaceae only develop following significant dilution of the upper water layers in the lake after very rainy winters. Such events occurred in 1980, and even more dramatically in 1992, when up to 3.5 × 10⁷ Archaea mL^?1 in the diluted upper 5–10 m of the water column coloured the lake red. Species isolated from the lake include Haloferax volcanii, Haloarcula marismortui, Halorubrum sodomense and Halobaculum gomorrense. Dunaliella was no longer observed in the lake after 1996, with prokaryote numbers remaining low. To characterize the residual microbial community in the lake, biomass was collected in February 2007 for environmental genomic analyses. The results were compared with the metagenome of microbial bloom material collected in 1992. The 16S rRNA archaeal phylotypes recovered from the 2007 sample were diverse, with phylotypes distantly related to the genera Halorhabdus, Haloplanus, Natronomonas and others. Halorhabdus sp. also was recovered in culture. The 1992 bloom sample was very homogeneous, however, with a single cluster remotely related to Halobacterium salinarum. These results illustrate that the microbial communities are dynamic, even in one of the most extreme environments on Earth, and exhibit strong shifts in species composition as conditions for life become increasingly adverse.     

The responses of Nile tilapia Oreochromis niloticus (Linnaeus, 1758) in Lake Wamala (Uganda) to changing climatic conditions

Changes in the catches of Nile tilapia, Oreochromis niloticus (Linnaeus, 1758), in Lake Wamala (Uganda) have been observed since its introduction. The factors contributing to these changes, however, are not well understood. This study examined changes in species composition, size structure, size at first maturity, length–weight relationship and condition factor of Nile tilapia in Lake Wamala, in relation to changes in temperature, rainfall and lake depth, to provide a better understanding of the possible role of changing climatic conditions. There was an increase in the minimum, maximum and average temperatures since 1980, but only the minimum (0.021 °C year^?1) and average temperatures (0.018 °C year^?1) exhibited a significant trend (P < 0.05). Rainfall increased by 8.25 mm year^?1 since 1950 and accounted for 79.5% of the water input into the lake during the period 2011–2013, while evaporation accounted for 86.2% of the water loss from the lake. The lake depth was above 4 m during the years when the rainfall exceeded the average of 1180 mm, except after 2000. The contribution of Nile tilapia to total fish catch and catch per unit effort (CPUE) increased with rainfall and lake depth up to the year 2000, after which they decreased, despite an increased rainfall level. The lake depth was positively correlated with the average total length and length at 50% maturity (r = 0.991 and 0.726, respectively), while the slopes of the length–weight relationships differed significantly between high and low lake depths [t₍₆₎ = 3.225, P < 0.05]. Nile tilapia shifted from an algal‐dominated diet during the wet season to include more insects during the dry season. The results of this study indicate Nile tilapia in Lake Wamala displays a typical r‐selected reproductive strategy, by growing to a small size, maturing faster and feeding on different food types, in order to survive high mortality rates under unfavourable conditions attributable to higher temperatures, low rainfall and low lake water levels.     

Advection,growth and nutrient status of phytoplankton populations in the lower River Murray,South Australia

To investigate the link between river flow, nutrient availability and development of algal blooms, growth rates of the major phytoplankton species were examined in situ in the lower River Murray, South Australia over the 1994/1995 summer. Eight sites were selected over a 54 km reach between Lock 1 and Nildottie and growth rates estimated by monitoring mean cell density in time‐aligned parcels of water as they travelled downstream. Discharge at Lock 1 during the period of study (3000–5000 ML day⁻¹) typified summer entitlement flows to South Australia. A large, shallow floodplain lake (lagoon), with an hydraulic connection to the river, supported a large population of cyanobacteria in summer, but inputs to the main channel did not substantially affect the abundance and composition of river phytoplankton. Mean net growth rates of Anabaena circinalis and A. flos‐aquae f. flos‐aquae were 0.132 and 0.176 day⁻¹, respectively, although individual rates varied from positive to negative. In contrast, the mean growth rate of the filamentous diatom Aulacoseira granulata was −0.15 day⁻¹, reflecting a decrease in population size with advection downstream. Mean cell densities of the three species did not exceed 5000 cells mL⁻¹ throughout the study. Growth bioassays conducted in the laboratory indicated that nitrogen was often the nutrient limiting algal growth, although it was not established whether nitrogen was limiting in situ. A conceptual model is presented, linking these findings with those of other work on the lower River Murray, to summarize the physical and chemical environmental factors governing the abundance of cyanobacteria in this reach of the river. Copyright © 2000 John Wiley & Sons, Ltd.     

Spatial distribution and habitat characterization of schistosomiasis host snails in lake and land habitats of western Kenya

Intermediate host snails of schistosomiasis were surveyed in this study to determine their abundance and distribution in the lake and land aquatic habitats of Lake Victoria basin of Kenya. Several sites were sampled at eight locations, both in the lake and on the land. The habitat and/or vegetation type (i.e. open water, hippo grass, hyacinth, ambatch trees, other vegetation, stream, swamp, pond, dam) of the sampled aquatic sites within the locations were also differentiated, water physicochemical parameters were determined, and the abundance of different species or taxa of phytoplankton and zooplankton were enumerated and correlated with the abundance of schistosomiasis snails in the sites. The results indicated significantly more Biomphalaria sudanica snails than Bulinus africanus snails in different physical habitats on land (Student's t‐test, P < 0.05), as well as in different locations on land (Student's t‐test, P = 0.026). Regression analyses revealed that several physicochemical parameters, including dissolved oxygen (R² = ?0.659; n = 8; P = 0.014), pH (R² = 0.728; n = 8; P = 0.007) and turbulence (R² = ?0.616; n = 8; P = 0.02), were predictive of Biomphalaria spp. abundance, while pH (R² = 0.610; n = 8; P = 0.02) and turbulence (R² = ?0.578; n = 8; P = 0.028) were predictive of Bulinus spp. abundance in different locations in the lake. Cyanobacteria (R² = 0.638; n = 8; P = 0.02) and chlorophyceae (R² = ?0.50; n = 8; P = 0.05) were shown to be predictive of both Biomphalaria spp. and Bulinus spp. abundance in different locations in the lake. Zooplankton abundance varied significantly between different locations in the lake (One‐way anova , P < 0.001). Bosmina spp. were found to be predictive of both Biomphalaria spp. (R² = ?0.627; n = 8; P = 0.01) and Bulinus spp. (R² = ?0.50; n = 8; P = 0.05) in different locations in the lake. The results from this study will help inform policy regarding control measures for schistosomias and intermediate snail hosts in Lake Victoria waters, as well as in adjacent terrestrial aquatic habitats and even beyond.     

Dynamics of photosynthetic pigments in an Andean lake in Colombia

Guatavita Lake is a small, sheltered tropical high mountain lake located in the Colombian Andes, with a closed watershed and a maximum depth of 25 m. It is the freshwater source for human consumption in nearby small towns, as well as being a site of cultural value for the country, as it was a sacred place to indigenous peoples until about five centuries ago. As the structure and function of this aquatic ecosystem is poorly understood, this study provides initial knowledge on its phytoplankton biomass dynamics, which should be useful in designing efficient management plans with environmental baseline information for similar lakes elsewhere. Physical and chemical data, and photosynthetic pigment concentrations, were measured for the period November 1999–November 2000 at the central vertical axis of Guatavita Lake. The vertical profile of the chlorophyll‐a concentration was closely related to the dissolved inorganic nitrogen concentrations and the thermal stratification characteristics. The maximum chlorophyll‐a concentration in the metalimnion was recorded for the thermal stratification period (November 1999–June 2000). Deepening of the maximum chlorophyll‐a concentration began in February, continuing to June, when it reached its lowest value at the 15 m depth. The phytoplankton biomass values showed an increment within the entire water column at the beginning of the mixing period (July). The relation between the metalimnetic peak of chlorophyll‐a and the dissolved inorganic nitrogen concentration suggests the growth of the phytoplankton community is limited mainly by the availability of nitrogen.     

湖泊水体营养状态对沉积物好氧氨氧化菌异质性的影响

采用荧光定量PCR(real-time fluorescence quantitative polymerase chain reaction)和末端限制性片段长度多态性(terminal restriction fragment length polymorphism,T-RFLP)方法,对江苏7个湖泊沉积物中好氧氨氧化微生物进行分析,研究湖泊水体营养状态对沉积物好氧氨氧化微生物空间异质性的影响。综合营养指数分析结果表明,23个采样点中,61%的湖区为中营养状态,39%的湖区为轻度富营养状态。荧光定量PCR分析结果显示,每克底泥中氨氧化古菌(ammonia-oxidizing archaea,AOA)和氨氧化细菌(ammonia-oxidizing bacteria,AOB)amo A基因拷贝数虽然分别从中营养湖泊的3.91×10~6和3.82×10~6上升到轻度富营养湖泊的1.30×107和6.07×10~6,但湖泊水体营养状态并未显著影响沉积物AOA和AOB的丰度。T-RFLP分析结果表明,湖泊水体营养状态对AOA和AOB的优势种属及群落多样性也未产生显著影响。典范对应分析结果表明,湖泊水体营养盐浓度能解释AOA的群落结构差异的56.3%,而仅能解释27.2%的AOB的群落结构差异,TN和NO-3-N浓度是影响沉积物AOA分布异质性的主要环境因子,湖泊水体营养盐浓度比综合营养状态指数更能影响AOA和AOB的群落结构组成。