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.     

Variations in zooplankton community structure and water quality conditions in three habitat types in northern Lake Victoria

Lake Victoria is vulnerable to increasing eutrophication, which has become manifested in ecological changes not yet fully understood. From October 2009 to January 2010, the influence of water quality on zooplankton community structure in three habitats in northern Lake Victoria, including sewage lagoons at the lake shores, Napoleon Gulf (NG) and the interface between the lake and the Nile River (also known as Source of the Nile River), was examined. Selected physico‐chemical parameters (dissolved oxygen concentration; water temperature; electrical conductivity; water depth) were measured in‐situ, while water samples were collected for chlorophyll‐a determinations. Zooplankton was sampled with conical plankton net (mesh size 60 μm; 0.25‐m mouth diameter). The NG and Source of the River Nile (SN) sampling sites exhibited significantly higher species richness, relative to the Sewage Lagoons (SL) site (F_2,69 = 68.533; P < 0.05). Higher mean densities and dry biomass of zooplankton was generally a characteristic of the SL site (8715 ± 3241 ind L^?1; 1862 ± 451 μg L^?1), compared to the NG (119 ± 24 ind L^?1; 53 ± 8 μg L^?1) and SN sites (151 ± 26 ind L^?1; 58 ± 9 μg L^?1). Copepoda constituted a high numerical composition of the zooplankton at the NG and SN sites (>90% for both sites), while Rotifera dominated the zooplankton community at the SL site (97%). The mean values (±SE) of soluble reactive phosphorus (4060.7 ± 776.6 μg L^?1) and nitrate–nitrogen (2121.7 ± 355.5 μg L^?1) were much higher for the SL site, compared with the SN (8.2 ± 1.1 μg L^?1; 28.6 ± 5.3 μg L^?1, respectively), and NG site (7.8 ± 0.8 μg L^?1; 32.7 ± 5.4 μg L^?1, respectively). This study indicated the nutrient‐rich conditions observed at the SL site suppress the zooplankton species diversity, but favour species‐specific abundance and biomass. These study results indicate the zooplankton community structure can be used as a biological indicator of water quality in the Lake Victoria region.     

Assessment of catfish (Clarias gariepinus) stock in Lake Baringo,Kenya

The management measures used for sustainable utilization of Clarias gariepinus in Lake Baringo do not have a stock assessment reference, attributable mainly to a lack of information on biological limits and target reference points. Assessment of Clarias gariepinus stock in Lake Baringo was carried out between August 2013 and July 2014. A total of 2772 fish were sampled from 25 boats (40%) for 5 days each week for length and weight measurements. Fish Stock Assessment Tools and yield model were used to estimate population parameters, exploitation rate and optimal fishing scenarios. Annual C. gariepinus standing biomass was estimated at 21 383 kg, L_∞ = 114.30, K = 0.37 year^?1, W = 0.0147L^2.81, Z = 1.14 year^?1, M = 0.61 year^?1, F = 0.53 year^?1, and exploitation rate = 0.46 year^?1. The relative yield‐per‐recruit (Y′/R) and biomass‐per‐recruit (B′/R) resulted in E_max = 0.44 and F_MSY = 0.50 year^?1. The yield‐per‐recruit ratio at maximum sustainable yield was 29.12%, and the SSB_MSY per recruit ratio = 56.10%. The steady‐state biomass, exploitation rate and optimal fishing scenario indicated a pristine fishery for the lake, suggesting the current fishing efforts should not be exceeded to enable sustainable economic utilization of C. gariepinus.     

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.     

Sediment greenhouse gases (methane and carbon dioxide) in the Lobo-Broa Reservoir, São Paulo State, Brazil: Concentrations and diffuse emission fluxes for carbon budget considerations

Carbon gases (methane, CH₄, and carbon dioxide, CO₂) were measured for the first time in sediments of the Lobo‐Broa Reservoir, near São Carlos in São Paulo State, Brazil. It is believed these are the first measurements of this kind in any of the many reservoirs located in Brazil. Even though the Lobo‐Broa Reservoir is classified as oligotrophic, the sediment gas concentrations were exceedingly high, ranging from 0.4–3 mmol L^?1 for CH₄ and 1–9 mmol L^?1 for CO₂. Both gases exceeded their in situ gas saturation values at these shallow water depths (7 m in central basin; 11 m at dam), resulting in numerous sediment bubbles. Organic matter was highly concentrated in the reservoir sediments, averaging 25.5% loss on ignition (LOI) (dam) to 26.9% LOI (central basin) for the 0–12 cm depth interval, with values as high as 29–30% LOI (12% organic carbon) in the surface 0–5 mm layer. The theoretical flux of dissolved pore water carbon gases to the sediment–water interface (SWI) averaged 3.4 mmol L^?1 m^?2 day^?1 CH₄ and 7.3 mmol L^?1 m^?2 day^?1 CO₂ for the surface 0–10 mm. From gas emission measurements at the water surface, it was calculated that 90% of CH₄ is consumed either at the SWI or in the water column, resulting in a loss of 0.31 mmol L^?1 m^?2 day^?1 of CH₄ to the atmosphere. However, only 20% of the total CO₂ gas transported across the water–atmosphere interface (36.3 mmol L^?1 m^?2 day^?1, or 1600 mg CO₂ m^?2 day^?1) was produced in the sediments. The remaining 80% of CO₂ probably comes from other carbon sources. With CH₄ oxidation in the aerobic water column, close to 30% of the carbon gas flux to the atmosphere could be accounted for by gas production of CO₂ and CH₄ in the sediments and their diffuse transport to the water column.     

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.     

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.     

Effects of environmental factors on cyanobacteria dynamics in Lake Baringo,Kenya

The dominance of cyanobacterial algae in light‐limited, shallow freshwater Lake Baringo is a major environmental concern in regard to Kenyan water quality and public health protection agencies. Accordingly, this study focused on determining the effect of different environmental factors on cyanobacteria dynamics in different sediment disturbance zones of the lake and in different seasons. This study also sought to bridge the knowledge gap regarding the influence of water clarity on cyanobacteria dynamics in the lake. Samples were collected from the field, stored in ice and transported to the laboratory for nutrient analyses. Cyanobacteria cultures isolated from the lake were grown under a 12:12 light/dark cycle. The frequency of dividing cells (FDC) technique, and a fluorescence microscopy technique, was used to count growing cyanobacteria cells. Specific cyanobacteria organic carbon synthesis was significantly negatively correlated with turbidity for the southern (r = ?0.6573; P < 0.05) and central sediment disturbance zones (r = ?0.6847; P < 0.05). This study indicated that water clarity is an environmental phenomenon that facilitates the movement of cyanobacteria into the turbid areas of the lake, where their production levels are significantly high, in contrast to the clear water along the edges of the water–land interface during the wet season (April to August) and dry season (September to March). Water clarity potentially enhances cell division inhibition and multiplication, thereby positively influencing cyanobacteria dynamics in Lake Baringo. Thus, it is concluded that the cyanobacteria growth rate resulting from use of the FDC technique suggests a link with inflowing water clarity that can be used to monitor in‐lake water quality, to better manage cyanobacteria blooms in Lake Baringo and in lakes and reservoirs elsewhere.     

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.     

A Seasonal Study Reveals the Occurrence of Exotic Rotifers,the River Antigua,Veracruz, Close to the Gulf of Mexico

Zooplankton studies in Mexican rivers are few despite the fact that Mexico has >200 rivers. We present data on the seasonal diversity of rotifers during 2013–2014 from the river La Antigua, near Veracruz. We collected samples from 15 stations along a horizontal gradient of ~5 km, from the upper reaches to about 2 km from the sea. The physico‐chemical variables analysed were temperature, pH, dissolved oxygen, conductivity, nitrates, phosphates, Secchi depth and salinity. From each site, 80 L of water was filtered with a 50‐μm plankton net. Rotifers were identified and quantified using a Sedgewick Rafter cell. The river is shallow (maximum depth 2.5 m during the rainy season). Our observations indicated that the phytoplankton community was dominated by diatoms; the chlorophyll a concentrations ranged from 0.55 to 26.1 μg L^?1 over the study period. We found >125 rotifer species belonging to 21 families, mostly from the Lecanidae, Brachionidae, Notommatidae and Lepadellidae. The density of rotifers was low, <40 ind. L^?1, but species diversity (Shannon–Wiener) ranged, except for a onetime minimum of 0.25, from 2.0 to 4.0 for most part of the year. We also encountered Notholca cf. liepetterseni , an endemic species to Europe and Lecane yatseni , endemic to Asia. Other new records included Lecane rhytida and Ptygura melicerta . The importance of long‐term studies in rivers is emphasized. Copyright © 2017 John Wiley & Sons, Ltd.     

Physicochemical characteristics of Lake IRAD, an artificial lake in Wakwa region, Cameroon

Wakwa is a region in north Cameroon characterized by intensive cattle production. This study evaluated the physicochemical characteristics of the waters in Lake IRAD, located near Wakwa, which is the main water source for cattle grazing in this area. Water samples were collected at four sampling sites during the rainy and dry seasons (April, July, October and February). The chemical composition of the water samples was analysed for various constituents, including nitrate (NO₃^–), chloride (Cl^?), phosphate (PO₄^3?), bicarbonate (HCO₃^?), calcium (Ca), magnesium (Mg), manganese (Mn), aluminium (Al), zinc (Zn), copper (Cu), iron (Fe), nickel (Ni), cadmium (Cd), ammonia–nitrogen (NH₄–N) and organic matter (OM). The mineral composition varied significantly (P < 0.05) with the sampling period. High concentrations of zinc (0.96 mg L^?1) and dissolved iron (1.23 mg L^?l) were observed during the dry season. Total iron (3.25 mg L^?1), OM (15.4 mg of O₂ L^?1), nitrate (28.82 mg L^?1) and NH₄–N (1.05 mg L^?1) concentrations were highest during the rainy season. The iron, OM and NH₄–N concentrations were higher than the USEPA‐recommended values (0.2 mg L^?1, 4 mg of O₂ L^?1 and 0.5 mg L^?1, respectively). The phosphate, copper, nickel and cadmium concentrations, considered as the polluting substances, were present in negligible concentrations, being below the detection limits of the analytical techniques used to measure them. The high iron, OM and nitrogen concentrations were attributed to water‐leached soil run‐off, as well as the activity of animals in the lake. Sampling sites 1 and 2, which were used mostly by cattle, were observed to have the highest concentrations of NH₄–N, compared with sites 3 and S (exit point). It will be necessary to delimit cattle access points to the lake to reduce this type of contamination of drinking water.     

Growth,mortality and stock status of mullets (Mugilidae) in Chilika Lake,India

The growth, mortality and stock status of grey mullets Chelon parsia (Ham. 1822), Chelon planiceps (Val. 1836) and Mugil cephalus (Linn. 1758) were investigated during December 2010 to November 2011 from Chilika Lake, Asia, largest brackish water lake. The length‐based analysis, using length frequency data collected from fish landing centres, formed the basic study methodology. Growth function and population parameters were studied using FAO ‐ICLARM Stock Assessment Tools‐II (FiSAT ‐II ). The von Bertalanffy growth function (VBGF ) was established as L _t  = 321 mm*(1 ? exp (‐0.98 year^?1 × (t + 0.085 year)) for C. parsia , L _t  = 315 mm × (1 ? exp (?0.80 year^?1 × (t  + 0.105 year)) for C. planiceps and L _t  = 700 mm*(1 ? exp (?0.70 year^?1*(t  + 0.097 year)) for M. cephalus . Lower K and higher L _∞ values for M. cephalus indicated slow growth and high longevity of the species, compared to other grey mullets. Length–weight relationships were derived, indicating isometric growth for grey mullets. Recruitment of mullets was observed throughout the year, with a peak during April–July. About 50% of the mullets were caught by fishing gear before reaching their first year of age. The level of exploitation (E  ≥ 0.60) was more than the optimum level (E  = 0.4 for pelagic stock), indicating overexploitation of grey mullets in Chilika Lake. Moreover, the average annual yields of three mullets were observed to be higher than the estimated maximum sustainable yields (MSY ), also indicating overharvesting of mullets. Thus, the mullets could be considered one of the highly overexploited resources in Chilika Lake. The findings of this study will facilitate the development of appropriate management strategies for the mullet fishery in Chilika Lake.     

Risk assessment for organochlorines and organophosphates pesticide residues in water and Sediments from lower Nyando/Sondu‐Miriu river within Lake Victoria Basin,Kenya

The objective of this study was to survey and document pesticide residue levels in the lower Nyando/Sondu‐Miriu catchment areas of Lake Victoria, Kenya, during the dry and rainy seasons of 2009. Water and sediment samples from the Nyando/Sondu‐Miriu Basin were analysed for selected pesticide residues, using gas chromatography equipped with Ni⁶³ and CP‐SIL 8CB‐15m and TSD detectors for organochlorine and organophosphorus pesticide residues, respectively. The findings indicated that banned organochlorines are still being used in the catchment. Dieldrin and p,p′‐DDD were notably higher (P < 0.05) in concentrations than their metabolically formed analogues of aldrin and DDT, respectively. Notably, organophosphorus was below detection levels in water samples, whereas diazinon and malathion were at higher levels in sediment samples. The total residues of DDT, HCH, methoxychlor and endrin generally were below WHO drinking water limits of 2, 2, 20 and 0.01 μg L^?1, respectively, whereas aldrin and dieldrin were above the recommended values of 0.03 μg L^?1. Agricultural activities in the Lake Victoria Basin are influencing accumulation of the pesticide residues in the basin rivers and the lake. The study recommends creation of buffer zones around the natural water bodies to reduce the inflow of pesticides into water bodies. An integrated pest management approach that encourages reduced usage of chemical compounds also should be encouraged.     

Growth and population parameters of Nile tilapia,Oreochromis niloticus (L.) in the open waters of Lake Victoria,Kenya

The Nile tilapia (Oreochromis niloticus) was introduced into Lake Victoria in the early 1950s and 1960s and has since become the dominant tilapiine in the lake. This study investigated the growth and population parameters of O. niloticus in Lake Victoria on the basis of length–frequency data collected during the period June 2014 and June 2015. The asymptotic length (L_∞) had a mean (±SE) value of 46.24 ± 0.04 cm TL, growth curvature (K) of 0.69 ± 0.25 year^?1, total mortality (Z) of 2.18 ± 0.80 year^?1, a natural mortality (M) of 1.14 ± 0.28 year^?1, a fishing mortality (F) of 1.05 ± 0.53 year^?1, an exploitation rate (E) of 0.46 ± 0.08, a growth performance index (?) of 3.14 ± 0.17 and a length at first capture (L_C50) of 20.31 ± 0.40 cm TL. Comparing the results of this study with previous studies indicates the parameters K, Z and M have increased, whereas ?, F, E and L_C50 have decreased. Changes in these parameters could be attributed to the existing high fishing capacity, and changing lake conditions. Thus, management measures should include continued restriction on illegal fishing methods and gears, such as the use of undersized gillnets (<5 in. mesh size) and beach seines. More attention also should be directed to the implementation of measures to control pollution of the lake from its various sources.     

Sediment phosphorus characteristics in the clearwater state of Lake Mogan, Turkey

This study examined the vertical distributions of total phosphorus (TP) and phosphorus fractions, and the iron and organic matter, in the littoral sediment in a macrophyte‐dominated, clearwater state in Lake Mogan between September 2005 and August 2006. Benthic macroinvertebrates and total bacteria in the sediment also were determined. No clear seasonal or depth‐related (0–20 cm) patterns were found in sediment concentrations for the measured parameters. The phosphorus release was quantitatively very low, and a negative phosphorus release (–0.132 µg m^?2 day^?1) was measured during the summer months. The TP concentrations of the sediment samples ranged between 675.00 and 1463.80 µg g^?1 dry weight (DW), and the trophic level of the lake was eutrophic. On average, inorganic phosphorus fractions comprised the largest fraction (63%), while organic‐bound phosphorus (Org ≈ P) constituted 37% of the TP in Lake Mogan. The most important phosphorus‐immobilizing factors are high iron content (14 200–47 750 µg g^?1 DW), the sediment's clay content (47.80–51.80%), and an abundance of macrophytes at the sampling station. The low abundance of benthic macroinvertebrates (510–850 individuals m^?2), which depend on sediments with high iron and low organic matter (5.42–13.30%), played a role in the sediment phosphorus retention. Although bacterial abundance in the surficial sediment appeared to be positively correlated to temperature, the overlying water did not experience anoxic conditions, supporting a state in which bacteria were able to retain phosphorus in their cell structures. Long‐term changes in the sediments of Lake Mogan must be monitored lake. In order to optimize the management of the lake, and to determine the longevity of a clearwater state following management measures and continued external phosphorus loading, long‐term changes in the sediments of Lake Morgan must be monitored.     

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.     

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.     

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.     

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.     

Occurence of Culturable Vibrio cholerae from Lake Victoria,and Rift Valley Lakes Albert and George,Uganda

Vibrio cholerae, a bacterium that causes cholera, poses a human health risk when consumed via untreated or contaminated water. Monthly investigations into the presence of V. cholerae from Lakes Albert, George and Victoria were conducted, with the goal being to examine the relationship between the occurrences of V. cholerae with various water quality parameters at fish landing sites in major water bodies in Uganda. The pH, temperature and electrical conductivity were measured at three fishing sites in each of the three study lakes; namely Gabba in Lake Victoria, Butiaba in Lake Albert and Kayanzi in Lake George. The pH values varied from 7.76 to 9.36 at Butiaba, 8.68 to 9.85 at Kayanzi and 6.6 to 9.88 at Ggaba. The temperature ranged from 17.9 to 32.3 °C at Butiaba, 22.5 to 29 °C at Kayanzi and 18.2 to 30.5 °C at Ggaba. The electrical conductivity ranged from 129.2 to 984 μS cm^?1 at Butiaba, 658 to 1090 μS cm^?1 at Kayanzi and 119 to 218 μS cm^?1 at Ggaba, for Lakes Albert, George and Victoria, respectively. Enrichment techniques were used to detect culturable V. cholerae on TCBS culture media. Seventy‐five (75%) of the samples (n = 90) were positive for V. cholera. The occurrence of V. cholerae was positively associated with water quality parameters over the 10‐month period of study. Vibrio cholerae was more frequently detected during the dry season (warmer) than during the wet season. These study results suggest the investigated study lakes are natural reservoirs for V. cholerae.