Land‐use effects on water quality of a first‐order stream in the Ozark Highlands,mid‐southern United States

Urban and agricultural land uses can alter the natural hydrologic conditions of streams and rivers and often degrade water quality. In the Ozark Highlands of the mid‐southern United States, the climate, topography, soil properties, karst limestone geology, agricultural practices and rapid urbanization make water quality of particular concern due to the increased potential for water quality degradation by contaminant leaching to groundwater and runoff to surface waters. The objective of this study was to evaluate the effects of season (i.e. dry/cool and wet/warm) and riparian land use (i.e. urban, grazed pasture, ungrazed pasture, wetland, cultivated agriculture and grassland) on surface water quality in a first‐order stream within a diverse agricultural watershed in the Ozark Highlands. Water samples were collected twice a month within each land use during base‐flow conditions from October 2006 through October 2007. Samples were also collected periodically during storm‐flow conditions from October 2006 through December 2007. The greatest in‐stream pH was adjacent to the grazed pasture. In‐stream NO₃‐N concentrations were greatest adjacent to the cultivated agriculture and grassland during the dry/cool season (i.e. October 2006 to March 2007) and averaged 2.67 mg L^?1. In‐stream soluble reactive P (SRP) concentrations were greatest adjacent to the grassland during the wet/warm season (i.e. April 2007 to October 2007) and averaged 0.81 mg L^?1. Concentrations of SRP, K, Mg and Zn were greater during storm‐ than base‐flow conditions and in‐stream As concentrations frequently exceeded 0.01 mg L^?1. Discharge and in‐stream NH₄‐N concentrations were unaffected by land use or season and averaged 0.003 m³ s^?1 and 0.10 mg L^?1, respectively, across all land uses and seasons. Results of this study clearly demonstrate the significant effect of adjacent land use on in‐stream water quality of a first‐order stream in a diverse agricultural watershed and highlight the importance of managing upstream land use in order to regulate downstream water quality. Copyright © 2010 John Wiley & Sons, Ltd.     

The Interplay between Environmental Conditions and Filamentous Algae Mat Formation in Two Agricultural Influenced South African Rivers

The regulation of nutrient inputs into rivers dominated by agriculture land use activities is an important aspect of ecological resilience of aquatic systems and the management of river eutrophication. The overabundance of benthic filamentous algae mats in river systems due to nutrient enrichment can modify the habitats of macroinvertebrate and fish communities as well as clogging irrigation crop sprayers of downstream water users. The current study examined over a period of 2 years (2013–2014) the interplay between physical and chemical river characteristics and epilithic filamentous algae biomass in two South African agricultural influenced rivers. The study area consisted of the Touw and Duiwe Rivers, which run into a proclaimed Ramsar site, namely, the Wilderness Lake System. A strong positive correlation was observed between the maximum filamentous algae biomass (97 chl‐a mg m^?2) observed during the dry season and the average water column alkalinity >30 mg l^?1. The benthic trophic status of the nine sampling sites during the dry seasons indicated the highest benthic algae biomass with mesotrophic (1.7–21 chl‐a mg m^?2) to hypertrophic (>84 chl‐a mg m^?2) conditions. During the dry season, only three sampling sites were below the suggested guideline value (35 µg l^?1) for total phosphorus (TP), while four sampling sites were below the total nitrogen guideline of 252 µg l^?1. In the wet season, two sites were below TP values with five sites below total nitrogen guideline values. From the data gathered, it was evident that water column alkalinity and hardness were the main drivers for the formation or absence of benthic filamentous algae mats in the two river systems and that nitrogen and/or phosphorus concentrations was overshadowed by the physical and chemical characteristics of the river systems at certain sites. Nutrient results for the river bottom sediments revealed that the sediment qualities were variable at the different sampling sites, but more specifically along the longitudinal paths of flow. It was apparent that the high TP concentrations in the water column and bottom sediment, which were lowest during the dry season, were associated with the highest epilithic filamentous algae mat formation. The outcome of the current study shows that a more holistic approach must be followed for the development of future eutrophication guidelines and nutrient thresholds in South African rivers influenced by agriculture land use activities. Copyright © 2016 John Wiley & Sons, Ltd.     

EFFECTS OF ARTIFICIAL FLOODING ON WATER QUALITY OF A FLOODPLAIN BACKWATER

Seasonal flooding of riverine backwaters is important in maintaining diverse aquatic habitats, but anthropogenic impacts have reduced the frequency and duration of such flooding. This study, conducted in a 2.5‐km‐long shallow floodplain severed meander backwater adjacent to the Coldwater River in Tunica County, Mississippi, USA, compared water quality during a late summer 30‐day artificial flooding period with 28‐day pre‐flood and 26‐day post‐flood periods. Flooding was simulated by pumping 0.22 to 0.35 m³ s^?1 from the river into the upstream portion of the backwater. In situ parameters (temperature, pH, dissolved oxygen, conductivity and fluorescent chlorophyll) were measured every 30 min at one site within the backwater. Solids (dissolved and suspended) and nutrients (phosphorus and nitrogen) were measured at three sites in the backwater and in the river every 3 to 5 days. Decreases in the amplitude of temperature, dissolved oxygen and pH diel cycles within the backwater were observed during flooding. Changes in patterns of solids and nutrients were also associated with flooding. Complex patterns in phosphorus and nitrogen emerged as a result of utilization by autotrophs (measured as chlorophyll) and seasonal changes. Artificial flooding in a shallow floodplain water body stabilized and improved water quality for aquatic biota and is a viable method for habitat rehabilitation in these systems. Copyright © 2011 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.     

基于水文学方法的大洋河河道生态需水量计算研究

河道生态需水量的确定对于河流生态系统健康至关重要,为计算大洋河河道内生态需水量,基于大洋河岫岩和沙里寨两个水文站1970-2012年流量资料,分别运用最小生态径流法以及本文提出的保证率为90%(枯水期)、75%(平水期)、60%(丰水期)的适宜生态径流法计算了大洋河河道内生态需水量,并运用Tennant法对计算结果进行评价分析并结合实测流量资料对大洋河生态径流满足度进行了分析,研究结果表明:适宜生态径流量可以使得大洋河河道内的生态环境状况在丰水期和枯水期达到极好的状态,而最小生态径流量也能使大洋河生态环境状况得到改善和稳定,大洋河在丰水期生态径流破坏程度要高于枯水期,本文的研究成果对于大洋河河道生态保护和治理可提供参考价值。     

Spatial variations in nutrients and other physicochemical variables in the topographically closed Lake Baringo freshwater basin (Kenya)

Spatial physicochemical parameters were determined from 39 sampling sites distributed throughout Lake Baringo during December 2010. Mean values of temperature, dissolved oxygen concentration and electrical conductivity decreased successively with depth, while the pH remained constant. Only the turbidity values increased marginally with depth. Of the surface water parameters, mean (range) values of dissolved oxygen (DO), pH, electrical conductivity, water transparency and turbidity were 6.9 (4.5–8.4) mg L^?1, 8.3 (7.8–8.5), 573 (556–601)μS cm^?1, 33 (28–37) cm and 43.3 (32.7–54.6) NTU, respectively. Mean and range values of total nitrogen (TN), nitrate‐nitrogen(NO₃‐N), ammonia nitrogen (NH₄‐N), total phosphorus (TP) and soluble reactive phosphorus (SRP) were 788.4 (278–4486) μg L^?1, 4.5 (2.4–10.0) μg L^?1, 42.6 (33.8–56.3) μg L^?1, 102.9 (20.3–585.3) μg L^?1 and 23.5 (15.2–30.5) μg L^?1, respectively. Dissolved silica concentrations ranged from 19.7 to 32.7 mg L^?1, with a mean value of 24.7 mg L^?1. The chlorophyll‐a concentrations were quite low, ranging from 1.4 to 4.9 μg L^?1, with a mean value of 4.2 μg L^?1. In contrast to previous reported values, a key finding in the present study is a relatively high water transparency, indicating a relatively clear water column, due possibly to the fact that the sampling was conducted during the dry period. The nutrient levels remained low, and the chlorophyll‐a concentration also was an almost all time low value. A TP value of 20 μg L^?1 and higher confirms strongly eutrophic conditions prevailing in the lake, with an extremely low potential for fish production and low species diversity, consistent with other studies. The results of the present study, therefore, reinforce the database for future management and monitoring plans for the Lake Baringo ecosystem, which lies adjacent to known geothermally active zones and a saline Lake Bogoria.     

Inter‐habitat variation in the benthos of the Upper Missouri River (North Dakota,USA): implications for Great River bioassessment

We examined inter‐habitat variation in benthic macroinvertebrate assemblages in the 180‐km Garrison Reach of the Upper Missouri River, North Dakota (USA) in 2001–2003. The Garrison Reach is unchannelized with a mostly rural setting. Flows are regulated by Garrison Dam. We sampled benthos from three habitats defined a priori: channel, shoreline, and backwater. Benthic assemblages were different in each habitat. Average Bray‐Curtis dissimilarity in assemblage composition ranged from 89% for backwater versus channel habitat to 70% for backwater versus shoreline habitat. There were distinct intra‐habitat groups within a priori habitats: channel assemblages included moving‐sand assemblages and other‐substrate channel assemblages; backwater assemblages included connected (to the river channel) and unconnected backwater assemblages; shorelines assemblages varied between natural (unprotected) and riprap (rock revetment) shorelines. Abundance and taxa richness were lowest and spatial variability highest for moving‐sand channel assemblages. Abundance was highest in backwaters. Taxa richness in backwaters and along channel shorelines were similar. Assemblages in all three habitats were dominated by Nematoda, Oligochaeta and Chironomidae. Taxa in these groups comprised at least 80% of mean abundance in all three habitats. Taxa that discriminated among habitats included the psammophilic chironomid Chernovskiia for moving‐sand channel substrates versus all other habitats; Hydroptila (Trichoptera) for riprap vs natural shorelines, Aulodrilus (Oligochaeta) for connected versus unconnected backwaters; and Nematoda for backwater versus channel and shoreline versus channel. Based on overlap patterns in benthic assemblages among habitats, we concluded that sampling main channel shorelines should also capture much of the natural and stressor‐induced variation in connected backwater and channel habitat exclusive of moving‐sand channel habitat. Published in 2006 by John Wiley & Sons, Ltd.     

Stratification,diel and seasonal energy transfers in Malilangwe Reservoir in the south‐eastern lowveld of Zimbabwe

This study provides a 9‐month record of Malilangwe Reservoir stratification and diel and seasonal energy transfers between February and October 2011. The reservoir was not strongly stratified during the hot‐wet season (February and March), exhibiting low dissolved oxygen concentrations of <2 mg DO L^?1 in the bottom layers (<6 m depth). During the cool‐dry season, when both the air temperature and solar radiation input decreased, the water temperatures in the epilimnion and across the thermocline decreased, and the water column was very weakly stratified. In this state, the reservoir could easily be mixed, even by light winds, resulting in overturning, thereby giving the bottom water an opportunity to become aerated, as observed during the cool‐dry season. The stability was greatest when the water level was high (during midday), while it was considerably lower during the predawn hours. The calculated annual Birgean heat budget for the reservoir was 110 MJ m^?2 year^?1 for February to October 2011. The diurnal Birgean heat budget was consistently associated with heat loss over a 24 hour period in all seasons, mostly between 0400 and 1000 h. Spearman's correlation suggests that the DO was significantly correlated with stability for the three study months (February r = 0.69, P < 0.05; June r = ?0.76, P < 0.001; October r = 0.96, P < 0.001). The results of this study suggests that warming temperatures (warmer months or seasons) will result in high Birgean heat budgets, which could potentially trigger some important changes in plankton population dynamics in small reservoir, with severe water quality implications. The nutrient concentrations varied throughout the seasons.     

Stream temperature dynamics within a New Zealand glacierized river basin

Knowledge of river thermal dynamics for glacierized basins is limited, despite the high sensitivity of these systems to climatic change/variability. This study examined spatio‐temporal water column and streambed temperature dynamics within a New Zealand glacierized river basin over two melt seasons. Water column temperature was recorded at three sites along the mainstem channel and four hillslope/groundwater‐fed tributaries. Air temperature, precipitation and stream discharge were monitored to characterize hydroclimatological conditions. Streambed temperature was monitored at the upper and lower main river sites at 0.05, 0.2 and 0.4 m depth. Water column temperature rose on average 0.6°C km^?1 along the glacier‐fed mainstem. Temperature was elevated during warmer periods but the downstream increase was reduced due to greater meltwater production (consequently a larger total stream flow volume for atmospheric heating) plus a proportional reduction in warmer groundwater contributions. Hillslope/groundwater‐fed tributaries yielded a range of temperature patterns, indicating variable sourcing (meltwater or rainfall) and residence times. In the upper basin, streambed temperature was warmer than the water column, suggesting groundwater upwelling; however, during high runoff events, water column and streambed temperature converged, indicating downwelling/heat advection by channel water. At the lower site, streambed temperature mirrored the water column, suggesting greater surface water/atmospheric influences. Key drivers of stream thermal regime were: (1) relative water source contributions, (2) prevailing hydroclimatological conditions, (3) distance from source, (4) total stream flow volume and (5) basin factors (specifically, valley/channel geomorphology and riparian forest). High magnitude precipitation events produced a contrasting stream thermal response to that reported elsewhere. In contrast to European alpine research, streams showed a reduced thermal range owing to the relatively mild, wet melt season climate. This New Zealand case study highlights the importance of understanding basin‐specific modification of energy and hydrological fluxes for accurate prediction of stream thermal dynamics/habitat and ecological response to climatic variability and change. Copyright © 2007 John Wiley & Sons, Ltd.     

Stratification and water quality of an abandoned opencast coal pit lake at Raniganj Coalfield Area,West Bengal,India

Stratification and assessment of the limnological conditions of an abandoned opencast coal pit (OCP), locally called Khadan, which is a potential water reserve located at the Raniganj‐Asansol Coalfield Areas of Burdwan, are the main objectives of this study. Water quality changes occurred with increasing depths in the OCP water and suggest a gradual and significant ecological restoration over time, which can create a condition of sustainable pisciculture. The physical and chemical variables of the OCP water changed with changing depths and seasons. The temperature and alkalinity decreased with depth throughout the year. The dissolved oxygen concentration exhibited a mixing characteristic in the summer and winter, but decreased with depth in the monsoon and postmonsoon seasons. The total hardness increased with depth in the monsoon season, being maximum in the winter (196.0 mg L^?1). The biochemical oxygen demand (BOD) and chemical oxygen demand (COD) increased with depth in the winter. The silicon concentration increased with depths, exhibiting a maximum value of 14.2 mg L^?1 in the winter and a minimum value of 1.24 mg L^?1 in the postmonsoon season. The ammonia‐nitrogen, nitrate‐nitrogen, potassium, chloride and sulphate concentrations only exhibited seasonal variations. The concentration of heavy metals (arsenic, chromium, cadmium, manganese) decreased with increasing depth. The fluoride concentration exhibited high concentrations in each OCP water layer. The results of this study indicated that limnological variables in thermally stratified lakes may be an important measure for better understanding the gradual process of developing an improved ecological conditions in the OCP and which can facilitate the possibility of pisciculture in the near future.     

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.     

黄土高原流域土地利用变化的水土流失效应

本文以黄土高原中部的油河流域为例,利用3个不同时期的遥感影像分析了1977年以来的讷河流域的土地利用变化特征,在此基础上,选择年径流深度、年侵蚀模数、汛期径流深度、汛期侵蚀模数和枯季径流深度为水土流失过程参数,通过基于流域降水与水土流失过程各参数的统计回归模拟分析,区分出了降水和土地利用等地表属性变化分别导致的流域水土流失变化过程.研究结果认为:1980年以来,流域水土流失明显趋缓,以10年为尺度得出的平均值,径流深度减少了32.75mm,侵蚀模数减少了778t/km2.水土流失的变化主要是由地表属性变化引起的,降水量变化的贡献很小.耕地面积变化和地表属性变化引起的水土流失变化具有显著的相关性,坡耕地改梯田的农田水利建设可能是导致水土流失变化的主要原因.     

Occurrence of residues of veterinary antibiotics in water,sediment and trout tissue (Oncorhynchus mykiss) in the southern area of Lake Titicaca,Peru

The city of Puno in Peru is the largest producer of trout (Oncorhynchus mykiss) using intensive floating cage systems installed in Lake Titicaca. As a result, the increase in diseases and the use of antibiotics to control them during the production cycle has been documented. We study the impact of antibiotics on drinking water, trout tissues and the lake's aquatic ecosystem. Nine antibiotics were monitored: tetracyclines, sulfonamides and fluoroquinolones. The samples were collected randomly and analyzed by liquid chromatography coupled to mass spectrometry and the solid-phase extraction system. The sediment samples and surface water samples contain high concentrations of antibiotics. All sediments contain fluoroquinolones (3.74 mg kg^?1) and tetracyclines (3.08 mg kg^?1) and the surface water contains fluoroquinolones of up to 408.2 and 652.7 ng L^?1 in the dry and rainy seasons respectively (P > 0.05). Drinking water samples from the city of Puno collected at sampling points with Lake Titicaca as a source of drinking water, reached an average of 188.1 and 222.2 ng L^?1 of ciprofloxacin in dry and rainy seasons respectively. Complementarily, in trout tissues, it reached 7.8 μg kg^?1 in oxytetracycline 8.7 μg kg^?1 in sulfatizole, 4.2 μg kg^?1 in ciprofloxacin and 3.6 μg kg^?1 in sarafloxacin. The presence of these antibiotics in surface water is attributed to the aquaculture activity, in addition to runoff and wastewater, and their presence can have detrimental effects on the aquatic ecosystem, and even affect public health due to the consumption of aquaculture products and drinking water contaminated with antibiotic residues.     

A dynamic model to predict responses of millets (Echinochloa sp.) to different hydrologic conditions for the Illinois floodplain–River

Millets grow on floodplain mud flats exposed when seasonal floods recede, and the seeds of this plant are an important food source for waterfowl during their spring and autumn migrations in the Mississippi Flyway. Productivity of millets along the Illinois River has declined because of unnaturally frequent floods that inundate the mud flats and drown the plants during the summer growing season. These small floods are caused by operation of the navigation dams on the main channel and by alterations of the floodplain and tributary watersheds and channels. Predictive models are needed to evaluate the most cost‐effective combination of approaches for restoring plant productivity. We developed a moist‐soil plant model that simulates millet growth on 1 m² in response to daily water levels during the summer growing season. The model responds to daily water depth, flood timing (within the growing season), and flood duration, and was qualitatively verified using historical (1938–1959) water levels and plant coverage for three areas along the Illinois River. In the absence of untimely floods, the model predicts net above‐ground primary productivity of ～500 g m^?2 yr^?1 and plant heights of up to 130 cm by the end of the growing season. As expected, growth declines with decreasing land elevation or with more frequent flooding (or a shorter duration of the dry period) at the same elevation. A dry period of >85 days is required to achieve at least 50% of maximum production during the growing season, which is somewhat longer than the 70‐day recommendation based on reported field observations. The model predictions of plant success or failure agree with historical observations, indicating that water regime is a major factor limiting plant success. Copyright © 2004 John Wiley & Sons, Ltd.     

Bank erosion in a macrotidal tropical river: Exploring the relative impact of boat wash on riverbank erosion

Macrotidal tropical rivers are dynamic systems where wet‐season floods and tidal flows cause significant riverbank erosion and sediment transport. This study aimed to explore patterns of riverbank erosion and deposition in a large, tropical, macrotidal river in Northern Australia; the Daly River. In particular, we aimed to determine if recreational boat use was impacting bank erosion in this dynamic river. Erosion pins were installed at multiple levels on both banks at 10 sites along a 34 km reach of the river. Measurements were made every four to six weeks during the low water dry season, and opportunistically during the wet season (flooding period) and seasonal transition periods. A bank geotechnical assessment, riverbed cross‐sections and site bathymetry were undertaken. Whilst the wet season was a period of substantial erosion (mean rate of 0.64 mm day^?1), the highest mean erosion rate (3.6 mm day^?1) was observed in the early dry season (April to May), a period of stabilizing water levels but greatest boat traffic. Bank erosion at this time was measured on both sides of the river and the inside of meander bends, which is atypical of normal riverine bank erosion patterns, and indicative of erosion due to boat wash. As the dry season progressed, significant spatial differences in erosion rates were evident, where erosion was observed at sites upstream of a large shallow sand bar, while sites downstream from the sand bar gained material through the deposition of tidally transported sediment. This study highlights the importance of understanding the significance and interaction of various erosive factors in tropical tidal rivers and has demonstrated that boat wash may be an important contributor to dry season bank erosion in these systems. We encourage management agencies to consider the role of boats in any future river management program in these systems.     

Declines in aquatic vegetation in navigation pool no. 8, upper Mississippi River between 1975 and 1991

The biomass and frequency of occurrence of aquatic macrophytes observed during 1975 and 1991 in Navigation Pool No. 8 of the upper Mississippi River were compared. Samples were collected from contiguous backwater, isolated backwater and impounded habitat types. Overall declines were observed in both frequency and biomass between the two years. Changes in depth, water clarity and bed sediment texture were also observed. About 50% of the samples collected in 1991 had no vegetation, compared with 20% in 1975. The greatest reduction in frequency occurred among submergent taxa (77% in 1975 to 31% in 1991), whereas only slight declines were observed for emergent taxa. However, the mean biomass of both lifeforms decreased substantially (48 to 1 g/m² for submergent and 61 to 27 g/m² for emergent lifeforms). The mean total biomass declined from 114g/m² in 1975 to 35 g/m² in 1991. Declines were most evident in the impounded habitat, where the frequency of occurrence of submergent taxa decreased from 83% in 1975 to 11% in 1991, and mean biomass decreased from 90 to 1 g/m². Similar but less extensive reductions in frequency and biomass of submergent taxa were observed in the contiguous backwater habitat. In contrast, both the frequency and biomass of floating-leaved macrophytes increased slightly in contiguous backwater habitats. Changes in frequency and biomass of macrophytes in isolated backwater habitats were neligible between 1975 and 1991. It is hypothesized that changes in the macrophyte community structure may be related to physical changes associated with 57 years of impoundment.     

Hydrochemistry of the hyporheic zone in salmon spawning gravels: a preliminary assessment in a degraded agricultural stream

Hydrochemical changes were monitored in a simulated, sea run salmon redd in a small agricultural stream in northeast Scotland following the 1998–1999 spawning season. Immediately after redd construction, the hydrochemical characteristics of hyporheic water, at depths of 0.1 m and 0.3 m, were very similar to stream water. These apparently well‐mixed waters were alkaline, well‐oxygenated and enriched in nutrients. In the weeks and months following redd construction, clear and statistically significant differences in the chemistry of stream and hyporheic waters were observed. Typically, hyporheic water had lower concentrations of dissolved oxygen (mean 7.35 mg L^?1 at 0.3 m depth) than stream waters (mean=11.26 mg L^?1). Alkalinity, calcium, sulphate and conductivity levels tended to be higher in hyporheic waters, with concentrations increasing with depth. These data implied an increasing influence of groundwater with depth in the hyporheic zone following redd construction; an inference supported by subsequent hydraulic head measurements, which revealed an upwards groundwater flux in the stream bed. However, groundwater–surface water interactions were dynamic and complex: road salts eluted into the stream during periods of snowmelt simulated tracer experiments that implied that a reversed hydraulic gradient may occur at high flows with deeper streamwater penetration and mixing in the hyporheic zone. High flows also result in the mobilization of fine sediments from the stream bed which subsequently infiltrated into spawning gravels. These appear to cause ‘capping’ of redds and probably reduce the hydraulic conductivity of the redd matrix. Infiltrating sediments also contain a small, but probably important organic component, the decomposition of which may contribute to oxygen consumption and nutrient mineralization in the hyporheic zone. Copyright © 2001 John Wiley & Sons, Ltd.     

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.     

长江荆南三口地区水系连通变异前后的径流变化特征研究

以长江荆南三口河系1956-2016年的实测径流量资料为基础,以1989年为水系连通变异分界点,运用趋势分析法和小波分析法研究基准期和变化期的径流量时空变化特征。结果表明：基准期和变化期年均径流量呈现下降趋势,下降速率分别为1.945 7×10⁸和0.768 2×10⁸ m³/a;径流主要集中在丰水季节,占比达60%~70%,变化期在4个水文季节的径流量比基准期下降了40%~80%,枯水季节下降幅度高达77%;径流量存在3类周期变化,变化期比基准期的小周期丰枯交替次数约增多2次,长周期范围和中心值约缩短6 a;新江口和管家铺两个水文站的径流量约为三口地区总流量的2/3,集中于松滋河的西支和藕池河的东支。研究结果可为荆南三口地区及洞庭湖的水资源调蓄提供科学技术参考。