Main River Sites Research Articles

Stream temperature dynamics within a New Zealand glacierized river basin

AbstractKnowledge 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.

Benefits to 0+ fishes of connecting man‐made waterbodies to the lower River Trent, England

AbstractFloodplain waterbodies are reputed to enhance recruitment of riverine fish populations via provision of spawning and nursery habitat, refuge from floods, and increased availability of planktonic food resources compared with main river channels. Notwithstanding, there have been few parallel studies of fishes and their food resources at both main river and floodplain sites. Thus, this study investigated the 0+ fishes, zooplankton and phytoplankton (chlorophyll a) at four main river and four (man‐made) floodplain sites on the lower River Trent, England, between May 1999 and October 2004 inclusive. All sites shared the same key fish species, and there were no consistent differences in the densities, growth or condition of 0+ fishes from main river and floodplain sites. Similarly, all sites shared the same key zooplankton taxa. However, zooplankton densities, notably of large‐bodied cladocerans, and chlorophyll a concentrations, were significantly higher at floodplain sites than at main river sites. Thus, connection of man‐made waterbodies has the potential to enhance recruitment of riverine fish populations via provision of important spawning and nursery habitat, and superior feeding opportunities for 0+ fishes compared with main river channels. Copyright © 2007 John Wiley & Sons, Ltd.

Distribution and sources of polycyclic aromatic hydrocarbons in Wuhan section of the Yangtze River, China

Polycyclic aromatic hydrocarbons (PAHs) are important organic contaminants with great significance for China, where coal burning is the main source of energy. In this study, concentrations, distribution between different phases, possible sources and eco-toxicological effect of PAHs of the Yangtze River were assessed. PAHs in water, suspended particulate matters (SPM) and sediment samples at seven main river sites, 23 tributary and lake sites of the Yangtze River at the Wuhan section were analyzed. The total concentrations of PAHs in the studied area ranged from 0.242 to 6.235 microg/l in waters and from 31 to 4,812 microg/kg in sediment. The average concentration of PAHs in SPM was 4,677 microg/kg, higher than that in sediment. Benzo(a)pyrene was detected only at two stations, but the concentrations were above drinking water standard. The PAHs level of the Yangtze River was similar to that of some other rivers in China but higher than some rivers in foreign countries. There existed a positive relationship between PAHs concentrations and the TOC contents in sediment. The ratio of specific PAHs indicated that PAHs mainly came from combustion process, such as coal and wood burning. PAHs may cause potential toxic effect but will not cause acute biological effects in sedimentary environment of the Wuhan section of the Yangtze River.

The effects of low pH and palliative liming on beech litter decomposition in acid-sensitive streams

The decomposition of allochthonous leaf litter is retarded by stream acidification, but few studies have evaluated whether this effect can be offset by liming – the palliative addition of calcium carbonate either to streams or their catchments. We assessed the response of litter decomposition to pH and experimental liming in Welsh upland streams. Small-mesh (<335 μm) litter-bags containing common beech (Fagus sylvatica L.) were submerged in main river sites along the River Wye, and in replicate acid, circumneutral and experimentally limed tributaries (all n = 3) for 20 days. Beech decomposition was inhibited in acid tributaries and main river sites compared to circumneutral tributaries. Despite having only moderately increased pH relative to acid streams, limed sites had increased decomposition rates that were indistinguishable from naturally circumneutral streams. Decomposition rates increased highly significantly with pH across all 12 sites studied, and values were near identical to those in more prolonged experiments elsewhere. There were no significant variations in shredder numbers with decomposition rate, and no evidence that sites with faster decomposition had smaller shredder proportions. Although based on short-term observations and leaves from just one tree species, these results are consistent with the well-known retardation at low pH of some aspect microbial decomposition (e.g. by hyphomycete fungi). They are among the first to suggest that stream liming to combat acidification might reverse such impacts of low pH. Further data are required on the microbiological causes and ecological consequences of altered detrital processing in acid-sensitive and limed streams.

Role of river bed sediments as sources and sinks of phosphorus across two major eutrophic UK river basins: the Hampshire Avon and Herefordshire Wye

The Equilibrium Phosphorus Concentration (EPC 0) of river bed sediments has been measured for a wide range of agricultural subcatchments and main river sites across two major eutrophic river basins: the Hampshire Avon and Herefordshire Wye catchments, to examine whether bed sediments are acting as sources or sinks of soluble reactive phosphorus (SRP) under low flows and times of greatest eutrophication risk. A conceptual reach-based model of interactions between bed sediment and river water has been used to estimate relative differences in bed-sediment SRP flux transfers. In this model, processes of SRP uptake and release are assumed to occur within an operationally defined 0.1 m river water ‘boundary layer’ and the uptake and release of SRP is assumed to be driven by a differential between the EPC 0 of the sediment and SRP concentration in the boundary layer. Most of the river monitoring sites in the Wye and Avon catchments had elevated SRP and boron (B) concentrations in the water column at low flows, linked to sewage effluent discharges. At these sites, bed sediments consistently acted as net sinks for SRP, demonstrating the role of bed sediments in riverine ‘self-cleansing’ mechanisms. In contrast, bed sediments were found to act as net sources of SRP under three circumstances: (i) where there was minimal sewage influence (in headwater streams of the Avon), (ii) where sewage inputs were subject to large hydrological dilution by water of low SRP concentration (in the main River Wye), (iii) where EPC 0 values were relatively high, as a result of deposition of particulates with high exchangeable P concentrations from diffuse sources or from effluents (immediately downstream of sewage treatment works (STWs)). Under baseflow conditions, high SRP concentrations from sewage effluent in the tributaries appear to ‘swamp out’ any potential release of SRP from the bed sediments. For rivers that are subject to effluent P-stripping, reductions in SRP in the overlying water could potentially result in changes to the in-stream P-cycling mechanisms, with bed sediments possibly switching from net sinks to net sources of SRP. This feature is of potential importance in relation to environmental management and phosphorus mitigation operations.

The organisation of fish assemblages in the regulated Lima basin, Northern Portugal

In order to understand the structure of fish assemblages in the modified Lima basin (Northern Portugal), two distinct datasets concerning the presence and abundance of fish species were subjected to multivariate analysis. On the River Lima two types of flow modification are present within kilometres of one another: (a) a reduced and constant flow due to hypolimnetic release; and (b) an intense and irregular flow. A comparison of their influence on fish assemblages revealed a gradient of assemblage types from tributaries to main river sites. The latter were characterised by a strong dominance of cyprinids, particularly Iberian barbel (Barbus bocagei). The former harboured two kinds of fish assemblages: those closer to the river mouth were dominated by the cyprinids Iberian chub (Squalius carolitertii) and Iberian nase (Chondrostoma polylepis), which were also frequently present in the main river; while in those further upstream the predominant species was the brown trout (Salmo trutta). Although explanatory variables such as distance from source, altitude, substrate coarseness and width were the primary correlates of fish assemblage composition, dam construction and flow regulation also had a significant effect upon assemblage structure, particularly by: i) reducing the importance of migratory species; ii) constraining the presence of trout in the regulated segments; and iii) simplifying the community, especially in the case of the constant and reduced flow regime.

Temporal and spatial variation in zooplankton populations in the River Great Ouse: an ephemeral food resource for larval and juvenile fish

The spatial and temporal dynamics of zooplankton populations were investigated in a highly engineered and regulated section of the River Great Ouse from 1989 to 1993. In addition to main river sites some secondary channels (back channels) and a marina were also included in the study. At main river sites Rotifera were strongly dominant, with peak numbers of 2000 1−1 to more than 18000 1−1, corresponding closely with spring and early summer maximum concentrations of chlorophyll a. Copepoda (mostly nauplii) were recorded at consistently lower densities (maxima, 20–125 1−1) whereas Cladocera were rarely recorded and never exceed 10 1−1. Most Cladocera belonged to species generally associated with macrophytes rather than truly planktonic species. The situation in main river and back channel situations contrasted markedly with that found in marinas, where densities of Rotifera, although extremely variable spatially, were generally higher than those in the adjacent main river. Copepoda and Cladocera were also much more abundant than at the river sites with taxa such as Bosmina and other typically planktonic taxa predominant. As rotifers are the main food for newly hatched cyprinids, spatial and temporal variation in their abundance relative to the spawning locations and hatching times of cypinid eggs may have important consequences for the growth and survival of young fish. © 1997 John Wiley & Sons, Ltd.

Distribution, growth and movement of River Usk brown trout (Salmo trutta)

The River Usk catchment in South Wales supports mainly freshwater resident brown trout, with few anadromous fish. Electric fishing and netting revealed that age‐class distribution differed between main river and tributary habitats, the latter environment acting as a nursery area for young fish. Few fry were found at main river sites. Age‐class distribution also differed between tributary systems, and possible reasons are discussed. Trapping experiments indicated that trout move to main river habitat at 2+ yr. Lengths at age (back‐calculated from scale reading) were similar for main river and tributary resident fish at 1 and 2 year, but main river fish were larger at 3 and 4 yr. This habitat shift and size difference is discussed with reference to current angling regulations.

Recruitment and turnover of populations of brown trout, Salmo trutta, in the upper River Wye, Wales

The age structure, abundance, net immigration and loss of trout populations were investigated in eleven tributary sites and two main river sites on the River Wye. Marking experiments indicated that recruitment to the site populations was mainly by immigration which began during the O‐group phase and continued, with seasonal fluctuations, throughout life. Immigrant fish had a higher probability of being lost from the populations than residents providing a statistical distinction between mobile and sedentary components of the population. The turnover of the site populations is described using a simple exponential model, and half‐times at different sites (stream length, 50 m) ranged from 140 to 430 days. Some factors affecting half‐times are briefly discussed.