I. A. Malcolm

Gamma distribution models for transit time estimation in catchments: physical interpretation of parameters and implications for time-variant transit time assessment.

In hydrological tracer studies, the gamma distribution can serve as an appropriate transit time distribution (TTD) as it allows more flexibility to account for nonlinearities in the behavior of catchment systems than the more commonly used exponential distribution. However, it is unclear which physical interpretation can be ascribed to its two parameters (?, ?). In this study, long?term tracer data from three contrasting catchments in the Scottish Highlands were used for a comparative assessment of interannual variability in TTDs and resulting mean transit times (MTT = ??) inferred by the gamma distribution model. In addition, spatial variation in the long?term average TTDs from these and six additional catchments was also assessed. The temporal analysis showed that the ? parameter was controlled by precipitation intensities above catchment?specific thresholds. In contrast, the ? parameter, which showed little temporal variability and no relationship with precipitation intensity, was found to be closely related to catchment landscape organization, notably the hydrological characteristics of the dominant soils and the drainage density. The relationship between ? and precipitation intensity was used to express ? as a time?varying function within the framework of lumped convolution integrals to examine the nonstationarity of TTDs. The resulting time?variant TTDs provided more detailed and potentially useful information about the temporal dynamics and the timing of solute fluxes. It was shown that in the wet, cool climatic conditions of the Scottish Highlands, the transit times from the time?variant TTD were roughly consistent with the variations of MTTs revealed by interannual analysis.

Groundwater–surface water interactions in upland Scottish rivers: hydrological, hydrochemical and ecological implications

Synopsis Contrary to previous hydrogeological assumptions, we now know that drift deposits and fracture systems in crystalline rocks can constitute important aquifers in the Scottish Highlands and other montane environments. Groundwater from these aquifers usually has an important influence on the hydrology, hydrochemistry and ecology of upland river systems. Tracer-based research in the Girnock burn catchment in the Cairngorms revealed that groundwater comprises at least 30% of annual runoff. Groundwater often enters stream channels via drift deposits in valley bottom areas, which appear to be fed from recharge areas on the catchment interfluves. A range of groundwater sources exist in the catchment reflecting the complex solid and drift geology. These account for spatial differences in stream hydrochemistry and the spatial delineation of groundwater discharges to rivers and riparian zones. Areas where groundwaters enter the stream channel directly can have profound ecological implications. Most obvious are low rates of salmonid egg survival where chemically reduced groundwater discharges through the hyporheic zone. However, it is argued that only further research will reveal the full significance of groundwater–surface water interactions to the ecological status of Scottish rivers.

Is the composition of dissolved organic carbon changing in upland acidic streams

The quantity and composition of dissolved organic carbon (DOC) exported from upland soils to surface waters is a key link in the global carbon cycle and economically important for treating potable waters. The relationship between ultraviolet (UV) absorbance and DOC concentrations can be used to infer changes in the proportion of hydrophobic (aromatic, recalcitrant) carbon and hence biodegradability of DOC. This study describes a significant change in the relationship between UV absorbance and DOC over 22 years at two upland moorland catchments in Scotland, UK. Despite increases in long-term DOC concentrations, analysis suggests that the proportion of hydrophobic material has declined. A statistical mixed-effect modeling approach was used to examine the likely mechanisms that could explain these observations. Annual nonmarine sulfate load was the only significant forcing factor that could explain the observed long-term trend in the UV absorbance-DOC relationship at both sites. It is hypothesized that enhanced heterotrophic decomposition of organic matter and increased solubility of carbon compounds in soils where sulfate driven acidification is being reversed are the dominant mechanisms behind this change in DOC composition. These trends will impact on carbon substrate dynamics by potentially increasing biodegradability of exported organic matter, influencing carbon cycling in terrestrial and aquatic ecosystems.

Does diurnal temperature variability affect growth in juvenile Atlantic salmon Salmo salar

This study investigated the effects of diurnal temperature variability (>7° C) on the growth of 1+ year Atlantic salmon Salmo salar. Experimental manipulation of water temperature was used to simulate: (1) constant and (2) naturally varying thermal regimes with similar daily mean values. Data from two replicates of four treatments (two thermal and two feeding regimes) were collected over 6 months corresponding to the main spring to summer growth period. Fish growth was assessed at fortnightly intervals. Small but significant differences in mean fork length (L(F) ) and mass were observed between temperature treatments, with smaller, lighter fish under the variable temperature regime. The effects of temperature regime on growth were independent of food ration. At termination of the experiment, the median L(F) and mass of fish exposed to the variable temperature regime were estimated, respectively, to be 2· 6 and 8· 0% less than those under the constant regime. Given the relatively small differences in growth attributable to variable temperature regime in these experiments, it is suggested that mean daily temperatures are adequate to inform juvenile growth models for field-based studies.

Can Conservation Stocking Enhance Juvenile Emigrant Production in Wild Atlantic Salmon

AbstractConservation stocking is frequently used by fishery managers to stabilize or increase production of depleted fish stocks. However, in the case of salmonids the benefits are increasingly questioned and generally poorly quantified. We investigated the effects of ova stocking on freshwater emigrant production in a declining Scottish population of “spring” Atlantic Salmon Salmo salar. The stocking program was designed to minimize risks, maximize expected benefits, and bypass population bottlenecks between the ova and fry stages. Long-term data (33 cohorts over 42 years, including 8 years of stocking) on numbers of ova and juvenile emigrant production were used to investigate whether the ova–emigrant stock–recruitment relationship differed between conditions of natural spawning and stocking. We considered Ricker and Beverton–Holt models that included terms for the effects of stocking, intercohort competition, and changes in trap efficiency on emigrant production. The “best model” was considered to be t...

Is Interstitial Velocity a Good Predictor of Salmonid Embryo Survival

Abstract Interstitial velocity within spawning bed substrates is frequently used to predict salmonid embryo survival and often forms the basis of habitat assessment or sediment-based predictive models. In this study, the relationships between interstitial velocity, hyporheic dissolved oxygen (DO), and survival of Atlantic salmon Salmo salar embryos were examined in an experimental field study that incorporated natural complexity in the hyporheic environment. Hyporheic DO and interstitial velocity were not clearly related over the observed range of velocities. There was strong evidence of a relationship between survival and DO but only weak evidence of a relationship with interstitial velocity. Conceptual models of hyporheic environmental variation and salmonid embryo survival should include the appropriate range of environmental processes. Given the findings of this study, we suggest that interstitial velocity should not be used to assess or predict spawning habitat quality; direct measurements of DO offe...

British and Irish Rivers

This chapter examines some representative rivers of the British Isles that encompass much of the geographical diversity. These include the northern rivers of the Spey, Tay and Tweed that drain mountainous parts of Scotland. In England and Wales, the focus shifts initially to rural rivers that have upland headwaters but also more marked lowland influences, including the Severn and Wye. The chapter assesses those rivers that drain catchments with more dominant urban influences. The small Frome–Piddle River in southern England shifts attention back to lowland catchments, albeit of a much more rural character. The river Shannon is the largest river system considered and lies within the Republic of Ireland. It has a distinct history and quite different character to the rivers on the British mainland. The chapter describes these rivers within a wider systematic consideration of the environmental characteristics of the British Isles. It describes the general geographical setting of the region and outlines the main topographic, climatic, and land use characteristics of this area. It also examines the way in which these factors produce a wide spectrum of geomorphic, hydrological, and biogeochemical characteristics and considers the manner in which this physico-chemical template affects the biodiversity of riverine and freshwater habitats. The chapter further reviews how a long legacy of increasingly intense human-environment interactions have affected these rivers and how future environmental changes are likely to be mediated by the increasingly sophisticated institutional arrangements that have evolved for their sustainable management.

Hydraulic modelling of the spatial and temporal variability in Atlantic salmon parr habitat availability in an upland stream

We show how spatial variability in channel bed morphology affects the hydraulic characteristics of river reaches available to Atlantic salmon parr (Salmo salar) under different flow conditions in an upland stream. The study stream, the Girnock Burn, is a long-term monitoring site in the Scottish Highlands. Six site characterised by different bed geometry and morphology were investigated. Detailed site bathymetries were collected and combined with discharge time series in a 2D hydraulic model to obtain spatially distributed depth-averaged velocities under different flow conditions. Available habitat (AH) was estimated for each site. Stream discharge was used according to the critical displacement velocity (CDV) approach. CDV defines a velocity threshold above which salmon parr are not able to hold station and effective feeding opportunities or habitat utilization are reduced, depending on fish size and water temperature. An average value of the relative available habitat () for the most significant period for parr growth - April to May - was used for inter-site comparison and to analyse temporal variations over 40years. Results show that some sites are more able than others to maintain zones where salmon parr can forage unimpeded by high flow velocities under both wet and dry conditions. With lower flow velocities, dry years offer higher values of than wet years. Even though can change considerably across the sites as stream flow changes, the directions of change are consistent. Relative available habitat (RAH) shows a strong relationship with discharge per unit width, whilst channel slope and bed roughness either do not have relevant impact or compensate each other. The results show that significant parr habitat was available at all sites across all flows during this critical growth period, suggesting that hydrological variability is not a factor limiting growth in the Girnock.

Metrics to assess how longitudinal channel network connectivity and in-stream Atlantic salmon habitats are impacted by hydropower regulation

Acknowledgements Thanks to the Scottish Governments Hydro Nation Scholars Program for funding WBB to do this research. Also, many thanks to colleagues at Marine Science Scotland and the James Hutton Institute for providing some of the datasets used in this work. The authors thank the anonymous referees for constructive feedback on the manuscript.

Ova fecundity in Scottish Atlantic salmon Salmo salar: predictions, selective forces and causal mechanisms

Ova fecundities of Scottish Atlantic salmon Salmo salar, predicted from log(10) regression of ova numbers and female fork length (L(F)), differed widely between upland and lowland stocks within the same river, whereas sea-age, river and year factors had insignificant effects on fecundity once L(F) was accounted for. For upland fish, the relationship between log(10)L(F) and log(10) ova mass (M(O)) was stable between two datasets collected 40 years apart. Although upland and lowland females both produced comparable log(10)M(O) (log(10)L(F))(-1), lowland females partitioned this into 45% more, but smaller ova, whereas upland females produced fewer, but larger, eggs. The possible causes and implications of this are discussed for evolutionary perspectives (lifetime production), population structure (local tributary v. large catchments; environmental effects), population dynamics and stability (density-dependent control mechanisms) and fisheries management (stock-recruitment; short and long-term stock sustainability).