J. Russell Manson

Impacts of Warming on the Structure and Functioning of Aquatic Communities : Individual-to Ecosystem-Level Responses

Environmental warming is predicted to rise dramatically over the next century, yet few studies have investigated its effects in natural, multi-species systems. We present data collated over an 8-year period from a catchment of geothermally heated streams in Iceland, which acts as a natural experiment on the effects of warming across different organisational levels and spatiotemporal scales. Body sizes and population biomasses of individual species responded strongly to temperature, with some providing evidence to support temperature size rules. Macroinvertebrate and meiofaunal community composition also changed dramatically across the thermal gradient. Interactions within the warm streams in particular were characterised by food chains linking algae to snails to the apex predator, brown trout These chains were missing from the colder systems, where snails were replaced by much smaller herbivores and invertebrate omnivores were the top predators. Trout were also subsidised by terrestrial invertebrate prey, which could have an effect analogous to apparent competition within the aquatic prey assemblage. Top-down effects by snails on diatoms were stronger in the warmer streams, which could account for a shallowing of mass-abundance slopes across the community. This may indicate reduced energy transfer efficiency from resources to consumers in the warmer systems and/or a change in predator-prey mass ratios. All the ecosystem process rates investigated increased with temperature, but with differing thermal sensitivities, with important implications for overall ecosystem functioning (e.g. creating potential imbalances in elemental fluxes). Ecosystem respiration rose rapidly with temperature, leading to increased heterotrophy. There were also indications that food web stability may be lower in the warmer streams.

Diagnostics and rubrics for assessing learning across the computational science curriculum

Abstract We describe our experiences with learning assessment in a new computational science program. We report on the development and pilot testing of assessment tools in both core and cognate courses. Specifically, we detail a diagnostic assessment that predicted success in our introductory computational science course with reasonable reliability; we give an account of our use of an existing assessment tool to investigate how introducing computational thinking in a cognate course influences learning of the traditional course material; and we discuss rubric development for project evaluation.

Assessing and refining an undergraduate computational science curriculum

Abstract We describe our experiences with curriculum development and learning assessment in a new undergraduate computational science program. We report on the development and pilot testing of assessment tools in both core and cognate courses. Specifically, we detail a diagnostic assessment that predicted success in our introductory computational science course with reasonable reliability; we give an account of our use of an existing assessment tool to investigate how introducing computational thinking in a cognate course influences learning of the traditional course material; and we discuss developing a pancurriculum rubric for scoring computational science projects.

A Comparison of Three Solute Transport Models Using Mountain Stream Tracer Experiments

Stream ecology may be influenced by the temporary trapping of solutes in geomorphologic structures, which is usually quantified by fitting the Transient Storage Model to tracer data. This paper explores the relationships between the parameters of this model and those of two simpler models, namely the Advection-Dispersion Model and the Aggregated Dead Zone model. It is motivated by the possibility of obtaining more reliable transient storage parameter values by correlating them with the parameters of the other models instead of evaluating them directly. Results were obtained by fitting all three models to a set of tracer data from mountain streams, predominantly in Iceland. Some strong correlations were found between some of the parameters of the transient storage model and the advection-dispersion model, but no strong correlations were found between the parameters of the transient storage model and the aggregated dead zone model. For all three models, combinations of the optimized parameters correctly described the bulk movement of the solute cloud, giving confidence in the optimized parameters.

Flow Dependence of the Parameters of the Transient Storage Model

Using 25 tracer experiments, parameters of the transient storage model (TSM) were evaluated for a reach of the river Brock in north-west England with the primary aim of investigating their dependence on flow rate. Since only a very few previous studies have considered this issue, these new results aid our understanding on how the TSM could be applied to a reach at flow rates beyond the range of flow rates for which observations of solute transport exist. Velocity increased with increasing flow rate in a manner consistent with current knowledge. In contrast, and unexpectedly, the dispersion coefficient reduced (weakly) with increasing flow rate and the values were rather scattered. The transient storage exchange rate increased with increasing flow rate, which corroborates some of the sparse existing knowledge of this parameter’s behaviour. The ratio of transient storage area to main channel area was essentially constant over the range of flow rates examined, which is consistent with some studies on single reaches.

Spatial Distribution of Dissolved Oxygen at Rapid Hydraulic Structures as an Indicator of Local-Scale Processes

This work aimed to examine the impact of rapid hydraulic structures on water temperature and dissolved oxygen concentration in the Porebianka mountain stream. This has been achieved by measurements of hydraulic characteristics and physiochemical properties of water such as water temperature and dissolved oxygen concentration. It has been shown that rapid hydraulic structures exhibit a large spatial diversity in morphology and flow paths, that manifests in the spatial heterogeneity of thermal conditions and oxygen concentrations at a single structure scale. The results have demonstrated that pools between the rapid have higher oxygen concentrations when compared to the rapid region. The highest concentrations of oxygen occured in pools located close to the upstream edge of the rapid ramp where the flow undergoes gradual acceleration. Elevated concentrations of dissolved oxygen were also observed in the dissipation basin. The lowest concentrations were observed at stream banks. The results emphasise the relative importance of site-specific characteristics on physiochemical properties of flow, which might help to understand multi-scale processes across rivers and improve future plans of restoration practices in mountain streams.

Hydraulic Influences on Dispersion and Reaeration in Rivers

An important application of environmental hydraulics is the prediction of the fate and transport of dissolved oxygen within fluvial systems. For rivers this requires knowledge of the principle hydrological processes such as advection and dispersion and the physico-chemical process of re-aeration. Currently, in the absence of appropriate field measurements quantifying the mixing or aeration processes in a river, we rely on semi-empirical predictive equations that attempt to relate these processes to global flow and channel parameters. Although there is some theoretical justification for the form of these equations, they are not particularly successful even for channels of simple shape. As more complex channel shapes (e.g. two-stage flood relief channels) are tackled the equations become increasingly inappropriate. To help address this concern, the chapter proposes a theoretical approach for evaluating both the longitudinal dispersion coefficient and the re-aeration coefficient in channels of arbitrary shape that is based on integral formulations and which uses theoretical predictions of the transverse flow structure that are based on Shiono and Knight’s (J Fluid Mech 222:617–646, 1991) momentum balance equation. The results for a simple channel (trapezoidal) are consistent with current knowledge, but they reveal unexpected patterns for a complex channel (two-stage, trapezoidal with active floodplains) that contains zones of distinctly different velocity and depth. The results also explore the role of the transverse turbulent transfer of momentum. For the simple channel, the dispersion coefficient was very small (being in the range 0–1 m2/s for flow rates between 0 and 35 m3/s and channel widths of approximately 15 m), and increased approximately linearly with flow rate. The influence of the transverse turbulent momentum exchange was relatively significant. For the complex channel, the dispersion coefficient was very large (being in the range 27,000–500 m2/s for flow rates between 35 and 175 m3/s and widths of approximately 55 m), and decreased with flow rate according to a power law with an exponent of about −4.7. The influence of the transverse turbulent momentum exchange was less than for the simple channel case. The predictions for both flow conditions are consistent with observed trends reported in Rutherford (River mixing. Wiley, Chichester, 1994). The very large dispersion coefficients found in the complex channel case could not be predicted using the existing semi-empirical equations proposed by Liu (J Environ Eng Div, Am Soc Civil Eng 103(EE1):59–69, 1977) and Deng et al. (J Hydraul Eng, Am Soc Civil Eng 127(11):919–927, 2001); neither could the rapid decrease with increasing flow rate. This is not surprising because the equations cannot represent the extremely strong transverse velocity shear that exists in these flows that contain zones of quite different velocity and depth. For the re-aeration coefficient in the simple channel we identified a power law decrease (exponent of about −0.5) with flow rate from about 40 to 10 per day up to the bank full condition. Once flows went over-bank the re-aeration coefficient jumped considerably (to about 100 per day) due to the small depths on the floodplains. It then reduced as a power law as flow rate increased (exponent of about −0.9). The influence of the transverse turbulent momentum exchange was not very significant for either channel case. Results from a semi-empirical equation proposed by Bennett and Rathbun (Reaeration in open-channel flow. United Sates Geological Survey, Washington, 74 pp, 1972) mirrored the computational results, but under-predicted the coefficient by about 50 % for both the simple and complex channel cases. Clearly, existing semi-empirical equations for the dispersion coefficient and the re-aeration coefficient should not be used for predicting non-conservative chemical transport for the over-bank case of a complex channel. A sensitivity analysis for the case of a steady oxygen demanding waste water discharge showed that the maximum dissolved oxygen sag and its location were insensitive to dispersion but were significantly sensitive to re-aeration for both channel cases. Hence, for this waste water scenario future work should focus on improving the prediction of re-aeration coefficients in both types of channel.

A massively parallel semi-Lagrangian algorithm for solving the transport equation

The scalar transport equation underpins many models employed in science, engineering, technology and business. Application areas include, but are not restricted to, pollution transport, weather forecasting, video analysis and encoding (the optical flow equation), options and stock pricing (the Black-Scholes equation) and spatially explicit ecological models. Unfortunately finding numerical solutions to this equation which are fast and accurate is not trivial. Moreover, finding such numerical algorithms that can be implemented on high performance computer architectures efficiently is challenging. In this paper the authors describe a massively parallel algorithm for solving the advection portion of the transport equation. We present an approach here which is different to that used in most transport models and which we have tried and tested for various scenarios. The approach employs an intelligent domain decomposition based on the vector field of the system equations and thus automatically partitions the computational domain into algorithmically autonomous regions. The solution of a classic pure advection transport problem is shown to be conservative, monotonic and highly accurate at large time steps. Additionally we demonstrate that the algorithm is highly efficient for high performance computer architectures and thus offers a route towards massively parallel application.