Lake Diefenbaker Research Articles

Sedimentation and erosion in Lake Diefenbaker, Canada: solutions for shoreline retreat monitoring.

This study looks into sedimentation and erosion rates in Lake Diefenbaker, a prairie reservoir, in Saskatchewan, Canada, which has been in operation since 1968. First, we looked at the historical data in all different formats over the last 70years, which includes data from more than 20years before the formation of the lake. The field observations indicate high rates of shoreline erosion, especially in the upstream portion as a potential region for shoreline retreat. Because of the great importance of this waterbody to the province, monitoring sedimentation and erosion rates is necessary for maintaining the quality of water especially after severe floods which are more common due to climate change effects. Second, we used Google Maps Elevation API, a new tool from Google that provides elevation data for cross sections drawn between two points, by drawing 24 cross sections in the upstream area extending 250m from each bank. This feature from Google can be used as an easy and fast monitoring tool, is free of charge, and provides excellent control capabilities for monitoring changes in cross-sectional profiles.

Sediment plume model-a comparison between use of measured turbidity data and satellite images for model calibration.

In this study, we built a two-dimensional sediment transport model of Lake Diefenbaker, Saskatchewan, Canada. It was calibrated by using measured turbidity data from stations along the reservoir and satellite images based on a flood event in 2013. In June 2013, there was heavy rainfall for two consecutive days on the frozen and snow-covered ground in the higher elevations of western Alberta, Canada. The runoff from the rainfall and the melted snow caused one of the largest recorded inflows to the headwaters of the South Saskatchewan River and Lake Diefenbaker downstream. An estimated discharge peak of over 5200m3/s arrived at the reservoir inlet with a thick sediment front within a few days. The sediment plume moved quickly through the entire reservoir and remained visible from satellite images for over 2weeks along most of the reservoir, leading to concerns regarding water quality. The aims of this study are to compare, quantitatively and qualitatively, the efficacy of using turbidity data and satellite images for sediment transport model calibration and to determine how accurately a sediment transport model can simulate sediment transport based on each of them. Both turbidity data and satellite images were very useful for calibrating the sediment transport model quantitatively and qualitatively. Model predictions and turbidity measurements show that the flood water and suspended sediments entered upstream fairly well mixed and moved downstream as overflow with a sharp gradient at the plume front. The model results suggest that the settling and resuspension rates of sediment are directly proportional to flow characteristics and that the use of constant coefficients leads to model underestimation or overestimation unless more data on sediment formation become available. Hence, this study reiterates the significance of the availability of data on sediment distribution and characteristics for building a robust and reliable sediment transport model.

Sympatric walleye Sander vitreus and sauger Sander canadensis in large reservoirs: Variable isotopic niche size and overlap across multiple time scales

AbstractWalleye Sander vitreus (Mitchill) and sauger Sander canadensis (Griffith & Smith) may occupy similar niches in sympatric populations, but their long‐term resource use is poorly studied. Multi‐tissue (liver, muscle and bone) isotopic niche analyses were used to compare resource use by these species in two large reservoirs in Saskatchewan, Canada. Population isotopic niches were larger for sauger (1.73‰2–3.60‰2) than walleye (0.51‰2–2.61‰2) in Lake Diefenbaker, but opposite in Tobin Lake (sauger = 0.54‰2–2.57‰2; walleye = 1.30‰2–2.94‰2). Analyses of population overlap suggest that these species co‐exist via niche partitioning, but partitioning was more pronounced in Lake Diefenbaker (overlap = 6%–38%; Tobin Lake overlap = 18%–62%). The size of individual niches and levels of intra‐ and interspecific overlap among individual fish also varied with reservoir. This study shows that walleye and sauger use different resources across different time scales, but the degree of partitioning can vary markedly with environment.

Assessment of nutrient loadings of a large multipurpose prairie reservoir

The relatively low water flow velocities in reservoirs cause them to have high capacities for retaining sediments and pollutants, which can lead to a reduction in downstream nutrient loading. Hence, nutrients can progressively accumulate in reservoirs, resulting in the deterioration of aquatic ecosystems and water quality. Lake Diefenbaker (LD) is a large multipurpose reservoir, located on the South Saskatchewan River (SSR), that serves as a major source of freshwater in Saskatchewan, Canada. Over the past several years, changes in land use (e.g. expansion of urban areas and industrial developments) in the reservoir’s catchment have heightened concerns about future water quality in the catchment and in the reservoir. Intensification of agricultural activities has led to an increase in augmented the application of manure and fertilizer for crops and pasture. Although previous research has attempted to quantify nutrient retention in LD, there is a knowledge gap related to the identification of major nutrient sources and quantification of nutrient export from the catchment at different spatial scales. Using the SPAtially Referenced Regression On Watershed (SPARROW) model, this gap has been addressed by assessing water quality regionally, and identifying spatial patterns of factors and processes that affect water quality in the LD catchment. Model results indicate that LD retains about 70% of the inflowing total nitrogen (TN) and 90% of the inflowing total phosphorus (TP) loads, of which fertilizer and manure applied to agricultural fields contribute the greatest proportion. The SPARROW model will be useful as a tool to guide the optimal implementation of nutrient management plans to reduce nutrient inputs to LD.

Impacts of Climate Change on the Water Quality of a Regulated Prairie River

Flows along the upper Qu’Appelle River are expected to increase in the future via increased discharge from Lake Diefenbaker to meet the demands of increased agricultural and industrial activity and population growth in southern Saskatchewan. This increased discharge and increased air temperature due to climate change are both expected to have an impact on the water quality of the river. The Water Quality Analysis Simulation Program (WASP7) was used to model current and future water quality of the upper Qu’Appelle River. The model was calibrated and validated to characterize the current state of the water quality of the river. The model was then used to predict water quality [nutrient (nitrogen and phosphorus) concentrations and oxygen dynamics] for the years 2050–2055 and 2080–2085. The modelling results indicate that global warming will result in a decrease in ice thickness, a shorter ice cover period, and decreased nutrient concentrations in 2050 or 2080 relative to 2010, with a greater decrease of nutrient concentrations in open water. In contrast to the effect of warmer water temperatures, increased flow through water management may cause increases in ammonium, nitrate, and dissolved oxygen concentrations and decreases in orthophosphate concentrations in summer.

Seasonal and inter-annual variability of phytoplankton in central Lake Diefenbaker (Saskatchewan, Canada) proximal to a large commercial aquaculture farm

Abstract Lake Diefenbaker, Saskatchewan, is a large reservoir (392 km 2 ) with multiple end users, including a commercial aquaculture farm, which rely on sustainable water quality. Phytoplankton samples were taken monthly at upstream, aquaculture, and downstream locations during August 2008 to November 2011. The phytoplankton community was primarily diatoms and cryptophytes. Diatom biomass was correlated with greater mixing depth, while cryptophyte biomass was correlated with higher dissolved nutrients. Phytoplankton biomass was characterized by date, season and year, and this information was used to test spatial differences between upstream, farm and downstream locations. There was no significant difference in chlorophyte, chrysophyte or dinoflagellate biomass spatially. Upstream sites (US) had higher diatom biomass in the fall (US-DS = 329 mg/m 3 ). Downstream sites (DS) had higher cryptophyte biomass in the summer (DS-US = 241 mg/m 3 ) and higher cyanobacteria biomass in the fall (DS-US = 15 mg/m 3 ). Spatial trends in diatoms, cryptophytes, and cyanobacteria between upstream and downstream locations were found to be significantly related to distance downstream, but failed to provide evidence of a fish farm effect. There was far greater inter-annual variability than spatial variability in phytoplankton biomass of Lake Diefenbaker, a lake prone to flooding and drought. For instance, flooding in 2011 resulted in elevated suspended solids, nutrient concentrations, and algal biomass (particularly cryptophytes and dinoflagellates) due to a La Niňa winter. Flood inputs of suspended solids to Lake Diefenbaker favour low-light adapted taxa (e.g., Rhodomonas minuta Skuja) over potentially toxic cyanobacteria.

Change in nutrient loading pattern due to coupled effect of change in concentration and hydroclimatic forces

ABSTRACTIncrease in nutrient loading into large riverine systems is of interest because of long-lasting negative effects on downstream water bodies. We know little of this phenomenon with respect to long-term relationships between water quality metrics and trends in river flow. Here we present a multi-decadal (1937–2014) examination of nutrient transport patterns of the South Saskatchewan and Red Deer Rivers, Canada. Using a multi-method approach, we show a slightly increasing trend in annual volume of water reaching a mid-watershed reservoir (Lake Diefenbaker) concomitant with a change in seasonal variability of flow events from the 1970s. Both total nitrogen (TN) and total phosphorus (TP) flux increased during the period of record. TN concentration increased, while TP concentration decreased. On average, the high-flow season transported 95% of annual TP flux, 75% of annual TN flux, and 65% of annual flow. Inter-annually, high-flow season contributions decreased with respect to water volume, but increased with respect to both TN and TP flux. We believe that changes in variability but also timing of hydroclimatic forces are responsible for observed patterns. Higher flows and more precipitation during the high-flow season are most likely responsible for increasing nutrient load regardless of overall decreases in nutrient concentration.

Parameter Sensitivity of a Surface Water Quality Model of the Lower South Saskatchewan River—Comparison Between Ice-On and Ice-Off Periods

Little is known about seasonal differences (ice-on vs. ice-off periods) and the sensitivity of in-stream processes to surface water quality constituents in rivers that have a persistent ice cover in winter. The goal of this study is to investigate the sensitivity of nutrient transformation processes on surface water quality, especially rivers in cold regions where ice-covered conditions persist for a substantial part of the year. We established a sensitivity analysis framework for water quality modelling and monitoring of rivers in cold regions using the Water Quality Analysis Program WASP7. The lower South Saskatchewan River in the interior of western Canada, from the Gardiner Dam at Lake Diefenbaker to the confluence of the North and South Saskatchewan rivers, is used as a test case for this purpose. The study reveals that parameter sensitivities differ between ice-covered and ice-free periods and biological model parameters related to nutrient-phytoplankton dynamics can still be sensitive during the ice-covered season. For example, sediment oxygen demand is an important parameter during the ice-on period, whereas parameters related to nitrification are more sensitive in the ice-off period. These results provide insight into important water quality monitoring aspects in cold regions during different seasons.

Phosphorus release from sediments in a river-valley reservoir in the northern Great Plains of North America

Aside from a companion investigation to this study, there are currently no peer-reviewed phosphorus (P) release rate data for northern North American (i.e., Canadian) reservoirs. Using Lake Diefenbaker, Saskatchewan, Canada as a case study, we tested the effect of variation in overlying water DO conditions on the P release rates from sediment cores. Sediment cores from four down-reservoir locations in Lake Diefenbaker were incubated under high (>8 mg l−1), low (2–3 mg l−1), or anoxic (<1 mg l−1) DO concentrations. Sediment cores were then analyzed for total P (TP) and three geochemical P fractions to assess how the DO regime influenced sediment P inventory. Maximum P release rates were highest under anoxic conditions and similar among sites (15.0–20.3 mg m−2 day−1), with the low-DO rates intermediate to the high-DO and anoxic P fluxes. Predictive internal P loading models considering only hypolimnetic anoxia may therefore oversimplify and thus underestimate P mobilization in situ. Non-apatite inorganic P (54 ± 10% across sites) from the top 1 cm of the sediment profile was the main source of P released during incubations, indicating that sampling on a coarser scale of resolution could obscure the relationship between sediment geochemistry and short-term P flux.

Do human activities affect phytoplankton biomass and composition in embayments on Lake Diefenbaker?

ABSTRACTLake Diefenbaker (LD) is an important source of water for southern Saskatchewan. LD is characterized by numerous embayments containing anthropogenic activities (e.g., housing, marinas, cattle watering). Many of these activities are increasing on this important reservoir in association with the rapidly developing economy of Saskatchewan. These activities may reduce water quality directly or indirectly by encouraging the growth of nuisance algae (i.e. cyanobacteria). Here, we examined phytoplankton biomass and composition in eight embayments exposed to anthropogenic activities, four unexposed embayments with no perceived human activities and six main channel sites adjacent to the embayments from June to October (2011 and 2012). Phytoplankton biomass and composition was not significantly different in exposed, unexposed embayments and main channel sites (p > 0.05), with the diatoms and cryptomonads constituting 87%–91% of the total phytoplankton biomass in both years. High flows from the South Saskatchewan River (SSR) in both years may have resulted in the rapid flushing of the embayments and dampened any localized impacts that could have resulted from anthropogenic activities as found in other studies. Hence, future study on LD should be conducted during years with low flow from the SSR when the rate of flushing of embayments will be reduced.

The 2013 flood event in the South Saskatchewan and Elk River basins: Causes, assessment and damages

In late June 2013, heavy rainfall and rapidly melting alpine snow triggered flooding throughout much of the southern half of Alberta. Heavy rainfall commenced on 19 June and continued for 3 days. When the event was over, more than 200 mm and as much as 350 mm of precipitation had fallen over the Front Ranges of the Canadian Rocky Mountains. Tributaries to the Bow River including the Ghost, Kananaskis, Elbow, Sheep and Highwood, and many of their tributaries, all reached flood levels. The storm had a large spatial extent causing flooding to the north and south in the Red Deer and Oldman Basins, and also to the west in the Elk River in British Columbia. Convergence of the nearly synchronous floodwaters downstream in the South Saskatchewan River system caused record high releases from Lake Diefenbaker through Gardiner Dam. Dam releases in Alberta and Saskatchewan attenuated the downstream flood peak such that only moderate flooding occurred in Saskatchewan and Manitoba. More than a dozen municipalities declared local states of emergency; numerous communities were placed under evacuation order in Alberta and Saskatchewan. The media reported that more than 100,000 people needed to evacuate their homes, and five people died. At CAD$6 billion, the Province of Alberta estimated that it was the costliest natural disaster in Canadian history. At their peak, the water levels were the highest in the past 60 years, and nearing those of historic events of the late 1800s and early 1900s where records exist. There was major damage to infrastructure roads, bridges and culverts. Downtown Calgary was inundated, as were many communities such as High River and Bragg Creek. Debris flows and debris torrents affected communities such as Canmore and Exshaw, and isolated many mountain communities including closing the TransCanada Highway for several days. Environment Canada called it the “Flood of Floods.” Insured losses of CAD$2 billion included thousands of cars and homes demolished and damaged by backed-up sewers. Flood damage losses and recovery costs from the flood are projected to exceed CAD$6 billion.

Species-specific mercury bioaccumulation in a diverse fish community.

Mercury bioaccumulation models developed for fish provide insight into the sources and transfer of Hg within ecosystems. Mercury concentrations were assessed for 16 fish species of the western reach of Lake Diefenbaker, Saskatchewan, Canada. For top predators (northern pike, Esox Lucius; walleye, Sander vitreum), Hg concentrations were positively correlated to δ(15)N, and δ(15)N to fish age, suggesting that throughout life these fish fed on organisms with increasingly higher trophic values and Hg concentrations. However, fish mass and/or age were the principal parameters related to Hg concentrations for most species. For 9 common species combined, individual variation in Hg concentration was explained in declining order of importance by fish mass, trophic position (δ(15)N), and fish age. Delta (15)N value was not the leading variable related to Hg concentration for the assemblage, probably because of the longevity of lower--trophic-level species (3 species ≥ 20 yr), substantial overlap in Hg concentration and δ(15)N values for large-bodied fish up to 3000 g, and complex relationships between Hg concentration and δ(15)N among species. These results suggest that the quantity of food (and Hg) consumed each year and converted to fish mass, the quantity of Hg bioaccumulated over years and decades, and trophic position were significant determinants of Hg concentration in Lake Diefenbaker fish.

The effects of the management of Lake Diefenbaker on downstream flooding

The impact of management of Lake Diefenbaker, Saskatchewan, the Canadian Prairies’ largest reservoir, on downstream flooding is examined over three large inflow events in 2005, 2011 and 2013. The reservoir stores inflows for water supply, recreational and ecological purposes and preferentially releases water in mid-winter for hydroelectricity generation. It can also have an important role in downstream flood mitigation. The analysis shows great uncertainty in inflows due to ungauged local inflows, and substantial errors in the mass balance of the reservoir associated primarily with high inflows. The management of the reservoir has been challenged by declining spring inflows since the 1960s with a trend for increasing minimum annual reservoir elevations over time. Management of the reservoir has undergone changes since 2011 to lower the minimum elevation so as to achieve the 1 July target level, resulting in very different effects on flooding in 2005, 2011 and 2013. In all years, the management of the reservoir reduced the maximum flooded area upstream of Saskatoon and the integrated flooded duration and area. In 2005 the area of flooding upstream of Saskatoon was reduced for all durations and in 2013 for durations shorter than 5 days, but in 2011 it was increased for durations between 5 and 20 days. Flooding on the Saskatchewan River, i.e. downstream of the confluence of the North and South Saskatchewan Rivers, was increased in 2011 due to the delay in the timing of the peak flow induced by reservoir operation. Elimination of downstream flooding in these years would have required continuous adjustment of outflows to optimize storage, which itself requires modelling the inflow hydrograph. Future operation of the reservoir should adopt such optimization and modelling, whilst considering the impact of non-stationarity due to climate change.

Dynamic water quality modelling and uncertainty analysis of phytoplankton and nutrient cycles for the upper South Saskatchewan River.

The surface water quality of the upper South Saskatchewan River was modelled using Water Quality Analysis Simulation Program (WASP) 7.52. Model calibration and validation were based on samples taken from four long-term water quality stations during the period 2007-2009. Parametric sensitivities in winter and summer were examined using root mean square error (RMSE) and relative entropy. The calibration and validation results show good agreement between model prediction and observed data. The two sensitivity methods confirmed pronounced parametric sensitivity to model state variables in summer compared to winter. Of the 24 parameters examined, dissolved oxygen (DO) and ammonia (NH3-N) are the most influenced variables in summer. Instream kinetic processes including nitrification, nutrient uptake by algae and algae respiration induce a higher sensitivity on DO in summer than in winter. Moreover, in summer, soluble reactive phosphorus (SRP) and chlorophyll-a (Chla) variables are more sensitive to algal processes (nutrient uptake and algae death). In winter however, there exists some degree of sensitivity of algal processes (algae respiration and nutrient uptake) to DO and NH3-N. Results of this study provide information on the state of the river water quality which impacts Lake Diefenbaker and the need for additional continuous monitoring in the river. The results of the sensitivity analysis also provide guidance on most sensitive parameters and kinetic processes that affect eutrophication for preliminary surface water quality modelling studies in cold regions.

Environmental factors influencing phytoplankton communities in Lake Diefenbaker, Saskatchewan, Canada

Abstract Lake Diefenbaker (LD) is a large reservoir located in the Canadian Prairies. Approximately 98% of the inflow is from the South Saskatchewan River (SSR). The composition and ecology of the phytoplankton community along the length of LD has not been reported since the 1980s. This is a potential problem for a reservoir that serves as an important source of water for indirect domestic consumption, irrigation, and recreation. Therefore, we examined epilimnetic whole water samples along the length of LD during the open-water season (June to October) in 2011 and 2012. We estimated the trophic status from chlorophyll a (TSI chl a ) and examined the phytoplankton community in relation to environmental factors. We observed high flow events that carried large nutrient loads and associated turbidity into LD from the SSR. Despite the high flow events, mean TSI chl a placed LD as a mesotrophic system with a highly diverse phytoplankton community (72 genera). The increased nutrient and non-algal turbidity associated with high flow events may be related to the dominance of the cryptophytes and the bacillariophytes (both constituted ~ 89% of the total phytoplankton biomass). Inflow and water temperature explained 41% of the variation in cryptophyte biomass, whereas mixing depth and particulate nitrogen to particulate phosphorus molar ratios explained 38% of the variation in bacillariophyte biomass. Despite the low cyanobacterial biomass (

Light and nutrient co-limitation of phytoplankton communities in a large reservoir: Lake Diefenbaker, Saskatchewan, Canada

The reliance on reservoirs to meet global water demands is increasing, as is the need to develop an understanding of factors influencing water quality in these understudied water bodies. This study assessed how the interaction between light and nutrients influences phytoplankton dynamics in a large, heterogeneous prairie reservoir; Lake Diefenbaker, Saskatchewan. A multiple indicator assessment of factors (light, phosphorus [P], and nitrogen [N]) influencing phytoplankton biomass, physiology, and gross primary productivity (GPP) was conducted. Light deficiency was assessed through the application of an indicator threshold for the mean daily mixed layer irradiance as well as the ratio of this irradiance to the light saturation parameter derived from photosynthesis-irradiance curves. Short-term physiological assays and long-term compositional nutrient status indicators were applied to assess the in situ nutrient deficiency of the epilimnetic phytoplankton communities. We observed regional differences in light conditions and nutrient chemistry related to incoming flow and nutrient load from the South Saskatchewan River. Light deficiency was detected 59% of the times measured during the open-water season of 2013. Both GPP and quantum efficiency of photosystem II (ϕPSII) were relatively high (108 ± 101 mmol O2 m− 2 day− 1 and 0.57 ± 0.07, respectively) across all regions and months. During conditions of light sufficiency, there was no evidence of N deficiency, and P was the limiting nutrient to phytoplankton communities 90% of the times measured. Nutrient use efficiency and GPP rates were significantly higher under light sufficient conditions. The asynchrony of light and nutrient supply influences production and biomass accrual in this reservoir.

Spatial and temporal trends in reservoir physicochemistry and phosphorus speciation within Lake Diefenbaker, a Great Plains reservoir, as inferred from depositional sediments

Previous research and observations by area residents suggested that the frequency of algal blooms in the Qu'Appelle arm of Lake Diefenbaker, Saskatchewan, Canada, has been increasing over time. However, limited historical data were available to assess long-term trends in the overall reservoir. Narrow river-valley reservoirs, such as Lake Diefenbaker, are known to display marked longitudinal physicochemical and biological gradients. To assess both spatial and temporal geochemical trends, sediment cores were collected from eight sites along the longitudinal axis of this reservoir and sectioned vertically for analysis of various parameters. Surficial sediments showed an increasing trend in organic carbon, total phosphorus concentrations, and δ15N values, and decreasing trends in δ13C values and in the carbon to nitrogen ratio, with distance down-reservoir. This was attributed to reduced deposition of allochthonous detritus and increased entrainment of nutrients as settled autochthonous materials, as the reservoir transitioned to less dynamic lacustrine conditions. A strong correlation was determined in the Qu'Appelle arm between chlorophyll a concentrations and the more biologically available forms of P (non-apatite inorganic P + organic P) in the sediment profile (r = 0.955, p < 0.001), suggesting an increase in autochthonous production in this region of the reservoir in the mid-1990s, which subsequently plateaued. The spatial and temporal trends illustrated in the sediment core profiles of this study demonstrate that paleolimnological investigations of narrow river-valley reservoirs need to consider geochemical and biological spatial gradients associated with reservoir zonation.

Spatial and temporal patterns in physical properties and dissolved oxygen in Lake Diefenbaker, a large reservoir on the Canadian Prairies

Abstract Lake Diefenbaker (LD) is a multi-purpose deep storage reservoir with complex longitudinal zonation. Despite the importance of LD to Saskatchewan, the last comprehensive evaluation of this large reservoir was completed 30 years ago. Therefore, an assessment of key features (patterns in stratification, turbidity, and dissolved oxygen (DO)) was undertaken during the ice-free period of 2011–2013 to characterize conditions that may affect water quality, including climate change. In addition, these features were compared with the expectations of the longitudinal zonation concept (LZC) of Kimmel and Groeger (1984). The reservoir was dimictic, and summer mixing depths (10–20 m) were similar between a drought year (1984) and the current study. Sections of the hypolimnia became hypoxic, or were sub-optimal for cold and cool water fish (≤ 5 mg/L). Volumetric hypolimnetic oxygen depletion rates were 0.034 to 0.12 mg L/day and overlapped with other lakes and reservoirs. Sections of LD would be prone to hypolimnetic anoxia had thermal stratification been prolonged for another 2 to 4 weeks. LD experienced three years of above average peak flows. The peak flow of 2013 brought high turbidity (e.g., 200 NTU) and organic matter that resulted in a metalimnetic DO minimum. Spatial patterns in thermal stratification and turbidity (but not DO) followed the expectations of the LZC. Drought conditions of 1984 resulted in reduced hypolimnetic volumes, anoxic conditions and algal blooms. Therefore, as the Northern Plains continue to warm, monitoring of Lake Diefenbaker during drought periods is warranted, especially when droughts follow high flow events.

Reconstructing diatom and chironomid assemblages to infer environmental spatiotemporal trends within Lake Diefenbaker, a narrow river-valley reservoir on the Canadian Prairies

Lake Diefenbaker (Saskatchewan, Canada) is a large reservoir in the southern Canadian Prairies. However, limited monitoring data were available to assess long-term trends in environmental quality. Therefore, a paleolimnological study was conducted to investigate temporal trends in the trophic status of the reservoir. Sediment cores were collected along the length of Lake Diefenbaker, and sediment subsections were analyzed for sub-fossil remains of diatoms and chironomid larvae. The compositions of diatom and chironomid communities were consistent over approximately the previous decade at up-reservoir locations, with a meso-eutrophic species, Stephanodiscus parvus, dominating the diatom communities. This taxon also dominated the early down-reservoir diatom communities along the main flow of the reservoir. However, during the 1980s, dominance of the diatom assemblage shifted from S. parvus to a combination of Aulacoseira ambigua and Tabellaria flocculosa in the majority of down-reservoir regions. Based on total phosphorus concentration optima for these species, this shift suggests a reduction in total phosphorus both spatially and temporally. However, these observed shifts could also have been due to increased water clarity or changes to thermal stratification regime both spatially (from up- to down-reservoir) and over time down-reservoir. An increase in relative abundance of the chironomid tribe Tanytarsini, both with distance down-reservoir and over time, was likely due to spatial and temporal changes in benthic habitat (i.e., sediment character). Results highlight the need to interpret temporal trends within the context of spatial gradients in physical processes, chemistry, and biology along the longitudinal axis of narrow river valley reservoirs.

Reactive silicon dynamics in a large prairie reservoir (Lake Diefenbaker, Saskatchewan)

Abstract There is an up-coming global surge in dam construction. River damming impacts nutrient cycling in watersheds through transformation and retention in the reservoirs. The bioavailability of silicon (Si) relative to nitrogen (N) and phosphorus (P) concentrations, in combination with light environment, controls diatom growth and therefore influences phytoplankton community compositions in most freshwater aquatic ecosystems. In this study, we quantified reactive Si cycling and annual Si retention in Lake Diefenbaker, a 394 km 2 prairie reservoir in Saskatchewan, Canada. Retention estimates were derived from 7 sediment cores combined with high-resolution spatiotemporal sampling of water column dissolved Si (DSi). Current annual DSi retention in the reservoir is approximately 28% of the influx, or 2.5 × 10 8 mol yr − 1 . The relative retention of DSi is higher than that of bioavailable N, but lower than that of bioavailable P, indicating a decoupling of the P, N, and Si cycles in the reservoir. The largest accumulation of reactive particulate Si (PRSi) is found in sediments deposited in the mid-reservoir region, which corresponds to the region of highest epilimnetic DSi decrease during the 2013 open water season. This region corresponds to the fluvial-to-lacustrine transition zone, and represents a hot spot for Si retention. Differences in retention efficiencies of macronutrients may not only affect the in-reservoir ecology and water quality, but also modifies the N:P:Si ratios exported downstream, which has the potential to alter ecosystem processes in receiving water bodies.