Journal of Great Lakes Research 41 Supplement 2 (2015) 1–7

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Journal of Great Lakes Research

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Lake Diefenbaker: The prairie jewel

Introduction with intakes that are 34 m below FSL. The proportion of inflowing water that flows out of Gardiner Dam varies from ~70% in low flow years (e.g., 2001 average flow was 84 m3 s−1)toover97%inhigh fl 3 −1 Fighting drought with a dam: Lake Diefenbaker, the reservoir of ow years (e.g., 2005; 374 m s ; Hudson and Vandergucht, 2015) fl life-giving water.CBC's Norman DePoe. with a median proportional out ow of ~94%. Additional water losses in- clude net evaporation, which can represent over 10% in dry years, irriga- On the broad expanse of flat land of the northern Great Plains of tion, and direct use from the reservoir. From the Qu'Appelle Dam, Lake Canada lies a large, narrow, river-valley reservoir, Lake Diefenbaker. Diefenbaker water is transferred out of the natural catchment and re- The reservoir was developed for multiple benefits including the irriga- leased down the Qu'Appelle River to supply southern regions of the tion of ~2000 km2 of land in Central Saskatchewan, the generation of province. With a mean depth of 22 m, the water residence time, though hydroelectricity, a source of drinking water, flood control, water for in- variable, is typically just over a year (Donald et al., 2015; Hudson and dustry and livestock, and recreation. This semi-arid region has an aver- Vandergucht, 2015). While generally considered to be dimictic age (1961–2003) annual precipitation of 352 mm (Chun et al., 2013). (Hudson and Vandergucht, 2015; North et al., 2015; Sadeghian et al., The reservoir produces ~15% of the province of Saskatchewan's electric- 2015) and mesotrophic (Abirhire et al., 2015; Dubourg et al., 2015), ity (Saskatchewan Water Security Agency [SWSA], 2012a). Lake Diefen- the stratification, mixing regime, and trophic state varies along the baker (51o1′53″N, 106o50′9″W; Fig. 1) is 182 km long with a surface length of the reservoir. area of 394 km2 and a volume of 9.03 km3 at full supply level (FSL; Sadeghian et al., 2015). Anthropogenic stressors Lake Diefenbaker is in the prairie pothole region of western Canada in the Saskatchewan River Basin. The Saskatchewan River Basin (Fig. 1) Upstream water use from major industrial centers and agriculture is home to 3 million people and represents a major source of water for can affect the quantity and quality of water. Within the reservoir, the prairie provinces of Alberta, Saskatchewan, and Manitoba. Located agriculture and industry also draw on the water supply with estimates in the Wheat Belt, 82% of Canada's irrigated agriculture is within this of the incremental increased gross benefit of irrigation versus dry land basin (Gober and Wheater, 2014). Situated along the South Saskatche- farming around the lake valued at ~$75 million yr-1 (SWSA, 2012b). wan River (SSR), the 7th longest river in Canada, Lake Diefenbaker Land use in Lake Diefenbaker's watershed is predominantly agriculture, was created by the construction of the Gardiner and Qu'Appelle Dams which includes cropping and livestock production (SWSA, 2012b). Wild in 1967. The SSR is fed by three principal tributaries, the Old Man, West Steelhead aquaculture is located within Lake Diefenbaker and is Bow, and Red Deer rivers in southern Alberta. These tributaries have one of Canada's largest freshwater rainbow trout (Oncorhynchus mykiss their headwaters in the Rocky Mountains (Fig. 1). The SSR basin has a Walbaum)producers(Fig. 1). It began its operations in 1994 and is gross drainage area of 152,700 km2 and an effective drainage area of currently licensed to produce 1450 metric tons of fish yr−1.Theyhave 82,300 km2 (SWSA, 2012b). Effective drainage area is the estimated received approval to expand into an adjacent embayment and increase area that contributes water to a receiving water body during a median production to 1750 metric tons yr-1 (Sweeney International runoff year. Because of its proximity to Alberta and its small effective Management Corp., 2010). drainage area within Saskatchewan, the majority (98%) of the water The SSR is defined by highly variable flows. Long-term historical re- that flows into Lake Diefenbaker originates in Alberta (SWSA, 2012b). construction suggests that natural flows from the Rocky Mountains The average annual flow rate of the SSR upstream of Lake Diefenbaker have varied substantially (Wolfe et al., 2011). Studies suggest that ranges from 200–300 m3 s−1 (Hudson and Vandergucht, 2015). Several there is a future risk of decreased flow in the SSR because of reductions hundred kilometers downstream of Lake Diefenbaker the SSR merges in mountain snowmelt resulting in lower base flows (St. Jacques et al., with the North Saskatchewan River to form the Saskatchewan River. 2010), greater evaporation rates (Tanzeeba and Gan, 2012), and The Saskatchewan River forms the largest North American inland diminishing glacial headwaters (Barnett et al., 2005). However, there delta, the Saskatchewan River Delta, at the Saskatchewan/Manitoba has also been evidence of recent change to the hydrologic cycle on the border upstream of Cedar Lake, Manitoba. Water then flows from prairies (Dumanski et al., 2015) and predictions of increased precipita- Cedar Lake into the north end of Lake Winnipeg, the world's 10th largest tion in the short- to medium term (Chun et al., 2013). This may result freshwater lake (Fig. 1). in increased extreme events, such as the 2013 Alberta flood that result- Lake Diefenbaker is 556.9 m above sea level at FSL with a maximum ed in four human deaths and the evacuation of 100,000 people from the depth (59 m) near the Gardiner Dam (Sadeghian et al., 2015). Outflow city of Calgary and its surrounding area (The Economist, 2013). Desig- from the Gardiner Dam occurs primarily via a hydroelectric station nated the “costliest natural disaster in Canada's history” damages

http://dx.doi.org/10.1016/j.jglr.2015.10.003 0380-1330/© 2015 International Association for Great Lakes Research. Published by Elsevier B.V. All rights reserved. 2 Lake Diefenbaker: The prairie jewel

Fig 1. Map of Lake Diefenbaker, Saskatchewan, Canada. The inflows and outflows of the reservoir are depicted in perspective with the Saskatchewan River Basin (illustrated with gray shading). exceeded $5 billion (Insurance Bureau of Canada, 2014). Increases in net and terrestrial ecosystems, particularly within the Saskatchewan River water use by human activity results in decreased river flows, with some Delta (MacKinnon et al., in press; Sagin et al., 2015; Fig. 1). The future predictions suggesting that by the year 2060, the SSR basin will eutrophication of Lake Winnipeg has received scientificstudy experience an 82% increase in total direct water demand (baseline sce- (e.g., Schindler et al., 2012) and the dubious designation by the Global nario) relative to 2010 water demands, 34% of which was attributed to Nature Fund (2013) as being that years “most threatened lake in the agriculture (Kulshreshtha et al., 2012). Such changes in future flow world”. Eutrophication remains a top water quality concern on the will have direct effects on the limnology and water quality of the reser- Canadian prairies. However, a recent study reported that the voir and warrants further study. Saskatchewan River only contributed 3.4% of the total phosphorus Lake Diefenbaker is managed for drought and flood control by the (TP) load, and 11.6% of the total nitrogen (TN) load to Lake Winnipeg Saskatchewan Water Security Agency. Winter drawdown to 9 m (Donald et al., 2015). In fact, of the 28 largest lakes and reservoirs in below FSL can occur as water is released to generate hydroelectricity the Lake Winnipeg basin, Lake Diefenbaker had the highest TP retention and create reservoir storage to capture spring inflows (Sadeghian rates and the 3rd highest TN retention (Donald et al., 2015). It also et al., 2015; SWSA, 2012b). The main challenge for reservoir appears that the loading has not changed significantly over the past operations is that future inflow volumes are not known months in 40 years. Trend analyses of the TP and TN loading were conducted in advance. The drawdown-filling cycle can affect rates of shoreline the SSR catchment from 1970–2010 at stations up- and down-stream erosion and result in changes to nutrient loading (discussed by of Lake Diefenbaker (Luis Morales-Mar´ın, University of Saskatchewan, Hewlett et al., 2015). This cycle can also affect infrastructure and access 2015, personal communication). They found that upstream of the reser- to the lake (Rescan, 2012). voir, TP loading exhibited a decreasing trend at a rate of ~0.5% yr-1 with no apparent trend at the downstream station. Both stations up- and Water quality down-stream of Lake Diefenbaker had decreasing long-term trends in TN loading at similar rates of ~1.5% yr-1. However, in the last decade The quality and quantity of water in Lake Diefenbaker are defined by (2000–2010) the lower SSR sub-catchment has exhibited increasing upstream sources. The operation of the Gardiner Dam outflow produces TN loads at a rate of 2.3% yr-1 (Luis Morales-Mar´ın, University of Sas- hydro-ecological stresses on downstream aquatic (Phillips et al., 2015) katchewan, 2015, personal communication). This may be reflected in Lake Diefenbaker: The prairie jewel 3 the public concern about the potential for declining Lake Diefenbaker productivity in river-reservoirs (Dubourg et al., 2015; North et al., 2015; water quality, especially related to occurrences of surface algal blooms Quiñones-Rivera et al., 2015). The reservoir pattern of phytoplankton (Rescan, 2012). productivity is consistent with the spatial distribution (Abirhire et al., Lake Diefenbaker stakeholders representing municipal, provincial, 2015) and temporal changes (Lucas et al., 2015a; Tse et al., 2015; Vogt and federal levels of government, industry, recreation, and environmen- et al., 2015) in phytoplankton biomass. Turbidity was consistently tal interests were consulted to identify their needs. Stakeholders negatively correlated with algal biomass (Abirhire et al., 2015; Tse identified eleven values that reflected the provision of services from et al., 2015; Yip et al., 2015) and primary productivity (Dubourg et al., the reservoir. The top three values were: protection of shoreline infra- 2015; Quiñones-Rivera et al., 2015), emphasizing its critical role in structure (including recreation, irrigation, and municipal infrastruc- Lake Diefenbaker and its effect on light. This suggests that reductions ture); effective and simplified communication of information for in algal/cyanobacterial abundance will not likely occur simultaneously stakeholder planning and decision-making; and water quality and with increases in water clarity, leading to interesting questions about treatment (Rescan, 2012). The impetus for undertaking a special issue reservoir sensitivity and response to changes in nutrient and total related to Lake Diefenbaker was the need to: 1) understand current suspended sediment inputs. and future vulnerability of water quality and aquatic ecosystem River reservoirs affect downstream ecosystems in many ways. While health to nutrient loading, 2) synthesize and integrate the knowledge many of these ecosystem altering effects such as river fragmentation of the reservoir from multiple academic and government agencies, and changes in flow have been documented elsewhere, nutrient reten- and 3) document baseline limnological information about the reservoir tion services of such large reservoirs have not been as well studied with- in anticipation of large and rapid change under changing climate and in a larger watershed context. Nutrient loading and retention studies water use demands. These knowledge gaps have strong local impor- reported in this special issue provide fundamentally important insight tance, but there is a clear need to better understand the structure and into sources, sinks, and effects on the ecology of Lake Diefenbaker. We ecological function of river-valley reservoirs globally and how have learned that Lake Diefenbaker acts as a net sink for P, N, and Si ubiquitous stressors can influence water quality in these longitudinally (Donald et al., 2015; Hewlett et al., 2015; Lucas et al., 2015b; Maavara variable systems. et al., 2015; North et al., 2015), but that sources of P within the reservoir (i.e., internal loading) could play an important role in its ecology and Integrated assessment perceived water quality (North et al., 2015). The sources versus sinks of carbon (C) appear to be dependent upon the hydrologic regime Insights into whether there have been any long-term changes in the (Quiñones-Rivera et al., 2015). reservoir's ecosystem function have been gleaned through multiple Given the predicted global surge in dam construction (Zarfl et al., lines of independent evidence from the studies in this special issue. 2015), combined with little information on “old” dams (i.e., N50 years; Historical monitoring data provide a means to assess whether a system Juracek, 2015), this comprehensive assessment of ecosystem function has changed over time, to understand the trajectory of such changes, to serves as a model system for large river-valley reservoirs. This special identify environmental thresholds preceding detrimental ecological ef- issue tests assumptions regarding the applicability of the longitudinal fects, or to craft informed mitigation goals (Smol, 2008). Upon initiation zonation concept (LZC) of Kimmel and Groeger (1984) that describes of this special issue, there was a paucity of reservoir-wide historical the physical and ecological patterns in river-reservoirs. The expecta- data. Long-term limnological information on the reservoir was scarce tions of the LZC were verified within the riverine-transitional- (SWSA, 2012b), temporally discontinuous (e.g., Saskatchewan lacustrine continuum for increases in thermal stratification (Hudson Environment and Public Safety and Environment Canada [SEPS and and Vandergucht, 2015), decreasing turbidity (Hudson and EC], 1988), or focused on one region of the reservoir (e.g., Hall et al., Vandergucht, 2015; Lucas et al., 2015a), decreasing nutrient concentra- 1999). tions (Abirhire et al., 2015; Dubourg et al., 2015; Maavara et al., 2015), Althoughtherewereincreasesintheautochthonousmaterialin increasing algal pigment abundance (Tse et al., 2015), and increasing more recent sediments (Lucas et al., 2015b), we found little light availability and primary productivity (Dubourg et al., 2015; Lucas evidence that supported the perception that water quality in Lake et al., 2015b). However, certain predicted patterns were not observed, Diefenbaker was in decline (Tse et al., 2015; Vogt et al., 2015; Yip such as for DO (Hudson and Vandergucht, 2015) and phytoplankton et al., 2015). The Qu'Appelle arm appears to be unique, with a strong biomass (Abirhire et al., 2015; Dubourg et al., 2015). The different pat- correlation found between particulate reactive silica (Si; Maavara terns from LZC found for DO and phytoplankton biomass in Lake Diefen- et al., 2015), biologically available forms of P (Lucas et al., 2015b), baker are believed to be related to incoming flow conditions and and chlorophyll a [Chl a] concentrations (Tse et al., 2015)insedi- relative changes in phytoplankton group contributions to overall bio- ment profiles. These relationships suggest an increase in autochtho- mass, respectively. nous production in this region of the reservoir in the early to mid- The LZC also described the physicochemical character of materials set- 1990s, which subsequently plateaued. Internal P loading rates tling to bottom sediments in Lake Diefenbaker. Classically, as turbulent (North et al., 2015) were highest in the polymictic (Sadeghian rivers flow into reservoirs, water velocity decreases and energy holding et al., 2015) Qu'Appelle arm and the synchronization of light and nu- materials diminish resulting in the settling out of materials (Harrison, trient supply (Dubourg et al., 2015) may be contributors to the oc- 1983). As described by McHenry et al. (1982), in narrow, elongate reser- currence of cyanobacterial blooms in this region (Vogt et al., 2015; voirs there is a particle size sorting of suspended materials. In Lake Diefen- Yip et al., 2015). In the remainder of the reservoir, cyanobacterial baker, the majority of this material settles in proximity to the highway 4 blooms are generally infrequent (Abirhire et al., 2015; Tse et al., bridge (Fig. 1) with coarse materials (e.g., sand) settling up-reservoir 2015; Vogt et al., 2015; Yip et al., 2015) and even less frequent and finer materials (e.g., clay) being carried further down-reservoir than other lakes with similar nutrient concentrations. This may re- until the deposition rate is reduced (Lucas et al., 2015b; Sadeghian et al., late to the immediate conditions of wind and mixing with suggestion 2015; SEPS and EC, 1988). Along this gradient of sediment deposition, tur- of the importance of low light and high P-deficiency (Dubourg et al., bidity decreased (Hudson and Vandergucht, 2015), resulting in higher 2015; Quiñones-Rivera et al., 2015)andhighflow (Abirhire et al., light conditions and greater algal biomass and productivity (Dubourg 2015; Hudson and Vandergucht, 2015; Yip et al., 2015). et al., 2015; Tse et al., 2015). In addition, once the majority of suspended Factors affecting spatial (Dubourg et al., 2015) and temporal allochthonous material settled from the water column, the autochtho- (Quiñones-Rivera et al., 2015) primary productivity were also studied. nous character of surficial sediment increased with distance down- While these are the first studies of primary production in Lake Diefenba- reservoir (Lucas et al., 2015b). It was also observed that in a system so ker, they also provide valuable insight into the dynamics and control of strongly influenced by the inflowing river (i.e., SSR), the location and 4 Lake Diefenbaker: The prairie jewel extent of the transition zone can vary spatially and temporally due to support the prevailing paradigm that predicts an initial upsurge in trophic changes in flow. The SSR carries an estimated load of ~6.3 million m3 of status upon reservoir formation. This pattern may result from limited material into Lake Diefenbaker annually, with storage volume of the res- penetration of light in the early years (Tse et al., 2015). ervoir being reduced by ~0.07% (SWSA, 2012b). The geochemical characteristics of Lake Diefenbaker sediments have shifted over time, likely due to a combination of reservoir ontogeny, In this issue both short and long-term changes to reservoir inflow or hydrological timing, and changing drawdown-fill regime (Lucas et al., 2015b). Ob- This special issue features 14 articles encompassing the physical, served trends in the vertical distribution of P in sediment profiles, chemical, and biological properties of Lake Diefenbaker. The goal of assessed in Lucas et al. (2015b), were consistent with increased autoch- these studies was to synthesize the current state of knowledge regarding thonous production over time and supported by trends in other geo- Lake Diefenbaker's past and present to understand the regional and na- chemical variables. Nevertheless, given the potential for various tional role it plays as a large reservoir, particularly in the face of increas- transformation processes within the sediment profile (Hupfer et al., ing water stress (Gober and Wheater, 2014). The overall objective of this 1995), natural diagenetic processes cannot be ruled out for the observed special issue is to bring attention to this large reservoir (as a narrow, vertical patterns of P distribution along the main flow of the reservoir. river-valley reservoir model), understand how it has changed since its Spatially, TP content in surface sediment, mainly as organic-bound P formation, and identify stressors affecting its water quality. and redox-sensitive P fractions increased with distance down-reservoir The issue cover photo is from Sadeghian et al. (2015) who present a as a function of settling organic matter and water depth (Lucas et al., current bathymetric map collected in 2012 and 2013. Temperature 2015b). This suggests that the potential for internal loading of P varies model animations from April 2011 to December 2012 show the strong in- among reservoir zones and hydrologically dissimilar regions such as the fluence of the inflowing SSR on the reservoir's temperature regime, and Qu'Appelle arm (Lucas et al., 2015b; North et al., 2015). role of upwelling from strong wind events (Sadeghian et al., 2015). Although no long-term trends are presently apparent, given the high Hudson and Vandergucht (2015) describe the spatial and temporal pat- concentrations of redox-sensitive P fractions in the sediments (Lucas terns in temperature, dissolved oxygen (DO), and turbidity over three et al., 2015b), internal P loading has been identified as a potential concern years (2011–2013) during the ice-free period. During this study, above that could affect future water quality (Hall et al., 1999; Vogt et al., 2015). average peak flows with high turbidity entered the reservoir in late spring Vogt et al. (2015) found no evidence of long-term trends in phytoplank- and early summer. Average rates of hypolimnetic DO depletion were sim- ton abundance nor water clarity in Lake Diefenbaker. They used a long- ilar to other lakes and reservoirs of comparable depth and size. Some re- term (nearly two decades) regression-based analysis in the Qu'Appelle gions of the reservoir were hypoxic by the end of summer and were arm to demonstrate that multiple interacting stressors control both phy- predicted to become anoxic if stratification had persisted for another 2– toplankton abundance and water clarity. Their results associated phyto- 4 weeks. Reduced hypolimnetic volumes, increases in anoxia, and signif- plankton abundance with soluble reactive phosphorus (SRP) icant algal blooms were reported in 1984, which was a low flow year concentrations and trophic control in the form of total zooplankton abun- (SEPS and EC, 1988). Thus, comparative assessment during contrasting dance, while water clarity was associated with climate systems and river inflows may provide further insight into the physical dynamics affecting inflow. They attribute the positive relationship between SRP and phyto- Lake Diefenbaker. plankton abundance to internal P loading. Vogt et al. (2015) forecast Contributing authors to this special issue have used a variety of that climate-induced reductions in flow will result in reduced turbidity, methods including historical satellite imagery and paleolimnology to and combined with predicted increases in anthropogenic nutrient in- recreate past reservoir conditions to put current conditions in perspec- fluxes from future economic development, the water quality of the reser- tive. Yip et al. (2015) successfully applied satellite imagery (Landsat) voir may decrease. Lake Diefenbaker has high TP retention (91–94%; analysis to Lake Diefenbaker. They illustrated that Chl a concentrations Donald et al., 2015; North et al., 2015), and North et al. (2015) investigat- and Secchi disk depths could be predicted with reasonable precision ed if this legacy sediment P has the potential to re-enter the water column in Lake Diefenbaker. They then applied their models to archived satellite through internal P loading from sediments. These are the first published data to hindcast past (1984–2012) Chl a concentrations and Secchi internal P loading rates for a Canadian reservoir. They report that internal depths. The relationship between predicted Chl a and Secchi depth P loading represented a quarter of the annual external TP load to the res- was positive; the reverse of the relationship typically found in lakes. ervoir. As one of only two manuscripts in this issue to report on the lon- The pattern of historical Chl a and Secchi depth values showed a de- gest season of the year in Saskatchewan- winter, North et al.'s winter crease from 1984–2012 associated with increasing flows and turbidity. rates (previously considered to be a season of minimal loading; Several paleolimnological studies were also conducted to assess Nürnberg et al., 1986) were higher than the estimated summer internal spatial and long-term trends in the environmental status of Lake Diefen- TP load. While overall not a significant source of P to the reservoir, internal baker within the context of the LZC. Lucas et al. (2015a) reported that the P loading does supply the limiting nutrient P (Dubourg et al., 2015; bacillariophyte (i.e., diatom) assemblage in the down-reservoir region of Quiñones-Rivera et al., 2015; SEPS and EC, 1988) to phytoplankton popu- Lake Diefenbaker was dominated by planktonic and tychoplanktonic lations within and downstream of Lake Diefenbaker on a year-round species. During the 1980s there was a shift in species dominance in the basis. majority of the down-reservoir sites. Spatially, the chironomid assem- Silica is frequently an important nutrient affecting bacillariophyte blage in the main channel and down-reservoir sites changed in response productivity. The study by Maavara et al. (2015) assessed inflowing to changes in habitat (i.e., decreasing clay content and increasing organic and outflowing mass loads, spatial water column concentrations, and matter content with distance down-reservoir; Lucas et al., 2015a). spatially distributed sediment core data collected along the length of As a result of multiple influences, reservoirs “age” rapidly compared to the reservoir to provide a comprehensive contribution to studies of riv- natural lakes (Kimmel and Groeger, 1984). Immediately following erine Si dynamics. The overall Si proportional retention rate in Lake Die- flooding there is an initial period of high productivity (i.e., trophic fenbaker was greater than N but less than P (Donald et al., 2015; North upsurge) due to the leaching of soluble nutrients from flooded soils, the et al., 2015). Shoreline erosion caused by high water levels and fluctua- decomposition of inundated litter and vegetation, and an increased avail- tions in this managed reservoir has been identified by stakeholders as a ability of benthic habitat. This is followed by a decline in productivity priority issue for the management of Lake Diefenbaker (Rescan, 2012). (Baxter, 1977; Benson, 1982; Ploskey, 1981), the magnitude and This concern is related to the fact that the mean minimum water levels duration of which is dependent upon reservoir-specificinfluences, have risen over 3 m since 1969 (Pomeroy and Shook, 2012). Given the including reservoir operations (Ploskey, 1981). In Lake Diefenbaker, pro- high sediment load from these eroding shorelines, Hewlett et al. files of sediment pigment concentrations exhibited no clear trends to (2015) determined if these sediments were an important source of Lake Diefenbaker: The prairie jewel 5 nutrients to the reservoir. Hewlett et al. (2015) filled an existing gap in longitudinal zonation. This is supported by a recent study (Luis Lake Diefenbaker's nutrient budget by assessing the total C, TN, and two Morales-Mar´ın, University of Saskatchewan, 2015, personal communi- types of bioavailable P nutrient content across bank classifications and cation) that found TP and TN loads at the Alberta/Saskatchewan border land uses along the entirety of the reservoir. They found that the contri- are similar to those entering Lake Diefenbaker (Donald et al., 2015). This bution of shoreline erosion to the nutrient budget of Lake Diefenbaker is suggests that land management practices and efforts to reduce nutrient minimal relative to the high external loads of both P and N from the SSR loading should be focused at sites upstream of the reservoir and not nec- (Donald et al., 2015; North et al., 2015). essarily along its shores. Expansion of anthropogenic activities in the Abirhire et al. (2015) examined the phytoplankton community in Saskatchewan watershed, combined with any increases in upstream in- relation to a series of environmental factors during the 2011 and 2012 puts and different flow conditions could promote cyanobacterial ice-free seasons. The reservoir had a diverse set of phytoplankton (72 blooms. genera) with the cryptophytes and bacillariophytes comprising the ma- There is an interest in increasing the delivery of water from Lake Die- jority of biomass (89%). The magnitudes of flow into the reservoir and fenbaker to downstream Buffalo Pound Lake (SWSA, 2012a), a drinking water temperature were significant factors that explained the pattern water reservoir serving over a quarter of a million people in the Sas- in cryptophyte biomass. However, the pattern in biomass of katchewan cities of Regina, Moose Jaw, and surrounding area. Buffalo bacillariophytes was partly explained by the mixing depth and to a less- Pound has recently experienced algal blooms that have interfered er extent by nutrient ratios. The authors indicate that although these with water treatment operations and significantly affected the water two high flow years were dominated by cryptophytes and supply (CJME, 2015). Due to increased demands from new potash bacillariophytes, historical data suggests that years characterized by mines, other industrial developments and irrigation projects, the Sas- low flow and long water residence times (i.e., drought) may be more katchewan government is currently evaluating the feasibility of an addi- prone to cyanobacterial dominance (SEPS and EC, 1988). Nutrient (P tional diversion to the Qu'Appelle River through an improved and N) and light status of Lake Diefenbaker phytoplankton communities conveyance system (SWSA, 2012a). were assessed by a variety of physiological assays in the open-water Aquatic invasive species will always pose a threat to Lake Diefenba- season of 2013. In addition to providing evidence of light and P co- ker, given its popularity with the boating and angling community. limitation, which supports findings of other studies in this issue Dreissenids (zebra mussels), one of the world's most problematic bio- (e.g., Lucas et al., 2015a,b; North et al., 2015; Yip et al., 2015), Dubourg logical invaders (Higgins and Vander Zanden, 2010), were recently dis- et al. (2015) provide a novel demonstration of the dependence of nutri- covered in Lake Winnipeg, which although downstream, is ent deficiency status on in situ light conditions. The application of light hydrologically connected to Lake Diefenbaker. The Department of limitation thresholds made it obvious that P inputs are of most concern Fisheries and Oceans Canada (2013) rated the probability of zebra mus- in the places and times when light is sufficient (e.g., not subjected to sel invasion of the lower South Saskatchewan basin as being “high” risk. high turbidity from the inflowing SSR and stratified conditions with shal- Climate change was identified as one of the anthropogenic low mixing depths). Rates of primary production were also reported in stressors currently operating on the reservoir, and its impact is like- Quiñones-Rivera et al. (2015) who investigated the relative influence of ly to increase in the future (Vogt et al., 2015). Warmer air temper- hydrology, water chemistry, and microbes on CO2 exchange in the atures (Cutforth and Judiesch, 2012) will cause increases in water Qu'Appelle arm of the reservoir over a four year period (2010–2013). temperatures, which may increase the prevalence of bacterial indi- Their water column metabolism estimates indicated that the Qu'Appelle cators (North et al., 2014). Half of the papers in this special issue arm was slightly autotrophic and had a net sequestration of atmospheric identify changes in hydrology as a principal factor affecting water

CO2. During periods of low flow, CO2 fluxes were reversed and respiration quality (Abirhire et al., 2015; Dubourg et al., 2015; Hudson and dominated the metabolic balance, indicating that hydrology and chemis- Vandergucht, 2015; Quiñones-Rivera et al., 2015; Sadeghian et al., try interact with water-column metabolism to influence CO2 fluxes. 2015; Vogt et al., 2015; Yip et al., 2015). Based in part on previous While CO2 flux was correlated with lake pH, water-column metabolism studies (SEPS and EC, 1988) and inference from recent results, it is was regulated by independent mechanisms (Quiñones-Rivera et al., predicted that the risk of cyanobacteria blooms may be more prev- 2015). alent during years of low flow. Finally, Phillips et al. (2015) examined the impact of the tail waters This special issue represents the most comprehensive study on Lake of the Gardiner Dam on downstream benthic invertebrates. Water leav- Diefenbaker to date and into the foreseeable future, and will be the ing Gardiner Dam in mid-summer was found to be ~9 °C colder than benchmark for future studies. The knowledge contained within will be that at a reference site upstream of Lake Diefenbaker. The temperature used to facilitate the preservation and guide the management of in the SSR became similar to the upstream reference site near the city Saskatchewan's prairie jewel. This study will also contribute to under- of Saskatoon, over 100 km downstream of Lake Diefenbaker (Fig. 1). A standing of impacts on river-valley reservoirs globally as they continue unique stenothermic benthic invertebrate community inhabited the to increase in number. river downstream of the dam, while certain temperature sensitive spe- cies were absent. Temperature was a significant factor in the differences in community compositions, although variation in flow and turbidity Acknowledgments may have also contributed. We would like to thank all of the contributing authors, What is coming down the pipe: Future threats to Lake Diefenbaker representing three universities and two government agencies, for their work to contribute new knowledge and understanding on While all potential threats and their risks to water quality cannot be Lake Diefenbaker's limnology. The guest editors and authors of definitely quantified, there are known risks that bear consideration and this volume are grateful to the University of Saskatchewan's Global reflection. Based on data to date, individual activities within the lake Institute for Water Security (GIWS) through the Canada Excellence such as the presence of the aquaculture facility, discharges of treated Research Chair in Water Security (CERC) for the initiation and fi- municipal wastewater, and in-lake use for agriculture do not appear to nancial assistance with regard to page charges for this issue. We ad- represent a significant risk (Abirhire et al., 2015; Lucas et al., 2015a,b). ditionally thank the Saskatchewan Water Security Agency for their In fact, there is no evidence within this special issue regarding concerns assistance with page charges, and Heather Wilson (GIWS) for the about nutrient contributions from within the province of Saskatchewan. map presentation. We greatly appreciate the support and guidance It appears that both trophic status and productivity are primarily dictat- provided by the journal Editors (Robert Hecky and Stephanie ed by what flows into the reservoir from the SSR and the influence of Guildford). 6 Lake Diefenbaker: The prairie jewel

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Lorne E. Doig School of Environment and Sustainability, University of Saskatchewan, Global Institute for Water Security, University of Saskatchewan, Saskatoon, Saskatoon, SK S7N 5C3, Canada SK S7N 3H5, Canada Toxicology Center, University of Saskatchewan, Saskatoon, SK S7N 5B3, Jeff J. Hudson Canada Dept. of Biology, University of Saskatchewan, Saskatoon, SK S7N 5E2, Canada Karl-Erich Lindenschmidt Global Institute for Water Security, University of Saskatchewan, Saskatoon, SK S7N 3H5, Canada