Canadian Journal of Fisheries and Aquatic Sciences
Influence of glacial flour on the primary and secondary production of Sockeye Salmon nursery lakes: a comparative modern and paleolimnological study
Journal: Canadian Journal of Fisheries and Aquatic Sciences
Manuscript ID cjfas-2018-0372.R1
Manuscript Type: Article
Date Submitted by the 14-Jan-2019 Author:
Complete List of Authors: Barouillet, Cécilia; Queen's University, Biology Cumming, Brian; Queen's University, Biology; Laird, Kathleen; Queen's University, Biology Perrin, Chris;Draft Limnotek Research and Development Inc., Selbie, Daniel; Fisheries and Oceans Canada, Science Branch
Glacial flour, Sockeye Salmon, CLADOCERA < Organisms, Bridge River Keyword: Diversion, HYDROPOWER < General
Is the invited manuscript for consideration in a Special Not applicable (regular submission) Issue? :
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1 Title: 2 Influence of glacial flour on the primary and secondary production of Sockeye Salmon nursery 3 lakes: a comparative modern and paleolimnological study 4 5 6 Authors: 7 Barouillet, Cécilia*,1, Brian F. Cumming1, Kathleen R. Laird1, Christopher J. Perrin2 and Daniel 8 T. Selbie1,3 9 1 Paleoecological Envrionmental Assessment and Research Laboratory, Department of Biology, 10 Queen’s University, 116 Barrie Street, Kingston, ON, Canada K7L 3J9 11 2 Limnotek Research and Development Inc. 4035 West 14 Avenue, Vancouver, BC, Canada V6R 12 2X3 13 3 Fisheries and Oceans Canada, Pacific Region, Science Branch, Cultus Lake Salmon Research 14 Laboratory, 4222 Columbia Valley Highway, Cultus Lake, BC, Canada V2R 5B6 15 Draft 16 Brian F. Cumming: [email protected] 17 Kathleen R. Laird: [email protected] 18 Christopher J. Perrin: [email protected] 19 Daniel T. Selbie: [email protected] 20 21 * Corresponding author 22 Phone: (+1) 343 333 5353 23 Email: [email protected] 24 25
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26 Abstract
27 The increasing rate of glacier retreat, and turbid glacial runoff can have a strong influence
28 on freshwater ecosystems. Seton and Anderson lakes (British Columbia, Canada) are Sockeye
29 Salmon (Oncorhynchus nerka) nursery systems. Since the 1940s, the Bridge River Diversion
30 (BRD) introduced glacially-turbid water into Seton Lake. To assess the impact of the BRD on
31 the production of Seton Lake, we combined data from limnological surveys with the analysis of
32 sub-fossil cladocerans and diatoms from sediment cores; using Anderson Lake as a reference.
33 The modern data indicate that the euphotic zone is 14m shallower, and the cladoceran density
34 and biomass are significantly lower in Seton Lake in comparison to Anderson Lake. The paleo- 35 data indicate that following the BRD, theDraft sedimentary fluxes of cladoceran and diatom declined 36 2- to 10-fold in Seton Lake and remained low thereafter. Together our data support declines in
37 primary and secondary producers following the BRD, likely due to changes in light penetration,
38 and/or other indirect influence, and provides insights into the impact of turbid meltwater on the
39 biological production of downstream lakes.
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40 Introduction
41 The increased rate of glacier retreat is a direct, visible, and perhaps one of the most
42 widely known outcomes of the rise in global air temperatures (IPCC 2014). The resultant glacial
43 runoff can have important ecological consequences for lakes situated downstream, including
44 through the transport of suspended glacial flour particles, which originate from the erosion of the
45 bedrock by glaciers (Fegel et al. 2016). Suspensions of glacial flour can reduce light penetration,
46 including the penetration of UVR and photosynthetically-active radiation (PAR) in the water
47 column (Rose et al. 2014). Additionally, through the colloidal binding of nutrients (Hodson et al.
48 2004, 2008), glacial flour can become a significant source or sink of biologically-available 49 nutrients (Hodson et al. 2004; Saros et al.Draft 2010; Slemmons and Saros 2012). Together, these 50 changes may limit primary producers and alter rates of primary production. As a result, lakes
51 receiving glacial meltwater may support relatively low abundances of primary and secondary
52 producers (Vinebrooke et al. 2010). Glacial flour can also directly or indirectly alter assemblages
53 within and among plankton communities (Kirk 1991; Saros et al. 2010; Slemmons and Saros
54 2012); influence trophic interactions (i.e. weakening or intensifying predator-prey interactions;
55 Horppila and Liljendahl-Nurminen 2005); and modify the vertical distribution of phytoplankton,
56 zooplankton (Hylander et al. 2011) and planktivores (Gregory 1993) within the water column.
57 Such limnological changes induced by glacial flour can thus result in the modification of energy
58 flows throughout food webs (Koenings et al. 1990; Laspoumaderes et al. 2013). Ecosystem
59 responses to glacial runoff can, however, be complex, as they vary spatially and temporally as a
60 function of the distance to and the magnitude of the glacial source (Saros et al. 2010; Bliss et al.
61 2014).
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62 In North America, many populations of Sockeye Salmon and kokanee (Oncorhynchus
63 nerka) rear in lakes influenced by glacial runoff. While the fish productivity in those turbid lakes
64 tend to be lower than in clear-water lakes (Koenings et al. 1985), the impacts of long-term inputs
65 of glacial flour on O. nerka freshwater productivity are not well known. Sockeye Salmon sustain
66 important commercial and recreational fisheries throughout the North Pacific, and understanding
67 the factors limiting the productive capacity of freshwater nursery ecosystems is critical for
68 effective conservation and management of salmon populations (Stockner and Macisaac 1996).
69 However, salmon populations and their nursery lakes are often exposed to multiple stressors that
70 can include overfishing, habitat destruction, and dams (Nehlsen et al. 1991; Healey 2011; Schoen
71 et al. 2017), making it hard to assess the impact of glacially-turbid water on nursery lakes
72 ecosystem dynamics. As juveniles O. nerkaDraft preferentially feed on cladoceran zooplankton,
73 particularly the energy-rich Daphnia (Mazumder and Edmundson 2002; Scheuerell et al. 2005),
74 the abundance of Daphnia (and other cladocerans such as Bosmina) is a key metric of the food
75 supply for O. nerka (Schindler et al. 2005b). In the absence of long-term monitoring data,
76 paleolimnological techniques (Smol 1992) have been used to reconstruct past ecological and
77 environmental changes in salmon nursery lakes (Finney 2000; Finney et al. 2002; Schindler et al.
78 2005a; Selbie et al. 2007; Gregory-Eaves et al. 2009). For instance, sedimentary cladoceran
79 remains have been used to reconstruct the secondary production over time in order to track
80 changes in predation (i.e. fish vs invertebrate planktivory) and primary production (Jeppesen et
81 al. 2001; Sweetman and Finney 2003; Korosi et al. 2013; Shi et al. 2016). Similarly, multi-
82 trophic paleolimnological analyses of sedimentary remains of diatoms, cladocerans, and stable
83 nitrogen isotopes (δ15N) have been successfully used to understand variations in past salmon runs
84 and the cumulative influences of stressors affecting existent salmon populations and their
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85 freshwater habitats, including overfishing, dams, watershed disturbance, and climate (Finney
86 2000; Selbie et al. 2007; Gregory-Eaves et al. 2009). Moreover, the integration of modern and
87 paleolimnological techniques can further help to understand the influence of different factors on
88 the productive capacities of nursery ecosystems to support O. nerka (Selbie et al. 2007, 2011).
89 In British Columbia, the Fraser River drainage basin hosts 377 individual populations of
90 Sockeye Salmon (Grant and Pestal 2012). Two of these populations are found in Seton and
91 Anderson lakes, two large and deep oligotrophic lakes of similar geomorphological, physical and
92 climatic conditions. These lakes also support the landlocked variant kokanee (O. nerka), also
93 known in Seton and Anderson lakes as “black” kokanee (Moreira and Taylor 2015) or “Gwenis” 94 in St’at’imc First Nations dialect (Geen Draftand Andrews 1961). Since the construction of 95 hydroelectric dams and a water diversion (i.e. the Bridge River Diversion, BRD) between the
96 1930s and 1960s, novel inputs of glacially-turbid water have been introduced to Seton Lake, a
97 historically clear-water system; whereas Anderson Lake, located upstream of the BRD, remains
98 undisturbed. Modern limnological studies have found that the primary and secondary production
99 of Seton Lake is currently lower than in Anderson Lake (Geen and Andrews 1961; Shortreed et
100 al. 2001), and according to the Photosynthetic Rate (PR) Model (Hume et al. 1996; Shortreed et
101 al. 2000), the rearing capacity of both lakes is underutilized (Shortreed et al. 2000). However,
102 existing limnological information on Seton Lake represents only a small portion of the post-
103 diversion period, making it difficult to assess the full impact of hydroelectric development on the
104 limnology and the rearing capacity of O. nerka in Seton Lake.
105 In the present study, we applied modern and paleolimnological techniques to assess
106 changes in the primary and secondary production of Seton and Anderson lakes, and to evaluate
107 the effects of the BRD on food availability for planktivorous fish in Seton Lake. We 5
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108 hypothesized that the introduction of glacially-turbid water would lead to a decline in the
109 primary (diatoms) and secondary (cladocerans) production in Seton Lake. A sampling design
110 similar to a Before-After-Control-Impact (BACI) layout (Stewart-Oaten et al. 1986) was used to
111 assess the cumulative effects of the water diversion and other regional factors that would be
112 unrelated to the BRD (e.g. climate), but influential upon Seton Lake primary and secondary
113 production, with Seton Lake being the impacted system, and Anderson Lake the reference site.
114 The results from our study are applicable to understanding variability in the food-web production
115 that supports valued fish populations in lakes located downstream from glaciers.
116 117 Materials and Methods Draft 118 Study Lakes
119 Seton and Anderson lakes are large and deep oligotrophic lakes located in the Fraser
120 River drainage basin, within the St’at’imc traditional territory in the central interior of British
121 Columbia, Canada (Fig. 1). Seton Lake (N 50°41.758’ W 122°08.007’) is located to the west of
122 the town of Lillooet, and Anderson Lake (N 50°38. 089’ W 122°23.577’) flows into the west end
123 of Seton Lake via the Seton Portage (Fig. 1). The geology of the region is composed of a mix of
124 volcanic, metamorphic and sedimentary rocks of Carboniferous to Jurassic origin (Geen and
125 Andrews 1961). Both systems are glacially-formed depressions surrounded by steep mountains.
126 Seton Lake and Anderson Lake have a mean water depth of 85 m and 140 m and a maximum
127 depth of 151 m and 215 m, respectively. The area is characterized by a semi-arid microclimate
128 with periodic influences of coastal weather and cold arctic air during the winter. The region is
129 hot and dry in the summer, with annual precipitation averaging only 300 to 400 mm, and
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130 consequently this region is sensitive to sporadic forest fires. The vegetation is dominated by
131 Ponderosa Pine (Pinus ponderosa). Seton and Anderson lakes contain anadromous populations
132 of Sockeye Salmon and resident populations of kokanee (O. nerka). Two populations of Sockeye
133 Salmon spawn in the tributaries of Anderson and Seton lakes, referred to as the Gates Creek and
134 the Portage Creek populations, respectively (Bell 1985). Although inter-annual variability in
135 these lakes is large, salmon populations have averaged ~33,770 ± 26,353 (1SD) of anadromous
136 Sockeye Salmon spawners over the past decade (Fisheries and Oceans Canada, unpubl.).
137
138 History of the Bridge River Diversion
139 The Bridge River Diversion (BridgeDraft River Power Project; BRD) was initiated by the 140 Bridge River Power Company in 1927. It was then taken over by the British Columbia Electric
141 Company, which was nationalized in 1961 to become part of the British Columbia Hydro and
142 Power Authority. The BRD was built to provide hydropower by diverting water from the Bridge
143 River into Seton Lake, a vertical drop of 1067m. The project started in the early 1930s when
144 water from the Bridge River was diverted for the first time into the upper part of Seton Lake at a
145 rate of ~2.7m3·sec-1 (Geen and Andrews 1961). The project stalled during the Great Depression
146 and World War II but was revived between 1946 and 1956 with the construction of the Seton
147 Dam at the outflow of Seton Lake, and the Terzaghi Dam, which flooded the Bridge River
148 Valley and created the Carpenter Reservoir (Fig. 1). This infrastructure allowed the flow from
149 the diversion to increase up to a mean annual flow rate of ~56.6 m3·sec-1 (Geen and Andrews
150 1961). By the end of the construction in 1962, two-thirds of the water coming into Seton Lake
151 originated from the Carpenter Reservoir, with a mean annual inflow rate of ~87 m3·sec-1±12 (data
152 courtesy of BC Hydro). The water from Carpenter Reservoir, rich in glacial flour, originates 7
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153 from the Bridge Glacier, a ~83 km2 lake-calving glacier which is rapidly retreating (Chernos
154 2014).
155
156 Modern limnological data
157 Modern limnological data were collected in 2014, 2015, and 2016 in order to obtain
158 knowledge of the present-day limnological state of the study lakes. Data were collected at 2
159 monitoring stations along the length of each lake to span a spatial gradient that could result from
160 the observed inflow of glacial flour from the BRD. The limnological data does not allow us to
161 directly assess the impact of the diversion on the primary and secondary production in Seton 162 Lake, as no pre-diversion limnological dataDraft exists for comparison. Instead, the modern data can 163 help us understand the present-day conditions, which provides context for the paleolimnological
164 study.
165 Over the three years of sampling, measurements of photosynthetically-active radiation
166 (PAR), turbidity, nutrient concentrations, rates of primary production (or gross primary
167 production, GPP), and zooplankton net hauls were completed monthly from May through
168 October (Fig. 1). A LiCor LI250A irradiance meter was used to estimate the depth of the
169 euphotic zone by measuring PAR in 1m intervals from the lake surface to a depth where PAR
170 was less than 1% of that at the surface. Turbidity measurements were collected using a Sea-Bird
171 Electronics SBE19plusV2 CTD along the entire water column. Total nitrogen (TN), total
172 phosphorus (TP), and bioavailable forms of nitrogen (NH4-N and NO3-N) and phosphorus
173 (soluble reactive phosphorus; SRP) were measured from water samples collected at a depth of
174 1m, and analysed using standard methods (APHA 2014).
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175 Measurement of Gross Primary Production
176 Gross primary production (GPP) was measured by the British Columbia Ministry of the
177 Environment in situ as the amount of 14C incorporated into particulate organic matter (Steeman
178 Nielsen 1952). Discrete water samples were collected for seven depths over the profile of the
179 euphotic zone. At each depth, water samples were transferred directly into two light and one dark
180 glass bottle, which were rinsed three times with the collected water before filling. Each water
14 181 sample was inoculated with 0.185 MBq (5 µCi) of NaH CO3 New England Nuclear (NEC-
182 086H). The cluster of bottles at each depth was incubated for 4-5 hours at the original collection
183 depth in the water column. Following retrieval, 100mL of each sample was filtered onto 0.2µm 184 polycarbonate filters, and stored in the dark.Draft The samples were then treated with 100µL of 0.5 N 14 Ò 185 HCl to eliminate the unincorporated inorganic NaH CO3, and five millilitres of Scintisafe
186 scintillation cocktail. The vials were counted using a BeckmanÒ Model #LS 6500 liquid
187 scintillation counter. Daily rates of primary production were vertically-integrated for each station
188 (Ichimura et al. 1980), and were calculated by multiplying the hourly primary production
189 (Parsons et al. 1984) by the incubation time, and were corrected for solar irradiance. For more
190 details about the method refer to Limnotek (2017).
191 Zooplankton Collection and Analysis
192 Vertical zooplankton hauls from 30 m depth to the surface were collected using a
193 Wisconsin net (153 µm mesh). The total volume of water that passed through the net was
194 recorded, and the samples were preserved in a 10% sugar-buffered formalin solution. Aliquots of
195 known sample volume were separated from each sample using a Folsom plankton splitter, and
196 species were identified and enumerated at 5-100x magnification. Biomass of zooplankton was
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197 determined from length-to-weight regressions (MacCauley 1984) using the average length
198 measurement of 25 random measurements taken for each taxon found in each sample. The
199 number of individuals counted and estimated biomass (µg dry weight) were extrapolated to the
200 total volume and the estimates were then integrated over the sampling depth (30m) to the number
201 of individuals or biomasses found in a 1 m2 (i.e. areal density). The monthly average over the
202 three sampling years of the zooplankton density and biomass was calculated for each of the
203 primary zooplankton groups (i.e. cladoceran, copepods, rotifers).
204 Data analysis was performed using R (v. 3.2.1). For all limnological variables (PAR,
205 turbidity, nutrient concentration, GPP, zooplankton density and biomass), the two monitoring 206 stations within Seton and Anderson lakesDraft were not significantly different (t-test; p>0.05) and 207 therefore, were treated as replicates. The Bartlett and Shapiro tests were used to evaluate whether
208 the modern limnological variables met the assumptions (homoscedasticity and normality) for a
209 parametric test, and the limnological variables that did not meet the assumptions were log-
210 transformed. A t-test was performed on the monthly average of each limnological variable to test
211 for inter-lake differences. Due to the high inter-annual variability of the GPP measurements, a 2-
212 way ANOVA was performed on GPP to test for differences between lakes, and years.
213
214 Paleolimnological Data
215 The paleo-data were used in this study to reconstruct the history of the primary and
216 secondary production of Seton and Anderson lakes, and assess the potential impact of the Bridge
217 River Diversion on the food supply for O. nerka in Seton Lake. The core chronologies relating
218 depth (cm) to sediment age in each core was developed to define the timing of the diversion and
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219 calculate fluxes. Core chronologies of the Anderson Lake cores was established using standard
220 methods of radioisotopic dating (Appleby 2002), while for the Seton cores, grain size analysis
221 was completed as a complement of the radioisotopic analyses . This intermediate step was
222 necessary due to the low 210Pb activities found in the Seton Lake cores. The history of the
223 primary and secondary production in the two study lakes was reconstructed using sedimentary
224 sub-fossils of cladoceran and diatom assemblages.
225 Sediment core collection
226 Five gravity cores were retrieved from Seton and Anderson lakes in August 2014 (Fig. 1)
227 using a Glew gravity corer (Glew et al. 2002). Sediment cores from both lakes were retrieved 228 from deep depositional basins, at 203 mDraft and 205 m in Anderson Lake (cores A1 and A2, 229 respectively), and at 128 m, 118 m, and 110 m in Seton Lake (cores S3, S2, and S1,
230 respectively). Cores A1 and A2 were 66.5 cm and 51.5 cm in length, respectively. Cores S3, S2,
231 and S1 were 74 cm, 66.5 cm, and 74.5 cm in length, respectively. Sediment cores S1-S3 were
232 collected along the central axis of Seton Lake to capture a gradient of influence from the BRD,
233 with core S3 located nearest to the discharge of the BRD, and cores S2 and S1 located at
234 increasing distances eastward from the BRD (Fig. 1). The two sediment cores from Anderson
235 Lake were collected along the length of the lake to span a similar spatial gradient as in Seton
236 Lake, and used as a spatio-temporal reference. All sediment cores were sectioned at 0.5 cm
237 intervals into sterile bags, and stored at 4 °C.
238 Grain size analysis & core chronology
239 Sediment samples for grain size analyses were prepared following the methods from
240 Jiilavenkatesa et al. (2001). Approximately, 100 to 150 mg of wet sediment was treated with the
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241 following chemical treatment: ~30 mL of 30% H2O2 for several weeks to digest the organic
242 matter; 2mL of 11M HNO3 for 10 minutes, to digest any remaining organic matter; 40mL of 1M
243 NaOH, placed in a water bath at ~70oC for a minimum of 20 minutes, to remove biogenic silica;
244 and ~8 mL of 0.5M HCl, to help neutralize the solution and flocculate the sediment. Between
245 each chemical treatment, the samples were rinsed three times as follows: the samples were
246 centrifuged at 3500 rpm for 30 minutes to 1 hour (until supernatant clear); the supernatant was
247 aspirated and; the remaining material was re-suspended in 40 mL of deionized water using a
248 vortex agitator. The grain size distribution was analyzed on a Malvern Mastersizer 2000,
249 assuming a reflective index of 1.54. The grain size results were then partitioned into three size
250 categories (clays, <2 µm; silts, 2-63 µm; and sand >63 µm) according to grain size classification
251 developed by Blott and Pye (2001), andDraft expressed as percent of total grain size.
252 Radioisotopic dating (210Pb, 137Cs, 214Pb, and 214Bi) was performed on all sediment cores
253 using an Ortec gamma spectroscopy counter (Schelske et al. 1994) and between 16 and 25
254 samples were analyzed per core. For the two cores from Anderson Lake (A1 and A2), a
255 Constant Rate of Supply (CRS) model was used to estimate the core chronology, based upon the
256 unsupported 210Pb activity in the core (Binford 1990). For the sediment cores from Seton Lake,
257 the 210Pb activity was very low (likely due to the presence of glacial flour in the core tops; Fig.
258 S1), and led to the use of independent markers to develop the chronologies, which included the
259 peak in 137Cs activity and changes in grain-size composition (Figs. S1, S2, S3). The peak in the
260 radioisotope 137Cs was used to identify the year 1963 (corresponding to the fallout maximum of
261 atmospheric nuclear testing; (Appleby 2001), while the sharp transition in grain sizes was used to
262 localize the first elevated inflows of glacially-turbid water coming from the BRD between the
263 late-1940s and the early-1950s. While the constructions of the diversion started in the 1930s, it is
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264 only around the late 1940s that the BRD started to divert important volumes of water (~56.6
265 m3·sec-1) down into Seton Lake. The sedimentation rates for the Seton Lake cores were
266 calculated using linear interpolation between the first increase in clay above background levels,
267 and the ca. 1963 horizon (137Cs peak), and from ca. 1963 to the top of each core, which
268 corresponded to 2014, the year of retrieval (Fig. S3). The average sedimentation rates from the
269 Anderson Lake sediment cores were used to estimate the sedimentation in Seton Lake prior to
270 the BRD (Fig. S3).
271
272 Sedimentary cladoceran and diatom analyses
273 The analysis of cladoceran sub-fossilsDraft were prepared following standard methods 274 (Korhola and Rautio 2001). Approximately 1 g of wet sediment was treated with 150 mL of 10%
275 KOH to deflocculate the sediment. The sediment-KOH mixture was then sieved through a 34 µm
276 mesh and backwashed with deionized water into a 12mL glass vial, to which several drops of
277 safranin glycerine solution (dye) and alcohol (preservative) were added. A 50µL aliquot of slurry
278 was deposited on a slide and allowed to dry. This process was repeated as necessary to achieve
279 samples of sufficient concentration for enumeration. Individuals on the entire slide were counted.
280 At high concentrations, a minimum count of 70 individuals per sample could be reached, but at
281 low concentrations a minimum count of 20 individuals per sample were enumerated, which is
282 sufficient to characterize a sedimentary cladoceran assemblages with low diversity (Kurek et al.
283 2010). For some intervals at the top of the Seton Lake cores, the minimum count of 20
284 individuals could not be reached because of the low concentrations, and limitations in
285 concentrating the slurry on the slides, due to high concentrations of clay (see intervals marked by
286 a dot in Fig. 5). Standard identification keys were used to identify the cladoceran remains 13
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287 (Szeroczyñska and Sarmaja-Korjonen 2007; Korosi and Smol 2012a, 2012b). Bosmina spp. was
288 only identified to the genus level because the head pores were difficult to localize due to the
289 presence of other small perforations.
290 The analysis of diatoms was completed following standard methods (Battarbee et al.
291 2002). Briefly, ~0.2-0.3 g of wet sediment was sub-sampled in 20 mL glass vials. To remove
292 organic matter, a 1:1 mixture by molar weight of concentrated nitric (HNO3) and sulphuric
293 (H2SO4) acid was added to each sample, which were then heated in a hot water bath for 6-7
294 hours. Samples were allowed to settle for 24 hours before the acid above the sample was
295 aspirated, and rinsed with deionized water; this rinsing procedure was repeated until the sample 296 had the same pH as deionized water. SedimentaryDraft concentrations of diatoms were determined 297 using known concentrations of microspheres added to each of the diatom samples (Battarbee and
298 Kneen 1982). Microspheres were then enumerated along with the diatoms.
299 Diatom and cladoceran fluxes were calculated using concentration data and estimated
300 sedimentation rates for each core. The fluxes were expressed as a net accumulation rate (#ind·
301 cm-2· year-1) and calculated by dividing the concentration (#ind·g dry weight-1) by the volume of
302 the sediment sample (cm-3), and by the number of years contained in the sediment sample (0.5
303 cm thickness). While the relative abundance (%) can give a good representation of the
304 cladoceran community composition present in the water column, fluxes give a better estimate of
305 the absolute density of individuals in the water column (Nykänen et al. 2009). However, it is
306 important to note that the calculation of flux data depends on a reliable core chronology with a
307 known estimates of core-specific sedimentation rates. The age-depth model developed for each
308 of the Seton Lake cores provides an average sedimentation rate for the three following periods:
309 prior to the diversion, from the start of the diversion until 1963, and from 1963 to present. The 14
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310 flux data thus provides a sufficient estimate of the magnitude of change in sedimentation rate
311 from before to after the diversion, and can be used in a comparative sense to the changes in the
312 Anderson Lake cores.
313
314 Results
315 Modern limnological data
316 Over the three sampling seasons, from 2014 to 2016, the depth of the euphotic zone was
317 significantly lower while turbidity was significantly higher in Seton Lake compared to Anderson
318 Lake (t-test, p<0.005, Fig. 2), TP and TN were not significantly different between the two lakes
319 (t-test, p<0.05, Fig. 2), when averaged acrossDraft sampling stations. GPP was significantly different
320 between years (t-test, p<0.05) and significantly greater in Anderson Lake than in Seton Lake (t-
321 test, p=0.05). The depth of euphotic zone and turbidity remained stable throughout the sampling
322 seasons in both lakes. The depth of the euphotic zone was on average ~14 m shallower in Seton
323 Lake compared to Anderson Lake, respectively (Fig. 2). The seasonal average turbidity in Seton
324 Lake was 3.3 NTU, and 1.9 NTU in Anderson Lake. The depth of the euphotic zone was
325 negatively correlated with turbidity in Seton Lake (r= -0.83, p<0.05), but was not correlated with
326 turbidity in Anderson Lake (r= -0.21, p>0.05). TP varied between 3 µg·L-1 in June and 2 µg·L-1
327 in October, and TN varied around a mean of 42 µg·L-1 in Seton Lake. In Anderson Lake, TP
328 remained relatively stable around a mean of 2 µg·L-1, and TN reached a maximum in May (76
-1 -1 329 µg·L ) and declined to a minimum in October (40 µg·L ). Ammonium (NH4-N) and SRP were
-1 330 below detection levels in both lakes (detection limits: 5 µg·L ). Nitrate (NO3-N) was low in both
331 lakes, seasonal average (May-Oct) of 7.6 µg·L-1 and 17 µg·L-1 in Seton Lake and Anderson Lake,
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332 respectively; with highest values reached in May in Seton Lake and in Anderson Lake (13.3
333 µg·L-1 and 44 µg·L-1, respectively). Seasonal mean TN:TP (molar ratio) averaged ~37 in Seton
334 Lake and ~54 in Anderson Lake. Gross primary production (GPP) of Seton Lake varied between
335 a maximum of 123 mg C·m-3·day-1 in June and a minimum of 71 mg C.m-3.day-1 in August, with a
336 seasonal average of 96.73 mg C·m-3·day-1. In Anderson Lake, GPP increased from May (96 mg
337 C·m-3·day-1) to September (255 mg C·m-3·day-1), and then declined in October (Fig. 2), with a
338 seasonal average of 157 mg C·m-3·day-1.
339 Modern zooplankton abundance and composition
340 Total zooplankton density and total cladoceran density were 2-fold lower in Seton Lake 341 than in Anderson Lake (t-test, p<0.005; DraftFigs. 2, 3). Cladoceran biomass was also significantly 342 different between the two lakes with a biomass 10-fold higher in Anderson Lake than in Seton
343 Lake. The density and biomass of the copepods (Fig. 3) and the density of the rotifers were not
344 significantly different between the two lakes (t-test, p>0.05). Total zooplankton biomass was
345 highly correlated with cladoceran biomass (r=0.9, p<0.005), while the total zooplankton density
346 was highly correlated with copepod density (r=0.9, p<0.005). In Seton Lake, copepod density
347 and biomass was highest in May, and cladoceran density and biomass was slightly higher in
348 August and September (Fig. 3). In Anderson Lake, copepod density and biomass was higher than
349 in Seton Lake, throughout the sampling season, with a maximum peak reached in May.
350 Cladoceran density and biomass was highest in June in Anderson Lake (Fig. 3).
351 In Seton Lake, the copepod community was numerically dominated by the cyclopoid
352 copepod, Cyclops scutifer Sars G.O (~84% percent of individuals; Fig. S5). The cladoceran
353 community was dominated by Daphnia ambigua Scourfield and Eubosmina longispina Leydig
354 (~76% and ~16%, respectively), and Eubosmina longispina was the only Bosmina sp. found in 16
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355 the modern samples. The rotifer assemblage was co-dominated by Kellicottia sp. and Asplancha
356 sp. (both ~50%). In Anderson Lake, the copepods were also dominated by Cyclops scutifer
357 (~50%; Fig. S5) and the cladoceran assemblage was dominated by Daphnia ambigua and
358 Eubosmina longispina (~68% and ~31%, respectively). The rotifers were dominated by
359 Kellicottia sp. (~98%).
360
361 Paleolimnological data
362 Grain size analysis & core chronology
363 Grain size analysis was used in conjunction with the radioisotope analysis of 137Cs in the
364 development of the core chronology forDraft Seton Lake. The results from the grain size analysis
365 were used as a marker of the completion of the BRD. In Seton Lake, clay (particle size <2 µm)
366 contributions to the sediment matrix began to increase around 50 cm, 44 cm, and 30 cm in cores
367 S3, S2, and S1, respectively, with percent composition of clay rising from ~5% to above 40%
368 (beginning of Zone A in Fig. S2). Clay contributions continued to rise until reaching a maximum
369 of ~80% around 37 cm, 26 cm, and 23 cm in cores S3, S2, and S1, respectively. Thereafter, the
370 contributions of clay to the sediment matrix was variable, but generally remained well above
371 background levels (Fig. S2). In contrast, the cores from Anderson Lake had a high (between 70%
372 and 90%) contributions of silt (i.e. particle of a size between 63-2µm) and low contributions of
373 clay (<1%) to the sediment matrix. Core S2 from Seton Lake was characterized by a discrete
374 sand horizon (63µm ≥) from 27 to 33 cm, which was treated as a single short-term event, and
375 excluded from the age-depth model (Fig. S2).
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376 The unsupported 210Pb activity, which derives from the atmospheric flux (versus
377 supported which derives from in situ decay, see Appleby 2001 for more information) in all cores
378 from Anderson and Seton lakes was generally low, but declined exponentially with depth in each
379 core (A1, r2=0.89; A2, r2=0.90; S1, r2= 0.85, S2, r2= 0.90 and S3, r2= 0.78), suggesting that the
380 cores were minimally disturbed (Fig. S1). The surficial total 210Pb concentration was below 100
381 Bq·Kg-1 in the Seton Lake cores, and just below 350 Bq·Kg-1 in the cores from Anderson Lake. A
382 peak in the 137Cs activity (> 50 Bq·Kg-1) was present at 9 cm and 8 cm in cores A1 and A2
383 (Anderson Lake). In Seton Lake, the peak in 137Cs activity occurred at 38 cm, 25 cm, and 20 cm
384 in cores S3, S2, and S1, respectively (Fig. S1), above the initial increase in clay, which was
385 associated with the time at which an important volume of glacially-turbid water was diverted for
386 the first time into Seton Lake ca. 1940 (DraftFigs. S2, S3). For the cores from Anderson Lake, 210Pb-
387 derived dates overestimated the age of the sediment down core and therefore were anchored to
388 the 137Cs dates in the CRS-model ( i.e. estimation of ca. 1960 from 210Pb was 1 cm below 137Cs
389 estimates; Appleby 2001).
390 Altogether, we were able to develop an age-depth model for the Seton Lake cores using
391 data from the grain size and radioisotope analysis, and for the Anderson Lake cores using the
392 radioisotope analysis. Results of the age-depth models are provided in the Supplemental material
393 (Fig. S4). It is important to note that while the age-depth model does not provide a sample-
394 specific date for each slice of sediment, it provides a reasonable estimate of the magnitude of
395 change of Seton Lake sedimentation rate.
396 Sedimentary cladoceran and diatom analyses
397 Total cladoceran and diatom fluxes in Seton Lake decreased post ca. 1960, whereas
398 fluxes increased or remained stable in Anderson Lake (Fig. 4). During this period, both Bosmina 18
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399 spp. and Daphnia longispina complex fluxes decreased in the Seton Lake cores (Fig. 5). Prior to
400 ca. 1960, core S3 had the lowest cladoceran and diatom fluxes of all cores and experienced an
401 ~2- to 3-fold decline post 1960. Cores S1 and S2 from Seton Lake displayed much higher fluxes
402 prior to 1960, and experienced ~3 to 10-fold declines post-1960 (Fig. 4). In Anderson Lake, the
403 total cladoceran and diatom fluxes remained constant relative to Seton Lake, with low variability
404 until ca. 1970-80, when they increased (Fig. 4), contributed to by an increase of both Bosmina
405 spp. and Daphnia longispina complex (Fig. 5). In both lakes, the concentration (Fig. S6) and flux
406 (Fig. 4) data show similar trends but different magnitudes. Bosmina spp. and the Daphnia
407 longispina complex were the dominant cladoceran groups (>90% cumulative relative abundance)
408 found in the sediments from both lakes (Fig. 5). In Seton Lake, Bosmina spp. was the dominant
409 taxonomic group prior to ca.1945. The relativeDraft abundance of the Daphnia longispina complex
410 then increased between the 1940s and 1950s. This change in relative abundance coincided with
411 an abrupt decrease in the flux of both Bosmina spp. and Daphnia longispina in S2 and S3 (Fig.
412 5). In the Anderson Lake cores, a shift from a Bosmina spp. to Daphnia longispina complex also
413 occurred, but started in the 1920s (Fig. 5).
414
415 Discussion
416 Together, our modern and paleo-data support substantial declines in the primary
417 (diatoms) and secondary (zooplankton) producers in Seton Lake following the establishment of
418 the BRD (Fig. 4). The modern limnological data enabled a direct comparison of the two lakes,
419 and the paleo-data allows a pre- and post-diversion perspective, that is not available through any
420 other means. Our modern limnological data indicate that the euphotic zone of Seton Lake is
421 approximately 10 m shallower than that of Anderson Lake. Similarly, secondary production (i.e. 19
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422 zooplankton density and biomass) is currently 3- to 10- fold lower in Seton Lake relative to
423 Anderson Lake, and gross primary production was lower in Seton Lake in July and August (Figs.
424 2, 3). Our analysis of the sedimentary remains of diatom and cladoceran sub-fossils allowed the
425 reconstruction of long-term spatial and temporal variation in the primary and secondary
426 production of these two systems over the last ~200 years, in order to determine whether the
427 lower primary and secondary production observed in the modern data between 2014-2016 in
428 Seton Lake is consistent with changes induced by the Bridge River Diversion (BRD). The fluxes
429 of diatom and cladoceran sub-fossils indicate that the production in Seton Lake declined greatly
430 following the establishment of the BRD (Fig. 4). Conversely, the fluxes in the cores from
431 Anderson Lake (reference system) exhibited stable or increasing trends over the last fifty years
432 (Fig. 4), which is consistent with pronouncedDraft regional warming, which is apparent in the mean
433 annual and winter temperatures (BC-MoE 2016). The large decline in primary and secondary
434 production in Seton Lake may be due to direct or indirect mechanisms related to the BRD,
435 including changes in the availability of light and nutrients, and/or modification of the water
436 residence time. Multiple lines of evidence suggest that the intrusion of glacial flour is the
437 primary mechanism by which the BRD negatively affected the production of Seton Lake.
438
439 Effects of glacial flour on lake production
440 The intrusion of turbid water can induce complex modifications to lake ecosystems.
441 Enhanced inputs of glacial meltwater can result in direct and indirect impacts on primary and
442 secondary production through several ecological processes which include changes in light
443 penetration, food-web interactions, nutrient availability, and water residence time (Hylander et
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444 al. 2011; Sommaruga 2015). The importance and potential significance of these various factors
445 are discussed below.
446
447 Change in light availability and effects on primary production
448 Glacial meltwater can induce important changes in light penetration, reducing the extent
449 of the euphotic zone. A survey of twenty-two lakes indicated that lakes receiving meltwater rich
450 in glacial flour had the lowest water clarity, along with lakes affected by humic coloration (Irwin
451 1974). Analogous to the findings of this study, our limnological data indicate that the depth of
452 the euphotic zone in Seton Lake is approximately half that in Anderson Lake (Fig. 2). Glacial 453 flour is often the primary factor responsibleDraft for light attenuation in glacially-fed lakes (Rose et 454 al. 2014). The high reflectance and scattering properties of glacial flour (Gallegos et al. 2008)
455 lessens the penetration of photosynthetically active radiation (Rose et al. 2014), thereby reducing
456 the depth at which photosynthesis can occur. Similarly, we found that water turbidity in Seton
457 Lake is 2-fold higher than in Anderson Lake, and measurements of turbidity and depth of the
458 euphotic zone were negatively correlated in Seton Lake (Fig. 2). The shallower euphotic zone in
459 Seton Lake is likely the direct result of the input of glacial flour from the BRD.
460 In lakes influenced by glacial flour, water clarity can become the main factor regulating
461 primary production, influencing plankton community structure and abundance (Sommaruga
462 2015). Joint and Pomroy (1981) found that turbid riverine inputs in the Bristol Channel (UK)
463 decreased primary production by up to 20-fold in regions of high turbidity. Prior to the diversion,
464 Seton Lake received its main inflow from Anderson Lake. Therefore, the changes in light
465 penetration induced by the BRD is likely the primary mechanism responsible for the decline in
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466 the fluxes of diatom and cladoceran observed in the sediment cores (Fig. 4). The paleo-data
467 support a reduction in the abundance of diatom algae in Seton Lake as a result of the BRD,
468 highlighting that the diatom flux declined by 3 to 10-fold in the Seton Lake cores following the
469 establishment of the diversion (Fig. 4). Interestingly, the diatom flux from Seton Lake during the
470 post-diversion period were on average 2 to 3-fold lower than the Anderson Lake flux, whereas
471 the present-day estimates of gross primary production (GPP) show that Anderson Lake has only
472 a slightly higher GPP (p= 0.05; Fig. 2). The discrepancy observed between our limnological data
473 and paleolimnological data may be explained by the fact that the diatoms are not the dominant
474 algal group in these lakes (Limnotek 2017). Nevertheless, the limnological survey shows that the
475 inter-annual variability of GPP is high in both lakes (Fig. 2), and previous surveys have shown
476 that Seton Lake tends to be less productiveDraft than Anderson Lake (Geen and Andrews 1961;
477 Shortreed et al. 2001). As the data from our sediment cores are an integration of several years,
478 our paleo-data may imply that on average Seton Lake primary production is lower than in
479 Anderson Lake, a trend not as apparent in the more limited modern data. Moreover, the paleo-
480 data indicate that the diatom flux in the cores from Seton Lake were higher than Anderson Lake
481 pre-BRD, suggesting that the primary production in Seton Lake was higher than in Anderson
482 Lake before the establishment of the diversion.
483
484 Influence on lake secondary production
485 The overall lower post-BRD zooplankton densities observed in the Seton Lake cores (Fig.
486 2) and the significantly lower cladoceran density and biomass (Fig. 3) could be related to the
487 decline of the primary producers following the establishment of the diversion, and subsequent
488 bottom-up food-web interactions. The reduction of the cladoceran flux in the cores from Seton 22
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489 Lake were coeval with the declines in diatom flux, suggesting that decline of the primary
490 producers, related to the increased turbidity, has negatively impacted the secondary producers in
491 Seton Lake. The adverse effects of turbid glacial meltwater on pelagic primary and secondary
492 producers of lacustrine systems have been previously recorded in paleolimnological studies of
493 the Holocene Late Glacial Maximum and Younger-Dryas (Birks and Birks 2008). The study
494 reported a decline in the phytoplankton and zooplankton abundance, corresponding to the time at
495 which the lakes received inputs of glacial runoff. Similar to these studies, our data indicate that
496 the timing of the decline of cladoceran and diatom fluxes in the sediment corresponded with the
497 novel increases in percent clay contributions to the sediment matrix of Seton Lake (Figs. 3, 5).
498 The currently lower secondary production found in Seton Lake could thus be a result of bottom-
499 up cascading food web effect induced byDraft the decline of lake primary production.
500 The input of glacial flour can also impair the filter-feeding apparatus of Cladocera
501 (Koenings et al. 1990; Kirk 1991). Often similar in size to food particles, filter-feeding
502 zooplankton are susceptible to ingest the suspended particles of clay. Koenings et al. (1990)
503 previously demonstrated that filter-feeding daphnid, bosmid, and taxa from the genus
504 Holopedium did not discriminate against glacial particles in glacially-turbid lakes at a threshold
505 turbidity of ≥5 NTU, which are similar to the turbidity levels reached in the early spring in Seton
506 Lake (Fig. 2). As the cladoceran assemblages from both Seton and Anderson lakes were almost
507 exclusively composed of Bosmina spp. and Daphnia spp. (Fig. 5, Fig. S5), two pelagic filter-
508 feeders commonly found in deep oligotrophic Sockeye Salmon nursery lakes, the input of glacial
509 flour to the system may have been detrimental to the overall cladoceran community. In fact, as
510 seen in Anderson Lake, spring appears to be the period at which zooplankton bloom (Figs. 2, 3).
511 However, corresponding with the high turbidity values, this spring zooplankton bloom is 3-fold
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512 lower in Seton Lake than it is in Anderson Lake (Fig. 2). More importantly, the cladoceran
513 density and biomass remain low during that period in Seton Lake, whereas they increase by 4-
514 fold in Anderson Lake (Fig. 3). Moreover, the competitive abilities of Daphnia can be
515 substantially lessened relative to other zooplanktons in glacial flour-rich waters. In fact, previous
516 laboratory studies found that Daphnia feeding rates was affected to a greater extent compared to
517 copepods when exposed to glacial flour (Hart 1988). The disproportionate susceptibility to
518 glacial flour amongst zooplankton groups may therefore explain the large declines in cladoceran
519 flux post-1940s in the cores from Seton Lake (Fig. 4), the overall lower modern cladoceran
520 density found in Seton Lake relative to Anderson Lake (Fig. 3), as well as the more diverse
521 copepod and rotifer communities found in Seton Lake (Fig. S5). Draft 522
523 Changes in nutrient availability
524 Glacial flour can impact the biological availability of nutrients in freshwaters. Through
525 the sequestration of nutrients by colloidal binding (i.e. Fe-bound and apatite-calcite bound),
526 glacial flour can alter nutrient stoichiometry in lakes located downstream from glaciers, acting as
527 a sink or source of phosphorus (Hodson et al. 2004; Sommaruga 2015). Investigations of the
528 phosphorus content of glacial flour have shown that glacial runoff containing weathered minerals
529 may furnish bioavailable inputs of phosphorus in P-limited freshwater ecosystems, whereas
530 phosphorus adsorption to the minerals tends to occur in ecosystems with higher phosphorus
531 concentrations (Hodson et al. 2004). Oligotrophic lakes can be especially sensitive to the
532 influence of glacial flour on nutrient bioavailability, as slight increases in available nutrients can
533 alter algal assemblages (Hylander et al. 2011; Slemmons and Saros 2012). The similar GPP
534 observed in the spring in Seton and Anderson lakes could therefore suggest that Seton Lake 24
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535 primary production may have benefited from an enhancement of nutrients, thereby compensating
536 for the lower light regime. While the bioavailable forms of phosphorus (SRP) were below the
537 detection levels in Seton and Anderson lakes, the stimulatory effect associated with an increase
538 in bioavailable nutrients can occur before nutrient levels are detectable (Hylander et al. 2011;
539 Slemmons and Saros 2012). As such it is difficult to assess whether the input of glacial flour
540 indeed enhanced the primary production through the transport of nutrients. Nonetheless, while
541 the GPP in Seton Lake remains low throughout the growing season, it increases from June to
542 September in Anderson Lake, and TN:TP molar ratios were above thresholds for P and N co-
543 limitation of autotrophic production in Seton Lake, supporting that phosphorus is the limiting
544 nutrient (Elser et al. 1990; Downing and McCauley 1992). As such, the stimulatory effect of
545 glacial flour is likely limited in Seton Lake.Draft It is also noteworthy that the colloidal binding
546 properties of glacial flour is relative to the type of bedrock underlying the originating glacier
547 (Hodson et al. 2004). The BRD glacial runoff comes from the Bridge Glacier which overlays on
548 Mesozoic and Cretaceous bedrocks rich in quartz diorite (BC digital Geology, Geospatial Data,
549 Government of British Columbia), a silica-rich mineral with low-phosphorus content and poor-
550 binding properties (Porder and Ramachandran 2013). The mineral properties of the glacial flour
551 may thus account for the lack of influence on the phosphorus levels in Seton Lake, and likely did
552 not have a stimulatory effect on primary production.
553 In regions where deposition of atmospheric reactive nitrogen is high, glacial runoff can
554 enrich freshwater ecosystems in nitrate, yet another mechanism by which glacial meltwater can
555 stimulate plankton production (Saros et al. 2010; Slemmons and Saros 2012). Although we did
556 not find data on atmospheric reactive nitrogen deposition in our study region, the concentrations
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557 of TN in Seton Lake were lower than in Anderson Lake discounting any prolonged nitrate
558 enrichment resulting from the input of the glacial flour (Fig. 2).
559
560 Changes in Seton Lake water residence times, water levels, & temperature
561 Water residence time can influence zooplankton community structure and recruitment
562 (DFO 2014). However, the post-diversion water residence time of Seton Lake is relatively
563 constant and long enough (~320 days) to not adversely affect the productivity (i.e. residence time
564 far exceeds biological replacement). In fact, Obertegger et al. (2007) showed that in a lake
565 experiencing large fluctuations in flushing rate (from 23 to 786 days), zooplankton abundance 566 and community structure were significantlyDraft affected by changes in water residence time below a 567 threshold value of 193 days, a much shorter water residence than Seton Lake, post-diversion
568 (~320 days). It is thus unlikely that the large reductions in cladoceran flux, coincident with the
569 onset of the BRD, were the direct result of higher flushing rates in Seton Lake.
570 Likewise, the BRD and the dam constructed at the outflow of Seton Lake could have
571 modified water levels. While we do not know by how much, if at all, the water levels of Seton
572 Lake have changed since the establishment of the hydroelectric management, it is unlikely to
573 have had a significant long-term influence on the biological composition of this deep fjord lake.
574 In fact, littoral cladoceran taxa were virtually absent from the sediment cores collected in both
575 Seton and Anderson lakes, supporting the fact that the littoral habitat is limited in those lakes
576 (Fig. 5).
577 Originating from the Bridge Glacier, the BRD could transport much colder waters into
578 Seton Lake, thereby influencing the primary and secondary production of Seton Lake.
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579 Temperature profiles were taken monthly during the three years of limnological sampling at 6
580 stations along the length of Seton Lake and 2 stations in Anderson Lake (Limnotek 2017). The
581 results indicated that there were no differences observed between the limnological stations within
582 Seton Lake, nor between the lakes, thereby providing evidence that the BRD did not significantly
583 change the water temperature of Seton Lake (Limnotek 2017).
584
585 Changes in Seton Lake sedimentation rate
586 Changes in sedimentation rate can influence the accumulation of subfossils in the
587 sediments (Battarbee et al. 2002). The results from the grain size and radioisotope analysis (Figs. 588 S1, S2) demonstrate that the influx of glacialDraft flour increased the sedimentation rate of Seton 589 Lake, which would dilute of the concentrations of cladoceran and diatom microfossils. In order
590 to correct for potential bias associated with changes in sedimentation rate, the paleo-data were
591 expressed as flux. However, flux data depend upon accurate estimates of sedimentation rates,
592 and the linear age-depth models developed for the Seton Lake cores present some challenges.
593 For instance, the linear model assumes a constant sedimentation rate between set ages (Fig. S3).
594 The main factors regulating the sedimentation rate of Seton Lake during the post-diversion
595 period are changes in the water flow coming from the BRD, and the delivery of inorganic
596 material (i.e. amount of glacial flour being discharged by the BRD). The annual inflow coming
597 from the BRD is regulated, and has been relatively constant since its completion (~87 m3.sec-1
598 ±12, data courtesy of BC Hydro). However, as the Bridge Glacier is receding (Chernos 2014),
599 the influx of glacial flour may have varied through time. The grain size analysis indicates that the
600 percent clay contribution during the post-diversion period indeed varies (Fig. S2); however, it
601 remains on average 3 to 4-fold higher in magnitude compared to the pre-diversion period in all 27
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602 cores from Seton Lake, as such the post-diversion sedimentation rate in Seton Lake remained on
603 average higher than the pre-diversion period. This is further supported by a first order
604 exponential decay of 210Pb in all cores from Seton Lake (Fig. S1), indicating an relatively
605 constant sedimentation rate. Moreover, the grain size analysis did not show any decline in the
606 percent of clay contribution in the top of the Seton Lake cores suggesting that the delivery of
607 glacial flour did not decline yet as a result of the Bridge Glacier retreating. Altogether, annual
608 water inflow and grain size data demonstrate that the main factors regulating the Seton Lake
609 sedimentation rate during the post-diversion period remained, on average, stable.
610 Another potential limitation to our linear age-depth model is the assumption that Seton 611 Lake sedimentation rate was similar to AndersonDraft Lake before the diversion was established. This 612 assumption is supported by the fact that Anderson and Seton lakes share similar morphological
613 properties, and are exposed to similar edaphic and climatic conditions. Various indigenous
614 accounts also refer to a landslide that separated the once single lake into two lakes (i.e. Seton and
615 Anderson lakes) at the Seton Portage about 8,000-20,000 year BP (Edwards 1985), further
616 supporting the assumption that the two lakes likely had a similar sedimentation rate before the
617 diversion was established. Nevertheless, the sedimentation rate can vary substantially within a
618 lake basin. This appears to be the case for the post-diversion period in Seton Lake, as illustrated
619 by the cesium peak (Fig. S1), and it is likely the result of the localized input of glacial flour from
620 the diversion which created a gradient of sedimentation rate within the deep depositional basin
621 (i.e. slower sedimentation rate in the cores located further from the diversion; Figs. S1, S2 & S4).
622 However, the age-depth model for the Anderson Lake cores developed from the radioisotopic
623 methods supports uniform sedimentation rate in the deep basin of Anderson Lake (Fig. S4), and
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624 thus, without the influence of the diversion, the Seton Lake pre-diversion sedimentation rate can
625 be assumed to be similar between the three cores.
626 Lastly, our paleo-data suggest that the fluxes of cladoceran and diatom were higher in
627 Seton Lake than in Anderson Lake before the establishment of the diversion, and this pattern was
628 reversed after the diversion was established. Although we do not have limnological data from the
629 pre-diversion period, the post-diversion limnological data, as well as that of previous studies
630 (Geen and Andrews 1961; Shortreed et al. 2001) agree with our paleo-data inferences, indicating
631 that the post-diversion cladoceran community is less abundant and GPP is lower in Seton Lake
632 than in Anderson Lake. Therefore, the magnitude of changes in the cladoceran and diatom fluxes 633 from before to after BRD is likely to beDraft real, and primarily driven by a decline in primary and 634 secondary production rather than an artifact of dilution due to an increase in sedimentation rates
635 in Seton Lake.
636
637 Recent increases in the biological production of Anderson Lake
638 In contrast to Seton Lake, the Anderson Lake cores suggest an enhancement of both
639 primary and secondary production in Anderson Lake over the last fifty years (Fig. 4). Starting
640 between the 1960s and 1980s, the increased primary and secondary production in Anderson Lake
641 could reflect an increase in mean annual temperature. In fact, the central interior of British
642 Columbia has warmed on average 1 °C per century from 1900 to 2013 (BC-MoE 2016). As most
643 of the warming has occurred during the winter months with an increased average winter
644 temperature of 1.6 °C per century (BC-MoE 2016), the increase in production observed in
645 Anderson Lake cores may be related to changes in the seasonality of the lake induced from
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646 warming atmospheric temperatures, including changes in the timing and magnitude of thermal
647 structure of the lake, lake ice phenology, and lake-water temperatures (Weyhenmeyer 1996). As
648 zooplankton density and biomass tended to be higher during the month of May and June in both
649 lakes (Figs. 2, 3), the increase in cladoceran flux observed in the Anderson Lake sedimentary
650 record could reflect longer ice-free periods and subsequent enhancement of the primary
651 production earlier in the spring (Schindler et al. 2005b). However, this hypothesis needs to be
652 further explored, as the diatom flux only increased in one of the Anderson Lake cores.
653
654 Effects of glacial flour on juvenile O. nerka food supply
655 Rates of primary production are Drafthighly related to fish biomass accrual (Downing et al. 656 1990) and the foundation for O. nerka productive capacity models throughout British Columbia
657 (Hume et al. 1996; Shortreed et al. 2000). Subsequent bottom-up control of the zooplankton
658 community strongly mediates trophic energy flows to O. nerka (Hyatt et al. 2004). Our results
659 suggest that the development of the BRD reduced the abundance of cladoceran, an important
660 food supply for O. nerka, relative to conditions prior to hydropower development.
661 The seasonal timing of zooplankton production can be an important factor regulating O.
662 nerka fry and smolt sizes. In lakes affected by glacial turbidity, juvenile Sockeye Salmon tend to
663 be smaller only in cold-water lakes where the peak of zooplankton production is delayed to later
664 in the summer (Koenings et al. 1985). In most Sockeye Salmon nursery lakes, peak fry
665 emergence occurs during the spring (Burgner 1991), a critical time for juvenile salmon to grow
666 and survive in freshwater. Our data indicate that zooplankton density peaks in May in both Seton
667 and Anderson lakes, suggesting that glacial flour does not induce a timing mismatch with
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668 foraging potential for juvenile O. nerka (Fig. 2). However, the cladoceran biomass remained low
669 in Seton Lake throughout the growing season while in Anderson Lake, the cladoceran biomass
670 peaked in June (Fig. 3). As cladocerans are considered as an important and efficient conduit of
671 energy to juvenile O. nerka (Shortreed et al. 2001; Mazumder and Edmundson 2002), and have
672 been found to be an important food supply for the juvenile O. nerka of the Seton-Anderson lakes
673 systems the higher cladoceran biomass observed in Anderson Lake suggests consistent foraging
674 opportunities sustaining the food web for O. nerka, whereas the foraging opportunities in Seton
675 Lake are not maintained to the same extent.
676 677 Implications for other glacially influencedDraft fisheries lakes 678 The BRD resulted in a substantial decline in the ecologically important primary (diatoms)
679 and secondary (Cladocera) producers in Seton Lake. Following the establishment of
680 hydroelectric management, the bioavailable nutrient concentrations in Seton Lake and the
681 enhanced flushing rates were unlikely to have had a measurable influence on the lake production.
682 Rather, the reduction of light penetration and the shallower euphotic zone in Seton Lake,
683 consistent with the influx of the glacial flour-rich meltwater, is likely the primary mechanism
684 that triggered the observed declines in production. The presence of glacial flour in the system
685 may have further contributed to the reduction of cladoceran filter-feeders in Seton Lake, thus
686 affecting the composition of the zooplankton community. Overall, our results suggest that the
687 long-term input of glacial flour can substantially reduce the primary and secondary production of
688 O. nerka nursery lakes. As no data exist on juvenile O. nerka population abundance and size
689 from the pre-diversion period, the direct impacts of glacial water intrusions on growth and
690 survival dynamics in populations of O. nerka in Seton Lake cannot be directly assessed. 31
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691 Nonetheless, the reduction of primary and secondary production in Seton Lake following the
692 BRD implicates large-scale changes to energy flow within the O. nerka food web.
693 Lake responses to glacial retreat may be region specific, presenting a spatio-temporal
694 dimension (Baraer et al. 2012; Bliss et al. 2014). Referred to as the peak water concept the early
695 stage of glacial retreat typically results in a peak of glacial meltwater runoff, followed by an
696 extended period of decline in discharge (Baraer et al. 2012). While most glaciers in British
697 Columbia may have passed this early stage and downstream rivers may experience a decline in
698 annual glacial runoff (Bliss et al. 2014), the upper Bridge River, which drains the Bridge Glacier,
699 has experienced continuous increase in flow from the Little Ice Age to the present (Moyer et al. 700 2016). As glacial retreat is accelerating,Draft the influences of turbid meltwater may thus vary greatly 701 among many lakes exposed to these upstream geomorphic processes. For instance, during the
702 early stage of glacial retreat, in the absence of potential compensatory mechanisms that enhance
703 upward trophic energy flows, such as changes in diel vertical migration of O. nerka permitting
704 longer daily foraging windows (Gregory 1993; Liljendahl-Nurminen et al. 2008; Hansen and
705 Beauchamp 2015), and/or reduced predation from piscivores (De Robertis et al. 2003; Pangle et
706 al. 2012), the expected increases in glacial flour delivery to O. nerka nursery lakes, and other
707 lakes experiencing similar influences, likely decrease their basal productive capacities to support
708 fish populations. This pattern may be slowly reversed at a later stage of glacial retreat as glacial
709 runoff declines (Bliss et al. 2014; Moyer et al. 2016).
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981 Acknowledgements
982 This study was funded by BC Hydro, with administration provided by Teri Neighbour,
983 Ahmed Gelchu, Darin Nishi, and Jeff Walker. We thank Bonnie Adolf, Gilda Davis, and Jude
984 Manahan of St’át’imc Eco-Resources Ltd who managed the study as part of a larger water use
985 planning project. Access to BC Hydro field facilities was provided by Dorian Turner. Field staff
986 included Garett Lidin for the cores collection; and Allison Hebert, Dani Ramos, Annika Putt,
987 Marc Laynes, Tyler Creasey, L.J Wilson, Caroline Melville, John Goes, Jessica Hopkins, Petra
988 Wykpis and Frank Richings for the limnological sampling. Limnological data compilations were
989 completed by Mike Chung. Lab support was provided by Shannon Harris, Danusia Dolecki, 990 Lech Dolecki, Dr. Scott Lamoureux, KeithDraft Harper, Ron Kerr, Shauna Bennett and staff of 991 Chemical Engineering Department, Queen’s University, Kingston, Ontario, Canada; Fisheries
992 and Oceans lab at Cultus Lake, British Columbia, Canada; ALS Environmental, Burnaby,
993 Canada.
994
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995 Figure Captions
996 Figure 1: Location of Seton Lake and Anderson Lake on the Fraser River watershed in British 997 Columbia, Canada. Location of the five sediment cores used in this study (A1 & A2 in Anderson 998 Lake; S1, S2 & S3 in Seton Lake), and limnological monitoring stations. Catchment area slope 999 rasters were generated from the NASA Shuttle Topography Radar Digital Elevation Model 1000 (Jarvis et al. 2008) using the slope tool in ESRI's Spatial Analyst extension. The water ways 1001 and water bodies shapefiles were retrieved from BCData Catalogue. 1002 Figure 2: Variation of the monthly average (over the three sampling years 2014, 2015 and, 2016) 1003 of the depth of the Euphotic Zone (estimated from the PAR measurements), turbidity, total 1004 phosphorus (TP), total nitrogen (TN), gross primary production (GPP) and total zooplankton 1005 density in Seton Lake (grey line), and Anderson Lake (blue, dotted line). Error bars represent the 1006 standard deviation. 1007 Figure 3: Monthly average of the cladoceran (left panel) and copepod (right panel) densities (top 1008 graphs) and biomasses (bottom graphs) in Seton Lake (grey circle) and Anderson Lake (blue 1009 triangle). Error bars represent the standard deviation. 1010 Figure 4: Total cladoceran flux (#individuals·cm-2·year-1 ×102) and total diatom flux (#valves·cm- 1011 2·year-1 ×108) for the Anderson Lake cores (black line and triangular symbols, A1 and A2) and 1012 the Seton Lake cores (grey line and squareDraft symbols, S1, S2 and S3) over the last 200 years. The 1013 blue shaded area illustrates the turbidity gradient of the two lakes (dark blue corresponding to 1014 clear water, and teal corresponding to turbid water rich in glacial flour) and indicates the timing 1015 of the construction of the diversion. 1016 Figure 5: Relative abundance (%, bar graph) and flux (#individuals·cm-2·year-1 ×102, grey shaded 1017 area) of the dominant cladoceran groups found in the cores from Seton Lake (cores S1, S2, S3) 1018 and Anderson Lake (cores A1 and A2) over the last 200 years. The dots along the Seton Lake 1019 cores correspond to the intervals where the minimum count of 20 individuals could not be 1020 reached. The red arrow corresponds to the time of establishment of the diversion.
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r oi C rv a e rp es enter R Ü
Alberta Seton Lake S3 British Columbia British ?!
S2 S1 &< Lillooet ?! &
A2 ?!
Legend Washington D'Arcy &< Monitoring Stations ! ?! Coring Sites 0 100 200 400 0 2.5 5 10 Bridge River Diversion Km Km
Draft
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May June July Aug Sept Oct 0 (m) 20 Euphotic ZoneEuphotic 6 Turbidity 40 (NTU) 3 0 6 ) 1 - L · 3 TP g μ ( 0 100 ( μ g TN · L - 1 60 ) ) 1 - 20 400 day day · 3 - m GPP · 200 mgC ( Zooplankton Zooplankton density 0 (
Draft# ind · 1000 m - 3 X 10 500 3 ) 0
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Cladocera Copepod ) 3 300 Seton Lake 1000
10 Anderson Lake x 3 - 150 500 m ⋅ Density 0 0 (#ind 600 150 ) 4 10 x 3 75 - 300 m ⋅ g Biomass μ 0 0 ( May June July Aug Sept Oct May June July Aug Sept Oct
Draft
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40 20 A1 0 40 ) 2 20 A2 10
x 0 1 - flux 10 5 S3 year year
⋅ 0 2 - 40 cm ⋅ Cladocera 20 S2 ind 0 (# 50 S1 25 0 1800 1840 1880 1920 1960 2000
100 Draft A1 0 ) 8 100
10 A2 x 0 1 - 20 year year
⋅ 10 S3 2 - 0 cm
⋅ 50 Diatom flux Diatom 25 S2
valves 0 (# 50 S1 25 0 1800 1840 1880 1920 1960 2000 Year (C.E.)
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S1 S2 S3 Bosmina spp. % Daphnia longispina Bosmina spp. % Daphnia longispina Bosmina spp. % Daphnia longispina complex % complex % complex % 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 0 50 100 ° ° ° 2000 ° ° ° ° ° ° ° ° ° ° 1960 ° ° °
) ° °
C.E 1920 Year ( Year
SetonLake 1880
1840
1800 0 25 50 0 5 10 0 10 20 0 5 10 0 5 10 0 5 A1 A2 0 50 100 0 50 100 0 50 100 0 50 100
2000
1960
1920
Year (C.E.) Year 1880 Anderson LakeAnderson 1840
1800 0 10 20 0 10 20 0 10 20 0 10 20 Flux (#ind⋅cm-2⋅year -1 x102) Draft
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