The introduction of salmonids into lakes of western North America for sport fishing is a widespread phenomenon. While numerous investigations have documented cascading trophic interactions upon the introduction of fish into naturally fishless systems, little research has been done to investigate the importance of natural fish status (fishless vs. fish bearing) in modulating historical food web response to dual forcing by bottom-up (resource regulation from nutrients) and top-down (planktivory from stocked fish) processes. We used the paleolimnological record to reconstruct food web changes in four lakes in interior British Columbia that have been stocked with rainbow trout since the early to mid-1900s. Analysis of pigments, diatoms, and Cladocera was undertaken in cores from all lakes. We predicted that if fish were important in structuring cladoceran abundance and composition, we would document a decline in the abundance of large daphnids post-stocking in our two naturally fishless lakes, and little change in the two fish-bearing lakes. Instead, we documented increased abundance of large daphnids after stocking in all lakes in the early to mid-1900s, a finding inconsistent with size-selective predation from planktivorous fish. Further, our data suggest that deep, low-oxygen refugia may be important in sustaining populations of large Daphnia, a process which was enhanced by increased nutrients and lake production according to sub-fossil diatom and pigment analyses. This study shows that fish stocking does not invariably result in a decrease in large-bodied Cladocera and that nutrients and lake type can modulate the response of invertebrate planktivores.

Several limnological and paleolimnological investigations have linked enhanced atmospheric nitrogen (N) deposition to nutrient enrichment and potentially increased primary production. The Athabasca Oil Sands Region (AOSR) in northeast Alberta is a significant regional source of N emissions, particularly since development intensified during the 1990's, and recent paleolimnological investigations have provided some evidence of increased lake production in the AOSR. However, the AOSR has also experienced atmospheric warming and hydrological change since ca 1900 AD, and therefore the relative effects of nutrient deposition and climatic mechanisms on lake production remain unclear. To address this problem, a factorial-designed paleolimnological assessment of 16 lakes in northwest Saskatchewan was undertaken to quantify changes in abundance and species composition of scaled chrysophytes over the past 100 years. Study sites included both N-limited and P-limited lakes within control regions as well as those receiving enhanced N deposition from the AOSR. We hypothesized that a change in algal communities within N-limited AOSR-impacted lakes, without concurrent changes in the other lake groups, would suggest AOSR-derived N as a driver of enhanced primary production. Instead, recent and marked increases in concentrations of scaled chrysophytes (mainly Mallomonas crassisquama) occurred in the recent sediments in cores from all four lake groups (N v. P limited, impacted v. control), suggesting that regional climate changes rather than N deposition was the paramount process enhancing chrysophyte production. However, because chrysophyte abundances tended to be higher in deep, lower pH lakes, and chrysophyte time series were fit best by lake-specific generalized additive models (GAMs), we infer that climate effects may have additionally been meditated by catchment and/or lake-specific processes.

Keywords: stable isotopes, invertebrate subfossils, sedimentary organic matter, energy and nutrient pathways, boreal lakes Declarations of interest: none 3 4 5 6 7 2 Abstract 8 Carbon and nitrogen stable isotope ratios of sub-fossil invertebrate remains are 9 potentially powerful indicators of nutrient flux, habitat-specific resource utilization, and trophic 10 interactions in lentic food webs, but are rarely estimated for multiple species within lakes. Here 11 we examined historical time series of δ 13 C and δ 15 N in remains of individual invertebrate taxa 12 representing pelagic, littoral and benthic habitats during the 20 th century in five boreal lakes of 13 central Canada. We applied a novel statistical approach based on Generalized Additive Models 14 (GAMs) to quantify the differences in centennial means and trends (i) between invertebrate 15 remains and sedimentary organic matter (SOM), and (ii) among different taxa within each lake to 16 evaluate the coherence of isotope signals during the 20 th century. Differences in mean δ 13 C and 17 δ 15 N were usually significant (p < 0.05) between SOM and invertebrate taxa, and among 18 individual taxa, reflecting selective feeding by invertebrates and differences in trophic position 19 within food webs. In contrast, patterns of historical variance in isotope values varied among 20 lakes with few consistent differences between long-term isotopic trends of SOM and invertebrate 21 remains. In particular, SOM and invertebrate isotopic trends were similar in relatively 22 dystrophic lakes, likely due to the importance of terrestrial carbon in both SOM and invertebrate 23 diets. However, significant SOM-invertebrate trend differences were observed for both δ 13 C and 24 δ 15 N in relatively clear-water lakes, possibly reflecting temporal variation in diets or tissue 25 fractionation. Comparison of historical trends in isotope values among taxa revealed few 26 consistent patterns, likely indicating uncoupled carbon and nitrogen fluxes through invertebrate 27consumers with different habitat specializations or feeding modes. Together, our findings 28 suggest that evaluation of taxon-specific δ 13 C and δ 15 N can provide valuable insights into 29 historical tropho-dynamics in lake food webs. 30

<p>As the industrial footprint of the Athabasca Oil Sands Region (AOSR) continues to expand, concern about the potential impacts of pollutants on the surrounding terrestrial and aquatic ecosystems need to be assessed. An emerging issue is whether recent increases in lake production downwind of the development can be linked to AOSR activities, and/or whether changing climatic conditions are influencing lake nutrient status. To decipher the importance of pollutants, particularly atmospheric deposition of reactive nitrogen (Nr), and the effects of climate change as potential sources of increasing lake production, lakes from both within and outside of the nitrogen deposition zone were analyzed for historical changes in diatom assemblages. Lake sediment cores were collected from <em>a priori</em> defined nitrogen (N) - and phosphorus (P) - limited lakes within and outside the N plume associated with the AOSR. Diatom assemblages were quantified at sub-decadal resolution since ca. 1890 to compare conditions prior to oil sands expansion and regional climate warming, to the more recent conditions in each group of lakes (Reference and Impacted, N- and P-limited lakes). Analyses of changes in assemblage similarity and species turnover indicates that changes in diatom assemblages were minimal both within and across all lake groups.  Small changes in percent composition of planktonic taxa, particularly small centric taxa (<em>Discostella </em>and <em>Cyclotella</em> species) and pennate taxa, such as <em>Asterionella formosa</em> and <em>Fragilaria crotonensis</em>, occurred in some of the lakes. While these changes were consistent with potential climate effects on algal growth, water column stability and other factors; the timing and direction of biotic changes were variable among sites suggesting that any apparent response to climate was lake dependent. The absence of a consistent pattern of diatom changes associated with receipt of reactive nitrogen or intrinsic nutrient-limitation status of the lake suggest that downwind AOSR emissions had no demonstrable effect on diatom composition. </p>

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