Peter Leavitt

Permanent URI for this collectionhttps://hdl.handle.net/10294/15874

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Open Access
Phytoplankton-Specific Response to Enrichment of Phosphorus-Rich Surface Waters with Ammonium, Nitrate, and Urea
(Public Library of Science, 2013-01-17) Donald, Derek B.; Bogard, Matthew J.; Finlay, Kerri; Bunting, Lynda; Leavitt, Peter R.
Supply of anthropogenic nitrogen (N) to the biosphere has tripled since 1960; however, little is known of how in situ response to N fertilisation differs among phytoplankton, whether species response varies with the chemical form of N, or how interpretation of N effects is influenced by the method of analysis (microscopy, pigment biomarkers). To address these issues, we conducted two 21-day in situ mesocosm (3140 L) experiments to quantify the species- and genus-specific responses of phytoplankton to fertilisation of P-rich lake waters with ammonium (NH 4+), nitrate (NO 32), and urea ([NH 2 ]2 CO). Phytoplankton abundance was estimated using both microscopic enumeration of cell densities and high performance liquid chromatographic (HPLC) analysis of algal pigments. We found that total algal biomass increased 200% and 350% following fertilisation with NO 32 and chemically-reduced N (NH 4+, urea), respectively, although 144 individual taxa exhibited distinctive responses to N, including compound-specific stimulation (Planktothrix agardhii and NH 4+), increased biomass with chemically-reduced N alone (Scenedesmus spp., Coelastrum astroideum) and no response (Aphanizomenon flos-aquae, Ceratium hirundinella). Principle components analyses (PCA) captured 53.2–69.9% of variation in experimental assemblages irrespective of the degree of taxonomic resolution of analysis. PCA of species-level data revealed that congeneric taxa exhibited common responses to fertilisation regimes (e.g., Microcystis aeruginosa, M. flos-aquae, M. botrys), whereas genera within the same division had widely divergent responses to added N (e.g., Anabaena, Planktothrix, Microcystis). Least-squares regression analysis demonstrated that changes in phytoplankton biomass determined by microscopy were correlated significantly (p,0.005) with variations in HPLC-derived concentrations of biomarker pigments (r2 = 0.13–0.64) from all major algal groups, although HPLC tended to underestimate the relative abundance of cyanobacteria. Together, these findings show that while fertilisation of P-rich lakes with N can increase algal biomass, there is substantial variation in responses of genera and divisions to specific chemical forms of added N.
Open Access
Pluses and minuses of ammonium and nitrate uptake and assimilation by phytoplankton and implications for productivity and community composition, with emphasis on nitrogen-enriched conditions
(Wiley, 2015-10-11) Glibert, Patricia M.; Wilkerson, Frances P.; Dugdale, Richard C.; Raven, John A.; Dupont, Christopher L.; Leavitt, Peter R.; Parker, Alexander E.; Burkholder, JoAnn M.; Kana, Todd M.
Anthropogenic activities are altering total nutrient loads to many estuaries and freshwaters, resulting in high loads not only of total nitrogen (N), but in some cases, of chemically reduced forms, notably NH1 4 . Long thought to be the preferred form of N for phytoplankton uptake, NH1 4 may actually suppress overall growth when concentrations are sufficiently high. NH1 4 has been well known to be inhibitory or repressive for NO- 3 uptake and assimilation, but the concentrations of NH1 4 that promote vs. repress NO- 3 uptake, assimi- lation, and growth in different phytoplankton groups and under different growth conditions are not well understood. Here, we review N metabolism first in a “generic” eukaryotic cell, and the contrasting metabolic pathways and regulation of NH1 4 and NO2 3 when these substrates are provided individually under equivalent growth conditions. Then the metabolic interactions of these substrates are described when both are provided together, emphasizing the cellular challenge of balancing nutrient acquisition with photosynthetic energy balance in dynamic environments. Conditions under which dissipatory pathways such as dissimilatory NO2 3 / NO2 2 reduction to NH1 4 and photorespiration that may lead to growth suppression are highlighted. While more is known about diatoms, taxon-specific differences in NH1 4 and NO2 3 metabolism that may contribute to changes in phytoplankton community composition when the composition of the N pool changes are pre- sented. These relationships have important implications for harmful algal blooms, development of nutrient criteria for management, and modeling of nutrient uptake by phytoplankton, particularly in conditions where eutrophication is increasing and the redox state of N loads is changing.
Open Access
Recording of climate and diagenesis through sedimentary DNA and fossil pigments at Laguna Potrok Aike, Argentina
(European Geosciences Union, 2016-04-27) Vuillemin, Aurèle; Ariztegui, Daniel; Leavitt, Peter R.; Bunting, Lynda; PASADO Science Team
Aquatic sediments record past climatic conditions while providing a wide range of ecological niches for microorganisms. In theory, benthic microbial community composition should depend on environmental features and geochemical conditions of surrounding sediments, as well as ontogeny of the subsurface environment as sediment degraded. In principle, DNA in sediments should be composed of ancient and extant microbial elements persisting at different degrees of preservation, although to date few studies have quantified the relative influence of each factor in regulating final composition of total sedimentary DNA assemblage. Here geomicrobiological and phylogenetic analyses of a Patagonian maar lake were used to indicate that the different sedimentary microbial assemblages derive from specific lacustrine regimes during defined climatic periods. Two climatic intervals (Mid-Holocene, 5 ka BP; Last Glacial Maximum, 25 ka BP) whose sediments harbored active microbial populations were sampled for a comparative environmental study based on fossil pigments and 16S rRNA gene sequences. The genetic assemblage recovered from the Holocene record revealed a microbial community displaying metabolic complementarities that allowed prolonged degradation of organic matter to methane. The series of Archaea identified throughout the Holocene record indicated an age-related stratification of these populations brought on by environmental selection during early diagenesis. These characteristics were associated with sediments resulting from endorheic lake conditions and stable pelagic regime, high evaporative stress and concomitant high algal productivity. In contrast, sulphate-reducing bacteria and lithotrophic Archaea were predominant in sediments dated from the Last Glacial Maximum, in which pelagic clays alternated with fine volcanic material characteristic of a lake level highstand and freshwater conditions, but reduced water column productivity. Comparison of sedimentary DNA composition with that of fossil pigments suggested that post-depositional diagenesis resulted in a rapid change in the initial nucleic acid composition and overprint of phototrophic communities by heterotrophic assemblages with preserved pigment compositions. Long DNA sequences (1400–900 bp) appeared to derive from intact bacterial cells, whereas short fragments (290–150 bp) reflected extracellular DNA accumulation in ancient sediments. We conclude that sedimentary DNA obtained from lacustrine deposits provides essential genetic information to complement paleoenvironmental indicators and trace post-depositional diagenetic processes over tens of millennia. However, it remains difficult to estimate the time lag between original deposition of lacustrine sediments and establishment of the final composition of the sedimentary DNA assemblage.
Open Access
Bias in Research Grant Evaluation Has Dire Consequences for Small Universities
(Public Library of Science, 2016-06-03) Murray, Dennis L.; Morris, Douglas; Lavoie, Claude; Leavitt, Peter R.; MacIsaac, Hugh; Masson, Michael E. J.; Villard, Marc-Andre
Federal funding for basic scientific research is the cornerstone of societal progress, economy, health and well-being. There is a direct relationship between financial investment in science and a nation’s scientific discoveries, making it a priority for governments to distribute public funding appropriately in support of the best science. However, research grant proposal success rate and funding level can be skewed toward certain groups of applicants, and such skew may be driven by systemic bias arising during grant proposal evaluation and scoring. Policies to best redress this problem are not well established. Here, we show that funding success and grant amounts for applications to Canada’s Natural Sciences and Engineering Research Council (NSERC) Discovery Grant program (2011–2014) are consistently lower for applicants from small institutions. This pattern persists across applicant experience levels, is consistent among three criteria used to score grant proposals, and therefore is interpreted as representing systemic bias targeting applicants from small institutions. When current funding success rates are projected forward, forecasts reveal that future science funding at small schools in Canada will decline precipitously in the next decade, if skews are left uncorrected. We show that a recently-adopted pilot program to bolster success by lowering standards for select applicants from small institutions will not erase funding skew, nor will several other post-evaluation corrective measures. Rather, to support objective and robust review of grant applications, it is necessary for research councils to address evaluation skew directly, by adopting procedures such as blind review of research proposals and bibliometric assessment of performance. Such measures will be important in restoring confidence in the objectivity and fairness of science funding decisions. Likewise, small institutions can improve their research success by more strongly supporting productive researchers and developing competitive graduate programming opportunities.
Open Access
Increased variability and sudden ecosystem state change in LakeWinnipeg, Canada, caused by 20thcentury agriculture
(Wiley, 2016-07-01) Bunting, L.; Leavitt, P.R.; Simpson, G.L.; Wissel, B.; Laird, K.R.; Cumming, B.F.; St. Amand, A.; Engstrom, D.R.
Eutrophication can initiate sudden ecosystem state change either by slowly pushing lakes toward a catastrophic tipping point beyond which self-reinforcing mechanisms establish an alternate stable state, or through rapid but persistent changes in external forcing mechanisms. In principle, these processes can be distinguished by determining whether historical changes in focal parameters (phytoplankton) exhibit transient (rising then declining) or continuously-elevated variability characteristic of alternate stable states or a “paradox of enrichment,” respectively. We tested this hypothesis in the south basin of Lake Winnipeg, Canada, a site with intense blooms of N2-fixing cyanobacteria since 1990, but for which little is known of earlier limnological conditions, causes of eutrophication, or whether modern conditions represent a alternate stable state. Paleolimnological analysis revealed that the basin was naturally mesotrophic (∼15–20 μg P L−1) with diazotrophic cyanobacteria, productive diatoms, and phosphorus-rich sediments. Eutrophication accelerated during ca.1900–ca.1990, when sedimentary nitrogen, phosphorus and carbon contents increased 10–50%, δ15N enriched 3–4‰, and concentrations of many fossil pigments increased 300–500%. Nearly 75% of 20th century variability was explained by concomitant increases in production of livestock and crops, but not by climate. After ca.1990, the basin exhibited a rapid threefold increase in akinetes from Aphanizomenon and Anabaena spp. and 50% declines in pigments from chlorophytes and cyanobacteria because of sudden socio-economic reorganization of agriculture. Phytoplankton variability quantified using Gaussian generalized additive models increased continuously since the onset of agriculture for bloom-forming taxa, did not decline after state change, and suggested that recovery should not be affected by stable-state hysteresis.
Open Access
Consequences of Fish Kills for Long-Term Trophic Structure in Shallow Lakes: Implications for Theory and Restoration
(Springer, 2016-07-22) Sayer, Carl D.; Davidson, Thomas A.; Rawcliffe, Ruth; Langdon, Peter G.; Leavitt, Peter R.; Cockerton, Georgina; Rose, Neil, L.; Croft, Toby
Fish kills are a common occurrence in shallow, eutrophic lakes, but their ecological consequences, especially in the long term, are poorly understood. We studied the decadal-scale response of two UK shallow lakes to fish kills using a palaeolimnological approach. Eutrophic and turbid Barningham Lake experienced two fish kills in the early 1950s and late 1970s with fish recovering after both events, whereas less eutrophic, macrophyte-dominated Wolterton Lake experienced one kill event in the early 1970s from which fish failed to recover. Our palaeo-data show fish-driven trophic cascade effects across all trophic levels (covering benthic and pelagic species) in both lakes regardless of pre-kill macrophyte coverage and trophic status. In turbid Barningham Lake, similar to long-term studies of biomanipulations in other eutrophic lakes, effects at the macrophyte level are shown to be temporary after the first kill (c. 20 years) and non-existent after the second kill. In plant-dominated Wolterton Lake, permanent fish disappearance failed to halt a long-term pattern of macrophyte community change (for example, loss of charophytes and over-wintering macrophyte species) symptomatic of eutrophication. Important implications for theory and restoration ecology arise from our study. Firstly, our data support ideas of slow eutrophication-driven change in shallow lakes where perturbations are not necessary prerequisites for macrophyte loss. Secondly, the study emphasises a key need for lake managers to reduce external nutrient loading if sustainable and long-term lake restoration is to be achieved. Our research highlights the enormous potential of multi-indicator palaeolimnology and alludes to an important need to consider potential fish kill signatures when interpreting results.
Open Access
Paleolimnological assessment of nutrient enrichment on diatom assemblages in a priori defined nitrogen- and phosphorus-limited lakes downwind of the Athabasca Oil Sands, Canada
(PAGEpress, 2017-04-14) Laird, Kathleen R.; Das, Biplob; Hesjedal, Brittany; Leavitt, Peter R.; Mushet, Graham R.; Scott, Kenneth A.; Simpson, Gavin L.; Wissel, Bjorn; Wolfe, Jared; Cumming, Brian F.
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 a priori 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 (Discostella and Cyclotella species) and pennate taxa, such as Asterionella formosa and Fragilaria crotonensis, 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.
Open Access
Effects of experimental nitrogen fertilization on planktonic metabolism and CO2 flux in a hypereutrophic hardwater lake
(Public Library of Science, 2017-12-12) Bogard, Matthew J.; Finlay, Kerri; Waiser, Marley J.; Tumber, Vijay P.; Donald, Derek B.; Wiik, Emma; Simpson, Gavin L.; del Giorgio, Paul A.; Leavitt, Peter R.
Hardwater lakes are common in human-dominated regions of the world and often experience pollution due to agricultural and urban effluent inputs of inorganic and organic nitrogen (N). Although these lakes are landscape hotspots for CO2 exchange and food web carbon (C) cycling, the effect of N enrichment on hardwater lake food web functioning and C cycling patterns remains unclear. Specifically, it is unknown if different eutrophication scenarios (e.g., modest non point vs. extreme point sources) yield consistent effects on auto- and heterotrophic C cycling, or how biotic responses interact with the inorganic C system to shape responses of air-water CO2 exchange. To address this uncertainty, we induced large metabolic gradients in the plankton community of a hypereutrophic hardwater Canadian prairie lake by adding N as urea (the most widely applied agricultural fertilizer) at loading rates of 0, 1, 3, 8 or 18 mg N L-1 week-1 to 3240-L, in-situ mesocosms. Over three separate 21-day experiments, all treatments of N dramatically increased phytoplankton biomass and gross primary production (GPP) two- to six-fold, but the effects of N on autotrophs plateaued at ~3 mg N L-1. Conversely, heterotrophic metabolism increased linearly with N fertilization over the full treatment range. In nearly all cases, N enhanced net planktonic uptake of dissolved inorganic carbon (DIC), and increased the rate of CO2 influx, while planktonic heterotrophy and CO2 production only occurred in the highest N treatments late in each experiment, and even in these cases, enclosures continued to in-gas CO2. Chemical effects on CO2 through calcite precipitation were also observed, but similarly did not change the direction of net CO2 flux. Taken together, these results demonstrate that atmospheric exchange of CO2 in eutrophic hardwater lakes remains sensitive to increasing N loading and eutrophication, and that even modest levels of N pollution are capable of enhancing autotrophy and CO2 in-gassing in P-rich lake ecosystem
Open Access
Regional versus local drivers of water quality in the Windermere catchment, Lake District, United Kingdom: The dominant influence of wastewater pollution over the past 200 years
(Wiley, 2018-05-10) Moorhouse, Heather L.; McGowan, Suzanne; Taranu, Zofia E.; Gregory-Eaves, Irene; Leavitt, Peter R.; Jones, Matthew D.; Barker, Philip; Brayshaw, Susan A.
Freshwater ecosystems are threatened by multiple anthropogenic stressors acting over different spatial and temporal scales, resulting in toxic algal blooms, reduced water quality and hypoxia. However, while catchment characteristics act as a ‘filter’ modifying lake response to disturbance, little is known of the relative importance of different drivers and possible differentiation in the response of upland remote lakes in comparison to lowland, impacted lakes. Moreover, many studies have focussed on single lakes rather than looking at responses across a set of individual, yet connected lake basins. Here we used sedimentary algal pigments as an index of changes in primary producer assemblages over the last ~200 years in a northern temperate watershed consisting of 11 upland and lowland lakes within the Lake District, United Kingdom, to test our hypotheses about landscape drivers. Specifically, we expected that the magnitude of change in phototrophic assemblages would be greatest in lowland rather than upland lakes due to more intensive human activities in the watersheds of the former (agriculture, urbanization). Regional parameters, such as climate dynamics, would be the predominant factors regulating lake primary producers in remote upland lakes and thus, synchronize the dynamic of primary producer assemblages in these basins. We found broad support for the hypotheses pertaining to lowland sites as wastewater treatment was the main predictor of changes to primary producer assemblages in lowland lakes. In contrast, upland headwaters responded weakly to variation in atmospheric temperature, and dynamics in primary producers across upland lakes were asynchronous. Collectively, these findings show that nutrient inputs from point sources overwhelm climatic controls of algae and nuisance cyanobacteria, but highlights that large-scale stressors do not always initiate coherent regional lake response. Furthermore, a lake's position in its landscape, its connectivity and proximity to point nutrients are important determinants of changes in production and composition of phototrophic assemblages.
Open Access
Spatial and temporal variation in nitrogen fixation and its importance to phytoplankton in phosphorus-rich lakes
(Wiley, 2018-11-27) Hayes, Nicole M.; Patoine, Alain; Haig, Heather A.; Simpson, Gavin L.; Swarbrick, Vanessa J.; Wiik, Emma; Leavitt, Peter R.
1. Limnological theory posits that phosphorus (P) limits primary production in freshwater lakes, in part because fixation of atmospheric nitrogen (N2) can compensate for limitations in nitrogen (N) supply to phytoplankton. However, quantitative estimates of the degree to which N2 fixation satisfies planktonic N demand are rare. 2. Here we used biweekly sampling during summer in seven lakes over 2 decades to estimate both planktonic N2 fixation and phytoplankton N demand. We further assessed the ability of biologically fixed N to satisfy N needs of primary producers in productive hardwater lakes. 3. Phytoplankton N requirements, derived from estimates of phytoplankton productivity and N content, were moderately synchronous (S = 0.41) among lakes (ca. 0.1–9.2 mg N m–3 hr–1). In contrast, rates of N2 fixation determined using isotopic natural abundance method (NAM; 0.002–3.2 mg N m–3 hr–1), or heterocyte-based calculations (0.10–1.78 mg N m–3 hr–1), varied asynchronously (SNAM = –0.03 and SHeterocyte = –0.11) among basins, accounted for a median of 3.5% (mean 11.3% ± 21.6) of phytoplankton demand, and were correlated to the abundance of Nostocales cyanobacteria when analysed using generalised additive models. 4. Overall, the total mass of fixed N accounted for a median of only 3.0% of the spring standing stock of total dissolved N in study lakes (mean 7.5 ± 12.1%), with higher relative importance of fixed N in highly productive downstream lakes. Thus, while fixed N helps sustain primary productivity, particularly in years with high rates of N2-fixation, it does not appear to eliminate N limitation of phytoplankton growth in these P-rich hardwater lakes.
Open Access
Differential stimulation and suppression of phytoplankton growth by ammonium enrichment in eutrophic hardwater lakes over 16 years
(Wiley, 2018-12-07) Swarbrick, Vanessa J.; Simpson, Gavin L.; Glibert, Patricia M.; Leavitt, Peter R.
Previous research suggests that fertilization of surface waters with chemically reduced nitrogen (N), including ammonium (NH4+), may either enhance or suppress phytoplankton growth. To identify the factors influencing the net effect of NH4+, we fertilized natural phytoplankton assemblages from two eutrophic hardwater lakes with growth-saturating concentrations of NH4Cl in 241 incubation experiments conducted biweekly May–August during 1996–2011. Phytoplankton biomass (as chlorophyll a) was significantly (p < 0.05) altered in fertilized trials relative to controls after 72 h in 44.8% of experiments, with a marked rise in both spring suppression and summer stimulation of assemblages over 16 yr, as revealed by generalized additive models (GAMs). Binomial GAMs were used to compare contemporaneous changes in physico-chemical (temperature, Secchi depth, pH, nutrients; 19.5% deviance explained) and biological parameters (phytoplankton community composition; 40.0% deviance explained) to results from fertilization experiments. Models revealed that that the likelihood of growth suppression by NH4+ increased with abundance of diatoms, cryptophytes, and unicellular cyanobacteria, particularly when water temperatures and soluble reactive phosphorus (SRP) concentrations were low. In contrast, phytoplankton was often stimulated by NH4+ when chlorophytes and non-N2-fixing cyanobacteria were abundant, and temperatures and SRP concentrations were high. Progressive intensification of NH4+ effects over 16 yr reflects changes in both spring (cooler water, increased diatoms and cryptophytes) and summer lake conditions (more chlorophytes, earlier cyanobacteria blooms), suggesting that the seasonal effects of NH4+ will vary with future climate change and modes of N enrichment.
Open Access
Widespread nitrous oxide undersaturation in farm waterbodies creates an unexpected greenhouse gas sink
(National Academy of Sciences, 2019-05-14) Webb, Jackie R.; Hayes, Nicole M.; Simpson, Gavin L.; Leavitt, Peter R.; Baulch, Helen M.; Finlay, Kerri
Nitrogen pollution and global eutrophication are predicted to increase nitrous oxide (N2O) emissions from freshwater ecosystems. Surface waters within agricultural landscapes experience the full impact of these pressures and can contribute substantially to total landscape N2O emissions. However, N2O measurements to date have focused on flowing waters. Small artificial waterbodies remain greatly understudied in the context of agricultural N2O emissions. This study provides a regional analysis of N2O measurements in small (<0.01 km2) artificial reservoirs, of which an estimated 16 million exist globally. We show that 67% of reservoirs were N2O sinks (−12 to −2 μmol N2O⋅m−2⋅d−1) in Canada’s largest agricultural area, despite their highly eutrophic status [99 ± 289 µg⋅L−1 chlorophyll-a (Chl-a)]. Generalized additive models indicated that in situ N2O concentrations were strongly and nonlinearly related to stratification strength and dissolved inorganic nitrogen content, with the lowest N2O levels under conditions of strong water column stability and high algal biomass. Predicted fluxes from previously published models based on lakes, reservoirs, and agricultural waters overestimated measured fluxes on average by 7- to 33-fold, challenging the widely held view that eutrophic N-enriched waters are sources of N2O.
Open Access
Bottom-Up Forces Drive Increases in the Abundance of Large Daphnids in Four Small Lakes Stocked with Rainbow Trout (Oncorhynchus mykiss), Interior British Columbia, Canada
(Springer, 2019-09-23) Mushet, Graham R.; Laird, Kathleen R.; Leavitt, Peter R.; Maricle, Stephen; Klassen, Andrew; Cumming, Brian F.
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.
Open Access
Regulation of carbon dioxide and methane in small agricultural reservoirs: optimizing potential for greenhouse gas uptake
(Copernicus Publications, 2019-11-08) Webb, Jackie R.; Leavitt, Peter R.; Simpson, Gavin L.; Baulch, Helen M.; Haig, Heather A.; Hodder, Kyle R.; Finlay, Kerri
Small farm reservoirs are abundant in many agricultural regions across the globe and have the potential to be large contributing sources of carbon dioxide (CO2) and methane (CH4) to agricultural landscapes. Compared to natural ponds, these artificial waterbodies remain overlooked in both agricultural greenhouse gas (GHG) inventories and inland water global carbon (C) budgets. Improved understanding of the environmental controls of C emissions from farm reservoirs is required to address and manage their potential importance in agricultural GHG budgets. Here, we conducted a regional-scale survey (∼ 235 000 km2) to measure CO2 and CH4 surface concentrations and diffusive fluxes across 101 small farm reservoirs in Canada's largest agricultural area. A combination of abiotic, biotic, hydromorphologic, and landscape variables were modelled using generalized additive models (GAMs) to identify regulatory mechanisms. We found that CO2 concentration was estimated by a combination of internal metabolism and groundwater-derived alkalinity (66.5 % deviance explained), while multiple lines of evidence support a positive association between eutrophication and CH4 production (74.1 % deviance explained). Fluxes ranged from −21 to 466 and 0.14 to 92 mmol m−2 d−1 for CO2 and CH4, respectively, with CH4 contributing an average of 74 % of CO2-equivalent (CO2-e) emissions based on a 100-year radiative forcing. Approximately 8 % of farm reservoirs were found to be net CO2-e sinks. From our models, we show that the GHG impact of farm reservoirs can be greatly minimized with overall improvements in water quality and consideration to position and hydrology within the landscape.
Open Access
Comparison of isotopic mass balance and instrumental techniques as estimates of basin hydrology in seven connected lakes over 12 years
(Elsevier, 2019-11-22) Haig, H.A.; Hayes, N.M.; Simpson, G.L.; Yi, Y.; Wissel, B.; Hodder, K.R.; Leavitt, P.R.
Mass-balance models using stable isotopes of hydrogen and oxygen provide useful estimates of the water balance of lakes, particularly in the absence of instrumental data. However, isotopic mass balances are rarely compared directly to measured water fluxes. Here we compared instrumental and isotope-based determinations of water fluxes in seven connected lakes over 12 years to quantify how agreement between the two approaches is affected by lake type and its position in the landscape. Overall, lake-specific ratios of evaporation to inflow (E/I) from instrumental measurements (median, x̃ = 0.06, median absolute deviation, MAD = 0.06) agreed well with isotopic estimates using headwater models (x̃ = 0.14, MAD = 0.08), with the exception of one lake with limited channelized inflow of surface waters (x̃ instrumental = 0.51 vs. x̃ headwater = 0.24). Isotope-instrument agreement improved (x̃ = 0.09 vs. x̃ = 0.03) when basin-specific (‘best-fit’) isotope models also considered local con- nectivity to upstream water bodies. Comparison among years revealed that mean isotopic E/I values were lowest in 2011 (mean, μ = 0.06, standard deviation, σ = 0.09) during a 1-in-140 year spring flood, and highest during a relatively arid year, 2003 (μ = 0.22, σ = 0.19), while interannual variability in E/I generally increased with distance downstream along the mainstem of the watershed. Similar patterns of agreement between methods were recorded for water-residence time. Isotope models also documented the expected low water yield from lake catchments (μ = 36.2 mm yr−1 , σ = 62.3) suggesting that isotope models based on late-summer samples integrate annual inputs from various sources that are difficult to measure with conventional methods. Overall, the strong positive agreement between methods confirms that water isotopes can provide substantial insights into landscape patterns of lake hydrology, even in ungauged systems.
Open Access
Paleo-environmental evidence of ecosystem change in Lake St. Clair region of Laurentian Great Lakes basin: contrasting responses to land-use change and invasive mussels
(Springer, 2020-01-23) Baustian, Melissa M.; Brooks, Yolanda M.; Baskaran, Mark; Leavitt, Peter R.; Liu, Bo; Ostrom, Nathaniel; Stevenson, R. Jan; Rose, Joan B.
The Laurentian Great Lakes have been subject to substantial modification from diverse anthropogenic stressors, including nutrient enrichment, climate change, chemical and biological pollutants, and invasive species, yet little is known of the relative historical influence of these factors. Here we analyze diverse fossil metrics from vibracores at two sites, a bay area (Anchor Bay) and a tributary (Clinton River) in the Lake St. Clair ecosystem to determine the ecological responses from land-use practices and invasive mussel invasions. Sediment cores spanning over 100 years indicated that the expansion of non-native Dreissena polymorpha and Dreissena rostriformis (dreissenid mussels) into Anchor Bay site after the mid-1990s was associated with ~ 60 to 95% reduction in algal and cyanobacterial abundances and twofold increase in sedimentary organic matter (SOM) and bioavailable phosphorus. These increases in SOM and bioavailable phosphorus are relatively similar to increases inferred from the late nineteenth century when large portions of the watershed were cleared and drained for agriculture. In contrast, the Clinton River site experienced a continuous increase in the influx of nutrients, organic matter, and elevated sedimentary phototrophic pigments during the twentieth century and into the twenty-first century. Site comparisons suggest different mechanisms inducing changes in primary production varied, where Anchor Bay was mainly affected by the comparatively recent (since ca. mid-1990s) endogenous influence of invasive species, while the Clinton River site was primarily influenced by the input of exogenous anthropogenic nutrients over the past 100 years. These new findings illustrate that watershed management and policies within large lakes with multi-jurisdictional (national) Area of Concerns should consider site-specific regulatory mechanisms.
Open Access
“New” cyanobacterial blooms are not new: two centuries of lake production are related to ice cover and land use
(Wiley, 2020-06-17) Ewing, Holly A.; Weathers, Kathleen C.; Cottingham, Kathryn L.; Leavitt, Peter R.; Greer, Meredith L.; Carey, Cayelan C.; Steele, Bethel G.; Fiorillo, Alyeska U.; Sowles, John P.
Recent cyanobacterial blooms in otherwise unproductive lakes may be warning signs of impending eutrophication in lakes important for recreation and drinking water, but little is known of their historical precedence or mechanisms of regulation. Here, we examined long-term sedimentary records of both general and taxon-specific trophic proxies from seven lakes of varying productivity in the northeastern United States to investigate their relationship to historical in-lake, watershed, and climatic drivers of trophic status. Analysis of fossil pigments (carotenoids and chlorophylls) revealed variable patterns of past primary production across lakes over two centuries despite broadly similar changes in regional climate and land use. Sediment abundance of the cyanobacterium Gloeotrichia, a large, toxic, nitrogen-fixing taxon common in recent blooms in this region, revealed that this was not a new taxon in the phytoplankton communities but rather had been present for centuries. Histories of Gloeotrichia abundance differed strikingly across lakes and were not consistently associated with most other sediment proxies of trophic status. Changes in ice cover most often coincided with changes in fossil pigments, and changes in watershed land use were often related to changes in Gloeotrichia abundance, although no single climatic or land-use factor was associated with proxy changes across all seven lakes. The degree to which changes in lake sediment records co-occurred with changes in the timing of ice-out or agricultural land use was negatively correlated with the ratio of watershed area to lake area. Thus, both climate and land management appeared to play key roles in regulation of primary production in these lakes, although the manner in which these factors influenced lakes was mediated by catchment morphometry. Improved understanding of the past interactions between climate change, land use, landscape setting, and water quality underscores the complexity of mechanisms regulating lake and cyanobacterial production and highlights the necessity of considering these interactions—rather than searching for a singular mechanism—when evaluating the causes of ongoing changes in low-nutrient lakes.
Open Access
Effects of lake warming on the seasonal risk of toxic cyanobacteria exposure
(Wiley, 2020-06-18) Hayes, Nicole M.; Haig, Heather A.; Simpson, Gavin L.; Leavitt, Peter R.
Incidence of elevated harmful algal blooms and concentrations of microcystin are increasing globally as a result of human-mediated changes in land use and climate. However, few studies document changes in the seasonal and interannual concentrations of microcystin in lakes. Here, we modeled 11 yr of biweekly microcystin data from six lakes to characterize the seasonal patterns in microcystin concentration and to ascertain if there were pronounced changes in the patterns of potential human exposure to microcystin in lakes of central North America. Bayesian time series analysis with generalized additive models found evidence for a regional increase in microcystin maxima and duration but recorded high variation among lakes. During the past decade, warmer temperatures, but not nutrient levels, led to a marked increase in the number of days when concentrations exceeded drinking and recreational water thresholds set by the World Health Organization and United States Environmental Protection Agency.
Open Access
Seasonal variation in effects of urea and phosphorus on phytoplankton abundance and community composition in a hypereutrophic hardwater lake
(Wiley, 2020-08-17) Swarbrick, Vanessa J.; Quiñones-Rivera, Zoraida J.; Leavitt, Peter R.
1. Urea accounts for half of global agricultural fertiliser applications, yet little is known of its role in eutrophication of freshwater ecosystems, nor how it interacts with phosphorus (P) in regulating phytoplankton composition, especially during spring and autumn. 2. To identify when and how urea and P inputs interact across the ice-free period, we conducted seven monthly fertilisation experiments in 3,240-L mesocosms from ice-off to ice-formation in a hypereutrophic lake. In addition, we ran bioassays with ammonium (NH 4 +) to compare the effects of urea with those of NH 4 +, the immediate product of chemical decomposition of urea. 3. Analysis of water-column chlorophyll a and biomarker pigments by high-perfor- mance liquid chromatography revealed that addition of inorganic P alone (100 μg P L–1 week–1) had no significant impact on either algal abundance or community composition in hypereutrophic Wascana Lake. Instead, fertilisation with urea (4 mg N L−1 week–1) alone, or in concert with P, significantly (p < 0.05) increased algal abundance in spring and much of summer, but not prior to ice formation in October. In particular, urea amendment enhanced abundance of cryptophytes, chlorophytes, and non-diazotrophic cyanobacteria during April and May, while fertilisation in summer and early autumn (September) increased only chlorophytes and non-diazotrophic cyanobacteria. 4. Comparison of urea mesocosms with NH 4 + bioassays demonstrated that urea lacked the inherent toxicity of NH 4 + in cool waters, but that both compounds stimulated production during summer experiments. 5. This study showed that urea pollution can degrade water quality in P-rich lakes across a variety of seasonal conditions, including spring, and underscores the im- portance of quantifying the timing and form of N inputs when managing P-rich freshwaters.
Open Access
Changes in coupled carbon‒nitrogen dynamics in a tundra ecosystem predate post-1950 regional warming
(Nature Research, 2020-10-28) Anderson, N. John; Engstrom, Daniel R.; Leavitt, Peter R.; Flood, Sarah M.; Heathcote, Adam J.
Arctic ecosystems are changing in response to recent rapid warming, but the synergistic effects of other environmental drivers, such as moisture and atmospheric nitrogen (N) deposition, are difficult to discern due to limited monitoring records. Here we use geo- chemical analyses of 210 Pb-dated lake-sediment cores from the North Slope of Alaska to show that changes in landscape nutrient dynamics started over 130 years ago. Lake carbon burial doubled between 1880 and the late-1990s, while current rates (~10 g C m−2 yr−1) represent about half the CO2 emission rate for tundra lakes. Lake C burial reflects increased aquatic production, stimulated initially by nutrients from terrestrial ecosystems due to late- 19 th century moisture-driven changes in soil microbial processes and, more recently, by atmospheric reactive N deposition. These results highlight the integrated response of Arctic carbon cycling to global environmental stressors and the degree to which C–N linkages were altered prior to post-1950 regional warming.

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