Kerri Finlay

Phytoplankton-Specific Response to Enrichment of Phosphorus-Rich Surface Waters with Ammonium, Nitrate, and Urea

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 (NH4 +), nitrate (NO3 −), and urea ([NH2]2CO). 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 NO3 − and chemically-reduced N (NH4 +, urea), respectively, although 144 individual taxa exhibited distinctive responses to N, including compound-specific stimulation (Planktothrix agardhii and NH4 +), 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 (r 2 = 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.

Daphnia control of the spring clear-water phase in six polymictic lakes of varying productivity and size

Phytoplankton biomass and community composition are regulated by interactions between light, nutrients, physical and chemical conditions (e.g., stratification, pH), and biotic mechanisms (e.g., herbivory, competition), although the relative strength of individual regulatory processes varies substantially among seasons, years, and lakes (VANNI & TEMTE 1990). Often phytoplankton communities exhibit a predictable temporal sequence of change in species abundance and composition (LAMPERT & SCHOBER 1978), as has been described in detail by the Plankton Ecology Group (SOMMER et al. 1986). In particular, development of the spring clear-water phase (CWP) is a characteristic event in the plankton phenology of many dimictic lakes within temperate regions. This sudden transition from turbid to clear water has been attributed to different causes, including elevated grazing by zooplankton (LAMPERT et al. 1986, TONNO et al. 2003), reductions in available nutrients during spring (HUPPERT et al. 2002), reduced turbulence and algal suspension following thermal stratification (REYNOLDS 1973), and increased suppression of low light adapted vernal taxa (ROMO & MIRACLE 1995). Similarly, it has proven difficult to evaluate whether subsequent collapses of herbivore populations are caused by food depletion and starvation (SHEI et al. 1988) or whether intense predation by fish (LUECKE et al. 1990) contributes to population crashes. Improved understanding of the controls of plankton succession during CWP development is essential to predict how plankton dynamics might be affected by climate warming in the future. We measured seasonal changes in algal and invertebrate abundance in 6 lakes for 12 years to quantify the role of herbivory, particularly by Daphnia, in regulating the onset and intensity of the CWP in productive lakes that lack pronounced thermal stratification. Further, we quantified Daphnia gut contents during 2006 to determine whether selective feeding by Daphnia contributed to increased water clarity. Although Daphnia are known to be important controls of CWP development in mesotrophic systems (ELSER & GOLDMAN 1990, JEPPESEN et al. 2003), the role of these grazers in more productive lakes is controversial and uncertain (DEGANS & DE MEESTER 2002).

An ecosystem management framework to maintain water quality in a macrophyte-dominated, productive, shallow reservoir

Loch Leven, SK, is a well-oxygenated, highly productive, clear-water lake dominated by a nuisance species of submergent macrophyte (Elodea canadensis Michaux) whose removal has been suggested to enhance recreational use. Previous empirical and theoretical work, however, has suggested that macrophytes offer an important ecosystem service in such lakes by sequestering nutrients, anchoring sediments, and providing shelter for filter feeding consumers. Macrophyte removal would introduce a risk of shifting the ecosystem to a less desirable turbid state, potentially dominated by toxic planktonic algal species. Here, we present the results of a contemporary (2014) spatio-temporal field survey of Loch Leven, which showed high water quality along several axes of assessment. Results are discussed within the context of historical sedimentary analysis, which indicated increases in algal biomass since 1980. Given the high potential for increases in planktonic biomass should Elodea be harvested, we propose only targeted macrophyte management for Loch Leven, and that large-scale Elodea removal programs would have to be paired with dredging of lake sediments to remove the source of internal nutrient loading. We further suggest that a long-term monitoring program be initiated to allow continued assessment of water quality in this shallow, macrophyte-dominated lake.

Effects of experimental nitrogen fertilization on planktonic metabolism and CO2 flux in a hypereutrophic hardwater lake

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 ecosystems.

Spatial and temporal synchrony of pCO2 in six hardwater lakes of central Canada

Water-column carbon dioxide (e02) concentrations are an important indicator of productivity and the relative importance of allochthonous and autochthonous carbon (C) sources to aquatic food webs. Furthermore, the partial pressure of e02 (pe02) determines whether a lake acts as a source or sink o f this greenhouse gas to the atmosphere. In softwater lakes, seasonal and interannual changes in pe02 arise mainly from variation in rates o f primary production (P) and ecosystem respiration (R), factors strongly influenced by subsidies o f inorganic nutrients an d organic material from the catchment (PRAIRIE et al. 2002, HANSON et al. 2003). In many instances, R exceeds P such that these lakes are heterotrophic, support food webs with subsidies of allochthonous e, are supersaturated with e02 (pe02.1ake > pe02.atm), and potentially release e02 to the atmosphere (HANSON et al. 2004). Although there is increasing evidence ofphysical and chemical drivers ofe02 concentration at the decadal scale in softwater systems (e.g., me loading from the watershed, HANSON et al. 2006; climate, KELLY et al. 200 l), lake metabolism is generally considered the primary driver of changes in pe02 in lakes. In contrast, relatively little is known o f the controls of temporal and spatial variability of the eo2 content of alkaline, saline, or hardwater lakes, despite recognition that both biological and chemical processes that regulate pe02 are more intense in these lakes (WANNINKHOF & KNOX 1996, MYRBO & SHAPLEY 2006). Hardwater lakes differ substantially from softwater systems in terms of productivity, pH, and alkalinity (WETZEL 200 l), all factors that influence pe02 concentration (Fig. l). For example, chemical equilibration at high pH (>8) results in low concentrations of e02 relative to those of He03and eo3 Zan d can lead to substantial ingassing o f atmospheric carbon (SCHINDLER et al. 1997, BADE & COLE 2006). Additionally, elevated nutrient influx and primary production can lead to depletion of water column eo2 by algae. Unfortunately, as yet relatively little is known ofhow seasonal variation in chemical conditions (e.g., pH, Die, eae03 precipitation) might alter net e fluxes. In this study, we measured seasonal and interannual changes in pe02 of 6 hardwater prairie lakes over a 12-year period to: (l) quantify the spatial and temporal variability o f potential e02 fluxes; (2) estimate the temporal coherence of pe02 among regionallakes; and (3) identify the main correlates of observed variance in eo2 content. Given that similar saline hardwater lakes account for ~so% of the total volume of inland waters (HAMMER 1986), improved understanding of the mechanisms regulating e flux in these systems is an important first-step to quantifying the ro le of inland waters in the global e budget (eoLE et al. 2007).

Generalized Additive Models of Climatic and Metabolic Controls of Subannual Variation in pCO2 in Productive Hardwater Lakes

Spatio-temporal variation in climate and weather, allochthonous carbon loads, and autochthonous factors such as lake metabolism (photosynthesis and respiration) interact to regulate atmospheric CO2 exchange of lakes. Understanding this interplay in diverse basin types at different timescales is required to adequately place lakes into the global carbon cycle, and predict CO2 flux through space and time. We analyzed 18 years of data from seven moderately hard lakes in an agricultural prairie landscape in central Canada. We applied generalized additive models and sensitivity analyses to evaluate the roles of metabolic and climatic drivers in regulating CO2 flux at the intra-annual scale. In all basins, at mean conditions with respect to other predictors, metabolic controls resulted in uptake of atmospheric CO2 when surface waters exhibited moderate primary production, but released CO2 only when primary production was very low (5 − 13 μg L−1) or when dissolved nitrogen was elevated (>2000 μg L−1), implying that respiratory controls offset photosynthetic CO2 uptake under these conditions. Climatically, dry conditions increased the likelihood of ingassing, likely due to evaporative concentration of base cations and/or reduced allochthonous carbon loads. While more research is required to establish the relative importance Pavillon des sciences biologiques (SB), Université du Québec à Montréal, Montréal (Québec ), H2X 1Y4, Canada c ©2018 American Geophysical Union. All Rights Reserved. of climate and metabolism at other time scales (diel, autumn/winter), we conclude that these hard fresh waters characteristic of continental interiors are mainly affected by metabolic drivers of pCO2 at daily-monthly timescales, are climatically controlled at interannual intervals, and are more likely to ingas CO2 for a given level of algal abundance, than are softwater, boreal ecosystems. Keypoints: • In Canadian hardwater prairie lakes, calculated CO2 fluxes correlate mostly with pH, not DIC • Intra-annual CO2 correlates with algal abundance (-CO2) and prolonged clearwater phases (+CO2) • CO2 influx increases with drier weather conditions, and is reduced with extreme N loading c ©2018 American Geophysical Union. All Rights Reserved.