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

Abstract

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.

Large‐scale biogeography and environmental regulation of methanotrophic bacteria across boreal inland waters

“…It is interesting that TN, and not TP, emerged as the strongest proxy of trophic status when considering the entire methanotrophic community. This result in no way diminishes the role of P in determining aquatic productivity, but underlines the importance of N in these boreal inland waters, as has been highlighted in a recent study (Bogard et al, ). Despite the fact that those environmental factors were selected statistically, their explanatory power remained generally low in both the partial Mantel and MRT analyses.…”

Inorganic carbon promotes photosynthesis, growth, and maximum biomass of phytoplankton in eutrophic water bodies

“…Light availability directly regulates phytoplankton photosynthesis and subsequent growth and is determined by incident solar irradiance, vertical light attenuation and mixing depth of the water column (Krause‐Jensen & Sand‐Jensen, ; Wofsy, ). Availability of nitrogen and phosphorus frequently limits formation of essential structural and catalytic cell pools and, thereby, influences rates of photosynthesis and growth as well as the maximum attainable biomass (Bogard et al., ; Phillips et al., ; Spijkerman, ).…”

“…However, it was never tested whether supplementary addition of DIC could have increased the biomass even further. Recent laboratory and modelling work (Verspagen, van de Waal, Finke, Visser, & Huisman, ; Verspagen, van de Waal, Finke, Visser, Van Donk et al., ) as well as mesocosm studies (Bogard et al., ; Low‐Décarie, Bell, & Fussmann, ) have shown that greater DIC supply stimulates short‐term phytoplankton biomass development and productivity in eutrophic or eutrophied lakes of low‐medium DIC content. Moreover, cyanobacterial surface blooms are stimulated by higher atmospheric CO 2 and water DIC concentrations (Ibelings & Maberly, ).…”

“…Ca 2+ + 2 → CaCO 3 (precipitation) + CO 2 (assimilation); McConnaughey et al., ; McConnaughey & Whelan, ). Photosynthetic use of CO 2 and , uncoupled to direct phototrophic calcification, may drive pH up in the free water, cause CaCO 3 precipitation with a resulting pH decrease and conversion to higher CO 2 concentrations (Bogard et al., ; Kranz, Wolf‐Gladrow, Nehrke, Langer, & Rosta, ).…”

“…pH 7 at 20°C (Bade & Cole, ). For this reason, DIC limitation may be most prominent in nutrient‐rich soft water lakes experiencing marked reduction of CO 2 and DIC during phytoplankton blooms, while hard water lakes may be less susceptible because of the larger DIC pool, the greater release of CO 2 from by chemical equilibration and carbonate precipitation as well as a higher uptake of atmospheric CO 2 at higher pH (Bogard et al., ; Kragh & Sand‐Jensen, ; Verspagen, van de Waal, Finke, Visser, & Huisman, ; Verspagen, van de Waal, Finke, Visser, Van Donk et al., ).…”