Nutrients associated with terrestrial dissolved organic matter drive changes in zooplankton:phytoplankton biomass ratios in an alpine lake

Abstract

Summary Dissolved organic matter (DOM) is increasing in many lakes due to climate change and other environmental forcing. A 21‐day microcosm experiment that manipulated terrestrial DOM was used to determine the effect of DOM on zooplankton:phytoplankton biomass ratios (z:p). We predicted that if DOM additions increase the amount of fixed carbon available for higher trophic levels through stimulation of the microbial loop and hence zooplankton, the z:p will increase. However, if DOM additions increase other nutrients besides fixed carbon, we predict stable or decreasing z:p due to nutrient stimulation of phytoplankton that subsequently enhances zooplankton. The effects of experimental additions of terrestrially derived DOM on zooplankton, phytoplankton, z:p and zooplankton net grazing were assessed in microcosms (sealed bags) incubated in the epilimnion (shallow; 1.5 m) and hypolimnion (deep; 8.0 m) strata of an alpine lake. DOM addition treatments (DOM+) had a 6.0‐ to 7.5‐fold increase in phytoplankton biomass relative to controls, but only a 1.3‐ to 1.5‐fold increase in zooplankton biomass, on day 21 of the experiment. The z:p was, thus, lower in the DOM+ treatments (ratios: 2.3 deep and 4.4 shallow) than in the control treatments (ratios: 13.4 deep and 17.5 shallow), providing evidence that DOM additions provide nutrient subsidies besides fixed carbon that stimulate phytoplankton biomass accumulation. The increase in zooplankton biomass during the experiment was similar in magnitude to the total amount of dissolved organic carbon (DOC) in the DOM added in the sealed bags at the beginning of the experiment, which suggests zooplankton biomass stimulation due to increased phytoplankton biomass, and not from DOM through the microbial loop, which would have greater trophic transfer losses. The consumer net grazing effect in the DOM+ treatments was reduced by 2.8‐fold in the shallow stratum and by 2.9‐fold in the deep stratum relative to the control treatments, indicating that zooplankton were unable to exert strong top–down control on the primary producers. The role of nutrients needs to be considered when examining the response of pelagic ecosystems to inputs of terrestrial DOM, especially in lakes with lower DOC concentrations.

Nitrogen and Phosphorus Loads to Temperate Seepage Lakes Associated With Allochthonous Dissolved Organic Carbon Loads

“…Yet results from similar leaching experiments support the trends that we found in wetland versus forest soils. Kissman et al () recorded C:N:P ratios of leachates from bog soils in montane forests ~167:11:1, supporting our conclusion that P enrichment relative to N may be more likely in wetland‐dominated landscapes. Qualls et al () found C:N:P ratios of soil leachates from the organic horizon in montane forests ~3601:76:1, supporting our conclusion that forest soils are relatively P poor.…”

Global change‐driven effects on dissolved organic matter composition: Implications for food webs of northern lakes

“…On the other hand, fish populations in lakes with higher allochthonous DOM inputs, especially those that are hydrologically connected to catchments with large C stocks, are to a larger degree supported by both (in variable proportion) bacterial and pelagic algal production (Karlsson et al., ; Solomon et al., ). Browning of lakes is leading to changes in basal production that propagate through food webs in lakes, including a shift from food webs dominated by autotrophs to food webs dominated by heterotrophic bacteria, and a shift from benthic to largely pelagic‐driven productivity (Ask et al., ; Karlsson et al., ; Kissman, Williamson, Rose, & Saros, ).…”

Biogeochemical Responses to Holocene Catchment‐Lake Dynamics in the Tasmanian World Heritage Area, Australia

“…Terrestrial and aquatic ecosystem dynamics are influenced by climatic change (e.g., Ball et al, 2010;Fritz & Anderson, 2013;Massaferro et al, 2013), land-use/vegetation alteration (e.g., Cooper et al, 2015;Kissman et al, 2017), and fire regime shifts (e.g., Araneda et al, 2013;Bixby et al, 2015;Brown et al, 2014) at multiple scales of space and time. Given the importance and the interconnectedness of both climate and fire (e.g., Emelko et al, 2016;Fletcher et al, 2014;Power et al, 2016), and terrestrial and aquatic ecosystems (Beck, Fletcher, Kattel, et al, 2018;Kissman et al, 2017;Strock et al, 2017), the paucity of research on how climate and fire drive terrestrial and aquatic ecosystem dynamics constitutes a critical knowledge gap that potentially undermines effective management and conservation endeavors. This lack of cross-cutting research is particularly salient in Australia, where fires are key ecological agent that both has shaped the unique flora of the region (Bowman, 2000) and threatens relict fire-sensitive plant systems with extinction (Holz et al, 2014).…”