Response of phytoplankton in an alpine lake to inputs of dissolved organic matter through nutrient enrichment and trophic forcing

Abstract

Inputs of terrestrially derived dissolved organic matter (DOM) are increasing in alpine lakes due to multiple drivers such as climate change, tree line advancement, and insect epidemics. A 21 d microcosm experiment investigated three potential mechanisms by which increased inputs of terrestrial DOM subsidies might affect phytoplankton density, growth, and species assemblage: (1) directly, by providing nutrients enhancing growth of select phytoplankton species (nutrient stimulation hypothesis); (2) indirectly, through trophic forcing of zooplankton uniformly increasing the total biomass of all zooplankton that selectively graze on phytoplankton (trophic intensity hypothesis); and (3) indirectly, through trophic forcing of zooplankton by favoring zooplankton species that selectively graze on phytoplankton (trophic shift hypothesis). We manipulated DOM (terrestrial DOM additions vs. unmanipulated control), zooplankton (presence vs. absence), and incubation depth (epilimnion vs. hypolimnion) in a full 3 3 3 factorial design. Phytoplankton density and growth increased substantially and species assemblage shifted to near dominance by Asterionella formosa in the presence of DOM. Zooplankton biomass and growth increased with the addition of DOM, yet the species assemblage remained stable across treatments, and contributed to selective grazing effects on phytoplankton. Our data support the nutrient stimulation and trophic intensity hypotheses. While DOM effects have been classically attributed to stimulation by addition of fixed carbon, our experiments indicate that nutrient stimulation is also important. Additionally, the indirect DOM effect of trophic forcing can occur in the absence of selective effects of DOM on zooplankton.

Effects of increased concentrations of inorganic nitrogen and dissolved organic matter on phytoplankton in boreal lakes with differing nutrient limitation patterns

“…Likewise increased algal biomass in the DOM Addition treatment in the N&P co-limited lake suggests that the N and P in DOM were largely bioavailable, with any P additions in particular likely facilitating the observed removal of nitrate in these treatments. The stimulation of algal biomass by DOM is consistent with other experiments that have demonstrated the availability of both N and P in DOM to phytoplankton in lakes (Berman and Chava 1999;Graneli et al 1999;Klug 2002;Kissman et al 2013) as well as those in estuarine and saline systems (Bradley et al 2010;Bronk et al 2010). It is possible that there are other components available in the DOM that are benefiting bacteria, which are then able to further breakdown or possibly mineralize DOM and increase the bioavailability of associated nutrients (Findlay and Sinsabaugh 1999).…”

“…Response to N addition is to the 40 lg NO 3 -N L -1 addition treatment, while the responses to DOM Shaded and DOM Addition are in the treatments without any N addition. Error bars indicate standard error Klug (2002) also found that chlorophytes as well as a species of Cryptomonas responded positively to DOM additions, while Kissman et al (2013) also found that the cell densities of several diatom taxa, including F. crotonensis, increased with a DOM addition. The responses of these different phytoplankton groups likely reflect multiple pathways by which increasing DOM can alter phytoplankton cell densities.…”

“…Dissolved organic matter spikes for each location were prepared following a modified approach from Kissman et al (2013). A concentrate was prepared for each lake from fallen leaves from each of the corresponding watersheds.…”

“…Similarly, the carbon subsidy supplied by DOM to heterotrophic plankton (bacteria and zooplankton) and mixotrophic phytoplankton is also well known ; Karlsson et al 2007). A third effect of DOM on phytoplankton that is increasingly recognized is the nutrient subsidy provided by this material (Graneli et al 1999;Klug 2002;Kissman et al 2013). DOM can be a source of nutrients such as P and N (Kortelainen et al 2006) but the biological availability of such nutrients is not well understood (Flynn 2008).…”

Absorption and fluorescence characteristics of rainwater CDOM and contribution to Lake Taihu, China

“…The close coupling between CDOM and nutrients further indicated that CDOM might be an important nutrient source for phytoplankton growth in lake water (Kowalczuk et al, 2010). For example, a 21 d microcosm experiment showed that the increasing inputs of terrestrial DOM caused an increase in biomass and a change in phytoplankton species assemblages (Kissman et al, 2013). Two mechanisms potentially explain how the nutrients from DOM stimulate phytoplankton growth.…”