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.

We investigated whether concentrations of the cyanobacterial toxin microcystin were positively associated with Dreissena polymorpha invasion by conducting surveys of 39 inland lakes in southern Michigan with low to moderate total phosphorus concentrations (≤20 µg·L–1). Lakes with D. polymorpha had 3.3 times higher microcystin concentrations and 3.6 times higher biomass of Microcystis aeruginosa (a major producer of microcystin) than comparable lakes without D. polymorpha. In contrast, the biomass of Anabaena spp. (another potential producer of microcystin) was 4.6 times higher in lakes without D. polymorpha. We also conducted a large-scale enclosure manipulation of D. polymorpha density in Gull Lake, a low-nutrient lake containing D. polymorpha. The experiment revealed a positive effect of D. polymorpha on microcystin concentrations and M. aeruginosa biomass. The congruence between survey and experimental results provides strong evidence that D. polymorpha invasion causes an increase in toxin concentrations in lakes with low to moderate nutrients. An increase in M. aeruginosa biomass may negatively impact food webs and public health because microcystins are known to be toxic to aquatic and terrestrial organisms.

Ecosystems are subsidized with inputs of mass and energy from their surroundings. These allochthonous inputs regulate many ecosystem characteristics. In inland waters, terrestrial inputs of organic matter regulate food-web structure, ecosystem metabolism, water clarity, and thermal stratification. Future changes in allochthony may be especially pronounced in high-elevation ecosystems due to increases in vegetation and precipitation associated with climate change. Several techniques exist to characterize the degree of allochthony of organic matter in aquatic systems, including metrics such as ΔH, the net isotopic discrimination between water and particulate organic matter (POM) of deuterium stable isotopes, and the fluorescence index (FI), which characterizes the fluorescence of dissolved organic matter (DOM). Despite the importance of allochthonous organic carbon inputs, little is known about either how allochthony varies across elevation gradients or whether different metrics are similarly related to allochthony. We measured AH, FI, and a suite of related water-quality characteristics in 30 lakes across a montane to alpine elevation gradient (2340 to 3205 m) in the Beartooth Mountains of Montana and Wyoming, USA, to understand how FI and AH varied with elevation, with one another, and with other allochthony-related water-quality characteristics. We hypothesized that allochthony of POM and DOM would decrease at higher elevations, with alpine lakes above treeline being more autochthonous compared with low-elevation lakes below treeline. We observed a significant inverse linear relationship between AH and Fl, with both metrics indicating a decrease in allochthony at higher elevations. Characteristics including the natural log of the ratio of concentrations of dissolved organic carbon to chlorophyll a (ln(DOC: Chl)), the spectral slope ratio between different spectra of two wavebands (SR, ratio of spectra at 275-295 to 350-400 nm), and a ratio of diffuse attenuation coefficients at 320 and 380 nm (KR, Kd320: Kd380) varied with both ΔH and FI while pH varied only with ΔH. High-elevation systems were characterized by low ln(DOC: Chl) and K(R), and high S(R) and pH. These results indicate that high-elevation lakes are more autochthonous than low-elevation lakes. The relationships among ΔH, FI, elevation, and other water-quality characteristics provide important insights to understand future changes in carbon cycling in mountain ecosystems.

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.

We conducted a survey of 50 thermally stratified lakes with similar nutrient concentrations and morphometries in Michigan to examine the direct and indirect effects of dreissenid mussels on the biomass and community composition of microzooplankton and macrozooplankton. Twenty-five lakes were infested with dreissenid mussels (invaded), while 25 lakes were dreissenid free (uninvaded). In invaded lakes, phytoplankton biomass was 24% lower, and water clarity was 21% greater. Total microzooplankton biomass was 44% lower, with ciliate and rotifer biomass 39% and 45% lower, respectively, in invaded lakes. Total macrozooplankton biomass was 33% lower, largely driven by a 40% lower biomass of Daphnia spp. in invaded lakes. In contrast, dreissenid status had no significant influence on total copepod biomass, calanoid biomass, or cyclopoid biomass. Our microzooplankton results are similar to those of previous studies conducted in shallow, well-mixed systems, although the magnitude of the dreissenid influence in our study was smaller, as might be expected in thermally stratified systems. On the other hand, ours is the first study to document lower biomass of Daphnia spp. and reduced rotifer diversity in invaded lakes. In contrast, we found no difference in macrozooplankton community structure. Dreissenids likely affected zooplankton directly through predation (microzooplankton) and indirectly through resource competition (micro-and macrozooplankton). Understanding how dreissenid mussels affect both microand macrozooplankton will help us to identify the potential mechanisms by which higher trophic levels (e.g., fish) are influenced by these invaders.