Shifts in microbial food web structure and productivity after additions of naturally occurring dissolved organic matter: Results from large‐scale lacustrine mesocosms

Abstract

Predicting the effects of dissolved organic matter (DOM) on pelagic food webs can be difficult because DOM modifies water column optics and can have contrasting effects on species across trophic levels. We combined large mesocosm, smaller-scale experiments and autoregressive modeling driven bu DOC concentration or DOM optical quality (colored DOM, or CDOM, measured as DOC-specific absorbance at 320 nm, SUVA 320 ) to assess how heterotrophic and phototrophic microbial populations were altered in a temperate oligotrophic lake. DOM additions yielded DOC concentrations of 1.6 mg L 21 (control) 2.5 mg L 21, 3.0 mg L 21, and 4.3 mg L 21. Primary (PP) and bacterial (BP) production as well as heterotrophic and autotrophic protist abundances were stimulated in the higher DOM additions. BP responded rapidly to DOM additions, but unlike PP, returned to the level of controls within 2-7 d. A bioassay showed that the DOM was a nitrogen source for phytoplankton. The two models revealed that BP and edible phytoplankton were stimulated by CDOM (SUVA 320 ), but only BP was stimulated by DOC concentration. Ultraviolet radiation (UV) inhibited protists in both models, but stimulated edible phytoplankton only in the SUVA 320 model runs. These results suggest that in transparent oligotrophic lakes large influxes of terrestrial (high SUVA 320 ) DOM will stimulate the microbial food web by providing a nutrient subsidy to bacteria and reducing exposure of protists to damaging UV. Nutrients associated with moderate DOM input may also stimulate PP relative to BP, as was observed in these and other experiments, rather than causing an overall system shift toward heterotrophy.

Bottom‐up and top‐down effects of browning and warming on shallow lake food webs

“…Until now, relatively few experimental studies of browning have been performed. These studies have reported negative effects of browning on pelagic primary production when shading by tDOM was high (Carpenter, Cole, Kitchell, & Pace, ; Jones & Lennon, ), but positive effects on primary and zooplankton production (Kelly et al., ; Kissman et al., ; Sanders et al., ) and neutral or positive effects on pelagic fish production (Hansson et al., ; Koizumi et al., ; Lefébure et al., ) when browning was moderate. While these observations are compatible with model predictions, it is generally difficult to compare these studies with our model, because of inconsistencies in the temporal and/or spatial scales between model assumptions and experimental conditions.…”

Direct and indirect effects of additions of chromophoric dissolved organic matter on zooplankton during large‐scale mesocosm experiments in an oligotrophic lake

“…However, the increase in Daphnia density during the final week of the experiment in low and medium CDOM treatments was preceded by a significant spike in edible phytoplankton in the low CDOM treatment as well as a 50% increase in the Daphnia composite food index in the medium CDOM treatment compared to the control. This increase in the Daphnia food index can probably be attributed to elevated bacterial production in the medium CDOM treatment on days 10 and 15 (Sanders et al., in press). Increases in phytoplankton and bacteria may have been caused by nitrogen and carbon subsidies, respectively, in the CDOM (Sanders et al ., in press).…”

“…This increase in the Daphnia food index can probably be attributed to elevated bacterial production in the medium CDOM treatment on days 10 and 15 (Sanders et al., in press). Increases in phytoplankton and bacteria may have been caused by nitrogen and carbon subsidies, respectively, in the CDOM (Sanders et al ., in press). A nutrient bioassay experiment conducted on lake and mesocosm water indicated that the CDOM contained biologically available nitrogen (Sanders et al ., in press).…”

“…Increases in phytoplankton and bacteria may have been caused by nitrogen and carbon subsidies, respectively, in the CDOM (Sanders et al ., in press). A nutrient bioassay experiment conducted on lake and mesocosm water indicated that the CDOM contained biologically available nitrogen (Sanders et al ., in press). Also, photochemical transformation of the CDOM may have increased biolability and stimulated both heterotrophic and autotrophic production.…”

“…Microplankton were also sampled on days 0, 7, 14 and 21, although these samples are a subset of a larger data set collected for a companion study focussed on microbial responses to CDOM (Sanders et al ., in press). Water to determine abundance of microplankton was collected from the surface mixed layer using a peristaltic pump and integrated into darkened containers maintained at in situ temperatures until preserved or incubated.…”