Autochthonous dissolved organic matter potentially fuels methane ebullition from experimental lakes

Shallow lakes are hotspots for carbon processing and important natural sources of methane (CH₄) emission. Ebullitive CH₄ flux may constitute the overwhelming majority of total CH₄ flux, but the episodic nature of ebullition events makes determining both quantity and the controlling factors challenging. Here we used the world's longest running shallow-lake mesocosm facility, where the experimental treatments are low and high nutrients crossed with three temperatures, to investigate the quantity and drivers of CH₄ ebullition. The mean CH₄ ebullition flux in the high nutrient treatment (41.5 ± 52.3 mg CH₄-C m^-2 d^-1) mesocosms was significantly larger than in the low nutrient treatment (3.6 ± 5.4 mg CH₄-C m^-2 d^-1) mesocosms, varying with temperature scenarios. Over eight weeks from June to August covered here warming resulted in a weak, but insignificant enhancement of CH₄ ebullition. We found significant positive relationships between ebullition and chlorophyll-a, dissolved organic carbon (DOC), biodegradable DOC, δ²H, δ¹⁸O and δ¹³C-DOC, autochthonous dissolved organic matter (DOM) fluorescent components, and a fraction of lipids, proteins, and lignins revealed using ultrahigh-resolution mass spectrometry, and a negative relationship between ebullitive CH₄ flux and the percentage volume inhabited of macrophytes. A 24 h laboratory bio-incubation experiment performed at room temperature (20 ± 2 °C) in the dark further revealed a rapid depletion of algal-DOM concurrent with a massive increased CH₄ production, whereas soil-derived DOM had a limited effect on CH₄ production. We conclude that eutrophication likely induced the loss of macrophytes and increase in algal biomass, and the resultant accumulation algal derived bio-labile DOM potentially drives enhanced outgassing of ebullitive CH₄ from the shallow-lake mesocosms.

Species-specific phenological trends in shallow Pampean lakes' (Argentina) zooplankton driven by contemporary climate change in the Southern Hemisphere

The relationship between the timing of recurrent biological events and seasonal climatic patterns (i.e., phenology) is a crucial ecological process. Changes in phenology are increasingly linked to global climate change. However, current evidence of phenological responses to recent climate change is subjected to substantial regional and seasonal biases. Most available evidence on climate-driven phenological changes comes from Northern Hemisphere (NH) ecosystems and typically involves increases in spring and summer temperatures, which translate into earlier onsets of spring population developments. In the Argentine Pampa region, warming has occurred at a much slower pace than in the NH, and trends are mostly restricted to increases in the minimum temperatures. We used zooplankton abundance data from Lake Chascomús (recorded every two weeks from 2005 to 2015) to evaluate potential changes in phenology. We adopted a sequential screening approach to identify taxa displaying phenological trends and evaluated whether such trends could be associated to observe long-term changes in water temperature. Two zooplankton species displayed significant later shifts in phenology metrics (end date of Brachionus havanaensis seasonal distribution: 31 day/decade, onset and end dates of Keratella americana seasonal distribution: 59 day/decade and 82 day/decade, respectively). The timing of the observed shift in B. havanaensis phenology was coincident with a warming trend in the May lake water temperature (4.7°C per decade). Analysis of abundance versus temperature patterns from six additional shallow Pampean lakes, and evaluation of previous experimental results, provided further evidence that the lake water warming trend in May was responsible for the delayed decline of B. havanaensis populations in autumn. This study is the first report of freshwater zooplankton phenology changes in the Southern Hemisphere (SH).