Experimental warming promotes CO2 uptake but hinders carbon incorporation toward higher trophic levels in cyanobacteria-dominated freshwater communities

• Published in: The Science of the total environment 2024-04, Vol.920, p.171029-171029, Article 171029
• Main Authors: Colina, Maite, Meerhoff, Mariana, Cabrera-Lamanna, Lucía, Kosten, Sarian
• Format: Article
• Language: English
• Subjects: Climate warming; CO2; cyanobacteria; Eutrophication; Ponds; Zooplankton
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/j.scitotenv.2024.171029

Shallow freshwaters can exchange large amounts of carbon dioxide (CO2) with the atmosphere and also store significant quantities of carbon (C) in their sediments. Current warming and eutrophication pressures might alter the role of shallow freshwater ecosystems in the C cycle. Although eutrophication has been widely associated to an increase in total phytoplankton biomass and particularly of cyanobacteria, it is still poorly understood how warming may affect ecosystem metabolism under contrasting phytoplankton community composition. We studied the effects of experimental warming on CO2 fluxes and C allocation on two contrasting natural phytoplankton communities: chlorophytes-dominated versus cyanobacteria-dominated, both with a similar zooplankton community with a potentially high grazing capacity (i.e., standardized density of large-bodied cladocerans). The microcosms were subject to two different constant temperatures (control and +4 °C, i.e., 19.5 vs 23.5 °C) and we ensured no nutrient nor light limitation. CO2 uptake increased with warming in both communities, being the strongest in the cyanobacteria-dominated communities. However, only a comparatively minor share of the fixed C translated into increased phytoplankton (Chl-a), and particularly a negligible share translated into zooplankton biomass. Most C was either dissolved in the water (DIC) or sedimented, the latter being potentially available for mineralization into DIC and CO2, or methane (CH4) when anoxic conditions prevail. Our results suggest that C uptake increases with warming particularly when cyanobacteria dominate, however, due to the low efficiency in transfer through the trophic web the final fate of the fixed C may be substantially different in the long run. [Display omitted] •Warming modifies CO2 flux and C allocation in fresh waters.•Phytoplankton structure and function change with both eutrophication and warming.•We experimentally compared cyanobacteria versus chlorophyte-dominated communities.•CO2 dynamics changed with warming, particularly under cyanobacteria.•Carbon transfer toward higher trophic levels decreased with cyanobacteria.