Contrasting greenhouse gas flux responses to ecological restoration: comparative insights from two shallow lakes.

Macrophyte-based lake restoration has effectively shifted lakes from turbid, phytoplankton-dominated conditions to a clearer state. Few studies have explored the impact of such restoration on greenhouse gas emissions in lakes. Through a year-long field study, we compared CO2 and CH4 diffusive fluxes between phytoplankton-dominated and macrophyte-restored regions in two shallow lakes. Annual mean CO2 fluxes were lower in restored than in unrestored regions (-3.603 ± 16.113 vs -0.934 ± 15.442 mol m-2 yr-1 in Lake Li; 0.952 ± 7.922 vs 3.630 ± 9.105 mol m-2 yr-1 in Lake Xuanwu). Nutrients, pH, and phytoplankton biomass were the key drivers for CO2 flux in both lakes. Macrophyte and zooplankton biomasses also constrained CO2 flux in Lake Xuanwu. In Lake Li, CH4 flux in the restored parts (1.327 ± 2.792 mol m-2 yr-1) exceeded that of the unrestored region (0.432 ± 0.511 mol m-2 yr-1) and was strongly impacted by macrophyte biomass. Contrarily, CH4 flux in the restored region of Lake Xuanwu (0.132 ± 0.113 mol m-2 yr-1) was significantly lower than in the unrestored region (0.476 ± 0.502 mol m-2 yr-1), influenced by nutrients and phytoplankton biomass. Lake Xuanwu has higher nitrogen and phosphorus concentrations than Lake Li. However, the magnitudes of nutrient concentration reductions in the restored areas of Lake Xuanwu were greater than those in Lake Li. Furthermore, annual CO2 equivalent flux reductions in the restored areas of Lake Xuanwu were greater than that of Lake Li. Our findings underscore the need for restoration strategies that simultaneously enhance biodiversity and reduce greenhouse-gas emissions.