Freshwater macrophyte type (macroalgae versus phanerogams) mainly determines detritus-derived greenhouse gases production: A microcosm experiment

Freshwater ecosystems are crucial in the global emissions of greenhouse gases (GHGs) such as CH₄. Macrophytes are the main organic matter (i.e., detritus) supplier to the sediment of these systems, thus controlling CH₄ production. However, species-specific differences (structure and composition) may determine contrasting patterns of detritus transformation into CH₄. Furthermore, eutrophication can affect the degradation and, consequently, CH₄ production. We performed a 64-day microcosm experiment with anoxic incubations of detritus from seven phylogenetically different macrophytes (two charophytes, filamentous algae –Spirogyra, Cladophora–, three submerged plants and an amphibious one), under two trophic conditions (oligo- versus eutrophic) and with/without sediment. We assessed the CH₄ and CO₂ production and the changes in the detritus quality at the end of the experiment. The ranking in the mean cumulative CH₄ production was: Chara hispida > Nitella hyalina > Najas marina ≈ Teucrium scordium > Stuckenia pectinata ≈ Myriophyllum spicatum > filamentous algae, and it was related to the detritus quality. GHGs maximum production rates were 1.6 (N. marina)-1.2 (C. hispida) mmol CH₄/(g OC·day) and 1.7 (N. marina)-1.5 (C. hispida) mmol CO₂/(g OC·day). The CO₂:CH₄ ratio was biased towards CO₂ during the first 10 days (average ratio of 200) and fell afterwards to about 1 for all macrophyte species and treatments. The sediment favored detritus decomposition (probably due to the “positive priming effect”), increasing GHGs production. The influence of nutrient enrichment was not evident. Delving into the macrophyte detritus quality-GHGs production relationship is needed to forecast the GHGs emissions in macrophyte-dominated systems.