Disproportionate Contribution of Vegetated Habitats to the CH4 and CO2 Budgets of a Boreal Lake

Many lakes have areas that are colonized by different types of macrophytes, including emergent plants that are rooted in sediments. Emergent macrophytes are known to influence local organic carbon (C) and greenhouse gas dynamics but are rarely considered in lake C assessments. Here, we assessed the direct and indirect effects of emergent macrophytes Typha latifolia and Brasenia schreberi on both CH4 and CO2 ambient concentrations and fluxes (diffusive, ebullitive, and plant-mediated) in a typical oligotrophic boreal lake and quantified the contribution of these vegetated habitats to the whole lake C emission budget. Surface water CO2 was generally elevated in vegetated habitats relative to unvegetated habitats. The largest differences between habitats, however, were observed in CH4 dynamics, with total CH4 emissions from vegetated habitats exceeding those from unvegetated littoral and pelagic habitats by one to two orders of magnitude. Direct plant transport dominated CH4 emissions in the T. latifolia habitat, whereas ebullition was the main pathway in the B. schreberi habitat, highlighting the importance of plant type. The average lake CH4 flux for the ice-free season would be 1.6 mmol C m(-2) d(-1) if only the unvegetated habitats were considered. This estimate increases by an order of magnitude (to 14.5 mmol C m(-2) d(-1)) when the vegetated littoral habitats, which contribute 81% of the total C emissions from 26% of the lake surface area, are factored in. Our results suggest that lake C gas budgets based only on pelagic and unvegetated littoral sampling may strongly underestimate total lake C emissions, especially in shallow lakes that develop extensive vegetated habitats.