At the global scale, inland waters are a significant source of atmospheric
carbon (C), particularly in the tropics. The active pipe concept predicts
that C emissions from streams, lakes and rivers are largely fuelled by
terrestrial ecosystems. The traditionally recognized C transfer mechanisms
from terrestrial to aquatic systems are surface runoff and groundwater
drainage. We present here a series of arguments that support the idea that
land flooding is an additional significant process that fuels inland waters
with C at the global scale. Whether the majority of CO2 emitted by
rivers comes from floodable land (approximately 10 % of the continents) or
from well-drained land is a fundamental question that impacts our capacity
to predict how these C fluxes might change in the future. Using classical
concepts in ecology, we propose, as a necessary step forward, an update of
the active pipe concept that differentiates floodable land from drained
land. Contrarily to well-drained land, many wetlands (in particular riparian
and littoral wetlands) combine strong hydrological connectivity with inland
waters, high productivity assimilating CO2 from the atmosphere, direct
transfer of litter and exudation products to water and waterlogged soils, a
generally dominant allocation of ecosystem respiration (ER) below the water
surface and a slow gas-exchange rate at the water–air interface. These
properties force plants to pump atmospheric C to wetland waters and, when
hydrology is favourable, to inland waters as organic C and dissolved
CO2. This wetland CO2 pump may contribute disproportionately to
CO2 emissions from inland waters, particularly in the tropics where
80 % of the global CO2 emissions to the atmosphere occur. In future
studies, more care must be taken in the way that vertical and horizontal C
fluxes are conceptualized along watersheds, and 2-D models that adequately
account for the hydrological export of all C species are necessary. In
flooded ecosystems, significant effort should be dedicated to quantifying
the components of primary production and respiration by the submerged and
emerged part of the ecosystem community and to using these metabolic rates in
coupled hydrological–biogeochemical models. The construction of a global
typology of wetlands that includes productivity, gas fluxes and hydrological
connectivity with inland waters also appears necessary to adequately
integrate continental C fluxes at the global scale.</p
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