G. Fauvet et al., Influence of benthic and interstitial processes on nutrient changes along a regulated reach of a large river (Rhone River, France), HYDROBIOL, 445(1-3), 2001, pp. 121-131
Benthic and hyporheic (i.e. the water-saturated interstitial zone beneath r
iver bed-sediments) processes together modify particulate and dissolved nut
rient fluxes in streams, but the relative importance of these processes rem
ains unstudied in large rivers. Changes in material and nutrient fluxes (to
tal suspended matter, biodegradable and refractory dissolved organic carbon
, and nitrate) were monitored along a reach with a regulated discharge (a b
y-passed section downstream of a dam). Mass balance diagrams highlight a co
ntrasting functioning between upstream and downstream parts of this sector
of the Rhone River (Pierre-Benite sector, France). At the upstream location
close to the water input, characterized by partially clogged sediments and
large lentic zones, decrease in total suspended matter (TSM) and biodegrad
able dissolved organic carbon (BDOC) fluxes resulted from high rates of ben
thic processes (i.e. sedimentation, respiration and mineralisation). At the
downstream location, characterized by strong vertical water exchanges in a
large riffle, nutrient changes resulted from the combination of processes
that took place at the surface of the stream bed and during water percolati
on through riffle sediments. Physical benthic processes (scouring of periph
yton, erosion and resuspension of sediment due to higher surface velocity)
lead to an increase of TSM and DOC fluxes. Within the riffle, decomposition
and mineralisation of organic matter in the first meter of the infiltratio
n zone, and physical entrapment of RDOC, make the riffle as a sink for DOC
and a source of nitrate. The significant contribution of the riffle to the
self-purification capacities of this large river reach shows the relevance
of including interstitial compartment in self-purification studies.