Abstract ID
nutdyn17
Type
Oral
Theme
nutdyn
Full title
Retention and processing of riverine nutrient and sediment loads by floodplains
Abstract text
River flood pulses deliver nutrients and sediment to hydraulically connected floodplains. This trapping of flood
transported material can have large implications for watershed transport processes and floodplain productivity.
However, few quantitative estimates exist for the percent retention of annual river loads of nitrogen (N),
phosphorus (P), and suspended sediment by wetlands. We measured depositional fluxes of nutrients and
suspended sediment onto floodplain soil surfaces (g m^-2 yr^-1; 1-6 yrs of
accumulation) over a sampling network that included the Coastal Plain portion of seven rivers in the
Chesapeake Bay watershed. For each river, the average N, P, and sediment depositional flux rates were
multiplied by an estimate of floodplain area to calculate floodplain trapping rates (kg yr^-1). We
then compared these rates to average river loads. Median material retention among the rivers was 24% of N
(range 4-156%), 59% of P (14-604%), and 119% of suspended sediment (30-690%). The percent retention of
N, P, and sediment among rivers increased with floodplain area and the duration of inundation and decreased
with river load. Uncertainty in retention estimates derive from several aspects related to permanency of
deposited nutrients and sediment in floodplains. Measurement of in situ mineralization of floodplain soil
nutrients in a Piedmont watershed indicates mean turnover times of 21 and 213 years for existing pools of N and
P, respectively. Sedimentation increased subsequent nutrient mineralization rates, which likely provides a
nutrient subsidy to floodplain vegetation. Nevertheless, depositional fluxes clearly exceeded mineralization
fluxes and suggests long-term storage of flood-derived nutrients in floodplain soils. Ongoing measurement of
floodplain deposition and erosion, as well as bank erosion, will determine net system sediment budgets. In
summary, riverine floodplains are dynamic systems with high capacity to influence watershed water quality.
Submission date
2009-10-05
Keywords
nutrient, retention, sediment, water quality
Will be submitting paper?
No