Nutrient Trapping by Sediment Deposition in a Seasonally Flooded Lakeside Wetland

Abstract

Sediment and nutrient retention was studied in a seasonally flooded lakeside wetland as a natural mechanism for preventing water quality deterioration. Both wetland and upland soils in the watershed had comparable concentrations of inorganic P on a per‐volume basis, while NH4+‐N and organic forms of N and P were much higher in the wetland soils. Nitrate concentrations expressed in a per‐volume basis were lower in the wetland soils than in the upland soils.The distribution of sediment and nutrients in the wetland was correlated with distance from a small stream flowing through the wetland. Deposition patterns were affected by recent stream channel migrations.The accumulation of nutrients and sediment delivered from the upland to wetland soils was estimated in two ways: (i) by calculating the volume of alluvium deposited in low natural levees adjacent to the stream; and (ii) by estimating nutrient and ash enrichment of histic surface soils farther away from the stream. Although the levees constituted only about 20% of the wetland surface area, they accounted for 81% of the sediment, 84% of the N, and 67% of the P retained by the wetland.The depth of Cs‐137 in the soil was used to estimate net sedimentation rates. Average annual accumulations over the wetland as a whole were: 2.0 kg sediment m−2 yr−1, 2.6 g P m−2 yr−1, and 12.8 N g m−2 yr−1. Since these values exceed those published for average annual storage by wetland plants, soil mechanisms are more important than vegetative uptake for long‐term nutrient and sediment retention in the White Clay Lake wetland.

Wetland nutrient removal: a review of the evidence

“…Many of the studies which recorded over 90% N reduction appear to have been conducted on a regular weekly or monthly sampling strategy (Baker and Maltby, 1995;Haycock and Pinay, 1993;Pinay and Decamps, 1988) or avoided sampling during high flow events (Cooper, 1990). Those studies that recorded relatively low N reduction (<50%) or highly variable N reduction were often those that were conducted daily (Cooper, 1994;Jordan et al, 2003) or sampled more regularly during high flows (Raisin and Mitchell, 1995;Brunet et al, 1994;Daniels and Gilliam, 1996), or sampled from many locations within the wetland (Johnston et al, 1984). This is also the case in some of the studies which found relatively low P reduction or variable P reduction (Raisin and Mitchell, 1995;Spangler, 1977;Jordan et al, 2003).…”

“…Wetlands reduce nutrients by encouraging sedimentation (Karr and Schlosser, 1978;Johnston et al, 1984), sorbing nutrients to sediments (see Khalid et al, 1977), taking up nutrients in plant biomass (Lee et al, 1975) and enhancing denitrification (Lowrance et al, 1984). While the evidence for this arises from a number of separate studies, some studies show that wetlands can be ineffective at reducing nutrient loadings, or might even increase nutrient loadings.…”

“…It is generally thought that direct nutrient reduction by plant uptake in wetlands is relatively unimportant in comparison with sediment and chemical transformation processes (Hammer, 1992). Johnston et al (1984) argue that nutrient removal by sediment may be more long term than uptake by plants which return nutrients as litter, a form that is more vulnerable to leaching and erosion. This can be reduced by plant harvesting before senescence, though it is important not to disturb the sediment and enhance erosion (Merritt, 1994).…”

Spatio-temporal distribution of nitrogen in the undulating littoral zone of Lake Taihu, China

“…In a eutrophic lake, the retentate contained abundant nutrients, which would enrich the riparian zone (Johnston et al, 1984). In the littoral, especially where there is a macrophyte cover, sediments carried by lake currents also may settle.…”

“…Finer suspended or resuspended sediment with abundant labile nutrient and absorbed pollutants (Naiman & Dé camps, 1997) carried by flowing water enrich riparian sediments proportionally (Johnston et al, 1984). Whether riparian zones will be a sink or source to open water is not clear.…”