Trophic State and Eutrophication - Freshwater Ecology: Concepts and Environmental Applications - page 362

The results of nutrient controls will be known in a few years, after the ef-
fects of a voluntary nutrient reduction program can be assessed.
EUTROPHICATION AND WETLANDS
Wetlands are parts of the landscape that have been largely ignored
when water quality problems are considered. Wetlands can have tremen-
dous benefits in nutrient immobilization and sediment trapping. In addi-
tion, use of wetlands is increasingly viewed as a way to remove nutrient
loads from sewage effluent. Finally, cases now exist in which eutrophica-
tion of wetlands can cause problems. For example, atmospheric deposition
of nitrogen in wetlands can cause eutrophication (Morris, 1991).
Wetlands as Nutrient Sinks
In natural systems, wetlands can have major impacts on flows of nu-
trients, sediments, and water through watersheds. As floodwater moves
through a wetland area, it spreads and slows, dropping sediments and sur-
rendering nutrients to the plants growing in the wetlands. Riparian wet-
lands may be particularly important in this regard.
Wetlands have been used successfully for some time for nutrient re-
moval and general sewage treatment in both North America and Europe
(Sloey et al., 1978; Brix, 1994). They can be used for nitrogen and phos-
phorus removal. When the wetlands are installed, they can have high ini-
tial rates of nutrient removal related to nutrient uptake by growing plants
and algae (Richardson and Schwegler, 1986). After the first few months of
heavy nutrient loads into the wetland, phosphorus removal can decrease
significantly, but nitrogen removal can remain at moderate levels. The rea-
son for the difference in nitrogen and phosphorus removal is that denitri-
fication can remove nitrogen in the form of N 2 gas, but when the system
becomes saturated with phosphorus little additional removal occurs. How-
ever, some phosphorus removal may continue because sedimentation in
wetlands can account for a significant loss of phosphorus from the incom-
ing water (Mitsch et al., 1995). If plant biomass is removed continuously
from the wetland, plants will add new growth and assimilate additional
nutrients. This process is necessary if nutrient removal is to continue. How-
ever, finding a use for removed plants, such as mulch or papermaking, may
be difficult because such uses are not always economically profitable.
One study demonstrated that wetlands that are used for nitrate re-
moval will release organic carbon (Ingersoll and Baker, 1998). Unfortu-
nately, increased organic carbon export increases the biochemical oxygen
demand draining from the wetland. Such demand can lead to water qual-
ity problems downstream.
The retention efficiencies of wetlands for various materials vary greatly.
Such variation is not surprising given the wide assortment of wetland types
that occur naturally. Wetlands have been shown to retain from 23 to 91%
of sediments coming in, from 12 to 1370 mg N m 2 year 1 , and from 1.2
to 110 mg P m 2 year 1 (Johnston, 1991). Essentially all studied wetlands
retain nitrogen (from 21 to 95% in those in which inputs and outputs have
been monitored) and therefore function as nutrient sinks . However, 9 of 24