Effect of reservoir discharge and sanitary effluent on an urban stream

Abstract

Eutrophication of aquatic ecosystems may occur from several sources, primarily
from anthropogenic inputs. Excess nutrient deposition can result in eutrophication and
community compositional shifts in organisms located in impacted ecosystems. In the Sangamon
River located outside Decatur, IL, discharge from the local reservoir, which receives inputs from
both a city landscape and agricultural landscape and a wastewater treatment plant (WWTP).
These may result in a large influx of nutrients into this system. Increased nutrient levels have the
potential to shift benthic periphyton, and macroinvertebrate communities and cause extinction of
low tolerance native species. To examine the impacts of inputs for the WWTP and reservoir,
sampling was performed above the WWTP (upstream sites) which encompassed the sampling
points from the discharge of the Decatur reservoir down to the WWTP discharge, and sampling
was performed downstream of the WTTP (downstream sites). PCA analyses show a significant
difference between upstream and downstream sites. During periods of high flow from the dam, a
detectable increase in available total oxidized nitrogen was found in the water column.
Additional water chemistry tests found significant differences between upstream and
downstream sites when examining nutrient levels based on high periods of discharge, and
seasonal variation. Our results show that there is a significant relationship between increased
TON, and NH3 levels upstream of the sanitation district during periods of high flow (high
reservoir discharge) and the downstream sites. This relationship shows that the reservoir is acting
as a sink of nitrogen (as TON and NH3) from the surrounding watershed, and a source of
eutrophication in upstream sites. Analyses also showed periods of no difference between
upstream sites and downstream sites as reservoir discharge increased, and a negative correlation
between water nutrient values of the downstream sites during periods of higher discharge. The
lack of difference seen between the sites at high discharge may be a result of dilution of the
WWTP discharge by reservoir water. Community structure metrics found significant differences
in MIBI (p = .00039), and IBI (p = .000839) scores between the two reaches with the UPS site
having lower scores compared to the DNS a finding that previous studies showed based on
diatom assemblages.
Overall, we found that high periods of discharge appear to mediate the influence of the
nutrient input from the WWTP, which in itself caused significant changes in the
macroinvertebrate, fish and benthic algal communities.

Date of Award	2012
Original language	American English
Awarding Institution	Eastern Illinois University
Supervisor	Charles L Pederson (Supervisor)