Abstract
The U.S. Geological Survey determined seasonal variability
in nutrients, carbon, and algal biomass in 22 wadeable
streams over a 1-year period during 2007 or 2008 within three
geographically distinct areas in the United States. The three
areas are the Upper Mississippi River Basin (UMIS) in Minnesota,
the Ozark Plateaus (ORZK) in southern Missouri and
northern Arkansas, and the Upper Snake River Basin (USNK)
in southern Idaho. Seasonal patterns in some constituent
concentrations and algal responses were distinct. Nitrate concentrations
were greatest during the winter in all study areas
potentially because of a reduction in denitrification rates and
algal uptake during the winter, along with reduced surface runoff.
Decreases in nitrate concentrations during the spring and
summer at most stream sites coincided with increased streamflow
during the snowmelt runoff or spring storms indicating
dilution. The continued decrease in nitrate concentrations
during summer potentially is because of a reduction in nitrate
inputs (from decreased surface runoff) or increases in biological
uptake. In contrast to nitrate concentrations, ammonia concentrations
varied among study areas. Ammonia concentration
trends were similar at UMIS and USNK sampling sites with
winter peak concentrations and rapid decreases in ammonia
concentrations by spring or early summer. In contrast, ammonia
concentrations at OZRK sampling sites were more variable
with peak concentrations later in the year. Ammonia may
accumulate in stream water in the winter under ice and snow
cover at the UMIS and USNK sites because of limited algal
metabolism and increased mineralization of decaying organic
matter under reducing conditions within stream bottom sediments.
Phosphorus concentration patterns and the type of
phosphorus present changes with changing hydrologic conditions
and seasons and varied among study areas. Orthophosphate
concentrations tended to be greater in the summer at
UMIS sites, whereas total phosphorus concentrations at most
UMIS and USNK sites peaked in the spring during runoff and
then decreased through the remainder of the sampling period.
Total phosphorus and orthophosphate concentrations in OZRK
streams peaked during summer indicating a runoff-based
source of both nutrients. Orthophosphate concentrations may
increase in streams in the late summer when surface runoff
composes less of total streamflow, and when groundwater
containing orthophosphate becomes a more dominant source
in streams during lower flows.
Seston chlorophyll a concentrations were greatest early in
the growing season (spring), whereas the spring runoff events
coincided with reductions in benthic algal chlorophyll a biomass
likely because of scour of benthic algae from the channel
bottom that are entrained in the water column during that
period. Nitrate, ammonia, and orthophosphate concentrations
also decreased during that same period, indicating dilution in
the spring during runoff events.
The data from this study indicate that the source of water
(surface runoff or groundwater) to a stream and the intensity
of major runoff events are important factors controlling instream
concentrations. Biological processes appear to affect
nutrient concentrations during more stable lower flow periods
in later summer, fall, and winter when residence time of water
in a channel is longer, which allows more time for biological
uptake and transformations. Management of nutrient conditions
in streams is challenging and requires an understanding
of multiple factors that affect in-stream nutrient concentrations
and biological uptake and growth.
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First posted June 29, 2012
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