Water Quality and Movement in Agricultural Landscapes - The Ecology of Agricultural Landscapes - page 295

with increasing NO 3 − loads, suggesting that the capacity of streams to remove NO 3 −
can become saturated, which then allows more NO 3 − to move downstream. High
NO 3 − loading was correlated with increased stream primary production (mainly by
algae), and NO 3 − removal by denitrification was correlated with increased stream
respiration (which, in turn, is driven by organic matter supply). The role of stream
denitrification as a source of atmospheric nitrous oxide, a potent greenhouse gas,
was also elucidated from these 15 N tracer experiments, providing evidence that
streams and rivers are more important to global nitrous oxide emissions than pre-
viously thought (Beaulieu et al. 2008, 2011). These cross-site studies show how
stream channels and riparian zones could be better managed to promote higher
rates of N removal by denitrification and thereby reduce eutrophication of down-
stream waters, but with the undesirable side effect of enhanced nitrous oxide emis-
sion to the atmosphere.
For Augusta Creek, like many streams in this glacial landscape, the large ground-
water inflow and the presence of lakes and wetlands along the stream system result
in a stable physicochemical environment compared to streams in other kinds of
landscapes (Allen et al. 1972, Fongers 2008). The water level of Augusta Creek
has been monitored since October 1964 by the U.S. Geological Survey (Station
04105700), and the long-term mean discharge (1964-1994) near its mouth is 1.28
m 3 s −1 . Because of the relatively slow movement of groundwater across the local
landscape, the water discharged by the creek is likely to reflect the recharge from
upland parts of the watershed over the past several decades (Stewart et al. 2010,
Tesoriero et al. 2013). Given this time lag, the stream water of Augusta Creek may
not be in equilibrium with or reflect changes in the current recharge of upland
areas. Bartholic et al. (2007) presented a groundwater flow model that estimates
spatial patterns of groundwater inflow and water temperature in Augusta Creek;
this model is used to show how large-scale groundwater withdrawals might affect
stream discharge and habitat for cool-water fishes. Augusta Creek and most other
local streams carry little inorganic material in suspension, but transport significant
quantities of bed load, mainly as sand, which may be affected by changes in stream
discharge.
Monitoring of NO 3 − concentrations in Augusta Creek by the KBS LTER since
1997 has shown increasing concentrations (Fig. 11.10A). However, when cor-
rected for discharge variation using the concentration-discharge relationship (Fig.
11.10B) and daily discharge records from the USGS station, NO 3 − fluxes appear
consistent and stable across 13 years of observation (Fig. 11.10C). Thus, the tem-
poral increase in concentrations is explained by changes in stream discharge rather
than by increasing amounts of NO 3 − exported from the watershed. This tendency for
little short-term change in stream NO 3 − export is not uncommon and likely reflects
the time lags and attenuation associated with groundwater movement through the
watershed (Basu et al. 2011, Sprague et al. 2011, Hamilton 2012) and possibly also
the delayed release of nitrogen from fertilized soils (Sebilo et al. 2013).
Total P concentrations are low and NO 3 − concentrations can be high in many
headwater streams, reflecting the importance of groundwater inflows and the high
mobility of NO 3 − relative to P in the soil-groundwater system, as discussed earlier.
When N-enriched groundwaters are discharged to surface waters, PO 4 −3 is further