Agriculture Reference
In-Depth Information
denitrification and NO
3
−
assimilation by plants and algae (Whitmire and Hamilton
2005). Denitrification can also be an important sink for NO
3
−
in groundwater-fed
lakes, even though concentrations remain quite high (Bruesewitz et al. 2012,
Finlay et al. 2013).
The limnology of three local lakes—Gull, Wintergreen, and Lawrence
(Fig. 11.1B)—was widely studied during the 1970s and 1980s, and these lakes
remain among the most studied inland water bodies in the world. Table 11.3 pres-
ents data on the hydrochemistry of these lakes based on a recent survey; detailed
information on the spatial and temporal variability of some of the variables can be
found in the earlier studies. These three lakes represent contrasts in morphometry,
hydrology, hydrochemistry, and ecology, but all are “hardwater” lakes that mix ver-
tically in the spring and fall and develop thermal stratification during the summer
and in ice-forming winters. The hardness of their water reflects the high Ca
2+
and
Mg
2+
concentrations in groundwater inflows.
Recent studies on Gull Lake include work on invasive zebra mussels (
Dreissena
polymorpha
), which thrive in alkaline waters and could potentially colonize most of
the lakes in the region. These mussels tend to promote dominance by
Microcystis aeru-
ginosa
, a phytoplanktonic cyanobacterium that often produces a potent toxin (Raikow
et al. 2004; Knoll et al. 2008; Bruesewitz et al. 2009; Sarnelle et al. 2005, 2012).
This is of particular interest because Gull Lake is only moderately productive (i.e.,
mesotrophic) and harmful cyanobacterial blooms, often produced by this species, are
traditionally associated with eutrophic lakes that have greater levels of P enrichment.
Summary
Landscape-level patterns in water quality in recently glaciated landscapes can be
ascribed to a combination of natural and anthropogenic influences. Weathering
of minerals—particularly the carbonate minerals calcite and dolomite—produces
marked changes in water quality as water from precipitation percolates through the
upper 1-2 m of the soil profile, and these changes influence underlying ground-
water and all downstream groundwaters and surface waters. Nitrate pollution
of infiltrating water is particularly apparent beneath conventionally N-fertilized
annual row crops, whereas perennial crops, poplar plantations, and natural succes-
sional vegetation leach comparatively little N. In contrast, P tends to be retained in
most upland soils, and hence groundwater N:P ratios are high, contributing to the
importance of P for limiting aquatic primary production in most groundwater-fed
surface waters.
Hydrologic exchanges between surface and groundwater systems are par-
ticularly important in the KBS landscape and have numerous ecological rami-
fications. In nonurban areas, most water is delivered to streams and rivers by
groundwater flow. Many lakes also receive a large fraction of their water from
groundwater and thus their chemistry resembles that of groundwater, although
calcium carbonate precipitation and biotic uptake in surface waters can reduce
concentrations of Ca
2+
, acid-neutralizing capacity, and labile nutrients (N, P,
and Si). Elevated concentrations of NO
3
−
present a water-quality issue in this
