Geoscience Reference
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frequent droughts predicted, irrigation demands will increase. Changes in land
use may require the construction of dams, with barrier effects for hydrological
conditions. This has implications for the size and spatial distribution of wetlands
(Brinson & Malvarez 2002; Pyke 2004; Perotti et al . 2005). Additionally, nutrient
enrichment and pollution are possible consequences of land-use changes from
intensified crop growth, prolonged growing seasons and increasing urbanization
and industrialization (van Breemen et al . 1998; Hudon 2004). Changes in
precipitation, evaporation and temperature determine the groundwater level,
which influences the wetland cover cycle, the transition between permanent and
temporary wetlands and hydrochemical variables (e.g. Johnson et al . 2004;
Lischeid et al . 2007). Water table height reflects the influences of climate change
in many wetland types.
Physicochemistry
Mineralization and release of nutrients are determined by hydrology and
temperature. Moisture and temperature influence microbial enzyme activity
and decomposition rates. Drier, warmer conditions could stimulate nutrient
mineralization and enhanced release from sediments to runoff water (Freeman
et al . 1996; Fenner et al . 2005). Mineralization of C, N, and P may differ
significantly among wetland types. In bog peats, nitrogen mineralization and
CO 2 production may decrease with increasing ambient temperatures and
lower water tables, whereas in fen peats, nitrogen mineralization may decrease
and methane production may increase with higher water tables (Keller et al .
2004), but the generality of these findings is questionable. Stored reduced sulphur
in anoxic zones of wetlands oxidizes during drought periods. Owing to the
subsequent efflux into streams and lakes, sulphate concentrations and acidity
can increase after droughts (Dillon et al . 1997; Aherne et al . 2004). Carbon
acquisition may increase under warmer and wetter conditions, whereas in
warmer and drier years, wetlands experience significant carbon losses (Carroll &
Crill 1997; Griffis & Rouse 2001). Climate change is predicted to cause a
doubling of net total C loss rates in wetlands (Clair et al . 2001). Maximum
soil temperatures are correlated with maximum CH 4 emission values, whereas
reduced water table levels suppress CH 4 emissions. Thus, long-term climatic
changes with less precipitation and decreased water tables may reduce the
incidence of CH 4 release from wetlands (Moore et al . 1998; Gedney & Cox
2003; Werner et al . 2003). Nonetheless, lowered groundwater changes due to
climate change may lead to increased nitrous oxide fluxes in natural
peatland soils (Regina et al . 1999). Under extreme drought, emissions may
increase exponentially with a linear decrease in the water table (Dowrick
et al . 1999).
Primary production
A key driver of changes in the community composition of wetland vegetation
is the altered hydrological regime. Drought results in a proportional loss of
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