Relationships Between Landscape, Spatial Scale, and Stream Water Chemistry in a Subtropical Karst System
Date
2009-08
Authors
Strickland, Jessica D.
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Abstract
Natural factors such as climate, geology, topography, and vegetation regulate biogeochemical processes that dictate important river characteristics, including surface water chemistry (Allan, 2004). Converting natural vegetation for urban and agricultural land use can alter water chemistry, creating a loss of biodiversity and reduced human consumptive and recreational use (Osborne and Wiley, 1988; Johnson et al., 1997). To evaluate landscape-water chemistry interactions in a subtropical context, landscape characteristics (natural and anthropogenic) were quantified at three spatial scales (sub-catchment, riparian, stream reach) and related to water chemistry variables collected over a 13-month sampling period. An additional dataset was incorporated into analysis to evaluate seasonal and long-term (1984-2008) temporal trends across contrasting stream flow conditions (low and high stream periods).
Landscape features accounted for slightly more variation in winter (52%) versus summer (48%) water chemistry (RDA, P < 0.0001). Nitrate (NO3-N) concentrations were found to be highest in the winter, near headwater and agricultural stream reaches, and in higher stream flow. Riparian areas with forested vegetation and clayey soils were negatively associated with NO3-N, which is consistent with terrestrial biological processes. Particulate substances (total phosphorus, chlorophyll a, and turbidity) were also associated with stream flow, clayey soils, and agricultural areas; however, concentrations were found to be highest during the summer in sub-catchments having steeper slopes and more urban land use. Specific conductivity and acid neutralizing capacity were highly correlated, and consistently negatively associated with sub-catchment urban land use and slope.
Seasonal temporal trends were found in most nutrient species, whereas long-term trends occurred only in NO3-N, soluble reactive phosphorus (SRP), and organic nitrogen (ON). Seasonal trends were consistent with the data collected throughout the watershed during 2007 and 2008, particularly in relation to the differences found across discharge periods. Nitrate concentrations have increased from 1984 to 2008 across both flow regimes (low and high stream flow) (GLS: β1 = 1.06, P = 0.03; β1 = 1.03, P = 0.02), whereas ON has decreased during low flow conditions (β1 = 1.02, P = 0.04). Atmospheric NO3-N deposition did not increase over the data period (t17 = -0.34, P = 0.74), which suggests that N-related trends are associated with land cover change. Soluble reactive P was highest (20 μg/L) in winter months characterized by low stream flows, which corresponded with the long-term increasing trend found only in the low flow period (β1 = 1.05, P = 0.03). These data suggest that the stochastic flow regime of the study area exerts a large influence on stream water chemistry through the regulation of landscape-aquatic connectivity across multiple spatial and temporal scales.
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Keywords
subtropical karst system, land use, water chemistry, stream measurements, watershed management, Pedernales River
Citation
Strickland, J. D. (2009). Relationships between landscape, spatial scale, and stream water chemistry in a subtropical karst system (Unpublished thesis). Texas State University-San Marcos, San Marcos, Texas.