You are here:
USE OF WATERSHED CLASSIFICATION IN MONITORING FRAMEWORKS FOR THE WESTERN LAKE SUPERIOR BASIS
Citation:
Detenbeck, N E. USE OF WATERSHED CLASSIFICATION IN MONITORING FRAMEWORKS FOR THE WESTERN LAKE SUPERIOR BASIS. Presented at Lake Superior Technical Non-point Pollution Abatement Committee, Ashland, WI, May 20, 2003.
Description:
In this case study we predicted stream sensitivity to nonpoint source pollution based on the nonlinear responses of hydrologic regimes and associated loadings of nonpoint source pollutants to catchment properties. We assessed two hydrologically-based thresholds of impairment, one for catchment storage (5-10%) and one for mature forest (<50 versus >60% of catchment in mature forest cover) across two different hydrogeomorphic regions within the Northern Lakes and Forest (NLF) Ecoregion: the North Shore (predominantly within the North Shore Highlands (NSH) Ecological Unit) and the South Shore (predominantly within the Lake Superior Clay Plain (LSCP) Ecological Unit). Water quality samples were collected and analyzed during peak snowmelt, baseflow, and event conditions from 24 second-order streams grouped as follows: three in each region x catchment storage x mature forest class. In the second phase of the study we selected 24 third-order watersheds in high and low mature forest classes (LMF, < 50% of watershed in mature forest cover) along gradients of watershed storage within each of the two hydrogeomorphic regions.
For second-order streams, water quality was affected by a combination of regional influences, catchment storage and mature forest. Regional differences were significant for suspended solids, phosphorus, nitrogen: phosphorus ratios, dissolved organic carbon and alkalinity. Catchment storage was significantly correlated with dissolved silica during the early to mid-growing season, and with dissolved organic carbon, specific conductance and alkalinity during all seasons. Total nitrogen and dissolved nitrogen were consistently less in low mature forest than in high mature forest catchments. Catchment storage interacted with the influence of mature forest for only two metrics: color and the soluble inorganic nitrogen:phosphorus ratio.
For second-order streams, significant interaction terms (region by mature forest or region by storage) suggest differences in regional sensitivity for conductance, alkalinity, total organic carbon, and color, as well as possible shifts in thresholds of impact across region or mature forest class.
For third-order streams, we detected differences between regions (North and South Shore) for particulates, nutrients, and pH, with all but silica values higher for South Shore streams (p < 0.05). We detected mature forest effects for turbidity, nutrients, color, and alkalinity, with higher values in the LMF watersheds. Dissolved N, ammonium, N:P, organic carbon and color increase while suspended solids, turbidity, and dissolved P decrease as a function of storage. We detected few region x mature forest or region x watershed storage interactions, thus we can apply threshold-based classification schemes to extrapolate effects across regions. Both regional differences in water quality and those associated with watershed attributes were more common for third-order streams in the western Lake Superior drainage basin as compared with second-order streams examined in the earlier study.
Use of the Northern Lakes and Forest Ecoregion alone as a basis for setting regional water quality criteria would lead to the misinterpretation of reference condition and assessment of condition. There were pronounced differences in background water quality between the North and South Shore streams, particularly for parameters related to differences in soil parent material and glacial history. A stratified random sampling design for stream water quality baseflow and loadings based on both hydrogeomorphic region and catchment attributes improves assessments of both reference condition and differences in regional sensitivity. Differences in regional sensitivity, once confirmed, can establish the basis for prioritizing unmonitored watersheds for subsequent monitoring and restoration activities.
This is an abstract or a proposed presentation and does not necessarily reflect EPA policy.