Stream Water Status and TrendsLand Use and Station Location |
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About the Data, Analysis and Interpretation Data used in this study were obtained from the Maryland Non-tidal Water Quality Monitoring Network, often referred to as the Non-tidal “Core/Trends” network. The discussion here is based upon monthly samples of 18-parameter take at 54 locations from 1986-2004. Collection and analytical methods are described in detail in the QA/QC plan (July 2004) and a copy may be down loaded from http://www.dnr.state.md.us/streams/pubs/106_qc_doc.pdf. The eighteen parameters reported here are: chlorophyll a, conductivity, dissolved oxygen, ammonium, nitrate, nitrite, nitrate plus nitrite, total Kjeldahl nitrogen, pH, orthophosphate, sulfate, total alkalinity, total organic carbon, total nitrogen, total phosphorus, total suspended solids turbidity and water temperature. Benthic macroinvertebrate community samples are obtained annually using either a Surber sampler or a Hester-Dendy multiplate sampler deployed for 8 weeks. Macroinvertebrates are identified to the genus level and community indices of Total Abundance, Total Taxa (TAXA #), Shannon-Wiener Diversity Index (Diversity Index) and the Hilsenhoff Biotic Index (HBI) were calculated for correlation with water quality data. Benthic data has been collected at 50 of the 54 stations where water chemistry is collected. Benthic macroinvertebrate collection methods are described in Friedman, 1996.
Status and Trends Determination of the status is based upon the median concentration of the most recent 3 years of data compared with the benchmark data set 1986-1996. The status for the current year (2002-2004) median concentration is reported to be in the lower 1/3 of the benchmark data set concentration, the middle 1/3 or the upper 1/3 (most concentrated). The categories, or terciles, provide a means of comparison for each station relative to the benchmark data. The null hypothesis of no trends is tested using the Season Kendall Tau procedure. Percent change is estimated using the Sen slope parameter. A test for non-linear trends was the result of parametric regression.
Correlations Benthic index temporal trends were determined using Spearman Rank Correlation (Gilbert, 1987) of the annual index value and the date on which the benthic sample was taken. Correlations with water chemistry data used the average of monthly concentrations for the 365 day period preceding the data on which the benthos were sampled.
Watershed Land Cover and Stream Flow Information Stream gauges with continuous records from 1986-2004 are associated with 24 of the 54 Core/Trend non-tidal stations. Information on streamflow at these locations is available from NWIS http://waterdata.usgs.gov/MD/nwis/. Station information was supplemented from USGS gauge location information and landcover information was obtained from Langland, 1995.
Bibliography Langland, Michael J., Patricia L. Lietman and Scott Hoffman. 1995. Synthesis of Nutrient and Sediment Data for Watersheds Within the Chesapeake Bay Drainage Basin. USGS Water – Resources Investigations Report 95-4233. Friedman, Ellen S. 1996. Status and Temporal Trends in Benthic Macroinvertebrate Communities as an Indicator of Water Quality at Maryland’s Core Monitoring Stations, 1976-1992. Gilbert, Richard O. 1987. Statistical Methods for Environmental Pollution Monitoring. Van Nostrand Reinhold, New York. pp320.
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