Open-File Report 99-219 contents page

WATER-QUALITY DATA

The MD DNR routinely analyzes water samples for a suite of water-quality parameters. Generally, two samples per month are collected during the SAV growing season (April through October). According to Batiuk and others (1992), five water quality parameters are considered to be particularly relevant for SAV habitat restoration and SAV survival -- Secchi depth (SECCHI), total suspended solids (TSS), chlorophyll-a concentration (CHLA), dissolved inorganic phosphorus (DIP), and dissolved inorganic nitrogen (DIN). We obtained data sets for these five parameters from the MD DNR Sampling Program for April through October for the period 1983 through 1997.

The following quality control procedures were followed in preparing the data for analysis. First, data obtained from the MD DNR was examined for outliers and any extraordinary values were discussed with MD DNR staff. In a very few cases, numbers were adjusted, but only in accordance with recommendations by MD DNR staff. Second, a small number of measurements were reported to be below the detection limit of the respective analytical procedure. After examination of the distribution of the seasonal data for each respective parameter for any season during which a value below the detection limit was reported, we decided to set those values equal to the detection limit because the seasonal distributions were not markedly affected. Third, in several cases, replicate samples were analyzed and two measurements were reported for the same parameter on the same day at the same time and location. In these cases, the two measurements were averaged and the average was used in the analysis, unless one of the measurements was reported as below the detection limit, in which case only the measurement not below the detection limit was used.

In discussing the data with staff at MD DNR, we discovered that from 1991 forward, analyses for DIP and DIN were run on filtered water samples, whereas prior to 1991, all analyses were performed on unfiltered samples. Unfortunately, no overlap sampling period existed during which both filtered and unfiltered samples were analyzed. Thus, it is possible that our results reflect changes in sampling procedures in addition to variations in river column conditions before and after 1990.

The median seasonal value for the set of measurements for each parameter for each year was computed. For the CBP segments, measurements made at each water-quality monitoring site within the respective segment were pooled and the season median was computed for the entire data set. Median values for the SAV growing season for the pertinent parameters for years 1983 through 1997 are given in Tables 4-8 for the UTR, LTR, TF2, POTOH and POTMH segments, respectively. Median values for the SAV growing season for the five parameters for years 1983 through 1997 for the nine water-quality monitoring station segments are given in Tables 9-17. Again, samples for DIP and DIN were analyzed differently after the 1990 season, and the contents of the tables are qualified by this observation.

Chesapeake Bay Habitat Criteria and Scores
Tidal Potomac River and Potomac Estuary segmentation and coverage by submersed aquatic vegetation
Open-File Report 99-219 contents page