Virginia Carter, Nancy Rybicki, Jurate Landwehr, Justin Reel and Henry Ruhl


The U.S. Geological Survey has been cooperating with other scientists under the auspices of the Interstate Commission on the Potomac River Basin to utilize existing data from the tidal Potomac River and Estuary for investigating linkages among living resources (primary producers, consumers) and abiotic components of the environment. Because the distribution and abundance of submersed aquatic vegetation in the tidal Potomac River and Estuary are controlled largely by light availability, the first step in investigating linkages with submersed aquatic vegetation is to examine the correlations that exist among vegetative cover, discharge, water quality and weather, all of which can affect light availability directly or indirectly. Growing season (April-October), spring (April-June), and summer (July-August) correlations are presented along with figures demonstrating the significant relationships among variables.


The ecosystem of the Chesapeake Bay, the Nation’s largest estuary, has been adversely affected during the past several decades by eutrophication caused by excessive nutrients entering the bay. Nutrient loading has been identified as the primary cause of periods of hypoxia that kill or stress living resources in parts of the bay. Additionally, high nutrient and sediment loads have decreased water clarity and consequently are largely responsible for the decline in submersed aquatic vegetation (SAV) that form the base of the food chain and provide critical habitat for finfish, shellfish, and waterfowl. The mission of the U.S. Geological Survey’s (USGS) Chesapeake Bay Ecosystem Program is to provide information to a broad community of policy makers, resource managers, scientists, and private citizens working on the restoration of the Chesapeake Bay. As part of this mission, USGS scientists collect and analyze data on current and historical nutrient and sediment loads in the drainage basin of the Chesapeake Bay and determine linkages between hydrologic parameters and the distribution and abundance of SAV in the Potomac River drainage basin.

Light is the primary factor controlling the distribution and abundance of SAV in the Chesapeake Bay and its tributaries (Batiuk and others, 1992; Carter and others, 1994; Carter and Rybicki, 1990). Light availability for SAV photosynthesis and growth is affected by water-column components such as total suspended solids (TSS) and chlorophyll-a (Carter and Rybicki, 1990). Additionally, epiphytic growths on the leaves and stems of SAV further reduce light availability for photosynthesis. Eutrophication causes an increase in the abundance of phytoplankton and thus increases chlorophyll-a, TSS, and epiphyte loads. Weather (precipitation, windspeed, available sunshine) also affects the amount of light available for photosynthesis and thus the distribution and abundance of SAV (Carter and others, 1994).

The USGS is cooperating with other scientists under the auspices of the Interstate Commission on the Potomac River Basin (ICPRB) to utilize existing data from the tidal Potomac River and Estuary for investigation of linkages among primary producers, consumers, water-quality and weather parameters, and discharge.

This this report presents statistical correlations among water quality, discharge, weather, and SAV coverage at mainstem stations in the tidal Potomac River and Estuary. The available data for 1983-95 include (1) biweekly water-quality monitoring data from the Maryland Department of Water Resources, the Virginia Water Control Board, and the District of Columbia, (2) daily and(or) monthly values for windspeed, wind direction, available sunshine (actual minutes of sunlight as a percent of possible sunlight), air temperature, and precipitation at National Airport from the National Oceanic and Atmospheric Administration (NOAA), (3) discharge data at Little Falls and sediment and nutrient loads at Chain Bridge from USGS, and (4) annual SAV coverage from the Chesapeake Bay Program.


Figure 1 is a map of the tidal Potomac River and Estuary showing water-quality monitoring stations and salinity related segments. . Before 1997, the Chesapeake Bay Program subdivided the tidal Potomac River and Estuary into 3 segments: tidal fresh (TF2)(fresh water), transition zone (RET2) (oligohaline to mesohaline), and lower estuary (LE2) (mesohaline) for the purpose of analyzing data and comparing tributaries baywide. Carter and Rybicki (1990) further divide the tidal river (TF2) into two zones, the upper tidal river (UTR) and the lower tidal river (LTR) (table 1; figure 1). In 1997, the Chesapeake Bay Program changed the boundaries of the segments to better reflect salinity, changing segment RET2 to POTOH (oligohaline) and segment LE2 to POTMH (mesohaline)(table 1, figure 1). SAV coverage for stations included all SAV within approximately 3 km upstream and downstream of each station (figure 1). SAV coverage for a segment included all SAV within the segment as depicted on Figure 1. SAV coverage for 1984-87 and 89-95 was provided by the Chesapeake Bay Program. SAV estimates for TF2 and stations therein for 1983 were made by Carter and Rybicki on the basis of extensive field work during the 1983 growing season. SAV estimates for TF2 and POTOH and stations therein for 1988 were made from 1:12,000-scale aerial photographs acquired for the Metropolitan Washington Council of Governments.

Water-quality data were collected biweekly by MD DNR at mainstem stations from Hatton Point to Point Lookout (figure 1). Growing season (April-October) means for each parameter for each year from 1983-95 were used in the correlations. Total nitrogen and total phosphorus were collected from 1986-95. Data collection at station NY (XDB3321) was discontinued in 1990, so this station was not used in the segment correlations. In 1990, analysis of nitrate plus nitrite (NO23), ammonia (NH4) , and phosphate (PO4) was changed from whole water samples to filtered water samples. Probability analysis showed that there was sufficient difference between filtered and unfiltered samples in the case of NH4 and PO4 to preclude including them in the correlations.

Correlations were done for Chesapeake Bay Program segments and for each mainstem station (figure 1) using SAS (PROC CORR)(SAS Institute 1986); correlations were done for the new segments only. Correlations were considered significant when probability ³ 0.05. Growing season precipitation, mean monthly wind speed, and mean monthly available sunshine for 1983-95 from National Airport and growing season mean monthly discharge from USGS were used in the correlations..


Tables 1-16 (Appendix 1) give all the growing season (April-October) Pearson Correlation Coefficients and probabilities for the mainstem stations and the Chespeake Bay Program segments. Tables 17-31 give all the spring (April-June) Pearson Correlation Coefficients and probabilities for the mainstem stations and the Chespeake Bay Program segments. Tables 32-46 give all the summer (July-September) Pearson Correlation Coefficients and probabilities for the mainstem stations and the Chespeake Bay Program Segments. Figures 2-16 (Appendix 2) show the significant growing season correlations for each mainstem station and segment and the relationship among these elements. Figures 17-21 show the significant spring correlations and their relationships for the salinity-related segments and Figures 22-26 show the significant summer correlations and their relationships for the salinity-related segments.

Carter, Virginia, Rybicki, N.B., Landwehr, J. M., and Michael Turtora, 1994, Role of weather and water quality in population dynamics of submersed macrophytes in the tidal Potomac River: Estuaries v. 17, no. 2, p. 417-426.

Batiuk, Richard, Heasley, Patsy, Orth, Robert, Moore, Kenneth, Stevenson, J.C., Dennison, William, Staver, Lori, Carter, Virginia, Rybicki, N.B., Hickman, R.E., Kollar, Stan, Bieber, Steven, and Peter Bergstrom, 1992, Chesapeake Bay submerged aquatic vegetation requirements and restoration goals: a technical synthesis: USEPA CBP/TRS 83-92, 162 p.

Carter, Virginia and N. B. Rybicki, 1990, Light attenuation and submersed macrophyte distribution in the tidal Potomac River and Estuary: Estuaries, v. 13, no. 4, p. 441-442.

                SAS Institute, Inc, 1990, SAS Procedures Guide, Version 6: SAS Circle, Cary, NC, 705p.

Appendix 1 Correlation tables by station and segment for the tidal PotomacRiver and Estuary, 1983-96

Appendix 2 Figures illustrating significant relationships among SAV, water quality parameters, weather parameters, and discharge in the tidal Potomac River and Estuary, 1983-96

For more information please contact Al Lombana

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