Linkages Among Submersed Aquatic Vegetation, Water Quality, Weather, and Discharge in the Tidal Potomac River and Estuary

   Submersed aquatic vegetation (SAV) is an important part of the food web in the Chesapeake Bay, providing shelter and nursery areas for shellfish and finfish and food for a variety of waterfowl, fish, and invertebrates.
    SAV essentially disappeared from the freshwater tidal Potomac River (TF2) and mesohaline lower Estuary (POTMH) in the late 1930's.  SAV has persisted in the oligohaline transition zone (POTOH) throughout the 1900's.  In the early 1980's, as water quality improved, many macrophyte species returned to the freshwater tidal river.  Populations have fluctuated widely in all three salinity segments of the tidal Potomac River and Estuary since 1983 (low-oblique aerial photos).  Recent declines of SAV in the Chesapeake Bay have been of concern to scientists and resource managers.  The tidal Potomac River and Estuary provide an ideal system for research leading to the restoration of SAV baywide.

Back to top

    This map shows the tidal Potomac River and Estuary from the Chain Bridge in Washington, DC, to Point Lookout, Md. (183 kilometers).  The Potomac River and Estuary is divided into three salinity zones (freshwater--TF2, oligohaline--POTOH, and mesohaline--POTMH).  The freshwater tidal river is further divided into the upper tidal river (UTR) and the lower tidal river (LTR).
    The graph below to the right shows SAV cover by year for the UTR and LTR; SAV coverage has been dynamic in these two reaches from 1983-96.

    The graph above to the left shows reemergence of SAV in TF2, continual presence of SAV in POTOH, and minimal vegetation in POTMH.

    This figure illustrates the increase in waterfowl numbers in the Upper Tidal Potomac River area with the increase in SAV coverage in the area.  The waterfowl numbers are from the National Audubon Society Christmas bird count for Ft. Belvoir, Va. and Washington, D.C..

Back to top

    In 1992, USGS researchers collaborated with the US EPA Chesapeake Bay Program (CBP) to establish water-quality criteria for restoration of SAV.  These criteria are reproduced in the following table, with one notable exception: the criteria for Dissolved Inorganic Phosphorus (DIP) shown below have been specifically established for the tidal Potomac River and Estuary and differ from those used regionally for the Chesapeake Bay.

    Recently, USGS investigators have developed a scoring system to determine whether water-quality criteria are met and whether compliance with these criteria does reflect the history of  SAV growth.  In the graphs below, hectares of SAV in Potomac River Segments are plotted by year, along with the SAV Habitat Scores.  The scores indicate to what extent the SAV habitat criteria were met or not met  for each year.   In general, compliance with the water-quality criteria corresponds closely to SAV coverage.  In POTMH, however, all water-quality criteria except DIN are met but SAV coverage has remained near zero although it is increasing.  USGS scientists have demonstrated that lack of available propagules (floating plant fragments, seeds, tubers) is one of the primary factors limiting the rate of SAV reestablishment in the lower part of this reach.

    In general, the water-quality criteria are closely related to SAV coverage in the UTR and LTR.

Back to top

    Light is the primary factor affecting the distribution and abundance of SAV in the Chesapeake Bay and its tributaries.  Light availability for SAV photosynthesis and growth is affected by water-column components such as total suspended solids (TSS) and phytoplankton, (indicated by chlorophyll-a).  Additionally, epiphytic growth on the leaves and stems of SAV further cuts down the amount of light available for SAV photosynthesis.  Increased nutrients cause an increase in the abundance of phytoplankton, TSS, and epiphyte loads.  Weather (precipitation, wind speed, available sunshine) can also affect the amount of light available for photosynthesis-high wind speeds result in resuspension of bottom sediments whereas low wind speed and high available sunshine encourage the growth of phytoplankton and epiphytes.  High river discharge increases erosion and turbidity and brings in nutrients while decreasing residence times for phytoplankton growth.  Below is a conceptual model which is a generalization of the inter- relations of these variables with SAV.

    Interactions among the suite of variables that affect SAV directly and indirectly are complex.  The LTR
interactions diagram below shows the relationships for all parameters in segment LTR.

 Back to top

Linear Correlation Studies
    Linear correlation studies were carried out for each of the Potomac River segments in order to see how the variables in the conceptual model  were related in the observations for the period 1983-1996.  The figure below shows the significant relationships of SAV, CHLA, TSS, and Secchi with other key parameters at the Potomac River and Estuary segments.   Pearson correlation coefficients shown have a probability < 0.05.   (SECCHI = Secchi depth; COND = specific conductivity; SAL = salinity; DISCH = flow; PRECIP = precipitation; TSS = total dissolved solids; TP = total phosphorus; NO23 = nitrate plus nitrite; TN = total nitrogen, CHLA = chlorophyll-a; WATEMP = water temperature; WINDSP = wind speed; AVSUN = available sunshine; SAV = coverage by SAV; SAV_D = difference in SAV coverage from previous year).

Light Threshold Studies
    By studying interannual changes in SAV coverage in relationship to mean seasonal Secchi depth,  the importance of light in the tidal fresh and oligohaline reach of the Potomac is clearly demonstrated.  When light penetration was high -- Secchi depth > 0.68 -- then SAV coverage increased.  At lower light levels, SAV coverage responded selectively to other stressing factors.   The change in SAV coverage from  the previous year  is shown in relation to the mean Secchi depth measured during the SAV growing season (April-October).

Relationships in the tidal fresh Potomac River (TF2)
    Light available to SAV is controlled by TSS and CHLA (an indicator of phytoplankton concentration) in the water column.  SAV coverage was negatively correlated with CHLA in the TF2 , and LTR.  Secchi depth is a measure of light penetration; it was negatively correlated with TSS in TF2, and LTR.  It is not linearly correlated with SAV coverage but rather sets a  threshold for SAV growth.  Secchi depth is not significantly correlated with CHLA in this segment.
    Nitrogen concentrations are high in the tidal fresh Potomac River (TF2, UTR, LTR) compared to the oligohaline (POTOH) and mesohaline (POTMH) segments.  NO23 constitutes >60% of TN. In the TF2 and LTR, NO23 and TN are negatively correlated with CHLA: the more phytoplankton there are, the more inorganic nitrogen they consume.  Phytoplankton growth as indicated by CHLA is sensitive to physical conditions, responding positively to available sunshine in the UTR and negatively to wind speed in the LTR and TF2.  Secchi depth was negatively correlated with wind speed in the UTR.

Relationships in the oligohaline Potomac River (POTOH)
    The oligohaline transition zone of the estuary is generally the location of the turbidity maximum.  There are rarely phytoplankton blooms and CHLA is a more minor part of water column light attenuation.  Secchi depth is negatively correlated with TSS.  Phosphorus is still the limiting nutrient; CHLA is positively correlated with TP.

Relationships in the mesohaline Potomac River (POTMH)
    Recent investigations by the authors have shown that lack of propagules, such as from Zostera marina, a salinity-tolerant species, rather than light has been the primary limiting factor on SAV coverage in the mesohaline Estuary.  SAV coverage is affected positively by higher discharge because more propagules are brought into the reach.  As discharge increases so do nitrogen and TSS concentrations which are brought in from upstream.  As discharge and nitrogen increase, Secchi depth decreases however, values consistently meet the water-quality criteria for restoration of SAV.  In years of higher wind speed, more DIP is resuspended into the water column with TSS, so that CHLA is positively correlated with wind speed.

Back to top

    In the TF2 and POTOH, SAV coverage generally reflects satisfaction of the Chesapeake Bay Program water-quality criteria for SAV restoration.  We have shown that a threshold light condition exists above which SAV has expanded in extent in these reaches but below this threshold, many other factors may affect population expansion or contraction.
    In the POTMH, all water-quality criteria for SAV restoration except DIN are met and SAV coverage has been slowly increasing over time. Transplant experiments and propagule flux studies indicate that lack of propagules is partially responsible for the slow revegetation rate.
    Correlations among the many variables affecting light availability and SAV suggest that these relationships are not simply linear.  The variation in relationships between phytoplankton and nutrients, TSS and Secchi depth, CHLA, and discharge, etc. illustrate how dynamic the river environment is from year to year and from reach to reach, and how complex the interactions are among the suite of variables that affect SAV directly and indirectly.

Back to top