George Mason Studies of the Tidal Potomac River

1
George Mason Studies of the Tidal Potomac River

• R. Christian Jones
• Department of Environmental Science and Policy
• Potomac Environmental Research and Education
  Center
• George Mason University
• Fairfax, Virginia, USA

2
Potomac Watershed

• Potomac is a subwatershed of the Chesapeake Bay
• The second largest encompassing parts of
  Virginia, Maryland, West Virginia, Pennsylvania
  and the entire District of Columbia

3
Potomac Watershed

• The Potomac traverses 383 miles (616 km)
• Its watershed is 14,679 mi2
• A major tributary is the Shenandoah which
  branches at Harpers Ferry

4
Potomac Watershed

• Human population is concentrated in the lower
  middle section of the Potomac near the fall line
• Above that is an area of intensive farming and
  beyond that forests are dominant

5
Potomac Watershed

• The watershed has 4 major physiographic (terrain)
  provinces
• The Coastal Plain consists of unconsolidated
  rocks (marine deposits that have never been
  compressed into rock)
• The boundary between the Coastal Plain and the
  Piedmont is called the Fall Line

6
Potomac Watershed

• Climatically, the Potomac Watershed is considered
  Human Subtropical to Humid Temperate
• Temperature varies seasonally, mean from near
  freezing in January to about 80oF in July and
  August
• Rainfall is fairly evenly distributed throughout
  the year with about 3-4 inches per month

7
Potomac Watershed

• Last 10 years of flow data
• Note annual pattern is generally consistent, but
  never exactly the same
• Generally, high flows in the winter and spring
  with lower amounts in summer and fall

8
Potomac Watershed

• Fall Line at Great Falls
• River changes from flowing to tidal

9
Tidal Potomac River

• Part of the Chesapeake Bay tidal system
• Salinity zones
• Tidal Freshwater (tidal river) lt0.5 ppt
• Oligohaline (transition zone) 0.5-6 ppt
• Mesohaline (estuary) 6-14 ppt

10
Tidal Freshwater Potomac

• Tidal freshwater Potomac consists of deep
  channel, shallower flanks, and much shallower
  embayments
• Being a heavily urbanized area (about 4 million
  people), numerous sewage treatment plants
  discharge effluent
• Note Blue Plains and Lower Potomac
• A major study area is Gunston Cove located about
  2/3 down the tidal fresh section of the river

11
Historic Distribution of Submersed Macrophytes in
the Tidal Potomac

• According to maps and early papers summarized by
  Carter et al. (1985), submersed macrophytes
  occupied virtually all shallow water habitat at
  the turn of the 20th century
• Gunston Cove was included

12
Macrophyte Distribution in 1980

• Anecdotal records indicate that by 1939,
  submersed macrophytes had declined strongly and
  disappeared from much of their original habitat
• An outbreak of water chestnut (floating
  macrophyte) was observed in the 1940s and in the
  lower tidal river, Myriophyllum spicatum invaded
  for a brief time
• Surveys done in 1978-81 indicate only very sparse
  and widely scattered beds
• Note no submersed macrophytes were found in
  Gunston Cove

13
P Loading and Cyanobacterial Blooms

• Fueled by nutrient inputs from a burgeoning human
  population and resulting increases in P inputs,
  phytoplankton took over as dominant primary
  producers by about 1930.
• By the 1960s large blooms of cyanobacteria were
  present over most of the tidal freshwater Potomac
  River during late summer months

• Point Source P Loading to the Tidal Potomac
  (kg/day)
• 32,200
• 7,700
• 1984 400

14
Tidal Potomac Food Web
Nutrients P
Light

• Understanding water quality and living resources
  in the tidal Potomac involves obtaining a more
  detailed understanding of the food web in the
  river

Phytoplankton
Submersed Macrophytes (SAV)
Invertebrates (zooplankton benthos)
Fish
15
George Mason Studies Doctoral Dissertations

• Jack Harper. 1988. Effects of summer storms on
  the phytoplankton of a tidal Potomac River
  embayment.
• James Coles. 1994. The effects of temperature and
  light intensity on the dynamics of dominant
  Potomac River phytoplankton.
• Steve Zylstra. 1994. Community structure
  indicators for monitoring tidal freshwater marsh
  systems on the Potomac River, Virginia.
• Daniel Sklarew. 2000. Tidal freshwater Potomac
  River eutrophication patterns and relations to
  climate change, nutrient management, and in situ
  factors.
• Nancy Rybicki. 2000. Relationships between
  environmental variables and species of submersed
  aquatic vegetation in the Potomac River,
  1985-1997.
• Saiful Islam. 2001. Seasonal dynamics of micro-,
  nanno-, and picoplankton in the tidal freshwater
  Potomac River in and around Gunston Cove.
• Leila J. Hamdan. 2003. Source, chemical
  composition and bacterial utilization of
  dissolved labile organic carbon in the Potomac
  River estuary.

16
George Mason Studies Doctoral Dissertations

• Joseph Ivers. 2003. Distribution and abundance of
  fishes relating to artificial aquatic macrophytes
  in the tidal Potomac River.
• Cassi L. Walls. 2004. Sedimentary fatty acids and
  sterols in the Potomac river.
• Cindy Smith. 2004. Distribution and abundance of
  macroinvertebrate communities in artificial beds
  of submerged aquatic vegetation (SAV).
• Theresa Connor. 2005. Temperature and food as
  factors affecting the population ecology of
  Bosmina longirostris (O.F. Muller, 1785) and
  Diaphanosoma brachyurum (Lieven, 1848).
• Phillip Ryan McEachern. 2005.Hydrophobic organic
  compounds in sediments of the Potomac River
  Watershed.
• Ryan Albert. 2007. The influence of past and
  future urbanization on watershed nitrogen export
  and hydrology dynamics in two mid-Atlantic
  watersheds in Fairfax, Virginia.

17
George Mason Studies Masters Theses

• James David Simons. 1984. Seasonal changes in
  primary production, physiognomy and flux of
  organic carbon in a freshwater tidal marsh.
• Sue Anne Touart. 1988. Seasonal and spatial
  distribution of ichthyoplankton of Gunston Cove,
  a freshwater tidal embayment.
• Kathy Monk. 1988. The influence of submerged
  aquatic vegetation on zooplankton abundance and
  diversity in the tidal freshwater Potomac River.
• Allan Hide. 1989. The effects of zooplankton
  grazing on phytoplankton biomass and species
  composition in an hypereutrophic tidal freshwater
  river.
• Susan R. Kircher. 1990. The effect of pH on the
  release of phosphorus from the sediments of
  Gunston Cove, Virginia.
• William Oerlein. 1990. Sediment phosphorus
  available to phytoplankton as a function of pH in
  Pohick Bay, Virginia.
• Steve Blumenshine. 1992. Spatial and temporal
  variation in the diet of the White Perch, Morone
  americana, in a tidal freshwater embayment.

18
George Mason Studies Masters Theses

• Angela Thorp. 1992. The role of submersed aquatic
  vegetation in structuring macroinvertebrate
  communities in the tidal Potomac River.
• Teresa Connor. 1992. Field and laboratory studies
  of the population dynamics, survival, and
  reproduction by Bosmina longirostris.
• Thomas H. Hollowell. 1992. Site profiles for
  tidal freshwater Potomac River candidates for
  inclusion in the Chesapeake Bay National
  Estuarine Research Reserve System in Virginia.
• Jessica Ann Hopple. 1994. Fate of hydrophobic
  organochlorine compounds in Hydrilla verticillata
  relative to sediments in the tidal Potomac River.
  MS. Chemistry.
• Steve Winesett. 1996. The impact of heavy metals
  on Diptera Chironomidae in the tidal freshwater
  Potomac River.
• Stephen Lynn Leathery. 1999. Lower non-tidal
  Potomac River fish monitoring and habitat
  characterization a study supporting American
  shad restoration.

19
George Mason Studies Masters Theses

• Leila J. Hamdan. 2000. Dissolved labile organic
  carbon and bacterial abundance in the Potomac
  River.
• Jeanet Ewing. 2004. A quantitative analysis of
  phytoplankton in the tidal freshwater Potomac
  River using high performance liquid
  chromatography.
• Jeanne M. Classen. 2004. Temporal and spatial
  variations in bacterial community composition in
  the mesohaline Potomac River.
• Christopher Ruck. 2006. An assessment of juvenile
  anadromous fishes (Alosa aestivalis, A.
  pseudoharengus, and Morone americana) in a tidal
  freshwater embayment of the Potomac River.
• Christopher R. Stone. 2006. Geology of a bayhead
  delta within a Potomac River tidal-freshwater
  estuary Pohick Bay, Virginia.
• Daemian Schreiber. 2006. Population genetics of
  American shad (Alosa sapidissima, Wilson 1811)
  and alewife (Alosa pseudoharengus, Wislon 1811)
  from the Potomac River and tributaries in
  Virginia.

20
Tidal Potomac Food Web
Nutrients P
Tributary Flow
Light

• The linkage between phytoplankton and its driving
  variables (light, nutrients, and tributary
  inflow) has been an important focus of our
  research
• Phytoplankton is a crucial food resource and has
  important effects on water quality

Phytoplankton
Submersed Macrophytes (SAV)
Invertebrates (zooplankton benthos)
Fish
21
Gunston CoveMajor Study Site

• Gunston Cove is a shallow embayment located about
  midway in the tidal freshwater part of the
  Potomac
• It receives runoff from the suburban Accotink
  Creek and Pohick watersheds
• It also receives treated sewage from the Noman
  Cole Pollution Control Plant of Fairfax County

22
Jackson Harper Dissertation Effects of summer
storms on the phytoplankton of a tidal Potomac
River embayment

• Phytoplankton and forcing factors measured on a
  daily basis over a 4 month period, mid-June to
  mid-Oct, 1986
• Focus on two cove stations
• Forcing factors from storms included
• Solar radiation
• Tributary input

23
Jackson Harper Dissertation Effects of summer
storms on the phytoplankton of a tidal Potomac
River embayment

• Solar radiation peaked in late June at time of
  summer solstice and declined through the study
  period
• Large variation from day to day reflected cloud
  cover
• Note the alternation between several days of
  relatively high light followed by 1-2 days of low
  light and then back to higher
• marks at the bottom indicate cold front
  passage, which many times corresponded to low
  light days

24
Jackson Harper Dissertation Effects of summer
storms on the phytoplankton of a tidal Potomac
River embayment

• Another factor that reflected summer storms is
  the volume of runoff entering the cove from
  tributary streams
• Note that during the early days of the study
  tributary flow into the cove was rather limited
• However, beginning in mid July and extending
  through the end of August there were frequent
  periods of high tributary flows indicating that
  cold front passage was accompanied by more
  precipitation and thus higher runoff
• In Sept and early Oct tributary flow declined to
  low levels

25
Jackson Harper Dissertation Effects of summer
storms on the phytoplankton of a tidal Potomac
River embayment

• Chlorophyll a which is an indicator of
  phytoplankton biomass increased steadily in late
  June-early July to a peak in mid July in the Cove
  (Sta710)
• A steady decline was observed through mid August
  and then a slight peak in mid September before
  finally dropping off in mid October
• Note that the values observed in the cove were
  generally over 100 ug/L which is very high
• Note that in the river values were much lower

26
Jackson Harper Dissertation Effects of summer
storms on the phytoplankton of a tidal Potomac
River embayment

• If we line up the stream flow graph and the
  chlorophyll graph, we see that the periods of
  high and increasing chlorophyll correspond to low
  tributary stream input (late June-early July
  mid-Sept)
• Likewise the period of higher tributary inflow
  from mid July to mid August witnessed a decline
  in phytoplankton
• These results suggest that tributary input from
  summer storms decreases phytoplankton

27
Jackson Harper Dissertation Effects of summer
storms on the phytoplankton of a tidal Potomac
River embayment

• Using the light and tributary flow data and dye
  studies to track the movement of water, Harper
  came up with a model describing the impact of
  summer storms
• From this, he developed a description of the
  parameters of an average cyclic response of the
  system to summer storms
• The cycle had an average period of about 5 days

28
Jim Coles Dissertation The effects of
temperature and light intensity on the dynamics
of dominant Potomac River phytoplankton.

• Jim Coles examined the temperature and light
  responses of 4 species of phytoplankton isolated
  from the Gunston Cove area
• Three were cyanobacteria and one was a diatom
• Microcystis
• Merismopedia
• Oscillatoria
• Melosira

Merismopedia
Microcystis
Melosira
Oscillatoria
29
Jim Coles Dissertation The effects of
temperature and light intensity on the dynamics
of dominant Potomac River phytoplankton.

• Algae typically show an increase in
  photosynthesis with increasing light until a
  certain level light saturation is reached, then
  level off or decrease at higher light
• These rates often increase with temperature

30
Jim Coles Dissertation The effects of
temperature and light intensity on the dynamics
of dominant Potomac River phytoplankton.

• In these experiments, the diatom showed a
  temperature effect only up to 20oC, but did not
  show high light inhibition (it is found in spring
  with high light and lower temps)
• The cyanobacteria continued to increase with
  temperature up to 30oC, but dropped off at high
  light (they are characteristic of late summer
  with high temps, but decreasing light)

31
Saiful Islam Dissertation Seasonal dynamics of
micro-, nanno-, and picoplankton in the tidal
freshwater Potomac River in and around Gunston
Cove.

• Phytoplankton come in a range of sizes
• Microplankton (gt20 um)
• Nannoplankton (2-20 um)
• Picoplankton (lt2 um)
• These can be separated using sequential
  filtration through ever small filters
• Islam looked at the seasonal dynamics of the
  different size fractions in Gunston Cove

32
Saiful Islam Dissertation Seasonal dynamics of
micro-, nanno-, and picoplankton in the tidal
freshwater Potomac River in and around Gunston
Cove.

• He found that nannoplankton were most important
  in spring
• Microplankton took over in late spring and
  remained dominant for most of the remainder of
  the year
• A peak in late August in nannoplankton was found
• Picoplankton were the least abundant always
• These patterns would affect food availability for
  invertebrates zooplankton who select food based
  on size

33
Tidal Potomac Food Web
Nutrients P
Light

• Another focus of our research has been to
  understand how the presence of SAV may alter the
  invertebrate community
• This would have indirect effects on the fish
  which utilize the invertebrates as a food source

Phytoplankton
Submersed Macrophytes (SAV)
Invertebrates (zooplankton benthos)
Fish
34
Angela Thorp ThesisThe role of submersed aquatic
vegetation in structuring macroinvertebrate
communities in the tidal Potomac River

• Thorpe compared the macroinvertebrates in two
  types of SAV beds with those in adjacent open
  waters
• Hypothesis was that SAV would harbor more
  invertebrates and a greater diversity

• The aquatic macrophyte community provides a
  different habitat than the open water which could
  lead to a greater abundance and diversity of
  invertebrates

35
Angela Thorp ThesisThe role of submersed aquatic
vegetation in structuring macroinvertebrate
communities in the tidal Potomac River

• She found a wide array of macroinvertebrates in
  the SAV bed and much more limited numbers and
  diversity in open water

36
Angela Thorp ThesisThe role of submersed aquatic
vegetation in structuring macroinvertebrate
communities in the tidal Potomac River

• Interestingly, the differences between the types
  of plants was small even when compared to
  differences between months
• Open water samples group on left
• On right SAV samples group not by SAV type, but
  by month
• These results substantiate the potential
  importance of SAV beds as a food source for fish

37
Kathy Monk Thesis The influence of submerged
aquatic vegetation on zooplankton abundance and
diversity in the tidal freshwater Potomac River

• Monk looked at zooplankton in a similar way that
  Thorpe looked at macroinvertebrates
• Zooplankton are smaller organisms, some of which
  do well in the open water
• However, SAV beds provide cover and possibly more
  or better food for zooplankton

• Like Thorp, Monk compared the macroinvertebrates
  in two types of SAV beds with those in adjacent
  open waters
• Hypothesis was that SAV would harbor more
  zooplankton and a greater diversity

38
Kathy Monk Thesis The influence of submerged
aquatic vegetation on zooplankton abundance and
diversity in the tidal freshwater Potomac River

• All groups were found in greater numbers in the
  SAV beds
• Some groups were much more common in the SAV
  beds, for example ostracods and some cladocera

39
Kathy Monk Thesis The influence of submerged
aquatic vegetation on zooplankton abundance and
diversity in the tidal freshwater Potomac River

• Monk also found a clear difference between the
  zooplankton community in the open water and that
  in the vegetation treatments
• This indicates the importance of the SAV beds as
  a habitat for many species of zooplankton
• And their potential importance as a feeding area
  for fish

40
Since 1983/84, water quality, plankton, fish and
benthos have been monitor-ed on a generally
semimonthly basis at a number of sites in the
Gunston Cove area.
Noman Cole PCP
Monitoring Site Key ? water quality and
plankton ?fish trawl fish seine
41
Water Quality and Plankton VariablesGunston Cove
Study

• Water Quality Variables
• Temperature
• Conductivity
• Dissolved oxygen
• pH
• N NO3-, NH4, organic N
• P PO4-3, Total P
• BOD
• TSS, VSS
• Chloride
• Alkalinity

• Plankton Variables
• Surface Chlorophyll a
• Chlorophyll a
• Photosynthetic rate (mgC/L/hr) at light sat.
• PBmax (mgC/mgChla/hr)
• Phytoplankton cell density by species
• Phytoplankton biovolume by species
• Microzooplankton (44 µm net) abundance
• Macrozooplankton (202 µm net) abundance

42
Water Quality and Submersed Macrophyte Variables

• Water Quality Variables
• Temperature
• Conductivity
• Dissolved oxygen
• pH
• N NO3-, NH4, organic N
• P PO4-3, Part. P,Total P
• BOD
• TSS, VSS
• Chloride
• Alkalinity
• Chlorophyll a
• Secchi depth

• Submersed Macrophytes
• 1994-2006
• Areal coverage using aircraft remote sensing
• Data collected by Virginia Institute for Marine
  Studies for the Chesapeake Bay program
• Pre 1994
• USGS field surveys
• GMU field surveys

43
Water Quality Data Analysis

• Summer data (June-September) utilized
• Utilized one cove station (Station 7) that has
  been sampled continuously over the period
  1983-2006
• Scatterplot by year over the study period
• LOWESS smoothing function applied
• Linear trends also tested over the study period
• Regression coefficients determined for
  significant linear trends
• Pre-1983 data were examined to place current
  study in context

44
Gunston Cove StationTotal Phosphorus

• P is limiting nutrient in this system
• Summer total phosphorus showed little change from
  1983 through 1988
• Summer total phosphorus decreased consistently
  from 1989 through 2006
• Linear trend highly significant with a slope of
  -0.0044 mg/L per yr or 0.10 mg/L over the period
  of record.
• P load decrease was complete by early 1980s. Yet
  TP decrease doesnt seem to start until 1990? Or
  was the 1983-88 period just a pause in a decline
  in TP that started earlier?

45
Gunston Cove StationChlorophyll a

• Chlorophyll a levels have decreased substantially
  over the period.
• In the mid to late 1980s chlorophyll a
  frequently exceeded 100 ug/L.
• Decline started in 1990 and quickened after 2000
• By 2006 values were generally less than 30 ug/L
  with a median of about 20.
• Linear regression yielded a significant linear
  decline at a rate of -3.8 ug/L per year or 84
  ug/L over the entire study
• Again, did the chlorophyll decline start in 1990
  or was this only part of a longer chlorophyll
  decline?

46
Gunston Cove StationTP Extended Record

• Limited data from 1969/70 indicates that TP was
  much higher at that time
• So, perhaps what appeared to be a lag or delayed
  response was actually just a pause in the
  loading-induced TP decline
• The pause was associated with high pH induced
  internal loading
• Total decline was from 0.8 mg/L to 0.06 mg/L over
  36 yrs or 0.02 mg/L/yr

47
Gunston Cove StationChlorophyll a Extended
Record

• In contrast to the TP and SRP, values of
  chlorophyll a from 1969/70 were not substantially
  higher than in the early 1980s
• This suggests that P levels had to be drawn down
  to at least the early 1980s levels (c. 0.15
  mg/L) before nutrient limitation of phytoplankton
  could begin to be a factor
• By 2000, TP was at about 0.10 mg/L and as it
  dropped further it began to cause a clear drop in
  chlorophyll a

48
TP response to decreased P Loading?

• Rate of TP decline was slow during 1980s period
  of internal loading
• Rate quickened in 1990 with apparent cessation of
  internal loading

49
Chla response to decreased TP in water column?

• Adding in historic data shows that before P
  loading reductions, chlorophyll was not sensitive
  to P in water column
• Presumably it was saturated with P, but by 1983,
  P and Chl were pretty closely related.
• Even with reductions, TP had to drop below 0.2
  mg/L, then Chl started to decline proportionately

50
Gunston Cove Light Environment

• Full restoration of Gunston Cove requires
  re-establishment of submersed macrophyte beds
• The primary requirement for this is light
  availability throughout the water column
• Light attenuation is due to algae, inorganic
  particles, and dissolved substances

51
Light Conditions Required for Submersed
Macrophyte Growth

• Batiuk et al. established minimum light
  requirements for SAV growth in Chesapeake Bay
• In tidal fresh region, 9-13 of incident light
  during SAV growing season was needed

52
Gunston Cove Station

• Secchi disk was fairly constant from 1984 through
  1995 with the trend line at about 40 cm.
• Since 1995 there has been a steady increase in
  the trend line from 40 cm to nearly 80 cm in
  2003.
• Linear regression was highly significant with a
  predicted increase of 1.51 cm per year or a total
  of 33 cm over the long term study period

53
Gunston Cove Light Environment over time

• Using the two time series of Kd, maximum depth of
  macrophyte colonization was predicted using the
  10 surface light criterion
• Predicted maximum macrophyte depth was well below
  1 m during the 1980s and 1990s
• But beginning in about 2000 it started to rise
  consistently and passed 1 m by 2003/04

Secchi-disk approx. Measured Kd
54
Reemergence of Submersed Macrophytes in Gunston
Cove

• 2000 Distribution

55
Reemergence of Submersed Macrophytes in Gunston
Cove

• 2005 Distribution

56
Summary of Phytoplankton, Light, Submersed
Macrophyte Response

• Improvements in water clarity related to
  P-limitation and decline of phytoplankton were
  correlated with an increase in submersed
  macrophyte coverage in Gunston Cove
• Since 1 m colonization depth was achieved (2004),
  macrophyte coverage has increased strongly

57
We have documented the partial restoration of
Gunston Cove to its pre-eutrophication conditions
including -Decrease in P loading -Decrease in TP
and phytoplankton chlorophyll -Increase in water
clarity -Reestablishment of submersed macrophyte
beds to a substantial portion of the cove
58
Acknowledgements

• Dr. Donald Kelso
• Fairfax County Department of Public Works
• Metropolitan Washington Council of Governments
• Interstate Commission on the Potomac River Basin
• U.S. Army Garrison Fort Belvoir
• Piedmont Environmental Council
• District of Columbia Government
• Many GMU graduate and undergraduate students have
  participated in field and lab data collections