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) • It’s watershed is 14,679 mi2 • A major tributary is the Shenandoah which branches at Harper’s 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) <0.5 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 1940’s 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 • Point Source P Loading to the Tidal Potomac (kg/day) • 32,200 • 7,700 • 1984 400 • 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 1960’s large blooms of cyanobacteria were present over most of the tidal freshwater Potomac River during late summer months

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 Master’s 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 Master’s 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 PotomacRiver fish monitoring and habitat characterization : a study supporting American shad restoration.

19. George Mason Studies Master’s 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 PotomacRiver and tributaries in Virginia.

20. Tidal Potomac Food Web Tributary Flow Nutrients P 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 DissertationEffects 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 DissertationEffects 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 DissertationEffects 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 DissertationEffects 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 (Sta7&10) • 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 DissertationEffects 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 DissertationEffects 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 DissertationThe 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 DissertationThe 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 DissertationThe 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 DissertationSeasonal 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 (>20 um) • Nannoplankton (2-20 um) • Picoplankton (<2 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 DissertationSeasonal 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. The aquatic macrophyte community provides a different habitat than the open water which could lead to a greater abundance and diversity of invertebrates 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 Angela Thorp ThesisThe role of submersed aquatic vegetation in structuring macroinvertebrate communities in the tidal Potomac River

35. She found a wide array of macroinvertebrates in the SAV bed and much more limited numbers and diversity in open water Angela Thorp ThesisThe role of submersed aquatic vegetation in structuring macroinvertebrate communities in the tidal Potomac River

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

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

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

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

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 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 Water Quality and Plankton VariablesGunston Cove Study

42. 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: Water Quality and Submersed Macrophyte Variables

43. 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 Water Quality Data Analysis

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 doesn’t 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 1980’s 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 1980’s • This suggests that P levels had to be drawn down to at least the early 1980’s 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 1980’s 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