Eos,Vol. 80, No. 21, May 25,1999 EOS
T R A N S A C T I O N S ,
A M E R I C A N
G E O P H Y S I C A L
VOLUME 80
U N I O N
NUMBER 21
MAY 25,1999 PAGES 237—244
Interdisciplinary Environmental Project Probes Chesapeake Bay Down to the Core
PAGES 2 3 7 , 2 4 0 - 2 4 1 Interrelated environmental c o n c e r n s about Chesapeake Bay are being addressed in an interdisciplinary project using paleoecological and g e o c h e m i c a l records from sediment cores to investigate H o l o c e n e climate and human e n c r o a c h m e n t . T h e research is looking at interannual through millennial-scale vari ability of bay salinity,sediment accumulation, and dissolved oxygen, temperature, and faunal and floral trends. Current and planned research is expected to result in better restora tion strategies by improving our understand ing of the linkages between the bay's ecosys tem, climate, and land use. Chesapeake Bay, the United States' largest and most productive estuary, faces several c o m p l e x environmental issues, including eutrophication and anoxia in the main channel and tributaries, high turbidity and rates of sed imentation, outbreaks of the toxic dinoflagellate Pfiesteria piscicida, and coastal erosion and s u b m e r g e n c e tied to sea-level rise. Such problems often are attributed to human activi ties in the bay's watershed, including pollu tion, urbanization, and deforestation, but it now is recognized that climatic factors also strongly influence bay salinity, temperature, and water quality Concern about the bay's future is especially acute for policy makers and land resource managers involved in bay restoration efforts but faced with a number of questions.Will the anomalously high and variable river discharge of the past d e c a d e continue? What impact will greater mean annual rainfall projected by climate models have on the bay? How will changes in river discharge and rainfall affect nutrient loads and anoxia in the bay? Have unique climatic and hydrological conditions contributed to recent toxic algal blooms?
A n o x i a , Salinity, a n d R i v e r D i s c h a r g e Chesapeake Bay is 320 km long, - 2 0 4 0 km wide, covers an area of 6,500 k m , and drains 166,000 k m of watershed, mainly in Maryland, Virginia, Pennsylvania, the District of Columbia, and New York (Figure l ) . T h e bay fills a den dritic river valley system consisting of the main channel (maximum depth - 5 3 m, aver age depth - 8 m ) and its tributaries that were drowned during the postglacial sea-level rise beginning about 10,000 years ago.The major sources of freshwater flow into the bay, all riv ers, are the Susquehanna (48% of total in2
2
Fig. 1. Satellite image showing Chesapeake Bay watershed and location of two sediment core sites (PTXT-2 and PTMC-3) under study for paleoclimatic history and the impact of anthropogenic changes on bay ecosystem. Major tributaries are labeled.
Eos,Vol. 80, No. 21, May 25,1999 Discharge
7/84
7/85
7/86
7/87
7/88
7/89
7/90
7/91
7/92
7/93
7/94
7/95
7/96 7/97
Salinity
cycling, high nitrogen demands by phytoplankton, bacteria, and benthos during sum mer are met by nitrogen regeneration through the decay of organic material in the water col umn and in bay sediments [Boynton and Kemp, 1985].Anoxia development in the bay is also influenced by wind-driven advection, aerobic heterotrophic and anaerobic bacteri al activity high June-August streamflow, tem perature, and other factors. It also is generally believed that 18th and 19th century deforesta tion and 20th century urbanization and fertil izer use contributed significantly to seasonal anoxia there [Cooper and Brush, 1991]. Such concerns led to large-scale restoration efforts beginning in 1983 with the initiation of the Chesapeake Bay Program to reduce the controllable load of nutrient influx from the watershed by 40% by the year 2000. Although nutrient dynamics and oxygen depletion are well known for seasonal timescales in the bay the impact of interannual and decadal vari ability in climate and streamflow on Chesa peake ecosystems has not b e e n studied and neither the duration nor the causes of pre-20th century anoxia are known. For example, since 1984, nitrogen and phosphorous loadings vary by factors of 2 and 4, respectively b e c a u s e of annual variability in streamflow. B e c a u s e pre cipitation and streamflow are highly correlat ed and there may b e teleconnections between regional climate and hemispheric and global climatic patterns [ Vega et al, 1999], climatic variability is a critical but poorly understood aspect of long-term ecosystem restoration and management [Cronin, 1997]. Salinity a n d Dissolved O x y g e n Variability
7/84
7/85
7/86
7/87
7/88
7/89
7/90
7/91
7/92
7/93
7/94
7/95
7/96 7/97
month/year Fig. 2. Seasonal and interannual salinity and dissolved oxygen in the Chesapeake Bay from 1984 to 1997. Record of river discharge (upper panel) from U.S. Geological Survey stream gauge station at Point of Rocks, Maryland, and monitoring record of Chesapeake Bay salinity and dissolved oxy gen (lower panels) from Chesapeake Bay Program station off the mouth of the Potomac River. Shown are both seasonal and interannual variability in discharge, salinity, and dissolved oxygen. Arrows in middle panel point out extremely low salinity during wet years. Arrows in lower panel show bottom waters remaining oxygenated during relatively dry, low discharge years in contrast to anoxia during most wet years.
flow), Potomac (33%), J a m e s (13%), Rappa h a n n o c k (3%), and Patuxent (1%) Rivers along the western shore, and the Choptank (1%) and Nanticoke ( 1 % ) along the eastern shore [Schubel and Pritchard, 1987]. As a partially stratified estuary, Chesapeake Bay experiences large seasonal and interan nual variability in salinity, temperature, and dissolved oxygen.This variability is influ e n c e d by precipitation and river discharge from the watershed, as well as nutrient con centrations in tributaries and ecological and physical processes in the estuary. Briefly, sea
sonal (June-September) anoxia develops below the pycnocline (steep vertical density gradient) in mesohaline portions of the bay after high spring river flows increase salinity stratification ( h a l o c l i n e ) , a n d fresher bay sur face water flows out over denser inflowing Atlantic water.This stratification minimizes oxygen replenishment to deeper water in northern bay basins [Boicourt, 1992]. In addition to physical processes, nutrient dynamics are also critical influences in the development of anoxia in the bay For exam ple, in s o m e models of the bay's nutrient
The relationship b e t w e e n river discharge, salinity, and dissolved oxygen levels over s e a s o n a l and interannual t i m e s c a l e s is evi dent in records from a Chesapeake Bay Program monitoring station off the mouth of the Potomac for the period 1984-1997 (Figure 2 ) . T h e most obvious trend o c c u r s over seasonal timescales when large spring discharge leads to a drop in both salinity and dissolved oxygen from high salinity, well-oxygenated winter conditions. Whether summer bottom oxygen reaches anoxic conditions ( 1.0 c m y r at PTMC-3. A sharp increase in ambrosia (ragweed) pollen, a stratigraphic event dated at about 1800-1850 when major land clearance took place, provides an additional time marker at core depths of 120 and 155 c m at sites PTXT-2 and PTMC-3, respectively With this chronology, downcore changes in the relative abundance of the marine benthic foraminiferal species Elphidium selseyense can provide a useful proxy of paleosalinity for the past 150 years. Although many factors influence species dis tribution in modern Chesapeake Bay this spe cies dominates benthic foraminiferal assem blages living in the bay when salinity e x c e e d s about 20 ppt; its a b u n d a n c e decreases in pro portion to decreasing salinity, down to a salini ty of 10 ppt. Both PTXT-2 and PTMC-3 sites have similar temporal trends in E. selseyense (Figure 3 c ) , reflecting large-scale variability in the bay's salinity related to climatic factors. Between 1850 and 1910, salinity levels as much as 8-10 ppt lower than those of recent decades are documented in these cores. 1
1
Rainfall a n d D r o u g h t This was a time of heavy rainfall, according to rainfall records from Washington, D.C., and tree ring records of summer precipitation in the Potomac River watershed and tidewater areas of southeastern Virginia and northeast ern North Carolina. Also, drought intervals dur ing the 20th century are recorded by high bay
salinity levels during the 1920s, 1930s, and 1960s.Tree ring records in the Potomac water shed confirm that extended summertime drought during the 1960s was unparalleled since at least 1730 [Cook andJacoby, 1983]. The dry 1960s contrast with the last few de cades of high and variable discharge and sa linity ( m e a n annual discharge often twice that of the 1960s). Paleosalinity history of the last few millen nia is being reconstructed from longer sedi ment cores dated by radiocarbon using foraminifera and other salinity proxies (ostracode, dinoflagellates, elemental shell chemistry). These results reveal several multidecadal periods of extremely high salinity indicat ing the o c c u r r e n c e of long-term droughts in the region.The most severe droughts resulting in high salinity levels in Chesapeake Bay were during the mid to late 16th century Tree ring records from the eastern United States also indicate 16th century droughts during these intervals [Stable et al., 1998] .The bay's sedi mentary record of late Holocene climate dur ing the periods known as the medieval warm period (9th through 14th centuries) and the little ice age (15th through 19th centuries) is described more fully elsewhere b y T C r o n i n et al. (unpublished manuscript, 1999). In addition to paleosalinity reconstruction, core samples have b e e n analyzed for geo c h e m i c a l and faunal indicators of paleoanoxia and productivity. Analyses include nitrogen isotopes ( 5 N ) , redox-sensitive metals (V,Zn, Cr, Ni, Cu), biogenic silica, and foraminiferal species tolerant of reduced levels of dissolv ed oxygen. Correlation of trends in metals with radiocarbon data suggests significant periods of seasonal anoxic conditions and high diatom productivity in the northern part of the bay between about 1200 and 1500 A.D.,as well as increasing overall oxygen de pletion and higher sediment diatom c o n c e n tration since approximately 1700.The ampli tudes of the fluctuations in the three geo c h e m i c a l parameters are stronger in the nor thernmost core, possibly b e c a u s e of bathyme try and greater proximity to the Susquehanna River mouth. Foraminiferal data also suggest that extended periods of anoxia since about 1970, probably owing to the c o m b i n e d effects of increased streamflow and nutrient influx, had a severe impact on Chesapeake Bay biota. 15
Sedimentation and Land Clearance Increased river discharge can also increase sediment flux. Suspended sediment increases particulate nutrient influx and turbidity, de creases light penetration, and disturbs subaquatic vegetation, a major habitat for bay organisms. Chesapeake Bay is a natural sedi ment sink; during the Holocene, a sedimenta ry wedge of variable thickness accumulated at rates that vary spatially b e c a u s e of estuarine circulation processes. It has been estimat ed that large-scale agriculture and deforesta-
Eos,Vol. 80, No. 21, May 25,1999 tion in the late 18th and 19th centuries increased sediment accumulation rates from about 0.1 c m yr" to 0.5 to 1.0 c m y r \ a 5- to 10-fold increase over rates prior to land clear a n c e [Cooper and Brush, 1991]. New C ages of fossil marine shells obtained from several coring sites indicate that precolonization sedi mentation rates in the main stem of the midbay ranged from 0.1 to > 0.5 c m yr" over the past 2000 years.These rates are somewhat higher than previous estimates based on C ages of bulk organic c a r b o n but are consis tent with stratigraphic data indicating a thick H o l o c e n e s e q u e n c e in many regions. Chronologies based on C s and P b sug gest that overall sediment rate increased at many sites in the mid-bay by two to four times following 18th and 19th century land clearance.These studies will b e used to improve our understanding of the sediment budget for the bay 1
1 4
1
graphic variability at o c e a n i c and cryospheric timescales in the Past Global Changes/Interna tional Geosphere-Biosphere Programme. Long coring in the bay is scheduled on an IMAGES cruise in the summer of 1999 to the Carib bean, the North Atlantic, and the Labrador and Nordic Seas and adjacent coastal regions. Researchers want to obtain cores containing as much of the Chesapeake Bay H o l o c e n e sedimentary record as possible.
1 4
1 3 7
2 1 0
Acknowledgments Research has been supported by the U.S. Geological Survey's ecosystems program, coast al and marine program, and climate change program.We are extremely grateful to Ray Najjar and Marcia Olson for a c c e s s to salinity and dissolved oxygen data, to Walt Boynton, Jerry Franks, and the crew of Aquarius, and Rick Younger and the crew of Discovery
for assis
tance in obtaining sediment cores. Comments Holocene History To more fully investigate the H o l o c e n e sedi mentary record of Chesapeake Bay a collabo rative program is being developed between the U.S. Geological Survey the U.S. Naval Research Laboratory, the Maryland Geological Survey and the U.S. Environmental Protection Agency The program is part of the Interna tional Marine Global Change Study (IMAGES), which is investigating climate and o c e a n o -
from Lynn Wingard and Walt Boynton consider ably improved the manuscript. Authors T. Cronin, S. Colman, Holmes,
D. Willard, R. Kerhin, C.
A. Karlsen, S. Ishman,
and J. Bratton
For more information, contact T. Cronin, U.S. Geological Survey, 2051 Eakins Court, Reston, VA 20191 USA; E-mail:
[email protected].
SPRING MEETING PREVIEW Researchers Explore Recent Advances in Understanding Gas Hydrates PAGES 2 3 7 - 2 3 8 "What will we get for the money?" That was o n e of the refrains asked by m e m b e r s of the U S . Congress during a May 12 House of Repre sentatives' s c i e n c e s u b c o m m i t t e e hearing on the Gas Hydrate Research and Development Act of 1999 (House Resolution 1753). But, impressed by the need for federal funding of key research into gas hydrates—a potential, abundant source of energy a major sink for glob al carbon, and an influence on seafloor stabili ty—committee members unanimously approved the legislation. It would earmark $42.5 million over five years to help fuel an multi-agency re search program by the U.S. Department of Ener gy (DOE),US.Geological Survey (USGS),US. Navy National Science Foundation (NSF),and other agencies, and also help fund researchers in academia and elsewhere. H.R. 1753, which is similar to a Senate bill (S.330) approved in April, currently is being further considered by the House S c i e n c e and
Resources Committees. S o m e researchers, while welcoming the legislation, had recom m e n d e d at least two or more times as much funding as the House bill would provide. Gas hydrate research, a topic that has gained support across party lines in Congress, also is a growing scientific concern. S o m e recent stateof-the-art geochemical, geophysical, and biolog ical research through modeling and from labo ratory and field experiments will b e presented during two oral sessions and one poster ses sion on hydrates at the upcoming AGU Spring Meeting in Boston, Mass. from J u n e 1 to 4. For more than a century, scientists have known about methane hydrates, which are naturally occurring crystalline solids of water and gas that look like ice, in which gas mole cules are surrounded by a cage of water mol ecules. Gas hydrates also are tremendous concentrators of methane, and researchers say that o n e unit volume of gas hydrate c a n con tain more than 160 volumes of gas at surface
References Boicourt,W. Confluences of circulation processes on dissolved oxygen in the Chesapeake Bay in oxygen dynamics in the Chesapeake Bay: A synthesis of recent results, edited b y D. E. Smith, M. Leffler, a n d G. M a c k i e r n a n , pp. 7-59, Maryland S e a Grant B o o k , C o l l e g e Park, Md., 1 9 9 2 . B o y n t o n , W. R., a n d W. M. Kemp, Nutrient regenera tion a n d oxygen c o n s u m p t i o n by s e d i m e n t s along an estuarine salinity gradient, Marine Ecol. Prog. Sen, 23,45-55,1985. Cook, E. R., a n d G. C. J a c o b y P o t o m a c River streamflow s i n c e 1 7 3 0 as r e c o n s t r u c t e d by tree r i n g s , i Clim.Appl. Meteorol., 22,1659-1672, 1983. Cooper, S. R., a n d G. S. Brush, Long-term history of C h e s a p e a k e Bay a n o x i a , Science, 254,992996,1991. Cronin,T. M., Climate Control, Chesapeake Bay J . , 7(3), 1,4-6. Najjar, R . , T h e water b a l a n c e of t h e S u s q u e h a n n a River Basin a n d its r e s p o n s e to c l i m a t e c h a n g e , J. Hydrol., in press, 1 9 9 9 . S c h u b e l , J. R., a n d D.W. Pritchard, A brief phys ical d e s c r i p t i o n of t h e C h e s a p e a k e Bay, in Contaminant problems and management of living Chesapeake Bay resources, edited by S. K. Majumdar, L.W. Hall Jr., a n d H. M.Austin, pp. 1-32, Pa. A c a d . Sci., P h i l a d e l p h i a , Pa., 1987. Stahle, D.W., M. K. Cleaveland, D. B. B l a n t o n , M. D.Therrell, a n d D. A. Gay,The lost c o l o n y a n d J a m e s t o w n drought, Science, 280,564-567, 1998. Vega, A., C.-H. Sui, a n d K.-M. Lau, Interannual to i n t e r d e c a d a l variations of t h e r e g i o n a l i z e d sur f a c e c l i m a t e of t h e United States a n d relation ships to g e n e r a l i z e d flow p a r a m e t e r s , Phys. Geogr.,'m press, 1 9 9 9 .
conditions. But the substance was largely dis missed as a laboratory curiosity until a Russian drilling crew in 1964 discovered m e t h a n e hy drates occurring naturally, which increased interest in locating other deposits. Now, gas hydrate research c a n involve scien tists from a number of disciplines investigating energy potential, geological hazards, and glob al climate change, according to Carol Ruppel, co-convener of the AGU sessions and assistant professor of geophysics at the Georgia Institute of Technology O n e driving reason for increasing r e s e a r c h into gas hydrates is the h o p e that perhaps the a b u n d a n t quantities of gas hydrates— w h i c h are found in m a r i n e e n v i r o n m e n t s at water depth greater than a b o u t 5 0 0 m and also in permafrost r e g i o n s — c o u l d help to m e e t future n e e d s for c l e a n e r energy. Ap proximately 9 8 % of all gas hydrates found in the world are m e t h a n e hydrates, a c c o r d i n g to researchers. "Wide production of methane from hydrates could reduce c a r b o n dioxide emissions by as much as 20% on a global basis without any reduction in energy consumption," Gerald Holder, dean of the s c h o o l of engineering at the University of Pittsburgh, testified at the May 12 Congressional hearing. USGS Research Biologist William Dillon, another hearing witness and the co-author of several papers and a poster to b e presented
