Distribution and Abundance of Submerged Aquatic Vegetation in Chesapeake Bay: An Historical Perspective

Estuaries Vol. 7, No. 4B, p. 531540 December 1964

Distribution and Abundance of Submerged

Aquatic Vegetation in Chesapeake Bay:

An Historical Perspective’

ROBERT J. ORTH, AND KENNETH A. MOORE Virginia Institute of Marine Science of the College of William and Mary Gloucester Point, Virgina 23062

ABSTRACT: An historical summary of the distribution and abundance of submerged aquatic vegetation (SAV) in the Chesapeake Bay is presented. Evidence suggests that SAV has generally been common throughout the bay over the last several hundred years with several fluctuations in abundance. The decline of Zosrera marina (eelgrass) in the 1930’s and the rapid expansion of Myriopl?yZlum spicutum (watermilfoil) in the late 1950’s and early 1960’s were two significant events involving a single species. Since 1965, however, there has been a significant reduction of all species in most sections of the bay. Declines were first observed in the Patuxent, Potomac and sections of other rivers in the Maryland portion of the Bay between 1965 and 1970. Dramatic reductions were observed over the entire length of the bay from 1970 to 1975. Particularly severe losses were observed at the head of the bay around Susquehanna Flats as well as in numerous rivers along Maryland’s eastern and western shores. Changes in the lower, Virginia portion of the bay occurred primarily in the western tributaries. Greatest losses of vegetation occurred in the years following Tropical Storm Agnes in 1972. Since 1975 little regrowth has been observed in the Chesapeake Bay. Other areas along the Atlantic Coast of the U.S. during the same period have experienced no similar widespread decline. It thus appears that the factors affecting the recent changes in distribution and abundance of submerged vegetation in the bay are regional in nature. Causes for this decline may be related to changes in water quality, primarily increased eutrophication and turbidity.

Introduction A distinct feature and one of Chesapeake

Bay’s important natural resources are the beds of submerged aquatic vegetation (SAV) that have occupied many of the shallow water areas (Anderson 1972; Stevenson and Confer 1978; Orth et al. 1979; Orth and Moore 198 1). In addition to its high pri- mary productivity, SAV is important because it is a food source for waterfowl, a habitat and nursery area for many species of commercially important fish and inver- tebrates, a shoreline erosion control mech- anism, and a nutrient buffer (Thayer et al. 1975).

Because of the recognized values of SAV beds to the Chesapeake Bay, and in partic-

’ Contribution No. 1187 from the Virginia Institute of Marine Science, School of Marine Science, Glouces- ter Point, Virginia.

ular its waterfowl populations, there has been some interest historically in the distribution and abundance of these vegetated areas. This has resulted in various field surveys by both state and federal agencies at different times and locations, utilizing, for the most part, different methodologies. Un- fortunately, much of the information from these surveys went unpublished or remained as state or federal reports. We have attempted here to summarize all the perti- nent information available and to provide a summarization of both the recent and past changes in SAV abundance.

We have organized the discussion of SAV distribution into three zones. The area between the mouth of the Chesapeake Bay to just above Smith Island is designated the lower bay zone; the area between Smith Is- land and Kent Island is designated the middle bay zone; and the area between Kent Island and Susquehanna Flats is designated

Q 1964 Estuarine Research Federation 531 016~6347/64/04BO531-1 0$01.50/O

532 FL J. Orth and K. A. Moore

the upper bay zone. These zones have distinct salinity regimes that influence the type of SAV community found within each area. The salinity within each zone roughly co- incides with the major salinity zones of the estuaries: polyhaline (1 B-25%), lower zone; mesohaline (5-l 8?@, middle zone; oligo- haline (0.5-5.0%), upper zone. Despite the fact that the major rivers (James, York, Rappahannock, Potomac, and Patuxent) as well as the smaller tributaries (e.g., Chop- tank, Chester, and Piankatank) of the Ches- apeake Bay have their own distinct salinity regimes, the distribution of the grasses in each river will be discussed within the zone where it connects to the bay proper.

HISTORICAL TRENDS (1700-1930) The pattern of SAV distribution and

abundance in the bay during this period was determined primarily from indirect evidence, pollen and seed analysis (Brush et al. 1980, 198 l), and qualitative observations. Aerial photographs were not generally available until the late 1930’s.

Biostratigraphic analysis of sediments for SAV seeds and pollen from Furnace Bay (Brush et al. 1980), a small embayment off Susquehanna Flats, indicated the continu- ous presence of SAV seeds from the 18th century. However, there appear to have. been some changes in species of SAV (for example, declines of Najas spp.) corre- sponding to changes in land use, such as deforestation. Increased erosion and sedi- mentation from these practices possibly resulted in more turbid water conditions and, thus, the eventual decline of species that re- quire higher light levels (Brush et al. 1980).

The Potomac River, the largest tidal trib- utary in the bay, historically contained numerous species of SAV that apparently were abundant. Several species ( Vallisneria americana, Ceratophyllum demersum, Na- jas spp. and Elodea canadensis) were reported in the vicinity of Washington, D.C. in one of the earliest accounts (Seaman 1875). Cumming et al. (19 16) showed the river below Washington, D.C. as having a narrow channel and wide shallow margins which were extensively vegetated with Pot- amogeton crispus, V. americana, and C. demersum. Many other pondweed species were reported at mouths of tributaries below

Washington, D.C. (Hitchcock and Standley 19 19), indicating the widespread presence of SAV species in the tidal portion of the Potomac River.

During the 1920’s and into the early 1930’s, Trapa natans (water chestnut) ex- panded very rapidly along a forty mile stretch in the upper Potomac River (Rawls 1964). The effects of this rapid growth on the other SAV species, was not known, but it conceivably could have outcompeted other native SAV species (Stevenson and Con- fer 1978). Local control of T. natans was apparently achieved by mechanical under- water mowing techniques, although the rea- son for its complete decline are unknown.

Zostera marina underwent some decline in the Chesapeake Bay in the late 19th century, although the magnitude of the decline was never quantified. Cottam (1934, 1935) stated that a guide from the Honga River Gunning Club reported on the decline of 2. marina in Dorchester County, Maryland in 1893-1894. Cottam also interviewed a member of the Maryland Game Commis- sion who commented on the decline of 2. marina in Chesapeake Bay in 1889 (at the time of the Johnstown Flood) and stated that it was 25 years before Z. marina fully recovered. Cottam documented other declines of Z. marina along the east coast of the U.S. -one as early as 1854. From these accounts, it appeared that Z. marina had undergone several fluctuations during this period.

In summary, evidence suggested that in the Chesapeake Bay: (1) SAV was apparently much more widespread from 1700 to 1930 than it is today; (2) SAV had been a persistent feature of shallow water habitats, although there may have been some localized shifts in species composition; and (3) abundance of Z. marina had apparently undergone several changes, although the magnitude of each change was not documented and cannot be compared.

RECENT PAST (1930-l 983) With an increased awareness of the value

of SAV as a food source for waterfowl win- tering in the bay and observations of major fluctuations of SAV in the bay and else- where, research focused on the distribution and abundance of SAV. During the last 50

Submerged Aquatic Vegetation 533

Fig. 1. Population fluctuations of watermilfoil compared to the dominant native species and total number of species found on the Susquehanna Flats from 1958-1975 (figure from Bayley et al. 1978). (Rat- ed abundance was calculated by assigning a value of 1 (rare) to 4 (abundant) multiplying this by the number of times the species occurred in each category.) (Re- printed with the permission of Estuaries.)

years, there have been several distinct periods in which significant changes occurred in SAV populations: (1) the 2. marina wasting disease in the 1930’s; (2) the Myrio- phyllum spicatum and Trapa natans proliferation in the 1950’s and early 1960’s, and (3) widespread changes in most SAV populations in the bay during the 1960’s and 1970’s. The following three sections discuss each of these periods.

THE ZOSTERA MARINA WASTING DISEASE (1931-1932)

The pandemic decline of Zostera marina in the early 1930’s was the most documented SAV change worldwide (Cottam 1934, 1935; Rasmussen 1977). Although a pathogen, Labyrinthula spp., was the suspected cause of the decline, more recent discussion has centered on the role of climatic factors (Rasmussen 1977). Cottam (1934) stated that “in the memory of man there has been no period of scarcity at all comparable to the present one (193 l-l 932 compared to other past periods).” The extent of the decline in Chesapeake Bay was never

a

5 0 -I

Fig. 2. Location of regions (cross-hatching) in the Bay area that were considered to be severely impacted by the growth of Eurasian watermilfoil from 1959- 1963.

quantified, but aerial photographs taken in 1937, five to six years after the height of the decline, are available for almost all of the shoreline in the lower bay. A review of these photographs and subsequent mapping of six sites (Orth et al. 1979) showed areas of shallow bottom with varying amounts of submerged vegetation. These beds are assumed to have been dominated by 2. marina based on knowledge of present day patterns and anecdotal information from long-time res- idents of these areas. Five of the six areas showed continuing increases in aerial coverage from 1937 up to 1972 while vegetation at the sixth site, which appeared very dense in 1937, remained stable during this period. Although quantitative information

534 R. J. Orth and K. A. Moore

is lacking for the period prior to the wasting affected by the M. spicatum was restricted disease, we can assume that the vegetation to the upper bay area and a large section of present in 1937 represented a partial recov- the Potomac River (Fig. 2). The intolerance ery from the height of the decline in 193 l- of M. spicatum to high salinity water limited 1932, Our conclusions from aerial photo- its downward expansion in the Chesapeake graphs confirmed Cottam’s (1935) report Bay and Potomac River. However, reasons that Chesapeake Bay 2. marina was show- for its decline by 1968 are not well under- ing “an encouraging change, with a few lo- stood, although disease was the suspected calized areas fast approaching the normal.” cause (Bayley et al. 1978).

THE MILFOILAND WATER CHESTNUT PROBLEMS (1950-1965)

Another major period of extensive SAV fluctuation in the Chesapeake Bay was the large increase in M. spicatum in the late 19 50’s and early 1960’s (Stennis 19 70; Bay- ley et al. 1968, 1978; Stevenson and Confer 1978). Until 1955, M. spicatum was found only sporadically in the bay, apparently introduced from Europe to the U.S. between 1880 and 1900 (Rawls 1978). Biostrati- graphic evidence substantiated its recent ar- rival to Chesapeake Bay (Brush et al. 1980). M. spicatum seeds were found in sections of sediment cores from Furnace Bay near Susquehanna Flats and dated only to ap- proximately 1935, though the cores recorded the presence of other SAV species to 1770.

According to Stevenson and Confer (1978), T. natans proliferated in the upper Potomac River, such that it was considered a problem by 1950. Although eradication was attempted, using both mechanical and chemical techniques, the ultimate reasons for its subsequent decline are not well documented.

THE BAYWIDE PROBLEM (1960-1983) In the 1960’s and 1970’s a number of field

The Susquehanna Flats typified the changes noted during the rapid expansion ofM. spicatum (Bayleyet al. 1978). In 1957, a survey of SAV found that M. spicatum did not occur at any sampling stations. Sub- sequently, it was found in one percent of these stations in 1958, 47 percent in 1959, 82 percent in 1960, and 89 percent in 1961 and 1962. After 1962, M. spicatum declined on the Flats, with slight increases in 1966 and 1967. The most serious effect associated with the rapid increase in M. spicatum was a decline in other native species such as E. canadensjs, N. guadalupensis, and V. americana (Fig. 1). Bayley et al. (1978) had suggested that the decline of native species in this area was due to competitive exclusion by M. spicatum.

and aerial surveys were conducted to esti- mate the distribution and abundance of SAV in the Chesapeake Bay. Data from these surveys showed a pattern of baywide change in all SAV species and in almost all sections of the estuary (Orth and Moore 1983a). Of significance is the fact that there is little evidence to suggest there had been changes in SAV distribution along the east coast of the United States comparable to those documented for Chesapeake Bay (Orth and Moore 1982a). The causes for this most recent decline have been discussed by Kemp et al. (1982, 1983), Wetzel et al. (1982) and Orth and Moore (198 3a) and appear to be related to changing water quality conditions in the bay (Heinle et al. 1980; U.S. EPA 1982), although biological factors may be important locally (Orth 1975; van Mont- frans et al. 1982).

On a baywide scale &f. spicatum increased from 20,200 ha (49,894 acres) in 1960 to 40,500 ha (100,035 acres) in 1961 (Rawls 1978). In creeks along the Potomac River, M. spicatum reached densities so high that it was considered a nuisance, and at- tempts were made to eradicate it with ap- plications of 2-4 D (Rawls 1978). The area

Figures showing the baywide distribution of SAV in 1965,’ 1970, 1975 and 1980 (Figs. 3 to 6) illustrate the major changes in SAV populations during the last 20 years. We chose 1965 as a starting point for three reasons: 1) the lack of information for a baywide determination prior to 1965; 2) the compounding problem of the proliferation of M. spicatum in the late 1950’s and early 1960’s; and 3) the extensive distribution of SAV during this time. These SAV distribution maps place the complex changes that have been observed in SAV populations over the last 20 years into perspective.


Fig. 3. Distribution of submerged aquatic vegetation in Chesapeake Bay- 1965.

Noticeably devoid of significant vegetation in 1965 was the James River, the largest of Virginia’s tributaries. Our examination of many earlier photographs revealed, except for small patches around the mouth of the river, the apparent absence of any vegetation along the main axis over the last 50 years.

1965-l 970 In 1965, SAV was quite abundant

throughout the bay and in all of the major tributaries (Fig. 3) despite the compounding effects of the M. spicatum. problem in the early 1960’s (Bayley et al. 1978). However, during this period, SAV in some areas had either begun to decline in abundance or had disappeared altogether.


One survey of 1,000 transects in the upper and middle bay zones from 1967 to 1969 (Stotts 1970) found large beds of vegetation in many areas, especially from the Chop- tank River to Smith Island. These large beds are evident in aerial photographs taken in September 1970. Stotts reported that abun- dances of SAV in several locations north of the Choptank declined in mid summer, several months earlier than what was expected. He suggested that these systems were being stressed by environmental factors.

SAV in some localized areas including Susquehanna Flats (Bayley et al. 1978) and the Chester River (Anderson and Macom- ber 1980), had increased in coverage from 1965 to 1970. The increase in these years may have been the result of the reemergence



of native SAV species in response to the decline of M. spicatum (Bayley et al. 1978).

The lower reaches of the Potomac contained abundant stands of vegetation in 1965 based on evidence from aerial photographs, personal accounts of local watermen, and an intensive benthic survey for the soft shell clam, Mya arenaria (Pfitzenmeyer and Dro- beck 1963). Much of the SAV that was observed in the middle and upper reaches of the Potomac River in the early 1900’s (Cumming et al. 1916) was, by this time, absent or present in very reduced stands. The timing of the disappearance of these beds is unclear but may have occurred in the 1920’s or 1930’s (Martin and Uhler 1939; Haramis and Carter 1983).

In the lower bay zone, 2. marina and Ruppia maritima were abundant and widely


distributed. Aerial photographs document that dense beds characterized much of the shoreline of the lower bay and its tributaries, and many areas showed a continued increase in coverage since the 1930’s (Orth and Gordon 1975; Orth 1976; Orth et al. 1979).

By 19 70 there were still substantial stands of SAV throughout the bay but evidence indicated some major losses had occurred in several areas (Fig. 4). Vegetation in the entire Patuxent River (R. Anderson, pers. commun.) and the lower Potomac River had all but completely disappeared by 1970.

1970-I 975 Between 1970 and 1975, the distribution

and abundance of SAV had changed baywide (Fig. 5). The decline of SAV that first


began in the mid-1960’s in some localized regions, was now evident in all sections of the bay, with some areas affected more than others. This shift also appeared to accelerate after Tropical Storm Agnes in June 1972.

A 644~station survey of SAV in 26 major areas in Maryland’s waters showed that in 197 1, 29.0% of the stations surveyed were vegetated. By 1973 only 10.5% of the stations were vegetated (Kerwin et al. 1977). SAV coverage fluctuated at comparatively low levels from 1974 to 1975, decreasing to 8.7% in 1975. The number of major areas devoid of SAV was five in 1971 and 11 in 1975 (see Fig. 2, Orth and Moore 1983a).

In the upper bay zone large reductions in vegetation were observed immediately after Tropical Storm Agnes in July and August. 1972 (Kerwin et al. 1977). The recovery of native SAV species on the Susquehanna Flats following the decline of M. spicatum in the late 1960’s ceased after Agnes (Fig. 1).

In the middle bay zone, SAV changes occurred in all sections. Species affected in these areas included many of the same species that were lost from the upper bay section following 1972 as well as 2. marina and R. maritima. These two species historically occurred in southern sections of this zone.

In the lower bay zone, vegetation adjacent to Smith Island and in the York, Rappa- hannock, and Piankatank rivers, as well as in many small tributaries, was substantially reduced during this period (Fig. 5). Losses of vegetation were greatest in areas where Z. marina formerly reached its up-river or up-bay limits. In six areas mapped from aerial photographs (Orth et al. 1979) SAV coverage generally increased from the 1930’s to 1970; however, there was a marked decline in the western shore sites beginning in the early 1970’s. Our data, especially for the York River, indicated that this decline of SAV occurred in the summer of 1973, as evidenced by the presence of large beds of SAV in April 1973 that were absent in April 1974.

1975-l 983

Between 1975 and 1983, SAV appeared

I to have reduced populations in almost all

areas (Fig. 6) with some regrowth in a few specific areas. A baywide survey conducted in 1978 (Orth et al. 1979; Anderson and Macomber 1980) showed that 16,044 ha (39,629 acres) had vegetation. Although es- timates of area1 coverage for the entire bay are not available for previous years, the amount recorded in 1978 appears substantially less than what existed in the 1960’s. Only 10 of the 26 areas surveyed in Mary- land’s waters had vegetation in 1980 compared with 21 in 1971 (Kerwin et al. 1977; Md. DNR data reports).

In the upper zone, where 2,098 ha were vegetated with SAV (Orth and Moore 1982a), the Chester River and Eastern Neck area had the greatest abundance (1,975 ha) and most diverse assemblage of SAV (7 species) in 1978. Only 110 ha were present in scattered beds in the Susquehanna Flats area. Two species, M. spicatum and V. americana, were present on the Flats in 1978 (Anderson and Macomber 1980), compared to 11 species that were present in 197 1 (Bay- ley et al. 1978).

In the middle zone (4,546 ha of SAV), the greatest concentration of vegetation in 1978 was located along the eastern shoreline in the Little Choptank River to the Eastern Bay area (3,500 ha). Very little vegetation was found in the Potomac River except in the region between the Wicomico River and Nanjemoy Creek. Surveys conducted by the U.S. Geological Survey during this time period confirmed the paucity of vegetation in the Potomac River (Carter and Haramis 1980; Carter et al. 1980; Haramis and Car- ter 1983).

In the lower bay zone (9,400 ha of SAV), the largest and most dense SAV areas were concentrated in several main regions: 1) along the western shore from just north of the James River to the Rappahannock Riv- er, especially in the region of Mobjack Bay; 2) along the bay’s eastern shore; and 3) in the shoal area between Tangier Island and Smith Island. The SAV bed between Tan- gier and Smith Islands was the single, most extensive vegetated area in the entire bay (2,394 ha). Vegetation abundance at most of the intensively mapped sites along the western shore continued to decline between 1974 and 1980. Evaluation of the SAV changes at the East River site as well as other


areas in the lower bay zone revealed that the SAV declined first in the deeper, off- shore sections rather than in the shallower, nearshore areas (Orth and Moore 1982a).

At one intensively sampled site in the York River (Allens Island), a general increase in vegetation abundance was observed from 1978 to 1981. Examination of this site revealed that numerous 2. marina seedlings were colonizing the shallower areas, most likely with seeds from adjacent beds (Orth and Moore 1983b). In addition, 2. marina transplanted in 1979 to a pre- viously vegetated site at Gloucester Point has persisted to the present time (Orth and Moore 1982b).

Field and aerial observations during the spring and summer of 1983 indicated that some SAV was revegetating areas in the York, Rappahannock and Potomac rivers (V. Carter, pers. commun.; Orth, unpubl. data). Whether this vegetation persists and indicates a renewal of favorable growing conditions remains for future surveys to de- termine. One species found only recently and apparently introduced by man in the Potomac River, Hydrilla verticillata, has the potential for very rapid growth, where it has become a problem in some areas of the United States. Only close examination of the Potomac River in subsequent years will demonstrate whether H. verticillata reaches high densities here.

Summary Although the data available on the his-

torical distribution and abundance of SAV in the Chesapeake Bay are rather fragmen- tary, evidence suggests that these communities were, until recently, widespread fea- tures of the shallow water areas in the bay subject to some specific fluctuations in abundance. Declines of Z. marina in the polyhaline and mesohaline zones and other low salinity species in the Potomac River in the 1930’s are suggested by qualitative reports. Subsequently, the Z. marina recovered while the SAV in the upper Poto- mat River did not. Conversely, there was rapid expansion of Trapa natans and Myr- iophyllum spicatum in the 1920’s and 1950’s to 1960’s in certain sections of the Potomac River and upper bay so as to create a nuisance to boating activities.

More quantitative information covering the period since the 1960’s reveals a widespread and major decline in the distribution and abundance of SAV species in most sections of the bay. Loss of SAV communities was first observed in the mid to late 1960’s in the upper bay areas, and in particular, the Patuxent, lower Potomac River and the upper reaches of some of the smaller tributaries (e.g., the Chester and Choptank rivers). By 1970, almost all the vegetation in the Patuxent River and lower Potomac River was gone. The decline of SAV in the bay accelerated in the early 1970’s and continued through 1980 with the most rapid decline occurring from 1972 to 1974 after the occurrence of Tropical Storm Agnes. Several sections in the bay that once contained abundant SAV had virtually none by 1980 (e.g., the Patuxent, Piankatank, and Rappahannock rivers); other sections had only small stands remaining (e.g., the Po- tomac and York rivers, and Susquehanna Flats). Since 1980 there has been some regrowth reported in certain areas but baywide the level of vegetation is still low.

Although this paper reviews the available information concerning the history of SAV in the bay and its tributaries, the picture is far from complete. Qualitative surveys, anecdotal information and unpublished reports provide most of the information. It appears that only when the loss or gain in the vegetative communities was perceived to be a problem by recreational or com- mercial interests were efforts made to in- vestigate the status of the resource. Ob- viously, regular quantitative surveys with standardized methodology combining both aerial photography and field sampling throughout the entire estuary are currently necessary for adequate determination of population levels. The analysis of this information along with appropriate water quality data and experimental investigations could yield important results necessary for an understanding of the factors con- trolling the growth and survival of these plant communities.

ACKNOWLEDGMENTS

This work was supported by grants R805951 X003201 and X003246 from the Environmental Pro tection Agency’s Chesapeake Bay Program. We than1


J. van Montfrans, Tom Fredette and two anonymous reviewers for helpful suggestions on earlier versions of the manuscript. We would especially thank H. Gordon, C. Alston and S. White who were key personnel in- volved in the VIMS program on the distributional as- pects of SAV, as well as W. Boynton for comments made during the synthesis phase of the Chesapeake Bay Program.

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Received for consideration, January 10, 1984 Accepted for publication, June 5, I984

Distribution and Abundance of Submerged Aquatic Vegetation in Chesapeake Bay: An Historical Perspective

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