Moore et al

Estuaries Vol. 23, No. 1, p. 115-127 February 2000

Analysis of the Abundance of Submersed Aquatic Vegetation

Communities in the Chesapeake Bay

KENNETH A. M O O R E ~

DAVID J. WILCOX ROBERT J. ORTH The Virginia Institute of Marine Sdence School of Marine Science College of William and Mary Gloucester Point, Virginia 23062

ABSTRACT: A p rocedure was deve loped us ing aboveg round field b iomass m e a s u r e m e n t s of Chesapeake Bay sub- m e r s e d aquat ic vegeta t ion (SAV), yearly species identification surveys, annual pho tograph ic m a p p i n g at 1:24,000 scale, and geographic in format ion sys tem (GIS) analyses to de t e rmine the SAV c o m m u n i t y type~ biomass~ and area of each m a p p e d SAV b e d in the bay and its tidal t r ibutaries fo r the pe r iod o f 1985 th rough 1996. Using species identif icat ions p rov ided th rough over 10,900 SAV g r o u n d survey observat ions , the 17 mos t a b u n d a n t SAV spec ies f o u n d in the baywere c lus tered into f o u r spec ies associations: ZOSTERA, RUPPIA~ P O T A M O G E T O N , a n d FRESHWATER MIXED. Monthly aboveground b i o u l a ~ values were then ass igned to each bed or bed sect ion based upon month ly biolnass mode l s deve loped for each COUllnunity. High salinity colnuluni t ies (ZOSTERA) were f o u n d to domina te total bay SAV aboveground b iomass du r ing winter, spring, and snlnlner. Lower salinity communi t i e s (RUPPIA, P O T A M O G E T O N , and FRESHWA- TER MIXED) d o m i n a t e d in the fall. In 1996~ total bay SAV s tand ing stock was nearly 22,800 metr ic tons at annual m a x i m u m biomass in July encompass ing an area o f approximate ly 25~670 hectares. Min imum b iomass in December a n d J anua ry o f that year was less than 5,000 metr ic tons. SAY annual m a x i m u m b iomass inc reased baywide f rom lows of less than 15~000 metr ic tons in 1985 a n d 1986 to nearly 25~000 metr ic tons du r ing the 1991 to 1993 per iod, while area increased f r o m approx imate ly 20,000 to nearly 59,999 hectares dur iug that s ame per iod. Year-to-year compar i sons of UlaXiUlUUl annua l colnulnuity abundance froul 1985 to 1996 indica ted that regrowth of SAV in the Chesapeake Bay f roln 1985-1993 occur red principally in the ZOSTERA COUllnunity~ with 85% of the baywide increase in b i o l n a ~ and 71% of the increase in area occur r ing in that COUllnunity. M a x i m u m b i o u l a ~ of FRESHWATER MIXED SAV beds also increased f rom a low of 3,200 inetric tOllk~ in 1985 to a high o f 6,650 metr ic tons in 1993, while inaxiulnul biolnas~ of both RUPPIA and P O T A M O G E T O N beds f luc tuated between 2,450 and 4,600 metr ic tons and 60 and 600 metr ic tons~ respectively, dur ing that s ame pe r iod with ne t decl ines o f 7% and 43%~ respectively, between 1985 and 1996. Dur ing the July pe r i od of annual , baywide, m a x i m u m SAV biomass, SAV beds in the Chesapeake Bay typically averaged approximate ly 0.86 metr ic tons of aboveg round d ry mass pe r hectare of bed area.

In t roduc t ion

Aerial p h o t o g r a p h y and m a p p i n g surveys have been used in a n u m b e r of regions to d e t e r m i n e the distr ibution of submersed aquatic vegeta t ion (SAV) popula t ions and changes in these popula- tions over t ime (Orth and Moore 1984; La rkum and West 1990; Coles et al. 1993; Bulthuis 1995; Ferguson and K o r f m a c h e r 1997; Robbins 1997). Aerial m a p p i n g surveys of Chesapeake Bay SAV have been conduc ted annually in the Chesapeake Bay and its sub-estuaries since 1985. Publ ished in r epor t f o rm (e.g., O r t h et al. 1997) as well as on the world wide web ( h t t p : / / w w w . v i m s . e d u / b i o / say) these data have proven to be useful for m a n y bay m a n a g e m e n t activities. Because of the strong relat ionships which have been developed be tween

1 Cor r e spond ing author ; tele: 804/684-7384; fax: 804/684- 7298; e-mail: [email protected] .

the occur rence of submersed ang iospe rms and water quality condi t ions (gat iuk et al. 1992; Dennison et al. 199S), the recovery of these communi t ies has been chosen as one the pr incipal indicators of the success of Chesapeake Bay clean-up efforts. T h e baywide annua l aerial surveys have therefore be- come a cost effective and comprehens ive tool with which to assess changes in this resource. However, the various SAV communi t i e s found in the Chesa- peake Bay system can r e spond differently to changing water quality condi t ions as communi t ies may differ in their capacity to withstand per iods of high turbidity, nu t r i en t enr ichment , or salinity ex t remes (Stevenson and Confer 1978; Car te r and Rybicki 1985; Stevenson et al. 199S; Moore et al. 1996). Since it is cost prohibi t ive to annual ly survey the SAV species composi t ion of each of the thousands of SAV beds in the bay, and there has been as yet no effective way to discriminate individual SAV beds into their dominan t species or communi ty

�9 2000 Estuarine Research Federation 115

116 K.A. Moore et al.

types f rom high al t i tude aerial pho tog raphy a lone (Or th and Moore 1983; Z ieman et al, 1989; gul- thuis 1995), a p r o c e d u r e was necessary to assign a classification type to each bed so that year-to-year changes in the various SAV communi t i e s could be assessed.

The aerial pho tog raph ic surveys of the Chesa- peake Bay shorel ines provide measures of SAV bed areas which are then subsequent ly pho to in te rp re t - ed into four density classes (Or th and Moore 1983). While these category-type data provide good measures of relative a b u n d a n c e they do not provide sufficient in fo rmat ion to de t e rmine SAV species biomass or s tanding crop. In addit ion, es- t ima tes of SAV a b u n d a n c e at v a r i o u s t imes t h r o u g h o u t the year are not directly available since the p h o t o g r a p h y is usually flown only once annually at t imes of est imated peak SAV aboveground biomass and these flight dates vary a m o n g the various regions of the bay. Typically, h igh salinity regions are p h o t o g r a p h e d in the late spring or early s u m m e r and low salinity and freshwater tidal areas in the late summer .

Estimates of spatial and tempora l variability of SAV biomass has b e c o m e increasingly i m p o r t a n t as the capacity of researchers and manage r s to effec- tively mode l coastal bay ecosystems improves. Dif- ferences between m e a s u r e m e n t s of SAV area coverage and actual b iomass can be impor tan t . Dur ing any par t icular year overall bay SAV area may increase, or decrease, or r emain the same f rom previous years, while SAV biomass may not vary line- arly with change in area. A c o m m o n metr ic such as biomass or s tanding crop is necessary to discriminate potent ia l changes over bo th spatial or temporal intervals, especially in context of the overall bay ecosystem. In addit ion, calibration and valida- tion of landscape scale, ecosystem simulat ion models (e.g., Boumans and Sklar 1990; Constanza et al. 1990; Cerco and Cole 1994) which are developed with an SAV c o m p o n e n t general ly require informat ion on SAV mass, no t a rea or relative abundance.

The overall goal of this study was to de t e rmine the biomass for all areas of SAV m a p p e d in the Chesapeake Bay over the per iod of 1985 th rough 1996 using SAV distribution and a b u n d a n c e informat ion available f rom annual reports , b iomass info rma t ion available f rom publ ished and unpublished studies by bay researchers , and species g round survey observat ions provided by researchers, t ra ined volunteers and others in the bay community. O u r specific objectives were to use previously collected survey in fo rmat ion to develop ap- p ropr ia te SAV species associations which could be used to classify the SAV beds found t h r o u g h o u t the bay into a small n u m b e r of c o m m u n i t y types; to

develop annual models of SAV biomass for each of these SAV communi ty types; and to develop a pro- cedure using geograph ic in fo rmat ion system (GIS) analyses to assign the appropr ia t e SAV c o m m u n i t y types and calculate the biomass of each SAV bed m a p p e d in the Chesapeake Bay annually f rom 1985 th rough 1996,

M e t h o d s

DEVELOPMENT OF SAV COMMUNITY TYPES

The identif icat ion of SAV communi ty types was based on an analysis of SAV ground survey data publ i shed in annua l SAV distribution repor ts f rom 1985 to 1996 (e.g., Or th et al. 1997). These repor ts d o c u m e n t the locations of all the SAV species which have been identified by p r e s e n c e / a b s e n c e censuses in field surveys conduc ted dur ing the growing season of each of the years by researchers , g o v e r n m e n t agencies, and trained individuals, including citizens groups. All eleven years of g round survey in fo rmat ion f rom 1985-1996 (no aerial m a p p i n g data in 1988) were digitized into a database using A R G / I N F O GIS for use in this analysis. Species in fo rmat ion was assigned to each of the individual locat ions which were identified on the SAV maps in each yearly repor t . Chara sp., Najas flexilis, Nitetta sp., Potamogeton epihyd'rus, Potamogeton nodosus, and Trapa natans were identified in twelve or fewer observat ions and therefore were not used in the de te rmina t ion of SAV communi ty types, Fig- ures l a - n presen t the recorded occur rences of each of the individual species f rom 1985-1996.

C o m m u n i t y types were developed f rom the entire g round survey database using numer ica l clus- t e r ing analysis. D ice ' s coe f f i c i en t o f s imi la r i ty (Boesch 1977) is a c o m m o n l y used quanti tat ive re- semblance measure which is useful for the numer - ical c l u s t e r i ng of b i n a r y ( p r e s e n c e / a b s e n c e ) g round truth data such as ga thered in the g round surveys (Clifford and Stevenson 1975). T h e grea ter the coefficient of similarity, the m o r e f requent ly paired species or g roups of species occur in the database. T h e overall bay SAV species spatial distributions, which are control led in most cases by salinity to lerance (Stevenson and Confer 1978), were then used a long with the results of the clustering analysis in the final ass ignment of individual species to specific communi ty types.

Figure 2 presents a s u m m a r y of the clustering analysis of all 11 yr of g round survey in fo rmat ion (10,023 observat ions) in dendrogra rn form. Dice's coefficients between individual species or species groups are r ep resen ted by the vertical lines. Table 1 presents a matr ix of the n u m b e r of observat ions repor t ing pairs of individual species as well as the n u m b e r of observat ions r epor t ing only a single

Chesapeake Bay SAV Communities l 17

~6' W 75" W 77" ",V %" W ~5" W

Z~,~tera ~.xirina leelgrass) Ruppia mariEma ( w id g e m grass)

~ ' w 7s 'w 75"w 7~'W ~ ' w

Potamoger (sag~ pondweed)

?TW 76"W ~5"W

potoa'aogettmI~e~E~l~ (redhe~J-grass)

77" W ?6' W 75' w

Vallisr~eriaat~erkueva (w~ldce]ery? Myr~olahylltonspicaUen [Eur~t~ia. wzmmfilfeil) ttydnlla vet~cillaca (hydfilla)

77" W 7~" W 75" W 77" W 76" W 35" W

]}" ~ " ~ " , ~ , ~, ~- ~. Ocean I ~ : $2~ .r~,.,,~,~,,o.:~,:~,~:~ Ocea~

P~t~uJgelOt~pv.~illt*$ (sic, haler pondw~e.d) a~ er&p~ (e~trly ptmdweed) Naj.a" sp

73" w

Ceratophyllwm demers~m (oooatail)

!

EIodea r (~lm~on elodea)

]7"N

Zantaiclaellbs paOaarrig ( h m c d ~ d ~ ) He~er~ther~ dubga (~ater-vr

Fig. 1. Ground survey observations of individual SAV species, 1985-1996.

1 1 8 K.A. Moore et al.

Z manna

R. ma~qtima

Z. ~atus~ris

P. perfoliatus

P~ pectinaO~

Nalas sp~

N. grac~tona

P.pusiltus

P, crispus

E. catmdens~s

ik IV. lttitlor

N. guadolupensi~

H. dubia

V. americana ~ j M. spl eotutn _ _ t ~

It. vematlata 1 } . . . . .. -- -/.' I C. donersura

I . . . . . . i I I I 0.5 0.4 0.3 0.2 o.1 o.o

Dice's Coefficient of Similarity

Fig. 2. Dendrogram of species association based upon all 1985-1996 ground survey information.

species. For example , Zannichetliapatustris (Zp) was f o u n d growing with Ruppia maritima (Rm) 134 t imes a n d Zostera marina (Zm) 11 times, b u t in m o n o s p e c i f i c s tands 874 times. Vallisneria americana (Va) was obse rved g rowing with 3/1. spicatum (Ms) 1,201 times, in m o n o s p e c i f i c s tands 209 t imes, bu t ne ve r with Z marina. A l t h o u g h Z marina a n d R. maritima are h ighly associated (Fig. 2), R. maritima is typically a m i n o r c o m p o n e n t of p o l y h a l i n e SAV beds in the lower bay, which are usual ly d o m i n a t e d by m o n o s p e c i f i c s tands of Z. marina in all bu t the shal lowest areas ( O r t h a n d M o o r e 1983; Moor e et al. 1995; Tab le 1). R. maritima, however, has a wide s a l i n i t y t o l e r a n c e a n d has also b e e n f o u n d t h r o u g h o u t the mid-bay as well as the Pa tuxen t , P o t o m a c , a n d R a p p a h a n n o c k Rivers i n m a n y m o n o s p e c i f i c beds (Table 1; Fig. l b ) , Based u p o n this a d d i t i o n a l i n f o r m a t i o n Z marina a n d R. m a r l tima were divided in to two species g roups with all beds c o n t a i n i n g Z marina ass igned to a Z O S T E R A c o m m u n i t y type a n d beds c o n t a i n i n g R. maritima, bu t n o t Z. marina ass igned to a RUPPIA c o m m u - ni ty type.

A s s i g n m e n t of lower sal ini ty species in to com- m u n i t y types was similarly d e t e r m i n e d . For example, Z. patustris, Potafaogeton pectinatus, a n d P. p e r fotiatus were f o u n d t h r o u g h o u t m a n y of the same mid-bay r eg ions a n d ove r l apped the d i s t r i b u t i o n of R. maritima, a l t h o u g h usual ly n o t at the same loca t ions (Fig. l b , c , d ,n ) . Z palustris can grow in m o n o s p e c i f i c beds early in the year ( H a r a m i s a n d Car te r 1983) a n d may n o t be f o u n d in some areas by la te-summer . In fact, some of the beds of Z. p a l ustris which are loca ted by g r o u n d t r u t h surveys early in the year (Fig. l n ) do n o t a p p e a r on the

TABLE 1. A matrix display of the number of observations out of 10,025 total bay wide observations from 1986-1996 reporting each pair of species. The number of observations reporting a single species is reported on the diagonal. Zp--Zanne&ellia palustris, Zm-- Zo~tera marina, Va Vallisrwria americana, Rm--R~ppia waritima, Ppu Potawogeton p~sil!,us, Ppf Potawogetor~ perfdiat'us, Ppc Yotamo- geton pectinatus, Ppc Potamogeton crispus, Nm--Najas minor, Ngu--Najas g~adalupensis, Ngr-Najas gracillima, N--Najas sp., Ms--Myric~ ph:ll'uv~ spicatuv~, Hv--H:drilla verticillata, Hd Heterar~thera dubia, Ec Elodea car~adensis; Cd Cerat@hylluv~ devwrs~v~.

Zp Zm Va Rm Ppu PpF PRo Per Nm Ns'u Nsr N Ms Hv Hd Ec @d

Cd 54 0 532 3 23 13 40 74 155 117 43 80 Ec 44 0 143 25 18 62 29 60 44 44 31 11 Hd 5 0 409 1 2 10 8 6 87 58 1 41 Hv 22 0 877 0 13 3 16 25 298 118 31 66 Ms 54 0 1,201 36 8 87 67 32 177 98 17 61 N 6 0 61 4 4 2 5 5 1 6 1 16 Ngr 17 0 13 0 6 0 0 17 26 19 8 Ngu 18 0 90 0 14 3 5 22 51 7 Nm 14 0 135 0 9 0 5 25 4 Pcr 26 0 40 ] 19 5 9 7 Ppc 51 1 77 53 2 57 79 Ppf 49 0 70 123 1 89 Ppu 18 0 13 0 0 Rm 134 699 25 2,387 Va 34 0 209 Zm 11 567 Zp 874

981 839 371 172 88 196 40 7 41 622 551 16

1,453 445 773

Chesapeake Bay SAV Communities 1 19

TABLE 2. SAV Species Associations. Species occurrence in community exceeds 10% of spades observations. * Dominant Species

�9 ZOSTERA Community

�9 RUPPIA Community

�9 POTAMOGETON Community

�9 F R E S H W A T E R M I X E D Community

Zostera marina* Ruppia maritima

Ruppia maritima* Potawogetor~ perfoliatus Potarnogeton pectinat~s Z_annichellia pal~stris

Pota~wgetor~ pectinat,~s* Potarnogeton perfoliatus* Pota~wgetor~ crisp~s Elodea canadensis

Myriophyll~rn spicaturn* H'ydrilla verticillata* Vallisneria americana* Ceratophyll,~m demers~w Heteranthera dubia Elodea canadensis Najas g~adalupensis Najas gTacilliv~a Najas minor Najas sp. Potarnogeton cri@us Potavwgetort pusillus

aerial pho tog raphy surveys of these regions in Au- gust (Or th et al. 1997). Al though Z. patustris has been found to be associated with a variety of species it was found mos t c o m m o n l y growing with R. rnaritima and is inc luded in that association. Since there were few beds of SAV which consist principally of Z pat'ust'r'is in the aerial m a p p i n g database (e.g., Or th et al. 1997) the a b u n d a n c e of this species may be underes t imated . P. perfoliatus and P. pectinatus in contrast, have been typically found as dominan t s in a variety of mixed and monospeci f ic stands (Table 1; Fig. 1c-d). There fo re , all beds repor t ed with ei ther P. perfoliatus or P. pectinatus, but no Z marina or R. maritima, were assigned to a P O T A M O G E T O N communi ty type.

Freshwater regions of the u p p e r bay and the up- pe r Po tomac River were vegeta ted with a diverse assemblage of SAV (Fig. 1e-m) which were clus- tered in a large g roup of 12 species ranging f rom Najas sp. to Ceratophyllum dernersum (Fig. 2). O f these 12 species Myriophytlum spicatum, Hyd'ritta re> ticittata, and Vattisneria americana were the most abundant . H. verticillata and M. spicatum had the highest co-occurrence of any two species r epor t ed with over 1,450 observat ions r epor t ing bo th species (Table 1). V. americana was found to co-occur with H. verticittata and M. spicatum 877 and 1,201 times, respectively. All beds no t assigned to the ZOSTERA, RUPPIA, or P O T A M O G E T O N communi ty types were assigned to a FRESHWATER MIXED c o m m u n i t y type.

Table 2 presents the species associations for all

four communi ty types including all species where occur rence exceeded 10% of observations. Figure 3a -d display the SAV bed field observat ions after ass ignment to communi ty type. FRESHWATER MIXED and P O T A M O G E T O N communi t i e s dom- inate the u p p e r bay and u p p e r tributaries, while RUPPIA was found t h r o u g h o u t m u c h of the bay excluding most freshwater tidal regions. ZOSTERA domina tes the lower bay.

ASSIGNMENT OF INDIVIDUAL SAV BEDS TO

COMMUNITY TYPES

Since yearly g round survey species in fo rmat ion was not available for each individual SAV bed a p r o c e d u r e was developed to classify each m a p p e d bed into a specific c o m m u n i t y type for each year of the aerial survey. In most areas of the bay and its tributaries, SAV beds which are located near one a n o t h e r tend to be composed of similar species. There fo re , to a certain extent , beds can be as- s igned to the communi ty type of the neares t po in t where field survey in fo rmat ion is available. This c o n f i d e n c e d e c r e a s e s wi th i n c r e a s i n g d i s t ance f rom a survey location. To de t e rmine the maxi- m u m distances that can be used with confidence, the distr ibution of field observat ions for 1994 and 1995 were analyzed spatially using A R C / I N F O GIS software. G r o u n d surveys for the years 1994 and 1995 were chosen because of the broad distribution and intensity (gg% of all beds surveyed) of g round survey observat ions made dur ing that period. On average, be tween 1985 and 1996, 29% of all the beds in the bay were g round surveyed each year.

First, the over-water distance between r epor t ed survey locations was compu ted and used to determine the pe rcen tage of observat ions within a particular distance of each o ther that share the same communi ty type. This distance re la t ionship can vary greatly t h r o u g h o u t the bay due to factors such as the local salinity gradient . There fo re , the CBP segmenta t ion scheme, an area compar tmenta l iza - tion of the Chesapeake Bay into subunits, which was developed based u p o n salinity distributions, natural geograph ic par t i t ions and o ther natural features (see O r t h et al. 1997), was used to apply this spatial analysis t h r o u g h o u t the ent i re bay. Each of 44 Chesapeake Bay P rog ram (CBP) segments was analyzed individually to estimate the m a x i m u m distance within which at least 90% of the g round survey observat ions within that s egment were of the same communi ty type. An example of this analysis for CBP S e g m e n t CB6 is p resen ted in Fig. 4. In this CB6 Segmen t area all SAV ground survey locations surveyed in 1994 and 1995 were found to be of the same communi ty type when they occurred within approx imate ly 8 km of each other.


77' W 76" W 75' W 77" W 76" W 75 ~ W

39= N

38" N

3 7 ' N

Z O S T E R A Communi ty R U P P I A Community 77" w 76" w 75- w 77" w 76' w 75" W

39' N

38' N

3T N

39' N

38" N

3 T N

39" N

3g" N

3 T N

POTAMOGETON C o m m u n i t y F R E S H W A T E R MIXED Community

Fig. g. Ground survey observations of SAV after assignment to community type, 1985-1996.

A 90% similarity was found up to a distance of approximately 11 km apart with a linear decrease in similarity with increasing distances up to g0 km. An increased similarity at distances beyond g0 km was likely due to comparisons between beds in separate tributaries within that segment area where salinity regimes were similar.

A step-wise p rocedure was used to assign a community type to each bed mapped in the annual aerial surveys from 1985 to 1996. If beds were directly surveyed in the current year they were assigned to a communi ty type based on the species reported. If a bed was not directly surveyed in a year assignment procedures were followed in the following order until assignment could be made.

Beds were assigned to the communi ty type of the nearest field observations of the current year which were located within the 90% similarity distance computed for the CBP segment where the bed was located. Beds that were directly surveyed in the preceding year were assigned to a community type based on the species repor t at that time. Beds were assigned to the communi ty type of the nearest field observations made the previous year within the 90% similarity distance computed for the CBP segment where the bed was located. Beds that were directly surveyed in the subsequent year were assigned to a communi ty type based on the species reported. Beds were assigned to the community type of the nearest field observations made

100

90

80 �9

e 70

r3 60

"~ 50

4O

Fig. 4.

m

t.

m \ %

0 10 20 30 40 50 60

Maximum Distance Between Observations (kin)

Analysis of similarity of SAV species repor ted in g round survey observations compm-ed to the over water distance between the observations for Chesapeake Bay Program Segment CB6. Arrows indicate distance at which pairs of individual species g round survey observations are classified in the same community type 90% of the time.


TABLE 3. Sources used in development of SAV biomass models for each community type.

FRESHWATER MIXED Community

Naylor and Kazyak 1995 Rybicld and Carter 1995 Carter et al. 1994 Carter and Rybicld unpubl i shed data Stevenson et al. 1993 Rybicki unpubl i shed data Kilgore et al. 1989 Rybicld et al. 1988 Rybicki et al. 1985 Staver 1986 Staver u n p u n i s h e d data Nichols et al. 1979

POTAMOGETON Community

Stevenson et al. 1993 Lubbers et al. 1990 Nichols et al. 1979

RUPPIA Communi ty

Moore et al. 1995 Stevenson et al. 1998 Or th and Moore 1986 Or th and Moore 1981

ZOSTERA Community

Moore et al. 1995 Or th and Moore 1986 Or th and Moore 1981

the subsequen t year within the 90% similarity distance compu ted for the CBP segmen t where the bed was located. Any remain ing SAV beds were individually assigned to a c o m m u n i t y type based on the spatial pa t te rns provided by the entire g round survey data set.

DEVELOPMENT OF SAV BIOMASS MODELS FOR EACH COMMUNITY TYPE

Published and unpubl i shed studies of SAV biomass f rom the Chesapeake Bay region (Table 3) were used to de t e rmine average mon th ly biomass values for the d o m i n a n t species of each communi ty type (Table 2). Only data f i om studies in which SAV aboveground biomass f i o m dense monotyp ic stands were r epo r t ed at least periodical ly in units of mass per a rea t h r o u g h o u t the growing season were selected for use. Belowground m e a s u r e m e n t s were not available for most species and therefore month ly models for this c o m p o n e n t of biomass were no t a t tempted. Aboveground biomass values were conver ted f iom wet weight or o ther r epor t ed units to dry mass per uni t a rea by first t ransform- ing each study's data to their p r o p o r t i o n of the r epo r t ed seasonal m a x i m u m of each species (cf., Nichols et al. 1979). These p ropor t i ons of seasonal m a x i m a were then appl ied to an overall average m a x i m u m seasonal value in units of g rams dry mass m -~ which was calculated using the subset of

studies that specifically r epor ted results in units of dry mass per area. In those studies where field biomass sampl ing was not conduc ted monthly, values for m o n t h s no t sampled were es t imated by l inear in terpolat ion. T h e m e a n month ly biomass values were de t e rmined by averaging the month ly values assuming equal area of each of the dominan t species (Table 2) compr is ing a c o m m u n i t y type.

Each of the four SAV communi t i e s demonst ra t - ed a distinctive pa t t e rn of shoot biomass (Fig. 5a- d). T h e ZOSTERA and RUPPIA communi t i e s were found to exhibit peaks of shoot biomass in the early and late summer , respectively, and bo th main- tained aboveground shoot biomass t h r o u g h o u t the winter. Shoot growth for ZOSTERA f iom average winter m i n i m u m s of 45 gdm m ~ was evident as early as February and rapid shoot dieback was ap- pa ren t beg inn ing in July after reaching an average m a x i m u m of 220 g d m m -~, with a second short per iod of growth in the fall. RUPPIA did not dem- onstrate a significant increase in shoot biomass until J u n e and it subsequent ly reached a m a x i m u m standing crop in August of approx imate ly 100 g d m m ~ after which it declined to winter levels of 20- 95 g d m m -~, Both the P O T A M O G E T O N and FRESHWATER MIXED communi t i e s were found to mainta in no shoot biomass f rom D e c e m b e r to April. Beginning at this time, however, shoot biomass of bo th communi t i e s rapidly increased. T h e


3 0 0 -

2513.

200

150

~ 3 0 0 -

~, - ,.~ 250 - <

~ 2 0 0 -

150 ]

R.

F R E S H W A T E R Community

/ i

c . R U P P I A C o m m u n i t y

300

25O

2(]O

150

f00

50

0

300

b.

P O T A M O G E T O N C o m m u n i t y

rl.

Z O S T E R A C o m m u n i t y

250

200 ..,~

15o

100 �9 ]00

" I ~ 50 - 50 .I." -,~

0 0 i i i i i i 1 i i F M A M J J A S O N D

M O N T H M O N T H

Fig. 5. Mean monthly (-+SE) SAV aboveground biornass by community type.

; V'~A" ~ J'J'A'S'O'N'rt

P O T A M O G E T O N c o m m u n i t y r e a c h e d a peak standing crop of 100 gdm m e or more by August with complete loss by December. In contrast, shoot biomass of the FRESHWATER MIXED communi ty increased th roughou t the summer and early fall, and reached an average max imum of nearly g00 gdm m -~ by October. A precipitous decline of shoot material typically followed with complete loss by December.

APPLICATION OF SAV BIOMASS TO AERIAL PHOTOGRAPHIC COVER GLASSES

Annual aerial pho tographic surveys of SAV coverage are summarized (e.g., Or th et al. 1997) as SAV areas which have been assigned to ranked density classes based upon photo- interpreta t ion using a Grown Density Scale adapted from Paine (1981) (Fig. 6). It was necessary to quantify how these density classes cor responded with measurements of SAV ground survey biomass so that the aerial survey data could be used to determine SAV biomass baywide. To accomplish this task unpublished field data obtained during the summer of 1990 at g5 locations th roughou t the bay were used. This data consisted of point-intercept measurements obtained by divers at 10 m intervals along t ransects o r i e n t e d p e r p e n d i c u l a r to the shore across SAV beds of different densities and species

P E R C E N T C O V E R D E N S I T Y C L A S S

85 m m m 95 !i / m

Very Sparse <10%

Sparse I0 -40 %

Moderate 40 70%

Dense 70 ~ 100%

Fig. 6. Crown density scale used for estimating density of 8AV beds from aerial photography. Rows of squares with black and white patterns represent three different arrangements of vegetated cover for a given percentage (Adapted from Paine 1981).

composition. Each point sample consisted of trip- licate estimates of bo t tom cover and depth within randomly placed 0.25 m e sampling rings. Such measurements have been previously demonst ra ted to provide very good estimates of SAV density and biomass (r e > 0.86; Or th and Moore 1988). The individual g round cover transects were then sepa- rated into segments based u p o n the published photo- interpreted density class zones comprising each area in 1990 (Orth et al. 1991).

Figure 7 illustrates the relationship between field g round cover measurements and the photo-interpreted density classes for all transect segments. The relationship was linear and significant (p < 0.001); however, the aerial photo- interpretat ion tended to underest imate g round cover at lower

;a.- C'

g o t,.9

80

70

60

50

4O

30

20

10 0

~ / r 2 = 0.99

. . . . I . . . . I . . . . I . . . . L . . . . I

20 40 60 80 1 O0

Mid Points of Density Classes (%)

Fig. 7. Comparison of SAV aerial density classification cate- gories to SAV groundcover measurements.

SAV densities and overes t imate at h igher densities. No consistent effects of c o m m u n i t y type or depths of SAV growth on the re la t ionship between g round cover and density class ass ignments could be de- termined. There fo re , density class to g r o u n d cover conversion was appl ied consistently across all SAV beds.

C h e s a p e a k e B a y S A V C o m m u n i t i e s '123

CALCULATION OF MONTHLY SAV BED BIOMASS

Monthly aboveg round biomass for each individual SAV bed, or bed segmen t where a bed had been pho to in t e rp r e t ed into subunits of different density class, was calculated by the following for- mula:

Monthly Biomass - Mb * Cc * Ba

Where Mb - mode l month ly biomass for assigned communi ty type (gdm m-Z), Cc - photo- inter- p re ted density class to g round cover conversion, and Ba - bed area (me).

Results

Results of the month ly shoot biomass calcula- tions for all SAV beds f rom 1985 th rough 1996 is summar ized in Fig. 8. During this per iod SAV max- i m u m s u m m e r biomass increased baywide f rom lows of 15,000 metr ic tons in 1985 and 1986 to highest levels of nearly 25,000 metr ic tons dur ing 1991 th rough 199S. The high salinity ZOSTERA communi ty domina t ed total bay SAV biomass during the winter, spring and summer . Lower salinity c o m m u n i t i e s (RUPPIA, P O T A M O G E T O N , a n d FRESHWATER MIXED) domina ted in the fall. At peak biomass in July, total bay system standing stock of SAV was approx imate ly 22,800 metr ic tons in 1996. M i n i m u m standing stock in D e c e m b e r and J a n u a r y of that year was less than 5,000 metr ic tons.

Year-to-year compar i sons of annual bay-wide

25000

20000

15000 ,2

t0000 > O <

5000 O N

1985

ZOSTERA ~ RUPPIA ~ POTAMOGETON[SSS] FRESHWATER MIXED

1986 1987 1988 1989 1990 1991 1992 1993 1994

Fig. 8. Tot~ monttflyChesapeake Bay SAV aboveground biomass by community type.

1995 1996


TABLE 4. BaywJde annual maximum SAV community total biomass (metric tons), total area (hectares), and mean biomass (tons/ hectare). Month when maximum occurred.

ZOSTERA RUPPIA POTAMOGETON FRESHWATER MI]~LED TOTAL ]BAY SAW" (Jul) (Aug) (Aug) (Oct) (Jul)

Total T~tal Mean Total Total Mean T~tal Total Mean Total T~tal Mean Total T~tal Mean Biomass k e a Biomass Biomass Area Biomass Biomass Area Biomass Biomass Area Biomass Biomass Area Biomass

Year (t) (ha) (t/ha) (t) (ha) (t/ha) (t) (ha) (t/ha) (t) (ha) (t/ha) (t) (ha) (t/ha)

1985 9 ,228 7,877 1 .17 3,501 7,552 1986 9 ,182 7,749 1 .18 3,486 6,900 1987 12,489 9,705 1 .28 2,897 5,902 1988 1989 15,540 10,084 1 .54 4,610 9,040 1990 17,190 13,406 1 .28 2,451 5,523 1991 17,814 15,565 1.81 3,091 6,531 1992 17,140 14,049 1 .22 3,886 9,253 1993 17,585 14,827 1 .17 3,611 9,803 1994 16,153 13,347 1.21 2,990 7,420 1995 16,678 18,477 1 .24 2,602 6,267 1996 16,605 13,385 1 .24 3,272 7,300

0.46 581 1,197 0 .49 8,208 8,248 0 .99 14,716 19,873 0.74 0.51 185 384 0 .48 4,529 4,154 1 .09 14,995 19,187 0.78 0.49 546 2,557 0 .25 4,079 2,154 1 .89 17,797 20,118 0.88

no mapping data for 1988 0.51 497 2,126 0 .28 4,546 2,901 1 .57 20,694 24,152 0.86 0.44 224 475 0 .47 6,385 4,887 1.31 23,060 24,292 0.95 0.47 600 947 0 .65 6,040 4,582 1 .82 24,442 25,625 0.95 0.42 357 802 0 .45 5,892 4,462 1 .32 24,206 28,566 0.85 0.37 61 162 0 .58 6,647 4,795 1 .39 24,525 29,587 0.82 0.40 609 995 0.61 5,291 4,723 1 .12 22,298 26,484 0.84 0.42 898 650 0 .60 4,683 3,857 1.21 21,986 24,252 0.90 0.45 334 564 0 .59 5,412 4,444 1 .22 22,783 25,669 0.89

maximum communi ty aboveground biomass and bed areas from 1985 to 1996 (Table 4) indicate that regrowth of SAV in the Chesapeake Bay has occurred principally in the ZOSTERA community. Rapid growth of ZOSTERA beds occurred between 1986 and 1991 with peak biomass increasing from approximately 9,200 to 17,800 metric tons, or a nearly 94% increase, during that five year period, but declining to 16,600 metric tons by 1996. Area similarly increased fi-om approximately 7,750 hectares to over 14,800 hectares by 1993 and declined to 13,390 by 1996. T h r o u g h o u t the 12 yr study period the overall biomass of the ZOSTERA community remained consistent with an average of 1.24 metric tons per hectare and a coefficient of variation (CV) of 5%.

Baywide annual m a x i m u m biomass of FRESH- WATER MIXED SAV beds also increased from a min imum of approximately 3,200 metric tons in 1985 to a m a x i m u m of 6,650 metric tons in 1998, nearly a 108% increase over eight years (Table 4). Year-to-year changes were quite large with a 40% increase in biomass between 1989 and 1990 alone. Mean biomass ranged from a low of 0.99 metric tons per hectare in 1985 to a high of 1.89 just two years later in 1987 with an average of 1.31 over the study period. This increase in baywide biomass from 1985 to 1987 was associated with marked decline in total communi ty area suggesting that much of the decline occurred in the lower density beds.

Baywide biomass of RUPPIA and POTAMOGE- TON beds fluctuated between 2,600 and 4,600 metric tons and 60 and 600 metric tons, respectively, during the study period with net declines in peak biomass of 7% and 48%, respectively, between 1985 and 1996 (Table 4). gaywide mean biomass of 0.45 and 0.47 metric tons per hectare during the study period were quite similar for the

RUPPIA and POTAMOGETON communit ies respectively, a l though year-to-year variability was larg- er for POTAMOGETON (30% versus 19% GV). Some of this large variability was related to a large decrease in POTAMOGETON biomass which occurred during the 1987 to 1989 period, due in part to a large increase in the area of low density beds.

In spite of year-to-year variability in the baywide annual maximum biomass of the individual SAV communi ty types (16% CV), the combined annual maximum biomass of Chesapeake Bay SAV was quite consistent f rom year-to-year (8% GV) and averaged approximately 0.86 metric tons per hectare (Table 4). This consistency was due, in large part, to the more constant annual maximum biomass of the ZOSTERA communi ty that domina ted the baywide SAV communit ies and averaged approximately 70% of the total bay annual maximum biomass th roughou t the 1985-1996 study period.

Discuss ion

In this study, aerial photography, g round survey observations, and biomass data from a variety of sources are integrated by GIS analysis to provide a summary of the changing SAV communi ty abundance in the Chesapeake Bay over a 12 yr period. These results demonstra te how new informat ion and insights can be developed for a complex system based u p o n existing data. Al though only a summary of the results of this application of GIS techniques are presented here, the direct avail- ability of this type of informat ion through mecha- nisms such as the world wide web are providing for a variety of applications ranging from ecosystem model ing to management . The emerging applications of geographic informat ion systems and remote sensing to aquatic botany (Ferguson and K o r f m a c h e r 1997; L e h m a n n and L a c h a v a n n e 1997; Robbins 1997) can provide for comprehen-

sive analysis of popula t ion level changes with greatly increased accuracy over traditional manual, g round survey techniques.

The annual biomass models presented here, since they are based directly upon published and unpubl ished measurements of SAV biomass for the region, reflect quite well the average annual patterns of aboveground biomass which have been observed in these communit ies locally (e.g., Or th and Moore 1986; Stevenson et al. 1993), as well as worldwide (e.g., Sand-Jensen 1975; Pulich 1985). However, they are by definition average models of biomass which have been adjusted by annual measurements of aerial coverage and density obtained by pho tography taken during annual periods of peak abundance for each community. During any particular year SAV seasonal abundances may be greater or less than model averages depend ing in part upon seasonal climatic or other condit ions (Carter and Rybicki 1986; Carter et al. 1994). For example, we have noted that after particularly hot summers the aboveground biomass of ZOSTERA beds may dieback more than usual and the fall biomass in some areas may be less than average (Orth and Moore 1986). However, the predicted assess- ments of interannual and intra-annual changes in SAV communi ty biomass presented here provide us with the best available measures of system spatial and temporal variability.

The results of this study (Table 4) demonstra te that in the period following the declines of SAV in the Chesapeake Bay and its tributaries which occurred from the early 1970s to the early 1980s (Haramis and Carter 1983; Or th and Moore 1983), there has been a nearly 66% increase in total bay SAV biomass from 1985 to 1991 followed by an ex- tended period of little or no change (1991-1996). Similarly, total bay SAV area increased approximately 49% from 1985 to 1993. One major tribu- tary, the Potomac, experienced a resurgence in freshwater SAV species beginning during the 1980s (Carter and Rybicki 1986; Carter et al. 1994) which was initiated by the spread of H. yettitillate. Due in large part to this regrowth, the annual maximum biomass of the FRESHWATER MIXED communi ty was observed to increase approximately 69% baywide over the 12 yr study period repor ted here. However, most of overall bay increase in total bay SAV abundance (85% of the 9,607 metric ton increase in annual m a x i m u m biornass and 71% of the 9,714 hectare increase in area) during this period occurred in the ZOSTERA community. The recovery in these communit ies may be due to a widespread improvement in habitat condit ions (Dennison et al. 1993) necessary for growth, or may simply be a recovery from the effects of ave ry large environmental stress of Hurr icane Agnes in


1972 which was associated with the initial decline (Orth and Moore 1983) with no real improvement in habitat quality since the 1970s. This storm pro- duced rainfall and runof f rates which were several times greater than those expected for re turn period f requency of 100 years and resulted in hydro- logical, geological, biological and water quality effects which might only occur at 100 to 200 yr intervals (CRC 1975). In contrast to the ZOSTERA and FRESHWATER M I X E D c o m m u n i t i e s , the RUPPIA and POTAMOGETON communit ies have not experienced a resurgence in abundance in most areas of the bay and its tributaries and, al- though there have been some localized increases (Orth et al. 1997) annual maximum biomass has declined 7% and 43%, respectively, since the mid 1980s. This suggests that habitat condit ions necessary for SAV regrowth of these species in most me- sohaline regions (Stevenson et al. 1993) remain poor, or alternatively some other factors may be limiting regrowth there.

Al though there has been regrowth of some SAV communities, with the most recent total bay abundances repor ted here ranging between 25,000 to 30,000 hectares, SAV in the Chesapeake gay system still represent only a fraction of the 250,000 hectares of bot tom, 2 m or less in depth, which at one time may have been capable of support ing SAV (Orth et al. 1994). The enormous potential for pri- mary and secondary produc t ion (Fredette et al. 1990) which could have been suppor ted by this 10- fold or greater abundance in SAV, especially in the mesohal ine and freshwater regions of the system, underscores the t remendous state change in the bay ecosystem which current condit ions represent.

ACKNOWLED GMENTS

Special thanks to Judith Nowak, Jennifer Whiting and Leah Nagey for their efforts in assemNing and organizing the ground survey information from a multitude of sources. Mso, our thanks to Britt-Anne Anderson for her assistance in compilation of file SAV biomass information and digitization of tile ground truth survey data. Funding for this research was provided by a grant from die United States Environmental Protection Agency, Chesapeake gay Program Office, Annapolis, Maryland. This is contribution 9285 from tile Vfrginia Institute of Marine Science, School of Marine Science, College of William and Mary.

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Received for consideratior~, December 22 1998 Accepted for p~ blication, A~g~st 1 Z 1999

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