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 r o c e d u r e was d e v e l o p e d u s i n g a b o v e g r o u n d field b i o m a s s m e a s u r e m e n t s of C h e s a p e a k e Bay subm e r s e d aquatic vegetation (SAV), yearly species identification surveys, annual p h o t o g r a p h i c m a p p i n g at 1:24,000 scale, a n d g e o g r a p h i c i n f o r m a t i o n system (GIS) analyses to d e t e r m i n e the SAV c o m m u n i t y type~ biomass~ a n d area of each m a p p e d SAV b e d in the bay a n d its tidal tributaries f o r the p e r i o d o f 1985 t h r o u g h 1996. U s i n g species identifications p r o v i d e d t h r o u g h over 10,900 SAV g r o u n d s u r v e y observations, the 17 m o s t a b u n d a n t SAV s p e c i e s f o u n d in the b a y w e r e clustered into f o u r s p e c i e s associations: ZOSTERA, RUPPIA~ P O T A M O G E T O N , a n d FRESHWATER MIXED. Monthly a b o v e g r o u n d b i o u l a ~ values were t h e n a s s i g n e d to each b e d or b e d section b a s e d u p o n m o n t h l y biolnass m o d e l s dev e l o p e d f o r each COUllnunity. High salinity colnulunities (ZOSTERA) were f o u n d to d o m i n a t e total bay SAV a b o v e g r o u n d b i o m a s s d u r i n g winter, spring, a n d snlnlner. 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 FRESHWAT E R MIXED) d o m i n a t e d in the fall. In 1996~ total bay SAV s t a n d i n g stock was nearly 22,800 metric tons at annual m a x i m u m b i o m a s s in July e n c o m p a s s i n g an area o f a p p r o x i m a t e l y 25~670 hectares. M i n i m u m b i o m a s s in D e c e m b e r a n d J a n u a r y o f that year was less than 5,000 metric tons. SAY a n n u a l m a x i m u m b i o m a s s i n c r e a s e d baywide f r o m lows of less than 15~000 metric tons in 1985 a n d 1986 to nearly 25~000 metric t o n s d u r i n g the 1991 to 1993 period, while area i n c r e a s e d f r o m a p p r o x i m a t e l y 20,000 to nearly 59,999 hectares d u r i u g that s a m e period. Year-to-year c o m p a r i s o n s of UlaXiUlUUl a n n u a l colnulnuity a b u n d a n c e froul 1985 to 1996 indicated that regrowth of SAV in the C h e s a p e a k e Bay froln 1985-1993 o c c u r r e d principally in the Z O S T E R A COUllnunity~ with 85% of the baywide increase in b i o l n a ~ a n d 71% of the increase in area o c c u r r i n g in that COUllnunity. M a x i m u m b i o u l a ~ o f FRESHWATER MIXED SAV beds also i n c r e a s e d f r o m a low of 3,200 inetric tOllk~in 1985 to a high o f 6,650 metric tons in 1993, while inaxiulnul biolnas~ o f both R U P P I A a n d P O T A M O G E T O N beds fluctuated between 2,450 a n d 4,600 metric tons a n d 60 a n d 600 metric tons~ respectively, d u r i n g that s a m e p e r i o d with n e t declines o f 7% a n d 43%~ respectively, between 1985 a n d 1996. D u r i n g the July p e r i o d of annual, baywide, m a x i m u m SAV biomass, SAV b e d s in the C h e s a p e a k e Bay typically a v e r a g e d a p p r o x i m a t e l y 0.86 metric t o n s of a b o v e g r o u n d d r y m a s s p e r hectare of b e d area.

Introduction

the o c c u r r e n c e of s u b m e r s e d a n g i o s p e r m s a n d water quality conditions (gatiuk et al. 1992; D e n n i s o n et al. 199S), the recovery of these c o m m u n i t i e s has b e e n chosen as one the principal indicators of the success of C h e s a p e a k e Bay clean-up efforts. T h e baywide a n n u a l aerial surveys have t h e r e f o r e bec o m e a cost effective a n d c o m p r e h e n s i v e tool with which to assess changes in this resource. However, the various SAV c o m m u n i t i e s f o u n d in the Chesap e a k e Bay system can r e s p o n d differently to changing water quality conditions as c o m m u n i t i e s m a y differ in their capacity to withstand periods of high turbidity, n u t r i e n t e n r i c h m e n t , or salinity e x t r e m e s (Stevenson and C o n f e r 1978; C a r t e r and Rybicki 1985; Stevenson et al. 199S; M o o r e et al. 1996). Since it is cost prohibitive to annually survey the SAV species c o m p o s i t i o n of each of the t h o u s a n d s of SAV beds in the bay, and there has b e e n as yet no effective way to discriminate individual SAV beds into their d o m i n a n t species or c o m m u n i t y

Aerial p h o t o g r a p h y a n d m a p p i n g surveys have b e e n used in a n u m b e r of regions to d e t e r m i n e the distribution of s u b m e r s e d aquatic vegetation (SAV) p o p u l a t i o n s a n d changes in these p o p u l a tions over time ( O r t h a n d M o o r e 1984; L a r k u m a n d West 1990; Coles et al. 1993; Bulthuis 1995; Ferguson a n d K o r f m a c h e r 1997; Robbins 1997). Aerial m a p p i n g surveys of C h e s a p e a k e Bay SAV have b e e n c o n d u c t e d annually in the C h e s a p e a k e Bay and its sub-estuaries since 1985. Published in r e p o r t f o r m (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 p r o v e n to be useful for m a n y bay m a n a g e m e n t activities. Because of the strong relationships which have b e e n developed b e t w e e n 1 C o r r e s p o n d i n g a u t h o r ; tele: 804/684-7384; fax: 804/6847298; e-mail: m o o r e @ v i m s . e d u . 9 2000 Estuarine Research Federation

115

116

K . A . Moore et al.

types f r o m high altitude aerial p h o t o g r a p h y a l o n e ( O r t h a n d M o o r e 1983; Z i e m a n et al, 1989; gulthuis 1995), a p r o c e d u r e was necessary to assign a classification type to each b e d so that year-to-year changes in the various SAV c o m m u n i t i e s could be assessed. T h e aerial p h o t o g r a p h i c surveys of the Chesap e a k e Bay shorelines provide m e a s u r e s of SAV b e d areas which are then subsequently p h o t o i n t e r p r e t ed into four density classes ( O r t h and M o o r e 1983). While these category-type data provide g o o d m e a s u r e s of relative a b u n d a n c e they do not provide sufficient i n f o r m a t i o n to d e t e r m i n e SAV species biomass or standing crop. In addition, est i m a t e s of SAV a b u n d a n c e at v a r i o u s t i m e s 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 a n n u ally at times of estimated p e a k SAV a b o v e g r o u n d biomass a n d these flight dates vary a m o n g the various regions of the bay. Typically, high 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 t e m p o r a 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 r e s e a r c h e r s a n d m a n a g e r s to effectively m o d e l coastal bay ecosystems improves. Differences between m e a s u r e m e n t s of SAV area coverage and actual biomass can be i m p o r t a n t . D u r i n g any particular year overall bay SAV area may increase, or decrease, or r e m a i n the same f r o m previous years, while SAV biomass may n o t vary linearly with c h a n g e in area. A c o m m o n metric such as biomass or standing crop is necessary to discriminate p o t e n t i a l changes over b o t h spatial or temporal intervals, especially in context of the overall bay ecosystem. In addition, calibration a n d validation of landscape scale, ecosystem simulation m o d els (e.g., B o u m a n s and Sklar 1990; C o n s t a n z a et al. 1990; Cerco a n d Cole 1994) which are d e v e l o p e d with an SAV c o m p o n e n t generally require inform a t i o n on SAV mass, n o t a r e a or relative a b u n dance. T h e overall goal of this study was to d e t e r m i n e the biomass for all areas of SAV m a p p e d in the C h e s a p e a k e Bay over the period of 1985 t h r o u g h 1996 using SAV distribution a n d a b u n d a n c e inform a t i o n available f r o m a n n u a l reports, biomass inf o r m a t i o n available f r o m p u b l i s h e d and u n p u b lished studies by bay researchers, and species g r o u n d survey observations p r o v i d e d by researchers, trained v o l u n t e e r s a n d others in the bay community. O u r specific objectives were to use previously collected survey i n f o r m a t i o n to develop app r o p r i a t e SAV species associations which could be used to classify the SAV beds f o u n d 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 a n n u a l m o d e l s of SAV biomass for each of these SAV c o m m u n i t y types; and to develop a procedure using g e o g r a p h i c i n f o r m a t i o n system (GIS) analyses to assign the a p p r o p r i a t e SAV c o m m u n i t y types a n d calculate the biomass of each SAV b e d m a p p e d in the C h e s a p e a k e Bay annually f r o m 1985 t h r o u g h 1996, Methods

DEVELOPMENT OF SAV COMMUNITYTYPES T h e identification of SAV c o m m u n i t y types was based on an analysis of SAV g r o u n d survey data p u b l i s h e d in a n n u a l SAV distribution r e p o r t s f r o m 1985 to 1996 (e.g., O r t h et al. 1997). T h e s e r e p o r t s d o c u m e n t the locations of all the SAV species which have b e e n identified by p r e s e n c e / a b s e n c e censuses in field surveys c o n d u c t e d during the growing season of each of the years by researchers, g o v e r n m e n t agencies, a n d trained individuals, including citizens groups. All eleven years of g r o u n d survey i n f o r m a t i o n f r o m 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 i n f o r m a t i o n was assigned to each of the individual locations which were identified on the SAV m a p s in each yearly report. Chara sp., Najas flexilis, Nitetta sp., Potamogeton epihyd'rus, Potamogeton nodosus, a n d Trapa natans were identified in twelve or fewer observations and t h e r e f o r e were not used in the d e t e r m i n a t i o n of SAV c o m m u n i t y types, Figures l a - n p r e s e n t the r e c o r d e d o c c u r r e n c e s of each of the individual species f r o m 1985-1996. C o m m u n i t y types were d e v e l o p e d f r o m the entire g r o u n d survey database using n u m e r i c a l clust e r i n g analysis. D i c e ' s c o e f f i c i e n t o f s i m i l a r i t y (Boesch 1977) is a c o m m o n l y used quantitative res e m b l a n c e m e a s u r e which is useful for the n u m e r ical c l u s t e r i n g of b i n a r y ( p r e s e n c e / a b s e n c e ) g r o u n d truth data such as g a t h e r e d in the g r o u n d surveys (Clifford a n d Stevenson 1975). T h e g r e a t e r the coefficient of similarity, the m o r e frequently paired species or g r o u p s of species occur in the database. T h e overall bay SAV species spatial distributions, which are controlled in m o s t cases by salinity tolerance (Stevenson a n d C o n f e r 1978), were t h e n used along with the results of the clustering analysis in the final a s s i g n m e n t of individual species to specific c o m m u n i t y types. Figure 2 presents a s u m m a r y of the clustering analysis of all 11 yr of g r o u n d survey i n f o r m a t i o n (10,023 observations) in d e n d r o g r a r n form. Dice's coefficients between individual species or species g r o u p s are r e p r e s e n t e d by the vertical lines. Table 1 presents a matrix of the n u m b e r of observations r e p o r t i n g pairs of individual species as well as the n u m b e r of observations r e p o r t i n g only a single

Chesapeake Bay SAV Communities ~6' W

Z~,~tera ~.xirina leelgrass) ~'w

7s'w

75"w

Vallisr~eriaat~erkueva (w~ldce]ery? 77" W

]}" ~ " ~ " , ~ ,

7~" W

~, ~-

~.

Potamoger

Ruppia mariEma ( w i d g e m grass) 7~'W

?TW

~'w

Myr~olahylltonspicaUen [Eur~t~ia. wzmmfilfeil) 75" W

Ocean

P~t~uJgelOt~pv.~illt*$ (sic,haler pondw~e.d)

77" W

I~:

76" W

$2~ .r~,.,,~,~,,o.:~,:~,~:~

a~

75" W

(sag~ pondweed)

76"W

ttydnlla vet~cillaca (hydfilla)

~5"W

77" ",V

%" W

potoa'aogettmI~e~E~l~ 77" W

l 17 ~5" W

(redhe~J-grass)

?6' W

75' w

Ceratophyllwm demers~m (oooatail)

35" W

Ocea~

!

er&p~ (e~trly ptmdweed)

Naj.a" sp

EIodea r

(~lm~on elodea)

73"w

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

]7"N

He~er~ther~ dubga (~ater-vr

Zantaiclaellbs paOaarrig ( h m c d ~ d ~ )

118

K.A. Moore et al.

Z manna R. ma~qtima Z. ~atus~ris P. perfoliatus P~pectinaO~ Nalas sp~

ik

N. grac~tona P.pusiltus P, crispus E. catmdens~s IV. lttitlor N. guadolupensi~

H.

dubia

V. americana

j

~

M. spl eotutn _ _

t~

It. vematlata 1 }

....

C. donersura I ...... 0.5

I

.. ---/.'

i 0.4

I 0.3

I o.1

0.2

I o.o

Dice's Coefficientof Similarity

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

species. F o r e x a m p l e , Zannichetliapatustris (Zp) was f o u n d g r o w i n g with Ruppia maritima (Rm) 134 t i m e s a n d Zostera marina (Zm) 11 times, b u t i n m o n o s p e c i f i c s t a n d s 874 times. Vallisneria americana (Va) was o b s e r v e d g r o w i n g with 3/1. spicatum (Ms) 1,201 times, i n m o n o s p e c i f i c s t a n d s 209 times, b u t n e v e r with Z marina. A l t h o u g h Z marina a n d R. maritima are h i g h l y 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 b e d s i n the l o w e r bay, w h i c h a r e u s u a l l y d o m i n a t e d by m o n o s p e c i f i c s t a n d s of Z. marina i n all b u t the shallowest areas ( O r t h a n d M o o r e 1983; M o o r e et al. 1995; T a b l e 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 h a s also b e e n f o u n d t h r o u g h o u t the m i d - b a y as well as the P a t u x e n t , Potomac, and Rappahannock Rivers in many m o n o s p e c i f i c b e d s (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 d i v i d e d i n t o two species g r o u p s with all b e d s c o n t a i n i n g Z marina a s s i g n e d to a Z O S T E R A c o m m u n i t y type a n d b e d s c o n t a i n i n g R. maritima, b u t n o t Z. marina a s s i g n e d to a R U P P I A c o m m u nity type. A s s i g n m e n t of l o w e r salinity species i n t o c o m m u n i t y types was similarly d e t e r m i n e d . F o r e x a m ple, 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 s a m e m i d - b a y r e g i o n s a n d o v e r l a p p e d t h e d i s t r i b u t i o n of R. maritima, a l t h o u g h u s u a l l y n o t at t h e s a m e loc a t i o n s (Fig. l b , c , d , n ) . Z palustris can grow i n m o n o s p e c i f i c b e d s early i n the y e a r ( H a r a m i s a n d C a r t e r 1983) a n d m a y n o t b e f o u n d i n s o m e areas by l a t e - s u m m e r . I n fact, s o m e of the b e d s of Z. p a l ustris w h i c h are l o c a t e d by g r o u n d t r u t h surveys early i n t h e y e a r (Fig. l n ) do n o t a p p e a r o n 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 Vallisrwriaamericana, Rm--R~ppia waritima, Ppu Potawogetonp~sil!,us, Ppf Potawogetor~perfdiat'us, Ppc Yotamogeton pectinatus, Ppc Potamogetoncrispus, 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~theradubia, Ec Elodea car~adensis; Cd Cerat@hylluv~devwrs~v~. Cd Ec Hd Hv Ms N Ngr Ngu Nm Pcr Ppc Ppf Ppu Rm Va Zm Zp

Zp

Zm

54 44 5 22 54 6 17 18 14 26 51 49 18 134 34 11 874

0 0 0 0 0 0 0 0 0 0 1 0 0 699 0 567

Va

532 143 409 877 1,201 61 13 90 135 40 77 70 13 25 209

Rm

3 25 1 0 36 4 0 0 0 ] 53 123 0 2,387

Ppu

PpF

PRo

Per

Nm

Ns'u

Nsr

N

Ms

Hv

Hd

Ec

@d

23 18 2 13 8 4 6 14 9 19 2 1 0

13 62 10 3 87 2 0 3 0 5 57 89

40 29 8 16 67 5 0 5 5 9 79

74 60 6 25 32 5 17 22 25 7

155 44 87 298 177 1 26 51 4

117 44 58 118 98 6 19 7

43 31 1 31 17 1 8

80 11 41 66 61 16

981 196 622 1,453 773

839 40 551 445

371 7 16

172 41

88

Chesapeake Bay SAV Communities TABLE 2. SAV Species Associations. Species occurrence in community exceeds 10% of spades observations. * D o m i n a n t Species 9 ZOSTERA C o m m u n i t y

Zostera marina* Ruppia maritima

9 RUPPIA Community

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

9 POTAMOGETON Community

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

9

FRESHWATER

Community

MIXED

aerial p h o t o g r a p h y surveys of these regions in August ( O r t h et al. 1997). A l t h o u g h Z. patustris has b e e n f o u n d to be associated with a variety of species it was f o u n d m o s t c o m m o n l y growing with R. rnaritima a n d is included 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., O r t h et al. 1997) the a b u n d a n c e of this species may be u n d e r e s t i m a t e d . P. perfoliatus a n d P. pectinatus in contrast, have b e e n typically f o u n d as d o m i n a n t s in a variety of m i x e d and m o n o s p e c i f i c stands (Table 1; Fig. 1c-d). T h e r e f o r e , all beds rep o r t e d with either 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 c o m m u n i t y type. Freshwater regions of the u p p e r bay and the upp e r P o t o m a c River were vegetated with a diverse assemblage of SAV (Fig. 1 e - m ) which were clustered in a large g r o u p of 12 species r a n g i n g f r o m Najas sp. to Ceratophyllum dernersum (Fig. 2). O f these 12 species Myriophytlum spicatum, Hyd'ritta re> ticittata, a n d Vattisneria americana were the m o s t a b u n d a n t . H. verticillata and M. spicatum had the highest c o - o c c u r r e n c e of any two species r e p o r t e d with over 1,450 observations r e p o r t i n g b o t h species (Table 1). V. americana was f o u n d to co-occur with H. verticittata and M. spicatum 877 a n d 1,201 times, respectively. All beds n o t assigned to the ZOSTERA, RUPPIA, or P O T A M O G E T O N comm u n i t y types were assigned to a FRESHWATER M I X E D c o m m u n i t y type. Table 2 presents the species associations for all

1 19

four c o m m u n i t y types including all species where o c c u r r e n c e e x c e e d e d 10% of observations. Figure 3 a - d display the SAV b e d field observations after a s s i g n m e n t to c o m m u n i t y type. FRESHWATER M I X E D and P O T A M O G E T O N c o m m u n i t i e s d o m inate the u p p e r bay a n d u p p e r tributaries, while RUPPIA was f o u n d t h r o u g h o u t m u c h of the bay excluding m o s t freshwater tidal regions. ZOSTERA d o m i n a t e s the lower bay. ASSIGNMENT OF INDIVIDUAL S A V BEDS TO COMMUNITY TYPES

Since yearly g r o u n d survey species i n f o r m a t i o n was not available for each individual SAV b e d 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 m o s t areas of the bay a n d its tributaries, SAV beds which are located n e a r one a n o t h e r tend to be c o m p o s e d of similar species. T h e r e f o r e , to a certain extent, beds can be assigned to the c o m m u n i t y type of the n e a r e s t p o i n t where field survey i n f o r m a t i o n is available. This c o n f i d e n c e d e c r e a s e s with i n c r e a s i n g d i s t a n c e f r o m a survey location. To d e t e r m i n e the maxim u m distances that can be used with confidence, the distribution of field observations for 1994 a n d 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 a n d 1995 were chosen because of the b r o a d distribution and intensity (gg% of all beds surveyed) of g r o u n d survey observations m a d e during that period. O n average, between 1985 and 1996, 29% of all the beds in the bay were g r o u n d surveyed each year. First, the over-water distance between r e p o r t e d survey locations was c o m p u t e d and used to determ i n e the p e r c e n t a g e of observations within a particular distance of each o t h e r that share the same c o m m u n i t y type. This distance relationship can vary greatly t h r o u g h o u t the bay due to factors such as the local salinity gradient. T h e r e f o r e , the CBP s e g m e n t a t i o n scheme, an a r e a c o m p a r t m e n t a l i z a tion of the C h e s a p e a k e Bay into subunits, which was developed based u p o n salinity distributions, natural g e o g r a p h i c partitions and o t h e r 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 entire bay. Each of 44 C h e s a p e a k e Bay P r o g r a m (CBP) segments was analyzed individually to estimate the m a x i m u m distance within which at least 90% of the g r o u n d survey observations within that s e g m e n t were of the s a m e c o m m u n i t y type. An e x a m p l e of this analysis for CBP S e g m e n t CB6 is p r e s e n t e d in Fig. 4. In this CB6 S e g m e n t area all SAV g r o u n d survey locations surveyed in 1994 a n d 1995 were f o u n d to be of the s a m e c o m m u n i t y type w h e n they occurred within a p p r o x i m a t e l y 8 km of each other.

120

K . A . Moore et al. 77' W

76" W

75' W

77" W

76" W

75 ~ W

39= N

39' N

38" N

38' N

37'N

3T N

ZOSTERA Community 77" w

76" w

RUPPIA 75- w

77" w

Community 76' w

75" W

39' N

39" N

38" N

3g" N

3TN

3TN

POTAMOGETON C o m m u n i t y

F R E S H W A T EMIXED R Community

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

A 90% similarity was f o u n d 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 b e y o n d 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 r o c e d u r e was used to assign a community type to each bed m a p p e d in the a n n u a l aerial surveys from 1985 to 1996. If beds were directly surveyed in the c u r r e n t year they were assigned to a c o m m u n i t y type based on the species reported. If a bed was not directly surveyed in a year assignment p r o c e d u r e s were followed in the following order until assignment could be made.

Beds were assigned to the c o m m u n i t y type of the nearest field observations of the c u r r e n t year which were located within the 90% similarity distance c o m p u t e d for the CBP s e g m e n t where the bed was located. Beds that were directly surveyed in the p r e c e d i n g year were assigned to a c o m m u nity type based on the species r e p o r t at that time. Beds were assigned to the c o m m u n i t y type of the nearest field observations m a d e the previous year within the 90% similarity distance c o m p u t e d for the CBP segment where the bed was located. Beds that were directly surveyed in the s u b s e q u e n t year were assigned to a c o m m u n i t y type based on the species reported. Beds were assigned to the community type of the nearest field observations m a d e

Chesapeake Bay SAV Communities

100

9

TABLE 3. Sources used in development of SAV biomass models for each c o m m u n i t y type.

m

t.

90

FRESHWATER MIXED C o m m u n i t y Naylor and Kazyak 1995 Rybicld and Carter 1995 Carter et al. 1994 Carter and Rybicld u n p u b l i s h e d data Stevenson et al. 1993 Rybicki u n p u b l i s h e d 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

80 m

e

70

r3

60

"~

50

1 21

\

%

POTAMOGETON C o m m u n i t y Stevenson et al. 1993 L u b b e r s et al. 1990 Nichols et al. 1979

4O 0

10

20

30

40

50

60

Maximum Distance Between Observations (kin) Fig. 4. Analysis of similarity of SAV species r e p o r t e d in g r o u n d survey observations compm-ed to the over water distance between the observations for Chesapeake Bay Program S e g m e n t CB6. Arrows indicate distance at which pairs of individual species g r o u n d survey observations are classified in the same c o m m u n i t y type 90% of the time.

the s u b s e q u e n t year within the 90% similarity distance c o m p u t e d for the CBP s e g m e n t where the bed was located. Any r e m a i n i n g SAV beds were individually assigned to a c o m m u n i t y type based on the spatial p a t t e r n s provided by the entire g r o u n d survey data set. DEVELOPMENT OF SAV BIOMASS MODELS FOR EACH COMMUNITY TYPE Published a n d u n p u b l i s h e d studies of SAV biomass f r o m the C h e s a p e a k e Bay region (Table 3) were used to d e t e r m i n e average m o n t h l y biomass values for the d o m i n a n t species of each c o m m u n i t y type (Table 2). Only data f i o m studies in which SAV a b o v e g r o u n d biomass f i o m dense m o n o t y p i c stands were r e p o r t e d at least periodically in units of mass p e r a r e a t h r o u g h o u t the growing season were selected for use. B e l o w g r o u n d m e a s u r e m e n t s were n o t available for m o s t species a n d t h e r e f o r e m o n t h l y m o d e l s for this c o m p o n e n t of biomass were n o t a t t e m p t e d . A b o v e g r o u n d biomass values were c o n v e r t e d f i o m wet weight or o t h e r r e p o r t e d units to dry mass p e r unit a r e a by first transforming each study's data to their p r o p o r t i o n of the r e p o r t e d seasonal m a x i m u m of each species (cf., Nichols et al. 1979). T h e s e p r o p o r t i o n s of seasonal m a x i m a were t h e n applied to an overall average m a x i m u m seasonal value in units of g r a m s dry mass m -~ which was calculated using the subset of

RUPPIA C o m m u n i t y Moore et al. 1995 Stevenson et al. 1998 O r t h and Moore 1986 O r t h and Moore 1981 ZOSTERA C o m m u n i t y Moore et al. 1995 O r t h and Moore 1986 O r t h and Moore 1981

studies that specifically r e p o r t e d results in units of dry mass p e r area. In those studies where field biomass s a m p l i n g was not c o n d u c t e d monthly, values for m o n t h s n o t sampled were estimated by linear interpolation. T h e m e a n m o n t h l y biomass values were d e t e r m i n e d by averaging the m o n t h l y values assuming equal area of each of the d o m i n a n t species (Table 2) c o m p r i s i n g a c o m m u n i t y type. Each of the four SAV c o m m u n i t i e s d e m o n s t r a t ed a distinctive p a t t e r n of shoot biomass (Fig. 5 a d). T h e ZOSTERA and RUPPIA c o m m u n i t i e s were f o u n d to exhibit peaks of s h o o t biomass in the early and late s u m m e r , respectively, and b o t h maintained a b o v e g r o u n d s h o o t biomass t h r o u g h o u t the winter. S h o o t growth for ZOSTERA f i o m average winter m i n i m u m s of 45 g d m m ~ was evident as early as F e b r u a r y and rapid shoot dieback was app a r e n t b e g i n n i n g in July after r e a c h i n g an average m a x i m u m of 220 g d m m -~, with a second short period of growth in the fall. RUPPIA did not d e m onstrate a significant increase in shoot biomass until J u n e and it subsequently r e a c h e d a m a x i m u m standing crop in August of a p p r o x i m a t e l y 100 g d m m ~ after which it declined to winter levels of 2 0 95 g d m m -~, Both the P O T A M O G E T O N a n d FRESHWATER M I X E D c o m m u n i t i e s were f o u n d to m a i n t a i n no shoot biomass f r o m D e c e m b e r to April. Beginning at this time, however, s h o o t biomass of b o t h c o m m u n i t i e s rapidly increased. T h e

122

K.A. Moore et al. R. FRESHWATER

b.

Community

POTAMOGETON

300-

300

2513.

25O

200

2(]O

150

150

PERCENT

COVER

DENSITY

CLASS

Community

Very Sparse <10%

f00

/

Sparse

50

I0 -40 % 0

i

c. ~

RUPPIA Community

300-

~,

rl. ZOSTERA

300

Community

250

,.~ <

250

~

200-

200

150 ]

15o

-

100 50 0

9

]00

"

50

-

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

0

..,~

I .I."

Moderate 40 70%

~

-,~

i i i i i i 1 i i FMAMJ J ASOND MONTH

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

POTAMOGETON community reached a peak standing crop of 100 gdm m e or m o r e by August with complete loss by December. In contrast, shoot biomass of the FRESHWATER MIXED c o m m u n i t y increased t h r o u g h o u t the s u m m e r and early fall, and r e a c h e d an average m a x i m u m of nearly g00 g d m 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 A n n u a l aerial p h o t o g r a p h i c surveys of SAV coverage are s u m m a r i z e d (e.g., O r t h et al. 1997) as SAV areas which have b e e n assigned to r a n k e d density classes based u p o n p h o t o - i n t e r p r e t a t i o n using a Grown Density Scale adapted from Paine (1981) (Fig. 6). It was necessary to quantify how these density classes c o r r e s p o n d e d with measurements of SAV g r o u n d survey biomass so that the aerial survey data could be used to d e t e r m i n e SAV biomass baywide. To accomplish this task u n p u b lished field data obtained during the s u m m e r of 1990 at g5 locations t h r o u g h o u t the bay were used. This data consisted of point-intercept measurements obtained by divers at 10 m intervals along t r a n s e c t s o r i e n t e d p e r p e n d i c u l a r to the s h o r e across SAV beds of different densities and species

85m m m 95 !i / m

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 p o i n t sample consisted of triplicate estimates of b o t t o m cover and d e p t h within r a n d o m l y placed 0.25 m e sampling rings. Such m e a s u r e m e n t s have b e e n previously d e m o n s t r a t e d to provide very g o o d estimates of SAV density and biomass (r e > 0.86; O r t h and M o o r e 1988). T h e individual g r o u n d cover transects were then separated into segments based u p o n the published p h o t o - i n t e r p r e t e d density class zones comprising each area in 1990 ( O r t h et al. 1991). Figure 7 illustrates the relationship between field g r o u n d cover m e a s u r e m e n t s and the photo-interpreted density classes for all transect segments. T h e relationship was linear and significant (p < 0.001); however, the aerial photo-interpretation tended to underestimate g r o u n d cover at lower

Chesapeake

80

g

4O

o t,.9

Communities

'123

Monthly a b o v e g r o u n d biomass for each individual SAV bed, or b e d s e g m e n t w h e r e a bed h a d b e e n p h o t o i n t e r p r e t e d into subunits of different density class, was calculated by the following formula:

60 50

SAV

CALCULATION OF MONTHLY SAV BED BIOMASS

70

;a.C'

Bay

Monthly Biomass - Mb * Cc * Ba W h e r e Mb - m o d e l m o n t h l y biomass for assigned c o m m u n i t y type ( g d m m-Z), Cc - photo-interp r e t e d density class to g r o u n d cover conversion, a n d Ba - bed area (me).

30 ~/

20 10

. . . .

0

Results

r2 = 0.99

I

20

. . . .

I

40

. . . .

I

. . . .

60

L

. . . .

80

I

1O0

Mid Points of Density Classes (%) Fig. 7. Comparison of SAV aerial density classification categories to SAV groundcover measurements.

SAV densities and overestimate at h i g h e r densities. No consistent effects of c o m m u n i t y type or depths of SAV growth on the relationship between g r o u n d cover and density class assignments could be determined. T h e r e f o r e , density class to g r o u n d cover conversion was applied consistently across all SAV beds.

ZOSTERA ~

RUPPIA ~

Results of the m o n t h l y shoot biomass calculations for all SAV beds f r o m 1985 t h r o u g h 1996 is s u m m a r i z e d in Fig. 8. During this period SAV maxi m u m s u m m e r biomass increased baywide f r o m lows of 15,000 metric tons in 1985 a n d 1986 to highest levels of nearly 25,000 metric tons during 1991 t h r o u g h 199S. T h e high salinity ZOSTERA c o m m u n i t y d o m i n a t e d total bay SAV biomass during the winter, spring and summer. Lower salinity communities (RUPPIA, POTAMOGETON, and FRESHWATER MIXED) d o m i n a t e d in the fall. At p e a k biomass in July, total bay system standing stock of SAV was a p p r o x i m a t e l y 22,800 metric tons in 1996. M i n i m u m standing stock in D e c e m b e r a n d J a n u a r y of that year was less than 5,000 metric tons. Year-to-year c o m p a r i s o n s of a n n u a l bay-wide

POTAMOGETON[SSS] FRESHWATER MIXED

25000

20000

,2

15000

t0000 > O

<

5000 O

N

1985

1986 Fig. 8.

1987

1988

1989

1990

1991

1992

1993

1994

Tot~ monttflyChesapeake Bay SAV aboveground biomass by community type.

1995

1996

124

K.A. Moore et al.

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

RUPPIA

(Jul)

(Aug)

POTAMOGETON

Total Biomass

T~tal kea

Mean Biomass

Total Biomass

Total Area

Mean Biomass

Year

(t)

(ha)

(t/ha)

(t)

(ha)

(t/ha)

1985 1986 1987 1988 1989 1990 1991 1992 1993 1994 1995 1996

9,228 9,182 12,489 15,540 17,190 17,814 17,140 17,585 16,153 16,678 16,605

7,877 1.17 7,749 1.18 9,705 1.28 10,084 13,406 15,565 14,049 14,827 13,347 18,477 13,385

1.54 1.28 1.81 1.22 1.17 1.21 1.24 1.24

FRESHWATER

(Aug)

MI]~LED

TOTAL ]BAY SAW"

(Oct)

(Jul)

T~tal Biomass

Total Area

Mean Biomass

Total Biomass

T~tal Area

Mean Biomass

Total Biomass

T~tal Area

Mean Biomass

(t)

(ha)

(t/ha)

(t)

(ha)

(t/ha)

(t)

(ha)

(t/ha)

3,501 7,552 0.46 3,486 6,900 0.51 2,897 5,902 0.49

581 1,197 0 . 4 9 8,208 185 384 0 . 4 8 4,529 546 2,557 0 . 2 5 4,079 no mapping data for 1988 4,610 9,040 0.51 497 2,126 0 . 2 8 4,546 2,451 5,523 0.44 224 475 0 . 4 7 6,385 3,091 6,531 0.47 600 947 0 . 6 5 6,040 3,886 9,253 0.42 357 802 0 . 4 5 5,892 3,611 9,803 0.37 61 162 0 . 5 8 6,647 2,990 7,420 0.40 609 995 0.61 5,291 2,602 6,267 0.42 898 650 0 . 6 0 4,683 3,272 7,300 0.45 334 564 0 . 5 9 5,412

m a x i m u m c o m m u n i t y a b o v e g r o u n d biomass and bed areas from 1985 to 1996 (Table 4) indicate that regrowth of SAV in the Chesapeake Bay has o c c u r r e d principally in the ZOSTERA community. Rapid growth of ZOSTERA beds o c c u r r e d 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 r e m a i n e d 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 FRESHWATER MIXED SAV beds also increased from a m i n i m u m 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 m a r k e d decline in total c o m m u n i t y area suggesting that m u c h of the decline o c c u r r e d in the lower density beds. Baywide biomass of RUPPIA and POTAMOGET O N 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 m e a n biomass of 0.45 and 0.47 metric tons per hectare during the study period were quite similar for the

8,248 0 . 9 9 4,154 1 . 0 9 2,154 1 . 8 9

14,716 19,873 0.74 14,995 19,187 0.78 17,797 20,118 0.88

2,901 4,887 4,582 4,462 4,795 4,723 3,857 4,444

20,694 23,060 24,442 24,206 24,525 22,298 21,986 22,783

1.57 1.31 1.82 1.32 1.39 1.12 1.21 1.22

24,152 24,292 25,625 28,566 29,587 26,484 24,252 25,669

0.86 0.95 0.95 0.85 0.82 0.84 0.90 0.89

RUPPIA and P O T A M O G E T O N c o m m u n i t i e s respectively, a l t h o u g h year-to-year variability was larger for P O T A M O G E T O N (30% versus 19% GV). Some of this large variability was related to a large decrease in P O T A M O G E T O N 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 m a x i m u m biomass of the individual SAV c o m m u n i t y types (16% CV), the c o m b i n e d a n n u a l m a x i m u m biomass of Chesapeake Bay SAV was quite consistent from 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 m o r e constant annual m a x i m u m biomass of the ZOSTERA c o m m u n i t y that d o m i n a t e d the baywide SAV c o m m u n i t i e s and averaged approximately 70% of the total bay a n n u a l m a x i m u m biomass t h r o u g h o u t the 1985-1996 study period. Discussion

In this study, aerial p h o t o g r a p h y , g r o u n d survey observations, and biomass data from a variety of sources are integrated by GIS analysis to provide a s u m m a r y of the c h a n g i n g SAV c o m m u n i t y abundance in the Chesapeake Bay over a 12 yr period. These results d e m o n s t r a t e how new i n f o r m a t i o n and insights can be developed for a c o m p l e x system based u p o n existing data. A l t h o u g h only a s u m m a r y of the results of this application of GIS techniques are presented here, the direct availability of this type of i n f o r m a t i o n t h r o u g h mechanisms such as the world wide web are providing for a variety of applications ranging from ecosystem m o d e l i n g to m a n a g e m e n t . T h e e m e r g i n g applications of g e o g r a p h i c i n f o r m a t i o n systems and rem o t e sensing to aquatic botany (Ferguson and K o r f m a c h e r 1997; L e h m a n n a n d L a c h a v a n n e 1997; Robbins 1997) can provide for c o m p r e h e n -

Chesapeake Bay SAV Communities

sive analysis of p o p u l a t i o n level changes with greatly increased accuracy over traditional manual, g r o u n d survey techniques. T h e a n n u a l biomass models presented here, since they are based directly u p o n published and u n p u b l i s h e d m e a s u r e m e n t s of SAV biomass for the region, reflect quite well the average annual patterns of a b o v e g r o u n d biomass which have b e e n observed in these c o m m u n i t i e s locally (e.g., O r t h and M o o r e 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 a n n u a l measurements of aerial coverage and density obtained by p h o t o g r a p h y taken during annual periods of peak a b u n d a n c e for each community. During any particular year SAV seasonal a b u n d a n c e s may be greater or less than m o d e l averages d e p e n d i n g in part u p o n seasonal climatic or o t h e r conditions (Carter and Rybicki 1986; Carter et al. 1994). For example, we have n o t e d that after particularly hot s u m m e r s the a b o v e g r o u n d biomass of ZOSTERA beds may dieback m o r e than usual and the fall biomass in some areas may be less than average (Orth and M o o r e 1986). However, the predicted assessments of i n t e r a n n u a l and intra-annual changes in SAV c o m m u n i t y biomass presented here provide us with the best available measures of system spatial and temporal variability. T h e results of this study (Table 4) d e m o n s t r a t e 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; O r t h and M o o r e 1983), there has been a nearly 66% increase in total bay SAV biomass from 1985 to 1991 followed by an ext e n d e d period of little or no c h a n g e (1991-1996). Similarly, total bay SAV area increased approximately 49% from 1985 to 1993. O n e major tributary, the Potomac, experienced a resurgence in freshwater SAV species b e g i n n i n g 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 m a x i m u m biomass of the FRESHWATER MIXED c o m m u n i t y was observed to increase approximately 69% baywide over the 12 yr study period r e p o r t e d here. However, most of overall bay increase in total bay SAV a b u n d a n c e (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 o c c u r r e d in the ZOSTERA community. T h e recovery in these communities may be due to a widespread i m p r o v e m e n t in habitat conditions (Dennison et al. 1993) necessary for growth, or may simply be a recovery from the effects of a v e r y large environmental stress of H u r r i c a n e Agnes in

1 25

1972 which was associated with the initial decline (Orth and M o o r e 1983) with no real i m p r o v e m e n t in habitat quality since the 1970s. This storm prod u c e d rainfall and r u n o f f rates which were several times greater than those expected for r e t u r n period f r e q u e n c y of 100 years and resulted in hydrological, geological, biological and water quality effects which might only occur at 100 to 200 yr intervals (CRC 1975). In contrast to the ZOSTERA a n d F R E S H W A T E R M I X E D c o m m u n i t i e s , the RUPPIA and P O T A M O G E T O N c o m m u n i t i e s have n o t e x p e r i e n c e d a r e s u r g e n c e in a b u n d a n c e in most areas of the bay and its tributaries and, alt h o u g h there have been some localized increases (Orth et al. 1997) annual m a x i m u m biomass has declined 7% and 43%, respectively, since the mid 1980s. This suggests that habitat conditions necessary for SAV regrowth of these species in most mesohaline regions (Stevenson et al. 1993) r e m a i n poor, or alternatively some other factors may be limiting regrowth there. A l t h o u g h there has b e e n regrowth of some SAV communities, with the most recent total bay abundances r e p o r t e d 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 bottom, 2 m or less in depth, which at one time may have b e e n capable of s u p p o r t i n g SAV (Orth et al. 1994). T h e e n o r m o u s potential for prim a r y and s e c o n d a r y p r o d u c t i o n (Fredette et al. 1990) which could have been s u p p o r t e d by this 10fold or greater a b u n d a n c e in SAV, especially in the mesohaline and freshwater regions of the system, u n d e r s c o r e s the t r e m e n d o u s state change in the bay ecosystem which c u r r e n t conditions represent. ACKNOWLEDGMENTS 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 SAVbiomass 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. LITERATURE CITED

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Chesapeake Bay SAV Communities SANI)-JENsEN, K. 1975. Biomass, net production and growth dynamics in an eelgrass (Z~seera marina L.) population in Vellerup Vig, Denmark. ophelia 14:185-201. STAVER, L. W. 1986. Competitive interactions of submerged aquatic vegetation under varying nutrient and salinity conditions. M.S. Thesis, University of Maryland, College Park, Maryland. STeVeNSON,J. C. AND N. M. CONFER. 1978. Summary of available information on Chesapeake Bay vegetation. U.S. Fisheries and Wildlife Service Report. FWS/OBS-78/66. Annapolis, Maryland. STeVeNSON,J. C., L. STAVER,AND K. W. STAVER.1993. Water quality associated with survival of submerged aquatic vegetation along an estuarine gradient. Est*zaries 16:346-361. ZIFXaAN,J. c . , j . w. FOURQUREAN,AND R. L. IVERs 1989. Dis-

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tribution, abundance and productivity of seagrasses and macroalgae in Florida Bay. Bulletin of Marine Science 44:292-811. SOURCES OF UNPUBLISHED MATERIALS CaRTF~, V. u n p u n i s h e d data. U.S. Geological Survey, 430 National Center, Reston, Virginia 22092. R~a~icxI, N. unpublished data. U.S. Geological Survey, 430 National Center, Reston, Virginia 22092. STAVER,g. unpublished data. Center for Environmental and Esmarine Studies, University of Maryland, Horn Point Laboratory, R O. Box 775, Cambridge, Maryland 21613.

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