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Seasonal and year-to-year variations in the growth of Zostera marina L. (eelgrass) in the lower Chesapeake Bay
Aquatic Botany, 24 (1986) 335--341 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
335
SEASONAL AND YEAR--TO--YEAR VARIATIONS IN THE GROWTH O F Z O S T E R A M A R I N A L. ( E E L G R A S S ) I N T H E L O W E R CHESAPEAKE BAY'
ROBERT J. ORTH and KENNETH A. MOORE
Virginia Institute of Marine Science and School of Marine Science of the College of William and Mary, Gloucester Point, VA 23062 (U.S.A.)
ABSTRACT Orth, R.J. and Moore, K.A., 1986. Seasonal and year-to-year variations in the growth of Zostera marina L. (eelgrass) in the lower Chesapeake Bay. Aquat. Bot., 24: 335--341. Seasonal and year-to-year variations in the growth of Zostera marina L. were measured at three sites in two locations in the lower Chesapeake Bay between 1978 and 1980. The maximum values for the 1979 above- and belowground standing crop ranged from 161-336 g dry wt m -2 and 61--155 g dry wt m -2, respectively, leaf length was 19.6---59.7 cm and shoot density 1418--2576 shoot m -2. Values for 1980 tended to be greater and may be related to climatical differences between the two years. Maximum values were usually recorded in the months of June and July when water temperatures were between 20 and 25°C. Significant loss of leaves occurred in July and August, when water temperatures ranged between 25 and 30°C, while new shoots began to appear more rapidly in late September as water temperatures dropped below 20°C. The greatest increase in all growth parameters occurred from April to June during which time reproductive shoots were present, and accounted for up to 25% of the total n u m b e r of shoots.
INTRODUCTION
Z o s t e r a marina L. is t h e m o s t w i d e l y d i s t r i b u t e d seagrass s p e c i e s i n t h e northern Pacific and the northern Atlantic regions (den Hartog, 1970). W i t h i n t h e C h e s a p e a k e B a y , Z. marina, as w e l l as o t h e r s p e c i e s o f s u b m e r g e d aquatic plants, had b e e n the focus o f a large i n t e r d i s c i p l i n a r y s t u d y because of the r e c e n t large-scale b a y - w i d e d e c l i n e ( O r t h a n d M o o r e , 1 9 8 3 a ) . In o u r r e c e n t p a p e r s , a s p e c t s o f t h e r e p r o d u c t i v e b i o l o g y o f Z. marina w e r e disc u s s e d ( O r t h a n d M o o r e , 1 9 8 3 b ; S i l b e r h o r n e t al., 1 9 8 3 } . H e r e , w e d e s c r i b e c h a n g e s i n t h e s t a n d i n g s t o c k o f Z. m a r i n a a t t h r e e sites i n t h e l o w e r C h e s a peak Bay over a 30-month period.
1Contribution No. 1284 from the Virginia Institute of Marine Science, College of William and Mary.
0304-3770/86/$03.50
© 1986 Elsevier Science Publishers B.V.
336 STUDY SITES
Z. marina was sampled f r o m two locations in the lower Chesapeake Bay: Brown's Bay located in the Mobjack Bay, and the Guinea Marshes located at the m o u t h of the York River (Fig. 1). Vegetation at Brown's Bay was a mixed assemblage of Z. marina and Ruppia maritima L. (sensu lato.). The depth of overlying water was approximately 0.5 m at mean low water (MLW). Sampling at this site c o m m e n c e d in October 1978. Vegetation at the Guinea Marsh site was p r e d o m i n a n t l y Z. marina, with R. maritima occurring only in the shallowest portions of the bed. Initially, one site was chosen for sampling, commencing in June 1978 (referred to as " G u i n e a Marsh offshore"). A second site (referred as " G u i n e a Marsh inshore"), 1000 m inshore of the first site, was added in April 1979. Water depths were similar at both Guinea Marsh sites (0.75 m at MLW); however, the inshore site, being protected on three sides by islands of Spartina alterniflora Lois., was less subject to wave action. Sediments at these three sampling sites were p r e d o m i n a n t l y sand (77-89%). The Guinea Marsh inshore area had a greater percentage of fine material than the offshore site (23 vs 17%) as well as smaller median grain size (3.1 vs 2.6¢).
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Temperature ranged f r o m 0 to 29°C during the 3-year period f r o m 1978 to 1980. Water temperatures for 1979 and 1980 are shown in Fig. 2. Data were obtained from a c o n t i n u o u s t e m p e r a t u r e recorder located 12 km upriver from the Guinea Marsh site. Temperatures obtained at each site at each sampling date were compared to those o f the continuous recorder and f o u n d to be similar. Data from National Oceanographic Atmospheric Administration for precipitation and sky cover, which were available for Norfolk, Virginia, a distance of 75 km away, were assumed to be representative of the sampling sites and were utilized here. M o n t h l y rainfall and the deviation in mean m o n t h l y sky cover from the 30-year average are shown in Fig. 2. Salinities at Brown's Bay and Guinea Marsh were usually identical. Salinities were always lowest in the spring and highest in the late summer or a u t u m n and ranged from 12.4 to 19.0°/00 in 1979 and from 15.8 to 24.5 °/o0 in 1980.
338 MATERIALS AND METHODS Monthly samples for above- and belowground standing crop, s h o o t length and s h o o t density (vegetative and reproductive) were taken from a section of the Z. marina bed where cover was uniform. Initially, a 0.1-m 2 ring was placed on the b o t t o m and all vegetation including the roots and rhizomes were removed b y hand to a depth of approximately 10 cm. F o u r samples were collected m o n t h l y at each sampling location. In June 1979, a 0.033-m 2 core was a d o p t e d for sampling the vegetation. A comparison of the data collected using these 2 m e t h o d s at 2 different sites showed no significant differences (P < 0.05) for the parameters being measured. Subsequently, 6 cores o f vegetation were taken at each site. After removal of the vegetation and sediment from the ring or core, all material was placed in a cloth mesh bag and washed free of sediment. Roots, rhizomes and leaves were then placed in another bag and held in running seawater until processed, normally within 24--48 h. Processing included: (1) separating the shoots from the living roots and rhizomes; (2) counting all shoots and measuring the longest leaf of 100 shoots; (3) counting reproductive shoots (when present) and recording total length; (4) drying roots, rhizomes and leaves for 48 h at 45°C; (5) placing the material in a desiccator, after drying to allow cooling to r o o m temperature; and (6) weighing the material to the nearest 0.01 g after removing from the desiccator. RESULTS Peak shoot standing crop at the three sites occurred in June or J u l y during both years with higher standing stocks recorded for 1980 compared to 1979 (Fig. 3A). Shedding o f older leaves occurrred b e t w e e n July and September when water temperatures generally exceeded 25°C, resulting in low standing stocks between S e p t e m b e r and March. Reproductive shoots were present b e t w e e n March and J u n e and accounted for 10--42% of the aboveground standing crop (Fig. 3A). R o o t and rhizome standing crop followed a seasonal pattern similar to the aboveground standing crop, with higher mean values in 1980 compared to 1979 (Fig. 3B). S h o o t densities were highest in J u n e or July and decreased to minimum values in S e p t e m b e r following the summer die-off (Fig. 3C). N e w shoots began to emerge in O c t o b e r when water temperatures had d r o p p e d below 20°C, and shoot production continued t h r o u g h o u t the winter m o n t h s to the following J u n e - - J u l y maximum. Newly germinated shoots between O c t o b e r and April partially a c c o u n t e d for the increase in s h o o t density. Reproductive shoots, when present, accounted for 11--25% o f the total n u m b e r of shoots in 1979 and 4--20% in 1980 (Fig. 3C). Leaves reached their m a x i m u m length in J u n e or July at the three sites (Fig. 3D). Among sites, shoots were shortest at Brown's Bay. At the Guinea Marsh Inshore site, shoots attained their greatest length in the summer of 1979 (59.7 cm), being almost twice as long as in 1980 (25.5 cm).
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Fig. 3. (A): Zostera marina shoot standing crop (g-m -2 +- SD) for each sampling site each season. Shaded areas represent additional contributions by reproductive shoots. (B): Zostera marina root and rhizome standing crop (g.m -2 -+ 1 SD) for each site each sampling period. (C): number of Zostera marina shoots, m -2 (-+ 1 SD) for each sampling site each sampling period. Shaded areas represent contribution by reproductive shoots. (D): shoot length (cm) (+- SD) for vegetative shoots of Zostera marina during each sampling period for each site. Unattached points represent length of reproductive shoots for the sampling period.
DISCUSSION Z o s t e r a m a r i n a in the C h e s a p e a k e B a y e x h i b i t e d distinct seasonal trends in its g r o w t h pattern at t h e three sampling sites. V i g o r o u s g r o w t h occurred f r o m March to J u l y w i t h m a x i m a l values for all m e a s u r e d parameters being recorded in J u n e or July. F l o w e r i n g t o o k place f r o m April t o J u n e w i t h seed release c o m p l e t e d b y m i d - J u n e (Silberhorn et al., 1 9 8 3 ) . Minimal
340 values for all measured parameters followed the extensive leaf loss that occurred in July and August when water temperatures approached 30°C. Increasing shoot p r o d u c t i o n in S e p t e m b e r or October, coinciding with a decline in temperatures below 20°C, resulted in the gradual increase in the density of shoots. However, because these shoots had very little biomass, the response in total standing crop (Fig. 3A) was n o t evident until April as water temperatures rose above 10°C. Also, because new shoots and leaves on existing shoots were continually produced, b u t grew slowly t h r o u g h o u t the winter, the mean length of the shoots actually declined (Fig. 3D) until the spring growth period (April--June), a characteristic also n o t e d for a European Z. marina bed (Jacobs, 1979). In addition, seed p r o d u c e d b y the preceding year's spring flowering began to germinate in O c t o b e r - - N o v e m b e r (Orth and Moore, 1 9 8 3 b ) and continued to germinate and grow t h r o u g h o u t the winter, adding to the total number of shoots. The differences in measured parameters between 1979 and 1980 may, in part, be related to different climatic conditions between the t w o years. Monthly mean surface water temperature, m o n t h l y precipitation and the deviation in m o n t h l y mean sky cover from the 30-year average (Fig. 2) indicated there was more rainfall (total a m o u n t each year = 165 vs 114 cm) and more sky cover in 1979 compared to 1980, as well as a difference in the heating pattern of surface water (Wetzel and Penhale, 1983). The increased sky cover and resulting lower solar insolation in 1979, especially in the spring and early summer seasons when Z. marina is in the most active growth phase, could have resulted in reduced vigor of the plants as suggested by SandJensen (1975), Jacobs (1979) and S a n d 4 e n s e n and Borum {1983). Yearly differences in growth parameters m a y also be related to the biology of the individual beds of Z. marina as well as specific short-term physical stresses which m a y initiate these biological changes. At the Guinea Marsh inshore site in early 1979, the bed was characterized by the presence of very long vegetative shoots (longest leaf lengths up to 60 cm) with 21.5% of the total n u m b e r of shoots being reproductive during the flowering season. During the summer, the plants became almost completely defoliated leaving only a few remaining shoots. This significant dieback did n o t occur in the other areas. One possible hypothesis is that the relatively p r o t e c t e d cove-like area in which the vegetation was growing was subject to extremely high water temperatures during certain short-term periods. Based on other observations in the area, it is n o t unlikely that during periods when low slack water and m a x i m u m midday solar insolation occurred water temperatures could exceed 35°C. In the other more exposed sites water temperatures generally do n o t rise above 30°C. Seedling recruitment (up to 66 m -2 ) at the inshore site accounted in the main part for the revegetation in 1980. Their vigorous growth in the spring of 1 9 8 0 resulted in a greater abundance of shoots (2597 vs 1418 m-2), b u t m a x i m u m length of the longest leaf never exceeded 30 cm compared to 60 cm in 1979.
341
Comparison of our data for Z. marina beds in Chesapeake Bay with other locations in the United States (Burkholder and Doheny, 1968; Penhale, 1977; Vaughan, 1982), Canada (Harrison, 1982), Japan (Aioi, 1980) and Europe (Sand-Jensen, 1975; Jacobs, 1979, Nienhuis and de Bree, 1980) indicated similar seasonal trends for growth and reproduction, but with a shift, depending on the latitude, in the period when maximal values were attained. Maximum values reported here for plant parameters fall within the range of maxima reported for other areas (Jacobs, 1984). ACKNOWLEDGEMENTS
This project was funded by the U.S. Environmental Protection Agency's Chesapeake Bay Program, Grant No. R805953. We would like to thank P. Gapcynski, J. Hauer, G. Markwith and P. Mauck for their assistance in processing the numerous samples collected during this project.
REFERENCES Aioi, K., 1980. Seasonal change in the standing crop of eelgrass (Zostera marina L.) in Odawa Bay, Central Japan. Aquat. Bot., 8: 343--354. Burkholder, P.R. and Doheny, T.E., 1968. The biology of eelgrass. Lamont Geol. Obs., Contrib. No. 1227, 120 pp. Den Hartog, C., 1970. The seagrasses of the world. North-Holland Publishing Co., Amsterdam, 275 pp. Harrison, P.G., 1982. Spatial and temporal patterns in abundance of two intertidal seagrasses, Zostera americana den Hartog and Zostera marina L. Aquat. Bot., 12: 305-320. Jacobs, R.P.W.M., 1979. Distribution and aspects of the production and biomass of eelgrass, Zostera marina L., at Roscoff, France. Aquat. Bot., 7 : 151--172. Jacobs, R.P.W.M., 1984. Biomass potential of eelgrass (Zostera marina L.). CRC Crit. Rev. Plant Sci., 2 : 49--80. Nienhuis, P.H. and de Bree, B.H.H., 1980. Production and growth dynamics of eelgrass (Zostera marina) in a brackish Lake Grevelingen (The Netherlands). Neth. J. Sea Res., 14: 102--118. Orth, R.J. and Moore, K.A., 1983a. Cheasapeake Bay: An unprecedented decline in submerged aquatic vegetation. Science, 222: 51--53. Orth, R.J. and Moore, K.A., 1983b. Seed germination and seedling growth of Zostera marina L. (eelgrass) in the Chesapeake Bay. Aquat. Bot., 15 : 117--131. Penhale, P.A., 1977. Macrophyte--epiphyte biomass and productivity in an eelgrass (Zostera marina L.) community_. J. Exp. Mar. Biol. Ecol., 26: 2 1 1 - - 2 2 4 . . Sand°Jensen, K., 1975. Biomass, net production and growth dynamics in an eelgrass (Zostera marina L.) population in Vellerup Vig. Denmark. Ophelia. 14: 185--201. Sand-Jensen, K. and Borum, J., 1983. Regulation of growth in eelgrass (Zostera marina L.) in Danish coastal waters. Mar. Technol. Soc. J., 17: 15--21. Silberhorn, G.M., Orth, R.J. and Moore, K.A., 1983. Anthesis and seed production in Zostera marina L. (eelgrass) from the Chesapeake Bay. Aquat. Bot., 15: 133--144. Vaughan, D.E., 1982. Production ecology of eelgrass (Zostera marina L.) and its epiphytes in Little Egg Harbor, New Jersey, Ph.D. Dissertation, Rutgers University, New Brunswick, NJ, 136 pp. Wetzel, R.L. and Penhale, P.A., 1983. Production ecology of seagrass communities in the lower Chesapeake Bay. Mar. Technol. Soc. J., 17: 22--31.
335
SEASONAL AND YEAR--TO--YEAR VARIATIONS IN THE GROWTH O F Z O S T E R A M A R I N A L. ( E E L G R A S S ) I N T H E L O W E R CHESAPEAKE BAY'
ROBERT J. ORTH and KENNETH A. MOORE
Virginia Institute of Marine Science and School of Marine Science of the College of William and Mary, Gloucester Point, VA 23062 (U.S.A.)
ABSTRACT Orth, R.J. and Moore, K.A., 1986. Seasonal and year-to-year variations in the growth of Zostera marina L. (eelgrass) in the lower Chesapeake Bay. Aquat. Bot., 24: 335--341. Seasonal and year-to-year variations in the growth of Zostera marina L. were measured at three sites in two locations in the lower Chesapeake Bay between 1978 and 1980. The maximum values for the 1979 above- and belowground standing crop ranged from 161-336 g dry wt m -2 and 61--155 g dry wt m -2, respectively, leaf length was 19.6---59.7 cm and shoot density 1418--2576 shoot m -2. Values for 1980 tended to be greater and may be related to climatical differences between the two years. Maximum values were usually recorded in the months of June and July when water temperatures were between 20 and 25°C. Significant loss of leaves occurred in July and August, when water temperatures ranged between 25 and 30°C, while new shoots began to appear more rapidly in late September as water temperatures dropped below 20°C. The greatest increase in all growth parameters occurred from April to June during which time reproductive shoots were present, and accounted for up to 25% of the total n u m b e r of shoots.
INTRODUCTION
Z o s t e r a marina L. is t h e m o s t w i d e l y d i s t r i b u t e d seagrass s p e c i e s i n t h e northern Pacific and the northern Atlantic regions (den Hartog, 1970). W i t h i n t h e C h e s a p e a k e B a y , Z. marina, as w e l l as o t h e r s p e c i e s o f s u b m e r g e d aquatic plants, had b e e n the focus o f a large i n t e r d i s c i p l i n a r y s t u d y because of the r e c e n t large-scale b a y - w i d e d e c l i n e ( O r t h a n d M o o r e , 1 9 8 3 a ) . In o u r r e c e n t p a p e r s , a s p e c t s o f t h e r e p r o d u c t i v e b i o l o g y o f Z. marina w e r e disc u s s e d ( O r t h a n d M o o r e , 1 9 8 3 b ; S i l b e r h o r n e t al., 1 9 8 3 } . H e r e , w e d e s c r i b e c h a n g e s i n t h e s t a n d i n g s t o c k o f Z. m a r i n a a t t h r e e sites i n t h e l o w e r C h e s a peak Bay over a 30-month period.
1Contribution No. 1284 from the Virginia Institute of Marine Science, College of William and Mary.
0304-3770/86/$03.50
© 1986 Elsevier Science Publishers B.V.
336 STUDY SITES
Z. marina was sampled f r o m two locations in the lower Chesapeake Bay: Brown's Bay located in the Mobjack Bay, and the Guinea Marshes located at the m o u t h of the York River (Fig. 1). Vegetation at Brown's Bay was a mixed assemblage of Z. marina and Ruppia maritima L. (sensu lato.). The depth of overlying water was approximately 0.5 m at mean low water (MLW). Sampling at this site c o m m e n c e d in October 1978. Vegetation at the Guinea Marsh site was p r e d o m i n a n t l y Z. marina, with R. maritima occurring only in the shallowest portions of the bed. Initially, one site was chosen for sampling, commencing in June 1978 (referred to as " G u i n e a Marsh offshore"). A second site (referred as " G u i n e a Marsh inshore"), 1000 m inshore of the first site, was added in April 1979. Water depths were similar at both Guinea Marsh sites (0.75 m at MLW); however, the inshore site, being protected on three sides by islands of Spartina alterniflora Lois., was less subject to wave action. Sediments at these three sampling sites were p r e d o m i n a n t l y sand (77-89%). The Guinea Marsh inshore area had a greater percentage of fine material than the offshore site (23 vs 17%) as well as smaller median grain size (3.1 vs 2.6¢).
~r
%
,..~i:~..
ro~',eoy ,
~
"< " ~
\
I
~
Gu.,.o M o , , . ,
"e,.~.e/,.
Fig. 1. Location of study sites used for standing stock studies (GMI -- Guinea Marsh Inshore; GMO -- Guinea Marsh Offshore).
337 30 u
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Fig. 2. C o m p a r i s o n o f 1 9 7 9 (solid circles) a n d 1 9 8 0 ( o p e n circles) s u r f a c e w a t e r t e m p e r a t u r e (Virginia I n s t i t u t e o f M a r i n e S c i e n c e pier), m o n t h l y rainfall a n d t h e d e v i a t i o n in m e a n m o n t h l y s k y cover (in t e n t h s ) f r o m t h e 30 y e a r average ( N o r f o l k , V i r g i n i a ) (rep r i n t e d w i t h t h e p e r m i s s i o n o f Mar. T e c h . Soc. J.).
Temperature ranged f r o m 0 to 29°C during the 3-year period f r o m 1978 to 1980. Water temperatures for 1979 and 1980 are shown in Fig. 2. Data were obtained from a c o n t i n u o u s t e m p e r a t u r e recorder located 12 km upriver from the Guinea Marsh site. Temperatures obtained at each site at each sampling date were compared to those o f the continuous recorder and f o u n d to be similar. Data from National Oceanographic Atmospheric Administration for precipitation and sky cover, which were available for Norfolk, Virginia, a distance of 75 km away, were assumed to be representative of the sampling sites and were utilized here. M o n t h l y rainfall and the deviation in mean m o n t h l y sky cover from the 30-year average are shown in Fig. 2. Salinities at Brown's Bay and Guinea Marsh were usually identical. Salinities were always lowest in the spring and highest in the late summer or a u t u m n and ranged from 12.4 to 19.0°/00 in 1979 and from 15.8 to 24.5 °/o0 in 1980.
338 MATERIALS AND METHODS Monthly samples for above- and belowground standing crop, s h o o t length and s h o o t density (vegetative and reproductive) were taken from a section of the Z. marina bed where cover was uniform. Initially, a 0.1-m 2 ring was placed on the b o t t o m and all vegetation including the roots and rhizomes were removed b y hand to a depth of approximately 10 cm. F o u r samples were collected m o n t h l y at each sampling location. In June 1979, a 0.033-m 2 core was a d o p t e d for sampling the vegetation. A comparison of the data collected using these 2 m e t h o d s at 2 different sites showed no significant differences (P < 0.05) for the parameters being measured. Subsequently, 6 cores o f vegetation were taken at each site. After removal of the vegetation and sediment from the ring or core, all material was placed in a cloth mesh bag and washed free of sediment. Roots, rhizomes and leaves were then placed in another bag and held in running seawater until processed, normally within 24--48 h. Processing included: (1) separating the shoots from the living roots and rhizomes; (2) counting all shoots and measuring the longest leaf of 100 shoots; (3) counting reproductive shoots (when present) and recording total length; (4) drying roots, rhizomes and leaves for 48 h at 45°C; (5) placing the material in a desiccator, after drying to allow cooling to r o o m temperature; and (6) weighing the material to the nearest 0.01 g after removing from the desiccator. RESULTS Peak shoot standing crop at the three sites occurred in June or J u l y during both years with higher standing stocks recorded for 1980 compared to 1979 (Fig. 3A). Shedding o f older leaves occurrred b e t w e e n July and September when water temperatures generally exceeded 25°C, resulting in low standing stocks between S e p t e m b e r and March. Reproductive shoots were present b e t w e e n March and J u n e and accounted for 10--42% of the aboveground standing crop (Fig. 3A). R o o t and rhizome standing crop followed a seasonal pattern similar to the aboveground standing crop, with higher mean values in 1980 compared to 1979 (Fig. 3B). S h o o t densities were highest in J u n e or July and decreased to minimum values in S e p t e m b e r following the summer die-off (Fig. 3C). N e w shoots began to emerge in O c t o b e r when water temperatures had d r o p p e d below 20°C, and shoot production continued t h r o u g h o u t the winter m o n t h s to the following J u n e - - J u l y maximum. Newly germinated shoots between O c t o b e r and April partially a c c o u n t e d for the increase in s h o o t density. Reproductive shoots, when present, accounted for 11--25% o f the total n u m b e r of shoots in 1979 and 4--20% in 1980 (Fig. 3C). Leaves reached their m a x i m u m length in J u n e or July at the three sites (Fig. 3D). Among sites, shoots were shortest at Brown's Bay. At the Guinea Marsh Inshore site, shoots attained their greatest length in the summer of 1979 (59.7 cm), being almost twice as long as in 1980 (25.5 cm).
339 A ~
LEAF STANDING CROP
C
SHOOT OENSITY
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|
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Fig. 3. (A): Zostera marina shoot standing crop (g-m -2 +- SD) for each sampling site each season. Shaded areas represent additional contributions by reproductive shoots. (B): Zostera marina root and rhizome standing crop (g.m -2 -+ 1 SD) for each site each sampling period. (C): number of Zostera marina shoots, m -2 (-+ 1 SD) for each sampling site each sampling period. Shaded areas represent contribution by reproductive shoots. (D): shoot length (cm) (+- SD) for vegetative shoots of Zostera marina during each sampling period for each site. Unattached points represent length of reproductive shoots for the sampling period.
DISCUSSION Z o s t e r a m a r i n a in the C h e s a p e a k e B a y e x h i b i t e d distinct seasonal trends in its g r o w t h pattern at t h e three sampling sites. V i g o r o u s g r o w t h occurred f r o m March to J u l y w i t h m a x i m a l values for all m e a s u r e d parameters being recorded in J u n e or July. F l o w e r i n g t o o k place f r o m April t o J u n e w i t h seed release c o m p l e t e d b y m i d - J u n e (Silberhorn et al., 1 9 8 3 ) . Minimal
340 values for all measured parameters followed the extensive leaf loss that occurred in July and August when water temperatures approached 30°C. Increasing shoot p r o d u c t i o n in S e p t e m b e r or October, coinciding with a decline in temperatures below 20°C, resulted in the gradual increase in the density of shoots. However, because these shoots had very little biomass, the response in total standing crop (Fig. 3A) was n o t evident until April as water temperatures rose above 10°C. Also, because new shoots and leaves on existing shoots were continually produced, b u t grew slowly t h r o u g h o u t the winter, the mean length of the shoots actually declined (Fig. 3D) until the spring growth period (April--June), a characteristic also n o t e d for a European Z. marina bed (Jacobs, 1979). In addition, seed p r o d u c e d b y the preceding year's spring flowering began to germinate in O c t o b e r - - N o v e m b e r (Orth and Moore, 1 9 8 3 b ) and continued to germinate and grow t h r o u g h o u t the winter, adding to the total number of shoots. The differences in measured parameters between 1979 and 1980 may, in part, be related to different climatic conditions between the t w o years. Monthly mean surface water temperature, m o n t h l y precipitation and the deviation in m o n t h l y mean sky cover from the 30-year average (Fig. 2) indicated there was more rainfall (total a m o u n t each year = 165 vs 114 cm) and more sky cover in 1979 compared to 1980, as well as a difference in the heating pattern of surface water (Wetzel and Penhale, 1983). The increased sky cover and resulting lower solar insolation in 1979, especially in the spring and early summer seasons when Z. marina is in the most active growth phase, could have resulted in reduced vigor of the plants as suggested by SandJensen (1975), Jacobs (1979) and S a n d 4 e n s e n and Borum {1983). Yearly differences in growth parameters m a y also be related to the biology of the individual beds of Z. marina as well as specific short-term physical stresses which m a y initiate these biological changes. At the Guinea Marsh inshore site in early 1979, the bed was characterized by the presence of very long vegetative shoots (longest leaf lengths up to 60 cm) with 21.5% of the total n u m b e r of shoots being reproductive during the flowering season. During the summer, the plants became almost completely defoliated leaving only a few remaining shoots. This significant dieback did n o t occur in the other areas. One possible hypothesis is that the relatively p r o t e c t e d cove-like area in which the vegetation was growing was subject to extremely high water temperatures during certain short-term periods. Based on other observations in the area, it is n o t unlikely that during periods when low slack water and m a x i m u m midday solar insolation occurred water temperatures could exceed 35°C. In the other more exposed sites water temperatures generally do n o t rise above 30°C. Seedling recruitment (up to 66 m -2 ) at the inshore site accounted in the main part for the revegetation in 1980. Their vigorous growth in the spring of 1 9 8 0 resulted in a greater abundance of shoots (2597 vs 1418 m-2), b u t m a x i m u m length of the longest leaf never exceeded 30 cm compared to 60 cm in 1979.
341
Comparison of our data for Z. marina beds in Chesapeake Bay with other locations in the United States (Burkholder and Doheny, 1968; Penhale, 1977; Vaughan, 1982), Canada (Harrison, 1982), Japan (Aioi, 1980) and Europe (Sand-Jensen, 1975; Jacobs, 1979, Nienhuis and de Bree, 1980) indicated similar seasonal trends for growth and reproduction, but with a shift, depending on the latitude, in the period when maximal values were attained. Maximum values reported here for plant parameters fall within the range of maxima reported for other areas (Jacobs, 1984). ACKNOWLEDGEMENTS
This project was funded by the U.S. Environmental Protection Agency's Chesapeake Bay Program, Grant No. R805953. We would like to thank P. Gapcynski, J. Hauer, G. Markwith and P. Mauck for their assistance in processing the numerous samples collected during this project.
REFERENCES Aioi, K., 1980. Seasonal change in the standing crop of eelgrass (Zostera marina L.) in Odawa Bay, Central Japan. Aquat. Bot., 8: 343--354. Burkholder, P.R. and Doheny, T.E., 1968. The biology of eelgrass. Lamont Geol. Obs., Contrib. No. 1227, 120 pp. Den Hartog, C., 1970. The seagrasses of the world. North-Holland Publishing Co., Amsterdam, 275 pp. Harrison, P.G., 1982. Spatial and temporal patterns in abundance of two intertidal seagrasses, Zostera americana den Hartog and Zostera marina L. Aquat. Bot., 12: 305-320. Jacobs, R.P.W.M., 1979. Distribution and aspects of the production and biomass of eelgrass, Zostera marina L., at Roscoff, France. Aquat. Bot., 7 : 151--172. Jacobs, R.P.W.M., 1984. Biomass potential of eelgrass (Zostera marina L.). CRC Crit. Rev. Plant Sci., 2 : 49--80. Nienhuis, P.H. and de Bree, B.H.H., 1980. Production and growth dynamics of eelgrass (Zostera marina) in a brackish Lake Grevelingen (The Netherlands). Neth. J. Sea Res., 14: 102--118. Orth, R.J. and Moore, K.A., 1983a. Cheasapeake Bay: An unprecedented decline in submerged aquatic vegetation. Science, 222: 51--53. Orth, R.J. and Moore, K.A., 1983b. Seed germination and seedling growth of Zostera marina L. (eelgrass) in the Chesapeake Bay. Aquat. Bot., 15 : 117--131. Penhale, P.A., 1977. Macrophyte--epiphyte biomass and productivity in an eelgrass (Zostera marina L.) community_. J. Exp. Mar. Biol. Ecol., 26: 2 1 1 - - 2 2 4 . . Sand°Jensen, K., 1975. Biomass, net production and growth dynamics in an eelgrass (Zostera marina L.) population in Vellerup Vig. Denmark. Ophelia. 14: 185--201. Sand-Jensen, K. and Borum, J., 1983. Regulation of growth in eelgrass (Zostera marina L.) in Danish coastal waters. Mar. Technol. Soc. J., 17: 15--21. Silberhorn, G.M., Orth, R.J. and Moore, K.A., 1983. Anthesis and seed production in Zostera marina L. (eelgrass) from the Chesapeake Bay. Aquat. Bot., 15: 133--144. Vaughan, D.E., 1982. Production ecology of eelgrass (Zostera marina L.) and its epiphytes in Little Egg Harbor, New Jersey, Ph.D. Dissertation, Rutgers University, New Brunswick, NJ, 136 pp. Wetzel, R.L. and Penhale, P.A., 1983. Production ecology of seagrass communities in the lower Chesapeake Bay. Mar. Technol. Soc. J., 17: 22--31.