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Detritus processing and mineral cycling in seagrass (Zostera) litter in an Oregon salt marsh
Aquatic Botany, 20 (1984) 97--108 Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands
97
D E T R I T U S P R O C E S S I N G AND M I N E R A L C Y C L I N G IN S E A G R A S S ( Z O S T E R A ) L I T T E R IN AN O R E G O N S A L T M A R S H
JOHN L. GALLAGHER College of Marine Studies, University of Delaware, Lewes, DE 19958 (U.S.A.) HAROLD V. KIBBY and KATHERINE W. SKIRVIN U.S. Environmental Protection Agency, Corvallis Environmental Research Laboratory, Corvallis, OR 97330 (U.S.A.) (Accepted for publication 21 May 1984)
ABSTRACT Gallagher, J.L., Kibby, H.V. and Skirvin, K.W., 1984. Detritus processing and mineral cycling in seagrass (Zostera) litter in an Oregon salt marsh. Aquat. Bot., 20: 97--108. In estuaries where seagrass beds adjoin marshes, the import and decomposition of seagrass litter in the marsh provide a mechanism for retaining nutrients within the wetlands and preventing loss to adjacent oceanic waters. Several aspects of the influence of seagrass litter on an Oregon salt marsh were studied. The quantity of Zostera litter in the marsh vegetation depended on elevation and on the marsh-plant canopy structure. Litter decomposition was most complete in the seagrass bed and in the highest marsh area. At intermediate elevations decomposition was very low after 40% of the material was degraded. The respiration rates of dead-plant communities (DPC s) and the release rates of dissolved organic carbon into the tidal waters depended on the location of the DPC in the marsh. Seagrass litter contributed between 14 and 31% as much dead material to the total litter in the marsh as did the marsh plants. Decomposition of this litter could release 6--8% of the nitrogen required for the growth of the marsh plants.
INTRODUCTION I n t e r a c t i o n s b e t w e e n a d j a c e n t w e t l a n d s y s t e m s m a y be greater t h a n t h o s e b e t w e e n similarly placed terrestrial units, b e c a u s e w e t l a n d s y s t e m s are inund a t e d either p e r i o d i c a l l y or c o n s t a n t l y b y a fluid m e d i u m with e x c e l l e n t b u o y a n c y a n d solvent qualities. In areas w h e r e seagrass beds are a d j a c e n t t o salt marshes, t h e r e m a y be i n t e r a c t i o n s o f p a r t i c u l a t e o r g a n i c c a r b o n (POC), dissolved o r g a n i c c a r b o n ( D O C ) and n u t r i e n t s (Gallagher, 1 9 7 8 ) . Since tidal w a t e r f l o w s o v e r these seagrass beds o n its w a y t o t h e m a r s h , p l a n t processes m a y m o d i f y t h e w a ~ r p r i o r t o its arrival in t h e m a r s h so t h a t i n o r g a n i c n u t r i e n t s , as well as various f o r m s o f D O C a n d P O C , m a y be a d d e d o r rem o v e d . W h e n t h e tide ebbs, similar c h a n g e s m a y have o c c u r r e d d u r i n g resid e n c y o f t h e w a t e r s in t h e m a r s h . R e p e a t e d e x c u r s i o n s o f t h e tide m a y amplif y t h e single-pass e f f e c t p r i o r t o flushing o f t h e w a t e r f r o m t h e p r o t e c t e d
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© 1984 Elsevier Science Publishers B.V.
98 basin where the emergent and submergent wetland systems coexist. Figure 1 shows the possible interactions between upland, marsh, seagrass-bed and other bay habitats, and the ocean. Netarts Bay, OR, in the Pacific Northwest of the U.S.A., has extensive seagrass beds in the subtidal and lower intertidal areas. Both Zostera marina L. and Zostera japonica Aschers. and Graebn. are found in the bay, the latter occurring more abundantly at higher elevations. Our observations indicate that the species density and vigor of the plants decrease markedly at higher elevations close to the lower marsh. The vegetative patchwork pattern of plant stands in the marsh appears to depend partly on elevation relative to the tides. The lower, gently sloping elevations of the Netarts marsh were occupied by Scirpus americanus Pers., which averaged --0.3 m relative to mean high water (MHW). Next in elevation was Distichlis spicata (L.) Greene (0.1 m), which occurred either in monotypic stands or mixed with Salicornia virginica L. (0.4 m) and formed a matrix into which were set patches of Triglochin maritimum L. (0.2 m). A mixed stand of Potentilla pacifica Howell (0.9 m) and Deschampsia cespitosa (L.) Beauv. formed the highest-elevation marsh type. Our purpose in this study was to examine one facet of the inter-
UPLAND]' MARSH
l
MARSH WATER
SEAGRASS BED BAY WATER OTHER BAY HABITATS
OCEAN Fig. 1. F l o w s o f e n e r g y and materials in seagrass t o o t h e r estuarine h a b i t a t s and to adjacent ecosystems.
99 action b e t w e e n beds o f the t w o Zostera species (which we will simply call Zostera) and several stands of marsh plants in Netarts Bay. The deposited dead plants and the associated assemblage of microbes were considered to be the dead-plant c o m m u n i t y (DPC) (Gallagher and Pfeiffer, 1977; Gallagher et al., 1984). The research focused on the deposition of dislodged seagrass in three t y p e s of marsh vegetation and on the dispersion of p h o t o s y n t h a t e s and minerals in the marsh. We addressed the following questions concerning the Zostera litter in the marsh. (1) What is the seasonal pattern of the quantity of Zostera litter in the marsh? (2) What are the Zostera decomposition rates in several wetland locations? (3) What are the aerial and aquatic respiration rates of the heterotrophic communities associated with the Zostera litter? (4) H o w much DOC is released from the decomposing dead-seagrass communities to be e x p o r t e d with the ebbing tide or consumed in the marsh? (5) H o w much detritus might be added to the marsh b y the decay of Zostera ? (6) What is an estimate of the nutrient input to the marsh from the grass bed? MATERIALS AND METHODS The Zostera litter in seven randomly selected plots in stands of Potentilla pacifica, Triglochin maritimum and Salicornia virginica was collected at sixweek intervals in the warm season, approximately March--October, and every eight weeks in the cool season (the study period ran from June 1977 to O c t o b e r 1978 in the P. pacifica and T. maritimum and to D e c e m b e r 1978 in the S. virginica stand); samples were dried to a constant weight at 60°C. The submerged (in water) respiration rates of the communities associated with the dead Zostera were measured using techniques described b y Gallagher and Pfeiffer (1977). Respiration in air (exposed) was measured b y CO2 evolution using syringes as incubators and an infrared gas-analyzer (Gallagher et al., 1984). DOC release from the communities was measured by placing litter in plastic Whirl-Pak ® bags containing seawater. After incubation for 3 h, DOC was measured using the methods of Menzel and Vacorro (1964). Net release was calculated as the difference b e t w e e n the DOC of litter-filled bags and the DOC of bags containing only water (Gallagher et al., 1976, 1984). Litter bags (plastic, 1 mm mesh, 20 cm square) containing recently stranded Zostera were set in the S. virginica marsh in August 1977. The bags were placed in the c a n o p y in an array around a series of stakes to which t h e y were held with nylon cord. Three to five bags were returned to the laboratory after 7, 16, 23, 59 and 72 weeks of incubation in the marsh. Similar Zostera litter bags were deployed in O c t o b e r 1978 and collected periodically for one year; these sets were placed in the Z. marina, S. virginica and P.
100
pacifica stands. Aerial and aquatic respiration rates and DOC losses were measured for the material in the litter bags as described above. Although amphipods were often seen in the litter, they were n o t included in the DPC because their high mobility made it difficult to take representative samples; therefore, the c o m m u n i t y was limited to microbes. The bags n o t used for these rate measurements were washed free of sediment and dried to constant weight at 60°C. A carbon--hydrogen--nitrogen (CHN) analyzer was used to determine the percentages of carbon and nitrogen. Spark-emission spectrometry (Jones and Isaac, 1969) was used for mineral analyses, while ash-free dry weights were calculated from data resulting from biomass ashing at 460°C in a muffle furnace for 3 h. Marsh elevations were determined through surveys c o n d u c t e d by the National Ocean Survey (NOS). The reported elevations of each plant stand are the mean of 50 determinations; these means were coupled to MHW through data collected b y using a water-level recorder o p e r a t e d in Netarts Bay for one year. F r e q u e n c y of inundation was calculated from tidal data and elevations. RESULTS
AND
DISCUSSION
All T. maritimum and S. virginica plots contained some Zostera litter each time they were sampled, b u t the litter distribution was very heterogeneous in both time and space in the P. pacifica stands (Table I). The quantities of litter in the three areas are shown in Fig. 2. Differences b e t w e e n sampling dates reflect both deposition and removal by the tides, and decomposition. TABLE I Percentage of Potentilla pacifica plots containing Zostera litter (n = 7) Date 1977 June July
August October December 1978 February March April
Percentage
0 43 0 43 0
* 100 100
June
29
July
14 0 0 0
September October December *No plots harvested.
101
The smallest mean a m o u n t of Zostera litter was f o u n d in P. pacifica (19 gm-:), the marsh with the highest elevation (0.9 m above MHW) of the intertidal zone and, consequently, the one with least frequent inundation (3%). In the lower areas, where the stands of S. virginica and T. maritimum were located, Zostera litter averaged 83 and 26 g m -2, respectively. Based on the elevations (0:4 and 0.2 m above MHW) and consequent inundation frequencies (15 and 25%), it might have been expected that less Zostera w o u l d be f o u n d in the S. virginica than in the T. maritimum stand. The effectiveness of plant stands as traps for Zostera litter depends n o t only on soil elevation b u t also on the structure o f the plant canopy. T. maritimum grows in tufts ~ 0 . 5 m taller than the surrounding marsh during the summer; however, early in the fall the c a n o p y collapses, altering the height of the plants which act as a filter to trap Zostera. The litter-trapping efficiency of a stand depends on the net effect of tide height and plant-height. When the water level is only slightly higher than the base of the tufts, the dense clumps o f stems act as barriers to the deposition of litter within the clumps. As the water level rises, the shoots act as traps for Zostera. By comparison, the S. uirginica c a n o p y is relatively uniform in height throughout the year because of the somewhat w o o d y stems of the plants. Unless the litter is tangled in the canopy, tides higher than the plant elevation tend to float the litter to a higher site. Kentula (1983) f o u n d Zostera leaf dynamics in Netarts Bay to resemble closely those reported at Roscoff, France by Jacobs (1979). There was an increase in the n u m b e r of leaves per shoot in the spring, with a decrease beginning in the summer and continuing into the fall. Zostera biomass in Netarts Bay is comparable to that found in other temperate regions (Kentula, 1983). Deposition of seagrass litter in the marsh (Fig. 2) is associated with periods of leaf-shedding and storms, and is particularly high when these t w o periods coincide. All of the deposition factors, coupled with the decomposition rates, resulted in the m a x i m u m mean quantity o f Zostera litter occurring &-Zostera in PotentiUo pacifca 0-Zostera in Trjlochi.~n nmrltlm~m r~- Zostera in Salicornia v~linlca
,2ot I00]
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. J
.
. A
. S 1977
0
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d 1978
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Fig. 2. Q u a n t i t i e s o f Zostera sp. l i t t e r in Potentilla pacifica, Salicornia virginica and Triglochin m a r s h - p l a n t s t a n d s in N e t a r t s Bay, O R ( b a r s r e p r e s e n t ± 1 S.E., n = 7).
102 in the S. virginica stand. The data for the q u a n t i t y of Zostera litter indicate t h a t there was a substantial input of detritus from the seagrass beds to the marsh. Litter turnover and microbial activity were assessed from respiration and leaching studies of unconfined litter and of the f o u r sets of litter bags. Decomposition rates in litter bags m a y be different from those occurring within similar unconfined litter because of differences in physical, chemical and biological factors in the two settings. These problems are complicated by the fact t h a t the degree of perturbation from the natural condition is n o t consistent across environmental gradients. All t h a t can be done is to design a study so as to minimize these differences and to recognize the shortcomings which m a y occur. In earlier work with unconfined litter of various marsh plant species, we f o u n d moisture, which differed among the three s t u d y sites, to be the most i m p o r t a n t factor in determining the respiratory rate of decomposers (Gallagher et al., 1984). To avoid problems in biasing the moisture regime away from natural conditions, we thus placed lightly packed litter bags into the canopies in positions where unconfined litter was found. Animal activity inside and outside of the bags was a second concern. The 1 m m mesh size would n o t allow adult amphipods (the only macrofauna observed) to enter or leave, but y o u n g animals could enter and grow there. If there was a bias, it m a y have been that predators could n o t feed on the amphipods; nevertheless, we did n o t observe differences in animal density between the bags and the open litter. Wave action was a factor only at the Zostera incubation site; the effect was n o t clear but m a y have been either to protect the plants or to enhance mechanical action, since the bags were tethered and hence n o t free to move with the waves. Although there are difficulties with the use o f litter bags as long-term integrators of decomposition, better techniques are n o t available (Gallagher, 1978); therefore, we deployed four sets of litter bags into the wetlands to estimate the decomposition rates of Zostera in several environments. Figure 3 illustrates the results of the 1977--1979 study, where all bags were incubated in the S. virginica marsh (mid-elevation), and of the 1978--1979 study, where bags were incubated in Zostera, S. virginica and P. pacifica (high~levation) stands. The two incubations in ~. virginica gave similar results, with a loss of 35% of the initial weight in the bags in approximately four months. Losses after t h a t time were very slow, and growth of organisms in and on the partly skeletonized Zostera tissue appears to have resulted in some gain in weight in the litter. In the S. virginica environment ~ 6 0 % of the Zostera litter was resistant to decomposition. In the Z. marina bed, decomposition was more rapid and continued until less than 5% of the original weight remained after one year. The portion of the seagrass bed where the litter bags were incubated was sandy and within the intertidal zone. Although no Eh values were obtained, sediment coloration and the lack of h y d r o g e n sulfide odor made it likely t h a t the decomposition conditions for the Zostera were aerobic even at times when the bags were covered with substrate. It is probable t h a t the more f r e q u e n t and longer immersion in
103 the seagrass beds stimulated more microbial activity and that wave action was responsible for mechanical breakdown of the tissue at this site. The seagrass litter in the P. pacifica site, that flooded least frequently, decomposed at a rate intermediate between those at the S. virginica and Zostera incubation sites. This site was very moist because of shading from tall clumps of D. cespitosa and dense growth of P. pacifica. Furthermore, the canopy o f P. pacifica collapsed soon after death to further reduce drying o f the Zostera litter.
I00,
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Fig. 3. Decomposition of Zostera litter communities in plastic-mesh bags incubated in stands of PotentiUa pacifica, Salicornia virginica and Zostera marina in Netarts Bay, OR ( 1 9 7 7 - - 1 9 7 9 ) (data based on ash-free dry weights (AFDW)).
As might be expected, the respiration of Zostera DPC was highest in summer (Fig. 4). The respiration rate in air, although generally lower than that in water, followed the same pattern as the aquatic rate. The moisture content of the Zostera DPC was high in the cool portion of 1 9 7 8 - - 1 9 7 9 , but l o w temperatures or other factors kept respiration rates low. Except for April 1978, the C/N ratio was always between 17 and 24. The average aerial and
104
aquatic respiration rates during the period March--October were 31 and 86 pg C (g dry wt.) -1 h -1, respectively. These rates were less than those for several DPCs with similar C/N ratios and may be associated with reduced availability o f nitrogen, as evidenced by the high concentration (59--67%) of crude fiber content. Harrison and Mann (1975) f o u n d t h a t one-half of the nitrogen in dead Zostera in shallow water along the coast of Nova Scotia was n o t protein but was trichloroacetic-acid-insoluble, residue-bound nitrogen. Godshalk and Wetzel (1978) also reported t h a t a large portion of the nitrogen in decomposing Zostera was not in a readily available form. Although the C/N ratio in the Zostera litter was favorable, the relatively low decomposition rates compared to several other marsh grasses m a y be associated with the unavailability of the nitrogen to microorganisms. oquotic re=wotion rote 0 - - 0 incubation temperotunl D--Q a~ iol respiration rot,
1
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Fig. 4. Aerial and aquatic respiration rates, incubation temperatures, C/N ratios, and percentage moisture contents of Zostera litter communities found in a Salicornia marsh at Netarts Bay, OR (1978--1979).
There was little change in aquatic respiration rate with time in the Zostera DPCs which were incubated in the S. virginica stands (Fig. 5). The aerial respiration rate was o f t e n lower than the aquatic rate by more than one-half. During the early stages of incubation in the marsh, aquatic rates at the P. pacifica site were higher than those in the S. virginica stand. Except for the first and last measurements the aerial DPC respiration rates at the P. pacifica site were lower than t h e aquatic rates. The aerial respiration rates of the DPC incubated at the P. pacifica site were higher than those of the DPC incubated in the S. virginica marsh. The aquatic rates for the DPC at the Z. marina site, although initially lower than those from P. pacifica, were very high during the last two samplings; however, the aerial rates were very low. Since the Z. marina incubation site was very wet most of the time, it is probable t h a t the microbes colonizing t h a t tissue were better adapted to aquatic t h a n to aerial respiration. The results of the respiration measurements are compatible with the litter-bag weight-loss studies, as the litter at the Z. marina site had the highest respiration rates and weight loss, t h a t in P. pacifica was intermediate, and that in S. virginica initially had the least. By the end o f the year the two marsh sites showed similar losses and respiratory rates.
105 In addition to decomposition associated with respiration, there is active DOC leaching from Zostera tissue a n d / o r release from attached microbes (Fig. 6). Leaching rates were high compared to those found for dead Spartina alterniflora Loisel. communities by Gallagher and Pfeiffer (1977) in Georgia. In Oregon, no correlation was f o u n d between leaching rate and temperature, although rates measured during the warmer March--August period were highest. For DPCs under continually submerged conditions in the laboratory, Godshalk and Wetzel (1978) did find a positive correlation with temperature. In the field this response was probably masked by other factors, such as time since last immersion, age of litter, and rainfall. The time elapsed since last immersion may be a factor because DOC could have accumulated on exposed Zostera litter as a result of activity by microbes. When the litter was submerged, the water m a y have received a pulse of DOC as the accumulated soluble c o m p o u n d s dissolved. We were interested in quantifying the DOC released when the n e x t tide covered the plants, and used a standard 3-h immersion for all our measurements.
,4°I 1201
LIJ i
tOO.
• imhollitter "~
or" 80
60
40
20-
0
0
N
D
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d
d
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Fig. 5. Aerial and aqudtic respiration rates of Zostera litter communities incubated in mesh bags in several wetland types in Netarts Bay, OR (bars represent ± 1 S.E., n = 3; data based on ash-free dry weight (AFDW)). In August we compared leaching rates f r o m Z. marina and Z. japonica litter from the marsh with similar samples from the mud flats where the plants grew (Fig. 6). In both cases Z. japonica released more DOC (~ = 0.05), but the difference bbtween the sites was even greater, with the litter in the marsh releasing the greatest a m o u n t of DOC. We attribute the relatively higher rates in the n~arsh site to the fact t h a t previous leaching frequency and inundation duration were greatest at the lower elevations of the seagrass beds. Differences in the age of the dead plant material could have been a factor, but we a t t e m p t e d to collect similar litter at the different sites.
106 ,3800 2880
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9~i978
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Fig. 6. DOC release rates (pg C (g dry wt.) -1 h-l) from Zostera litter communities in a Salicornia virginica marsh at Netarts Bay, OR. August data are also presented for litter collected from the seagrass bed (bars represent + 1 S.E.,n = 4;data based on dry weights). T h e leaching rate o f DOC f r o m t h e litter bags was negative at t h e Z. marina site at the end o f t he first i n c u b a t i o n period (Fig. 7). This DOC uptake f r o m th e water m a y have been by microbes which colonized t he n e w l y placed litter. Conversely, the u p t a k e could simply have been t he result o f physical sorption at dead-plant surfaces. A relatively low b u t positive rate was measured f or t he r e m a i n d e r o f t h e year. T h e litter incubated in t he S. virginica stand exhi bi t ed a similar p a t t e r n e x c e p t t h a t t he rate was n o t negative after th e first i n c u b a t i o n period. T h e rate f o r t he litter from t h e bags in the P. pacifica zone was initially high and remained so t h r o u g h o u t t he year. Tidal water inundates these areas i n f r e q u e n t l y and it is probable t h a t soluble organic car b o n from microbial activity and litter degradation which accumulated b etween floodings was released as a pulse o f DOC when t h e tides reached th e stranded litter. As an indication o f t he c o n t r i b u t i o n o f the Zostera litter t o t h e detritus pool, we c o m p a r e d t he m e a n annual q u a n t i t y o f seagrass litter with t h a t o f dead marsh plants in t he t h r e e marsh types. In t he S. virginica plant stand the seagrass litter averaged 31% o f t he marsh-plant dead biomass. T h e values f o r the T. m a r i t i m u m and P. pacifica areas were 27 and 14%, respectively. T h er ef o r e, seagrass litter appears t o be c o n t r i b u t i n g substantially t o t h e detrital p o o l o f these t h r e e wetland p l a n t types in Netarts Bay. At t he time we were studying n u m e r o u s aspects o f the ecology o f these t hree marsh types, we were also examining Scirpus americanus and Distichlis spicata stands (--0.3 and 0.1 m above MHW, respectively). In these lower-elevation
107
stands our sampling methods for other purposes did not allow us to collect seagrass litter, but the quantity appeared to be much larger than that reported for the areas which we could sample. A significant part of the plant material decomposing in marsh regions in estuaries such as Netarts Bay, where large, productive grassbeds border the marshes, is allochthonous. 600w
P-
_
<~
Zostero
in P. poci f i ca
- - - - Zostera in ~. virginica ......... Zostera in Z. marina 300
'l-
'. "N.
LiJ (.3 •.
...... "e-.""
0
N 1978
D
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d
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Fig. 7. DOC release rates (ug C (g dry w t . ) - ' h -1) from Zostera litter c o m m u n i t i e s incubated in mesh bags in PotentT"lla pacifica, Salicornia virginica and Zostera marina stands in Netarts Bay, OR (data based on ash-free dry weights (AFDW)).
Our first approximation of nutrient input into the marsh by Zostera litter decomposition was calculated using the litter biomass data, the respiration rates and the chemical composition data. The average annual respiration rates of the Zostera litter in air and in water were multiplied by the mean litter standing crops in the three plant zones to give the amounts of carbon which would have been consumed if the litter had decomposed while having been continually either exposed or submerged. These quantities were multiplied by the percentages of the durations of exposure or submergence at each of the sites, and the total biomasses respired were obtained by summing the two. Measurements of the nitrogen, potassium and phosphorus contents of the litter confined in bags did not change appreciably over the year between October 1977 and October 1978 (N, 2.1--1.6%; K, 0.18--0.15%; P, 0.18--0.17%); thus, it appeared that carbon, nitrogen, potassium and phosphorus were being liberated from the litter in proportion to the composition of the litter. The inorganic-nutrient releases shown in Table II were calculated by multiplying the quantity of litter respired by the elemental composition of the litter. Using data to be reported elsewhere, we also measured marsh-plant nutrient requirements. We were thus able to calculate the potential annual contribution of nutrients released during Zostera decomposition to the marsh-plant nutrient requirement. Respectively 6, 7 and 8% of the nitrogen required for one year's growth of S. virginica, T. maritimum, and P. pacifica could be supplied by the decomposition of Zostera if all the released nutrients were consumed by plant growth. It is unlikely that all the imported nutrients would be absorbed by the plants, since a great deal may enter other marsh components. The important result is that some of the nutrients from decomposing Zostera are cycled to the marsh where they may
108 TABLE II Annual o u t p u t s of nutrients (rag m -2 ) from decomposition o f Zostera litter in the marsh Nutrient
Salicornia
Triglochin
Potentilla
Nitrogen Phosphorus Potassium
1100 130 130
400 40 50
460 40 30
either be used or exported back to the donor system (the seagrass bed) in tidal runoff; thus, these nutrients are not quickly lost from the seagrass-bed-marsh complex to the ocean as water flushes from the estuary. The present data set provides evidence for coupling of the adjacent wetland sites. This connectivity tends to retain the nutrients within the two-system complex rather than to lose them to another ecosystem. ACKNOWLEDGEMENTS
This work was supported by and conducted at the U.S. Environmental Protection Agency laboratory in Corvallis, OR, with partial technical support from the National Science Foundation under Grant No. DES72-0165-A02. The authors wish to thank all those who helped gather data, especially C. Humphreys, J.McCrady and D. Seliskar along with M. Mills who edited the manuscript. REFERENCES Gallagher, J.L., 1978. Estuarine angiosperms: productivity and initial p h o t o s y n t h a t e dispersion in the ecosystem. In: M. Wiley (Editor), Estuarine Interactions. Academic Press, New York, pp. 131--143. Gallagher, J.L. and Pfeiffer, W.J., 1977. Aquatic metabolism o f the communities associated with attached dead marsh plants. Limnol. Oceanogr., 22: 562--565. Gallagher, J.L., Pomeroy, L.R. and Pfeiffer, W.J., 1976. Leaching and microbial utilisation of dissolved organic carbon from leaves o f Spartina alterniflora. Estuarine Coastal Mar. Sci., 4: 467--471. Gallagher, J.L., Kibby, H.V. and Skirvin, K.W., I 9 8 4 . C o m m u n i t y respiration o f decomposing plants in Oregon estuarine marshes. Estuarine Coastal Shelf Sci., 18: 421--431. Godshalk, G.L. and Wetzel, R.G., 1978. Decomposition o f aquatic angiosperms. III. Zostera marina L. and a conceptual model of decomposition. Aquat. Bot., 5: 329-354. Harrison, P.G. and Mann, K.H., 1975. Chemical changes during the seasonal cycle o f growth and decay in eelgrass (Zostera marina) on the Atlantic coast of Canada. J. Fish. Res. Board Can., 32: 615--621. Jacobs, R.P.W.M., 1979. Distribution and aspects o f the p r o d u c t i o n and biomass o f eelgrass, Zostera marina L., at Roscoff, France. Aquat. Bot., 7: 151--172. Jones, J.B. and Isaac, R.A., 1969. Comparative elemental analysis of plant tissue b y spark-emission and atomic absorption spectroscopy. Agron. J., 61: 393--394. Kentula, M.E., 1983. Production dynamics of a Zostera marina L. bed in Netarts Bay, Oregon. Ph.D. Thesis~ Oregon State University, 158 pp. Menzel, D.W. and Vacarro, R.F., 1964. The measurement o f dissolved organic and particulate carbon in seawater. Limnol. Oceanogr., 9: 138--142.
97
D E T R I T U S P R O C E S S I N G AND M I N E R A L C Y C L I N G IN S E A G R A S S ( Z O S T E R A ) L I T T E R IN AN O R E G O N S A L T M A R S H
JOHN L. GALLAGHER College of Marine Studies, University of Delaware, Lewes, DE 19958 (U.S.A.) HAROLD V. KIBBY and KATHERINE W. SKIRVIN U.S. Environmental Protection Agency, Corvallis Environmental Research Laboratory, Corvallis, OR 97330 (U.S.A.) (Accepted for publication 21 May 1984)
ABSTRACT Gallagher, J.L., Kibby, H.V. and Skirvin, K.W., 1984. Detritus processing and mineral cycling in seagrass (Zostera) litter in an Oregon salt marsh. Aquat. Bot., 20: 97--108. In estuaries where seagrass beds adjoin marshes, the import and decomposition of seagrass litter in the marsh provide a mechanism for retaining nutrients within the wetlands and preventing loss to adjacent oceanic waters. Several aspects of the influence of seagrass litter on an Oregon salt marsh were studied. The quantity of Zostera litter in the marsh vegetation depended on elevation and on the marsh-plant canopy structure. Litter decomposition was most complete in the seagrass bed and in the highest marsh area. At intermediate elevations decomposition was very low after 40% of the material was degraded. The respiration rates of dead-plant communities (DPC s) and the release rates of dissolved organic carbon into the tidal waters depended on the location of the DPC in the marsh. Seagrass litter contributed between 14 and 31% as much dead material to the total litter in the marsh as did the marsh plants. Decomposition of this litter could release 6--8% of the nitrogen required for the growth of the marsh plants.
INTRODUCTION I n t e r a c t i o n s b e t w e e n a d j a c e n t w e t l a n d s y s t e m s m a y be greater t h a n t h o s e b e t w e e n similarly placed terrestrial units, b e c a u s e w e t l a n d s y s t e m s are inund a t e d either p e r i o d i c a l l y or c o n s t a n t l y b y a fluid m e d i u m with e x c e l l e n t b u o y a n c y a n d solvent qualities. In areas w h e r e seagrass beds are a d j a c e n t t o salt marshes, t h e r e m a y be i n t e r a c t i o n s o f p a r t i c u l a t e o r g a n i c c a r b o n (POC), dissolved o r g a n i c c a r b o n ( D O C ) and n u t r i e n t s (Gallagher, 1 9 7 8 ) . Since tidal w a t e r f l o w s o v e r these seagrass beds o n its w a y t o t h e m a r s h , p l a n t processes m a y m o d i f y t h e w a ~ r p r i o r t o its arrival in t h e m a r s h so t h a t i n o r g a n i c n u t r i e n t s , as well as various f o r m s o f D O C a n d P O C , m a y be a d d e d o r rem o v e d . W h e n t h e tide ebbs, similar c h a n g e s m a y have o c c u r r e d d u r i n g resid e n c y o f t h e w a t e r s in t h e m a r s h . R e p e a t e d e x c u r s i o n s o f t h e tide m a y amplif y t h e single-pass e f f e c t p r i o r t o flushing o f t h e w a t e r f r o m t h e p r o t e c t e d
0304-3770/84/$03.00
© 1984 Elsevier Science Publishers B.V.
98 basin where the emergent and submergent wetland systems coexist. Figure 1 shows the possible interactions between upland, marsh, seagrass-bed and other bay habitats, and the ocean. Netarts Bay, OR, in the Pacific Northwest of the U.S.A., has extensive seagrass beds in the subtidal and lower intertidal areas. Both Zostera marina L. and Zostera japonica Aschers. and Graebn. are found in the bay, the latter occurring more abundantly at higher elevations. Our observations indicate that the species density and vigor of the plants decrease markedly at higher elevations close to the lower marsh. The vegetative patchwork pattern of plant stands in the marsh appears to depend partly on elevation relative to the tides. The lower, gently sloping elevations of the Netarts marsh were occupied by Scirpus americanus Pers., which averaged --0.3 m relative to mean high water (MHW). Next in elevation was Distichlis spicata (L.) Greene (0.1 m), which occurred either in monotypic stands or mixed with Salicornia virginica L. (0.4 m) and formed a matrix into which were set patches of Triglochin maritimum L. (0.2 m). A mixed stand of Potentilla pacifica Howell (0.9 m) and Deschampsia cespitosa (L.) Beauv. formed the highest-elevation marsh type. Our purpose in this study was to examine one facet of the inter-
UPLAND]' MARSH
l
MARSH WATER
SEAGRASS BED BAY WATER OTHER BAY HABITATS
OCEAN Fig. 1. F l o w s o f e n e r g y and materials in seagrass t o o t h e r estuarine h a b i t a t s and to adjacent ecosystems.
99 action b e t w e e n beds o f the t w o Zostera species (which we will simply call Zostera) and several stands of marsh plants in Netarts Bay. The deposited dead plants and the associated assemblage of microbes were considered to be the dead-plant c o m m u n i t y (DPC) (Gallagher and Pfeiffer, 1977; Gallagher et al., 1984). The research focused on the deposition of dislodged seagrass in three t y p e s of marsh vegetation and on the dispersion of p h o t o s y n t h a t e s and minerals in the marsh. We addressed the following questions concerning the Zostera litter in the marsh. (1) What is the seasonal pattern of the quantity of Zostera litter in the marsh? (2) What are the Zostera decomposition rates in several wetland locations? (3) What are the aerial and aquatic respiration rates of the heterotrophic communities associated with the Zostera litter? (4) H o w much DOC is released from the decomposing dead-seagrass communities to be e x p o r t e d with the ebbing tide or consumed in the marsh? (5) H o w much detritus might be added to the marsh b y the decay of Zostera ? (6) What is an estimate of the nutrient input to the marsh from the grass bed? MATERIALS AND METHODS The Zostera litter in seven randomly selected plots in stands of Potentilla pacifica, Triglochin maritimum and Salicornia virginica was collected at sixweek intervals in the warm season, approximately March--October, and every eight weeks in the cool season (the study period ran from June 1977 to O c t o b e r 1978 in the P. pacifica and T. maritimum and to D e c e m b e r 1978 in the S. virginica stand); samples were dried to a constant weight at 60°C. The submerged (in water) respiration rates of the communities associated with the dead Zostera were measured using techniques described b y Gallagher and Pfeiffer (1977). Respiration in air (exposed) was measured b y CO2 evolution using syringes as incubators and an infrared gas-analyzer (Gallagher et al., 1984). DOC release from the communities was measured by placing litter in plastic Whirl-Pak ® bags containing seawater. After incubation for 3 h, DOC was measured using the methods of Menzel and Vacorro (1964). Net release was calculated as the difference b e t w e e n the DOC of litter-filled bags and the DOC of bags containing only water (Gallagher et al., 1976, 1984). Litter bags (plastic, 1 mm mesh, 20 cm square) containing recently stranded Zostera were set in the S. virginica marsh in August 1977. The bags were placed in the c a n o p y in an array around a series of stakes to which t h e y were held with nylon cord. Three to five bags were returned to the laboratory after 7, 16, 23, 59 and 72 weeks of incubation in the marsh. Similar Zostera litter bags were deployed in O c t o b e r 1978 and collected periodically for one year; these sets were placed in the Z. marina, S. virginica and P.
100
pacifica stands. Aerial and aquatic respiration rates and DOC losses were measured for the material in the litter bags as described above. Although amphipods were often seen in the litter, they were n o t included in the DPC because their high mobility made it difficult to take representative samples; therefore, the c o m m u n i t y was limited to microbes. The bags n o t used for these rate measurements were washed free of sediment and dried to constant weight at 60°C. A carbon--hydrogen--nitrogen (CHN) analyzer was used to determine the percentages of carbon and nitrogen. Spark-emission spectrometry (Jones and Isaac, 1969) was used for mineral analyses, while ash-free dry weights were calculated from data resulting from biomass ashing at 460°C in a muffle furnace for 3 h. Marsh elevations were determined through surveys c o n d u c t e d by the National Ocean Survey (NOS). The reported elevations of each plant stand are the mean of 50 determinations; these means were coupled to MHW through data collected b y using a water-level recorder o p e r a t e d in Netarts Bay for one year. F r e q u e n c y of inundation was calculated from tidal data and elevations. RESULTS
AND
DISCUSSION
All T. maritimum and S. virginica plots contained some Zostera litter each time they were sampled, b u t the litter distribution was very heterogeneous in both time and space in the P. pacifica stands (Table I). The quantities of litter in the three areas are shown in Fig. 2. Differences b e t w e e n sampling dates reflect both deposition and removal by the tides, and decomposition. TABLE I Percentage of Potentilla pacifica plots containing Zostera litter (n = 7) Date 1977 June July
August October December 1978 February March April
Percentage
0 43 0 43 0
* 100 100
June
29
July
14 0 0 0
September October December *No plots harvested.
101
The smallest mean a m o u n t of Zostera litter was f o u n d in P. pacifica (19 gm-:), the marsh with the highest elevation (0.9 m above MHW) of the intertidal zone and, consequently, the one with least frequent inundation (3%). In the lower areas, where the stands of S. virginica and T. maritimum were located, Zostera litter averaged 83 and 26 g m -2, respectively. Based on the elevations (0:4 and 0.2 m above MHW) and consequent inundation frequencies (15 and 25%), it might have been expected that less Zostera w o u l d be f o u n d in the S. virginica than in the T. maritimum stand. The effectiveness of plant stands as traps for Zostera litter depends n o t only on soil elevation b u t also on the structure o f the plant canopy. T. maritimum grows in tufts ~ 0 . 5 m taller than the surrounding marsh during the summer; however, early in the fall the c a n o p y collapses, altering the height of the plants which act as a filter to trap Zostera. The litter-trapping efficiency of a stand depends on the net effect of tide height and plant-height. When the water level is only slightly higher than the base of the tufts, the dense clumps o f stems act as barriers to the deposition of litter within the clumps. As the water level rises, the shoots act as traps for Zostera. By comparison, the S. uirginica c a n o p y is relatively uniform in height throughout the year because of the somewhat w o o d y stems of the plants. Unless the litter is tangled in the canopy, tides higher than the plant elevation tend to float the litter to a higher site. Kentula (1983) f o u n d Zostera leaf dynamics in Netarts Bay to resemble closely those reported at Roscoff, France by Jacobs (1979). There was an increase in the n u m b e r of leaves per shoot in the spring, with a decrease beginning in the summer and continuing into the fall. Zostera biomass in Netarts Bay is comparable to that found in other temperate regions (Kentula, 1983). Deposition of seagrass litter in the marsh (Fig. 2) is associated with periods of leaf-shedding and storms, and is particularly high when these t w o periods coincide. All of the deposition factors, coupled with the decomposition rates, resulted in the m a x i m u m mean quantity o f Zostera litter occurring &-Zostera in PotentiUo pacifca 0-Zostera in Trjlochi.~n nmrltlm~m r~- Zostera in Salicornia v~linlca
,2ot I00]
~
8o
_j i:~
2
~
60. \\ \
4020. J
. J
.
. A
. S 1977
0
N
// D
J
F
M
J
d 1978
A
S
0
N
O
Fig. 2. Q u a n t i t i e s o f Zostera sp. l i t t e r in Potentilla pacifica, Salicornia virginica and Triglochin m a r s h - p l a n t s t a n d s in N e t a r t s Bay, O R ( b a r s r e p r e s e n t ± 1 S.E., n = 7).
102 in the S. virginica stand. The data for the q u a n t i t y of Zostera litter indicate t h a t there was a substantial input of detritus from the seagrass beds to the marsh. Litter turnover and microbial activity were assessed from respiration and leaching studies of unconfined litter and of the f o u r sets of litter bags. Decomposition rates in litter bags m a y be different from those occurring within similar unconfined litter because of differences in physical, chemical and biological factors in the two settings. These problems are complicated by the fact t h a t the degree of perturbation from the natural condition is n o t consistent across environmental gradients. All t h a t can be done is to design a study so as to minimize these differences and to recognize the shortcomings which m a y occur. In earlier work with unconfined litter of various marsh plant species, we f o u n d moisture, which differed among the three s t u d y sites, to be the most i m p o r t a n t factor in determining the respiratory rate of decomposers (Gallagher et al., 1984). To avoid problems in biasing the moisture regime away from natural conditions, we thus placed lightly packed litter bags into the canopies in positions where unconfined litter was found. Animal activity inside and outside of the bags was a second concern. The 1 m m mesh size would n o t allow adult amphipods (the only macrofauna observed) to enter or leave, but y o u n g animals could enter and grow there. If there was a bias, it m a y have been that predators could n o t feed on the amphipods; nevertheless, we did n o t observe differences in animal density between the bags and the open litter. Wave action was a factor only at the Zostera incubation site; the effect was n o t clear but m a y have been either to protect the plants or to enhance mechanical action, since the bags were tethered and hence n o t free to move with the waves. Although there are difficulties with the use o f litter bags as long-term integrators of decomposition, better techniques are n o t available (Gallagher, 1978); therefore, we deployed four sets of litter bags into the wetlands to estimate the decomposition rates of Zostera in several environments. Figure 3 illustrates the results of the 1977--1979 study, where all bags were incubated in the S. virginica marsh (mid-elevation), and of the 1978--1979 study, where bags were incubated in Zostera, S. virginica and P. pacifica (high~levation) stands. The two incubations in ~. virginica gave similar results, with a loss of 35% of the initial weight in the bags in approximately four months. Losses after t h a t time were very slow, and growth of organisms in and on the partly skeletonized Zostera tissue appears to have resulted in some gain in weight in the litter. In the S. virginica environment ~ 6 0 % of the Zostera litter was resistant to decomposition. In the Z. marina bed, decomposition was more rapid and continued until less than 5% of the original weight remained after one year. The portion of the seagrass bed where the litter bags were incubated was sandy and within the intertidal zone. Although no Eh values were obtained, sediment coloration and the lack of h y d r o g e n sulfide odor made it likely t h a t the decomposition conditions for the Zostera were aerobic even at times when the bags were covered with substrate. It is probable t h a t the more f r e q u e n t and longer immersion in
103 the seagrass beds stimulated more microbial activity and that wave action was responsible for mechanical breakdown of the tissue at this site. The seagrass litter in the P. pacifica site, that flooded least frequently, decomposed at a rate intermediate between those at the S. virginica and Zostera incubation sites. This site was very moist because of shading from tall clumps of D. cespitosa and dense growth of P. pacifica. Furthermore, the canopy o f P. pacifica collapsed soon after death to further reduce drying o f the Zostera litter.
I00,
\
,
\\
80
\ \ \
-
(.9
\ 60
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w
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- incuboted in Potenlilla pocifica O - i n c u b a t e d in Z o s t e f o rnorino 0 - incubated in SolK:ornio vj~rginico --1977-9 --1978-9
o
~
study study
~o
~
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Fig. 3. Decomposition of Zostera litter communities in plastic-mesh bags incubated in stands of PotentiUa pacifica, Salicornia virginica and Zostera marina in Netarts Bay, OR ( 1 9 7 7 - - 1 9 7 9 ) (data based on ash-free dry weights (AFDW)).
As might be expected, the respiration of Zostera DPC was highest in summer (Fig. 4). The respiration rate in air, although generally lower than that in water, followed the same pattern as the aquatic rate. The moisture content of the Zostera DPC was high in the cool portion of 1 9 7 8 - - 1 9 7 9 , but l o w temperatures or other factors kept respiration rates low. Except for April 1978, the C/N ratio was always between 17 and 24. The average aerial and
104
aquatic respiration rates during the period March--October were 31 and 86 pg C (g dry wt.) -1 h -1, respectively. These rates were less than those for several DPCs with similar C/N ratios and may be associated with reduced availability o f nitrogen, as evidenced by the high concentration (59--67%) of crude fiber content. Harrison and Mann (1975) f o u n d t h a t one-half of the nitrogen in dead Zostera in shallow water along the coast of Nova Scotia was n o t protein but was trichloroacetic-acid-insoluble, residue-bound nitrogen. Godshalk and Wetzel (1978) also reported t h a t a large portion of the nitrogen in decomposing Zostera was not in a readily available form. Although the C/N ratio in the Zostera litter was favorable, the relatively low decomposition rates compared to several other marsh grasses m a y be associated with the unavailability of the nitrogen to microorganisms. oquotic re=wotion rote 0 - - 0 incubation temperotunl D--Q a~ iol respiration rot,
1
~.--~
./."~ --~
l_
f i
\',
,,/.,'A
',,
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w
70
w
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0
~o o
~: ~
/~
I,--_
<~T ~o
~oo
\
/ / /
~ 1°°t 0
A
M
~1 J
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0
N
D
d
F
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M
,I ,I 1978
A
S
0
N
D
,,I
F M 1979
A
0
Fig. 4. Aerial and aquatic respiration rates, incubation temperatures, C/N ratios, and percentage moisture contents of Zostera litter communities found in a Salicornia marsh at Netarts Bay, OR (1978--1979).
There was little change in aquatic respiration rate with time in the Zostera DPCs which were incubated in the S. virginica stands (Fig. 5). The aerial respiration rate was o f t e n lower than the aquatic rate by more than one-half. During the early stages of incubation in the marsh, aquatic rates at the P. pacifica site were higher than those in the S. virginica stand. Except for the first and last measurements the aerial DPC respiration rates at the P. pacifica site were lower than t h e aquatic rates. The aerial respiration rates of the DPC incubated at the P. pacifica site were higher than those of the DPC incubated in the S. virginica marsh. The aquatic rates for the DPC at the Z. marina site, although initially lower than those from P. pacifica, were very high during the last two samplings; however, the aerial rates were very low. Since the Z. marina incubation site was very wet most of the time, it is probable t h a t the microbes colonizing t h a t tissue were better adapted to aquatic t h a n to aerial respiration. The results of the respiration measurements are compatible with the litter-bag weight-loss studies, as the litter at the Z. marina site had the highest respiration rates and weight loss, t h a t in P. pacifica was intermediate, and that in S. virginica initially had the least. By the end o f the year the two marsh sites showed similar losses and respiratory rates.
105 In addition to decomposition associated with respiration, there is active DOC leaching from Zostera tissue a n d / o r release from attached microbes (Fig. 6). Leaching rates were high compared to those found for dead Spartina alterniflora Loisel. communities by Gallagher and Pfeiffer (1977) in Georgia. In Oregon, no correlation was f o u n d between leaching rate and temperature, although rates measured during the warmer March--August period were highest. For DPCs under continually submerged conditions in the laboratory, Godshalk and Wetzel (1978) did find a positive correlation with temperature. In the field this response was probably masked by other factors, such as time since last immersion, age of litter, and rainfall. The time elapsed since last immersion may be a factor because DOC could have accumulated on exposed Zostera litter as a result of activity by microbes. When the litter was submerged, the water m a y have received a pulse of DOC as the accumulated soluble c o m p o u n d s dissolved. We were interested in quantifying the DOC released when the n e x t tide covered the plants, and used a standard 3-h immersion for all our measurements.
,4°I 1201
LIJ i
tOO.
• imhollitter "~
or" 80
60
40
20-
0
0
N
D
J
F
M
A
M
d
d
A
S
0
N
D~
Fig. 5. Aerial and aqudtic respiration rates of Zostera litter communities incubated in mesh bags in several wetland types in Netarts Bay, OR (bars represent ± 1 S.E., n = 3; data based on ash-free dry weight (AFDW)). In August we compared leaching rates f r o m Z. marina and Z. japonica litter from the marsh with similar samples from the mud flats where the plants grew (Fig. 6). In both cases Z. japonica released more DOC (~ = 0.05), but the difference bbtween the sites was even greater, with the litter in the marsh releasing the greatest a m o u n t of DOC. We attribute the relatively higher rates in the n~arsh site to the fact t h a t previous leaching frequency and inundation duration were greatest at the lower elevations of the seagrass beds. Differences in the age of the dead plant material could have been a factor, but we a t t e m p t e d to collect similar litter at the different sites.
106 ,3800 2880
2400
1920 '
l/Ill\\\\
~ ~. , . o -
~ ~
1977
9~i978
~/
\
/
,'
480-
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F
\ } Z. joix~l¢o (marsh)
1~ M
A
_--~M
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Fig. 6. DOC release rates (pg C (g dry wt.) -1 h-l) from Zostera litter communities in a Salicornia virginica marsh at Netarts Bay, OR. August data are also presented for litter collected from the seagrass bed (bars represent + 1 S.E.,n = 4;data based on dry weights). T h e leaching rate o f DOC f r o m t h e litter bags was negative at t h e Z. marina site at the end o f t he first i n c u b a t i o n period (Fig. 7). This DOC uptake f r o m th e water m a y have been by microbes which colonized t he n e w l y placed litter. Conversely, the u p t a k e could simply have been t he result o f physical sorption at dead-plant surfaces. A relatively low b u t positive rate was measured f or t he r e m a i n d e r o f t h e year. T h e litter incubated in t he S. virginica stand exhi bi t ed a similar p a t t e r n e x c e p t t h a t t he rate was n o t negative after th e first i n c u b a t i o n period. T h e rate f o r t he litter from t h e bags in the P. pacifica zone was initially high and remained so t h r o u g h o u t t he year. Tidal water inundates these areas i n f r e q u e n t l y and it is probable t h a t soluble organic car b o n from microbial activity and litter degradation which accumulated b etween floodings was released as a pulse o f DOC when t h e tides reached th e stranded litter. As an indication o f t he c o n t r i b u t i o n o f the Zostera litter t o t h e detritus pool, we c o m p a r e d t he m e a n annual q u a n t i t y o f seagrass litter with t h a t o f dead marsh plants in t he t h r e e marsh types. In t he S. virginica plant stand the seagrass litter averaged 31% o f t he marsh-plant dead biomass. T h e values f o r the T. m a r i t i m u m and P. pacifica areas were 27 and 14%, respectively. T h er ef o r e, seagrass litter appears t o be c o n t r i b u t i n g substantially t o t h e detrital p o o l o f these t h r e e wetland p l a n t types in Netarts Bay. At t he time we were studying n u m e r o u s aspects o f the ecology o f these t hree marsh types, we were also examining Scirpus americanus and Distichlis spicata stands (--0.3 and 0.1 m above MHW, respectively). In these lower-elevation
107
stands our sampling methods for other purposes did not allow us to collect seagrass litter, but the quantity appeared to be much larger than that reported for the areas which we could sample. A significant part of the plant material decomposing in marsh regions in estuaries such as Netarts Bay, where large, productive grassbeds border the marshes, is allochthonous. 600w
P-
_
<~
Zostero
in P. poci f i ca
- - - - Zostera in ~. virginica ......... Zostera in Z. marina 300
'l-
'. "N.
LiJ (.3 •.
...... "e-.""
0
N 1978
D
~
d
F
M
A
M
d 197'9
d
A
S
0
Fig. 7. DOC release rates (ug C (g dry w t . ) - ' h -1) from Zostera litter c o m m u n i t i e s incubated in mesh bags in PotentT"lla pacifica, Salicornia virginica and Zostera marina stands in Netarts Bay, OR (data based on ash-free dry weights (AFDW)).
Our first approximation of nutrient input into the marsh by Zostera litter decomposition was calculated using the litter biomass data, the respiration rates and the chemical composition data. The average annual respiration rates of the Zostera litter in air and in water were multiplied by the mean litter standing crops in the three plant zones to give the amounts of carbon which would have been consumed if the litter had decomposed while having been continually either exposed or submerged. These quantities were multiplied by the percentages of the durations of exposure or submergence at each of the sites, and the total biomasses respired were obtained by summing the two. Measurements of the nitrogen, potassium and phosphorus contents of the litter confined in bags did not change appreciably over the year between October 1977 and October 1978 (N, 2.1--1.6%; K, 0.18--0.15%; P, 0.18--0.17%); thus, it appeared that carbon, nitrogen, potassium and phosphorus were being liberated from the litter in proportion to the composition of the litter. The inorganic-nutrient releases shown in Table II were calculated by multiplying the quantity of litter respired by the elemental composition of the litter. Using data to be reported elsewhere, we also measured marsh-plant nutrient requirements. We were thus able to calculate the potential annual contribution of nutrients released during Zostera decomposition to the marsh-plant nutrient requirement. Respectively 6, 7 and 8% of the nitrogen required for one year's growth of S. virginica, T. maritimum, and P. pacifica could be supplied by the decomposition of Zostera if all the released nutrients were consumed by plant growth. It is unlikely that all the imported nutrients would be absorbed by the plants, since a great deal may enter other marsh components. The important result is that some of the nutrients from decomposing Zostera are cycled to the marsh where they may
108 TABLE II Annual o u t p u t s of nutrients (rag m -2 ) from decomposition o f Zostera litter in the marsh Nutrient
Salicornia
Triglochin
Potentilla
Nitrogen Phosphorus Potassium
1100 130 130
400 40 50
460 40 30
either be used or exported back to the donor system (the seagrass bed) in tidal runoff; thus, these nutrients are not quickly lost from the seagrass-bed-marsh complex to the ocean as water flushes from the estuary. The present data set provides evidence for coupling of the adjacent wetland sites. This connectivity tends to retain the nutrients within the two-system complex rather than to lose them to another ecosystem. ACKNOWLEDGEMENTS
This work was supported by and conducted at the U.S. Environmental Protection Agency laboratory in Corvallis, OR, with partial technical support from the National Science Foundation under Grant No. DES72-0165-A02. The authors wish to thank all those who helped gather data, especially C. Humphreys, J.McCrady and D. Seliskar along with M. Mills who edited the manuscript. REFERENCES Gallagher, J.L., 1978. Estuarine angiosperms: productivity and initial p h o t o s y n t h a t e dispersion in the ecosystem. In: M. Wiley (Editor), Estuarine Interactions. Academic Press, New York, pp. 131--143. Gallagher, J.L. and Pfeiffer, W.J., 1977. Aquatic metabolism o f the communities associated with attached dead marsh plants. Limnol. Oceanogr., 22: 562--565. Gallagher, J.L., Pomeroy, L.R. and Pfeiffer, W.J., 1976. Leaching and microbial utilisation of dissolved organic carbon from leaves o f Spartina alterniflora. Estuarine Coastal Mar. Sci., 4: 467--471. Gallagher, J.L., Kibby, H.V. and Skirvin, K.W., I 9 8 4 . C o m m u n i t y respiration o f decomposing plants in Oregon estuarine marshes. Estuarine Coastal Shelf Sci., 18: 421--431. Godshalk, G.L. and Wetzel, R.G., 1978. Decomposition o f aquatic angiosperms. III. Zostera marina L. and a conceptual model of decomposition. Aquat. Bot., 5: 329-354. Harrison, P.G. and Mann, K.H., 1975. Chemical changes during the seasonal cycle o f growth and decay in eelgrass (Zostera marina) on the Atlantic coast of Canada. J. Fish. Res. Board Can., 32: 615--621. Jacobs, R.P.W.M., 1979. Distribution and aspects o f the p r o d u c t i o n and biomass o f eelgrass, Zostera marina L., at Roscoff, France. Aquat. Bot., 7: 151--172. Jones, J.B. and Isaac, R.A., 1969. Comparative elemental analysis of plant tissue b y spark-emission and atomic absorption spectroscopy. Agron. J., 61: 393--394. Kentula, M.E., 1983. Production dynamics of a Zostera marina L. bed in Netarts Bay, Oregon. Ph.D. Thesis~ Oregon State University, 158 pp. Menzel, D.W. and Vacarro, R.F., 1964. The measurement o f dissolved organic and particulate carbon in seawater. Limnol. Oceanogr., 9: 138--142.