THE MOVEMENT
OF PHOSPHORUS THROUGH THE SALT
MARSH CORD GRASS, SPART1NA ALTi!iRNIFLORA
LOISEL’
Robert J. Reimold
University
of Georgia
Marinc
Institute,
Sapelo Island
31327
ABSTRACT
A pathway by which phosphorus is transferred
from the sediment to Spartina to the
estuarine waters is described : Spartina can serve as a nutrient
pump and translocate
measurable quantities of phosphorus from the salt marsh sediment to the leaves; then, with
tidal inundation,
an average of 9.84 mg-atom P/m is released in the marsh waters at each
tidal cycle, Seasonal data indicate the flux of phosphorus through this system is closely
associated with the productivity
of the plant material in the marsh.
INTRODUCTION
The salt marsh cord grass, Spartina alterniflora Loisel, is the predominant
vegetation of the salt marshes of the southeastern
coast of the United States, and together
with its resultant detritus serves as a food
source for populations of organisms inhabiting the waters of the marsh and adjoining waters of the Atlantic Ocean (Odum
1961; Odum and de la Cruz 1967). Pomeroy ct al. (1969) h ave demonstrated that
Spartina is a dominant feature in the
cycling of phosphorus in the salt marsh
ccosystcm, removing phosphorus from sediments and rclcasing it back into the water via bacterial degradation of the dead
plant.
My study was conducted near the Uniat
versity of Georgia Marine Institute
Sapelo Island in the Duplin estuary watershed, which has a nearly monospecific
stand of S. alterniflora.
Questions raised
by Williams and Murdoch ( 1969)) Pomcroy et al. ( 1969), and Pomeroy ( 1970)
point to a need for studies of the movement of phosphorus through Spartina. The
purpose of my experiments was to seek
pathways for the flux of phosphorus between sediments, Spurtinu, and water in
the salt marsh.
I extend thanks to the staff at the Ma1 Contribution
No. 231 from the University
of
Georgia Marine
Institute.
This work was supported by U.S. Atomic Energy Commission Contract No, ORO-3238.
LIMNOLOGY
AND
OCEANOGRAPIIY
rinc Institute and to L. R. Pomeroy
assistance during this study.
for
METIIODS
A simple method was devised for placing the radionuclides in the sediments. A
solid rubber stopper was inserted into the
end of a 1.6-m length of PVC pipe, which
was pushed into the sediments to the dcsired depth and left in place throughout
the experiment with its top above high
water level. The stopper was removed by
running a plastic rod down the pipe, and
a solution of 10-20 &i of carrier-free a2P
in a small amount of membrane-filtered
estuarine water was poured into the pipe.
At varying intervals from several hours to
40 days, Spartina shoots (i.e. portions of
the plant projecting above the sediments)
were harvested, ashcd in a muffle furnace
for 4 hr (SSOC), and the s2P content
determined.
All water samples used in the cxperimcnts were prcparcd by filtration through
a membrane filter (0.45-p pore size). Phosphorus was precipitated
from the water
and collcctcd on a second membrane filtcr (Johannes 1964). Inorganic phosphorus
concentrations were determined according
to Strickland and Parsons (1968). Planchcts were counted automatically
to lo5
counts
in a gas-flow counter for 32P activity and all counts corrected for counting
efficiency and background.
For field investigations
to evaluate the
effect of immersion of the plants in sea-
606
JULY
1972,
V. 17(4)
1’IIOSPHORUS
MOVEMENT
water for varying lengths of time, sediments around tall Spartina plants in the
streamsidc marsh were labeled at loo-cm
depth with 20 ,uCi of 32P. Six days later
when the plants showed detectable radioactivity with a survey meter, several of
the upper leaves were immersed ( in situ)
in membrane-filtered
estuarine water for
3 hr. This experiment was designed to
simulate tidal inundation of the vegetation
and consequently was conducted just before actual inundation by high tide. The
water in which the plant was immersed
dissolved the salts that visibly coat the
leaves; this solution was then precipitated
and analyzed for a2P in the suspended and
soluble fraction as above.
In another experiment plants were transferred to plastic pots containing 32P-labeled
sediment (So-100 &i/liter
of marsh mud)
and left in the field. At irregular intervals
leaves were washed with 250 ml of mcmbranc-filtered
estuarine water, which was
collected in a basin, the phosphorus precipitated, and the wash water counted for
32.P activity.
Thcsc studies lasted for 30
days with as many as 30 replicates for
each day.
To confirm the amount of total inorganic
phosphorus being lost during tidal inundation, the harvested plants from square
meter quadrats were washed in membranc-filtered
estuarine water, the wash
water again membrane filtered, and the
dissolved
inorganic
phosphorus
determined. In other experiments the plants
were washed immediately before harvesting instead of immediately after.
Finally, experiments were conducted to
evaluate seasonal uptake of 32P by Spartina from salt marsh sediments. Sediments
were routinely labeled at l-m depth with
20 &i of P, with each measurement rcpresenting six replicate observations. These
experiments were designed to correspond
generally with the six ecological seasons of
the year ( Allec ct al. 1949)) with the actual times modified to be relevant to the
Georgia coast and the actual flowering
and growth of Spartina. Each experiment
began within 1 week of the beginning of
TIIROUGIl
607
S. ALTERNZFLORA
1200
1
/
I
.
.’
/
i
DAYS AFTER INJECTION
FIG. 1. Depth
injected related to
nifhu.
-=
20-cm depth; = lOO-cm depth.
in sediment at which label was
uptake of “P by Spcwtimt d&rO-cm depth; - - - - - =
- = SO-cm depth; - s s . Vertical bars = 1 SD.
an ecological season; at varying intervals
(from l-33 days after the introduction
of
the label), aerial portions of the Spartina
were harvested and counted as described
earlier. The study was further subdivided
into plants labeled on the creek bank (tall
cord grass) and plants growing on the high
marsh (short cord grass). This study, over
1 year, provided adequate data for scasonal interpretation.
RESULTS
The greatest uptake of :V was from the
loo-cm depth in the sediment ( Fig. Y).
On the day of introduction
of the Iabel,
leaves were collected at the end of 4 hr;
these wcrc the samples used for dctermining activity on day zero. The label applied
GO8
HOBERT
J. REIMOLD
TABLE 1. Seasonal effects of phosphorus uptake
hy Spartina from salt marsh sediments labeled at
a deptlz of 1 m with 20 &i
of “P-POJ.
Each
measurement reported is the mean of six obseruations. The activity is reported as count min-’ mg
ash-l +1 SE of the mean. Stems and leaves have
been combined
Days after
introduction
of label
Low
marsh
Hibernal
1
G
11
27
(15 December-15
622
5-t-2
3+-l
5c2
Prevernal
(I 6 February-Z
79 -+ 6
300 It 11
502 -t- 21
608 2 19
1
6
10
25
Vernal
(26 March-31
1,772
523 &
t- 29
11
i
10
26
High
marsh
Aestival
Swotinal
1
10
16
33
Autumnal
1
6
10
25
5 March)
52 5
762
91 +84 2
7
8
12
10
May)
1,938
633 2f 24
18
1,157 c 27
1,915 & 25
July)
(15
688
1,936
3,115
2,827
August)
(1 September-14
5O3 It 9
857 2 11
1,225 -+ 10
1,120 A 9
N-25
4+2
61~3
321
4-1-l
(1 June-Z4
548k14
1,168 -I 18
2,573 zk 23
1,476 -+- 20
July-31
iz 16
-I 19
+ 23
+- 20
I
February)
989 -+ 18
2,874 +- 33
1
3
6
16
IIQO-
826
2,676
1,598
1,433
2
-c
2
-+
19
17
19
18
1,176
1,189
2,022
2,331
e
-t-t
+-
20
19
20
24
December)
533
539
864
868
-+
2
+
-I
10
11
8
9
at the surface showed the fastest initial
uptake, but after 6 days, the differences
in labeling at greater depths became evident. Plants on sediment labeled at the
loo-cm depth all demonstrated high activity at days 6 and 11, with lower activities
in shallower depths. Since the plant was
most effectively
labeled by tagging the
sediments at 100 cm, all subsequent labeling was done at this depth. The results
also demonstrated that water percolated
through the more shallow sediments, and
the label was widely dispersed. At 100 cm
IOO-
N=25
I N=20
- IN=15
-tN= I5
011,11,,1,,,11~
I,,,,
20
25
0
5
IO
I5
DAYS AFTER
INITIATION
OF EXPERIMENT
30
FIG. 2. Effects of repcatcd washing of Spnrtina lcaves (previously
labeled with “P-PO,)
with
membrane-filtered
estuarine water. Values are reported as the mean of the number of observations
( N) el SE of the mean.
it remained in contact with the root svstern and rhizomes of the Spartina, the iajor portion of which was between 0.5 and
1.5 m deep.
The results of the seasonal study demonstrate maximum uptake of s2P by Spartinn
during the vernal, aestival, and scrotinal
seasons (Table 1). Little activity was
found during the hibernal season. An increase in the uptake of 32P in the plants
in the prevernai was then continued during the following three seasons, when flowering occurs. Activity decreased with the
onset of the autumnal season and was
again at a minimum during the following
hibernal season. The low marsh strcamside plants ( tall SpurtZna) appear to bc
more active than the high marsh (short
Spartina) in the uptake of the radioactive
label during the more productive seasons.
A maximum quantity
of the label was
released from nearby Spartina shoots into
estuarine water nearly lo-15 days after the
introduction
of a2P (Fig. 2). By the end
of 30 days most of the available 32P had
PHOSPHORUS
MOVEMENT
TABLE 2. Comparison of wet weight of Spartina
with loss of inorganic phosphorus front the lemes
of the plant due to washing the plant in membrane-filtered
seawater”
Qnnclrat
Marsh
type
I-1:u3x%t
Weight
DIP
I? loss
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
High
High
High
High
High
Low
Low
Low
Low
Low
High
High
High
Low
Low
After
After
After
Aftes
After
After
After
After
After
After
Before
Before
Before
Before
Before
600.5
716.9
511.4
412.6
497.6
1,616.4
1,234.1
1,028.5
1,046.l
1,164.l
864.0
866.9
758.3
1,874.3
1,647.2
65.7
53.8
70.0
89.0
95.6
76.1
91.2
88.3
91.8
87.4
75.6
80.1
69.8
85.2
87.6
9.14
13.32
7.30
4.63
5.20
21.24
13.53
11.64
11.39
13.31
11.42
10.82
10.86
21.99
18.80
THROUGII
S. ALTERNIFLORA
GO9
organic phosphorus in the wash water.
The theoretical loss of phosphorus from
the plant to the water (during tidal inundation, based on water volume) and the
biomass of the plant are summarized in
Table 2. This thcorctical loss is based on
the assumption that each gram of living
Spartina would rclcase an average of 12.3
pg-atom DIP ( dissolved inorganic phosphorus) and is computed as the mean of
the values of thcorctical loss of phosphorus
from the plants. *
1)ISCUSSION
Thcsc results dcmonstratc that an undisturbed salt marsh, such as the Duplin estuary, is a source of phosphorus for the
coastal waters of the ocean. Similar conclusions were derived by Pomeroy ct al.
* Before and after refer to whether the plants were
( 1972) from a consideration of the phoswashed hcfore or after harvest; weight refers to wet weight
phorus concentrations
in the same salt
of Spartina (g/m”);
DIP refers to concentrations
of clissolved inorganic phosphorns in the leaf wash water (pgmarsh ecosystem, Cord grass plays an imatom/liter);
P loss refers to theoretical loss of phosphorus
portant role in the flux of phosphorus in
from the plant to the water each tidal cycle (fig-atom P/g
plant biomass ),
the salt marsh. My results are also consistcnt with those of McRoy and Barsdatc
apparently moved through the plant and (1970) in relation to phosphate absorption
was no longer present. This expcrimcnt
by cclgrass ( Zostera marina ) .
was carried out during the 30 days in the
The cxpcrimcnt designed to dc tcrminc
middle of the serotinal season, and the the optimum depth for introduction of the
somewhat lower values toward the end of label demonstrated maximum uptake at
the experiment may rcflcct the tcrmina100 cm; greater depths should be invcstition of the actual growing period. A loss gatcd. The loss of label from the upper
of .72P into the depths of the clay lattice
50 cm of scdimcnt probably occurred bcmay also occur, further supporting
the cause the many crab burrows (mainly Ucn
ideas of dispersion by percolation of water
sp.) allow the label to dispcrsc within the
through the shallow sediments suggested
sediment. The roots and rhizomes of the
by the data in Fig. 1.
Spurtinu appear to be distributed primarIn another study, Spnrtinn
leaves im- ily below 50 cm and conscqucntly are able
mcrscd in membrane-filtered
water for 4 to absorb the label from the grcatcr depth.
hr rcleascd enough 32P for the water to
The seasonal uptake of 32P by Spartintc
acquire an activity nearly equal to that oE is closely associated with actual productivthe mud, detritus, and suspended material
ity of the grass. The maximum uptake of
washed from the leaves of the plant. Thus
label occurred during summer months
Spartina not only incorporates 32P label
when productivity
was at its maximum
from the sediment into the plant tissue, (Odum and Smalley 1959). Several cspcribut with tidal inundation
the halophyte
mcnts in which I introrduccd 6”Zn and 90Sr
releases 32P label into the water.
at loo-cm depth in 10&i
amounts during
Comparisons of the effect of leaf wash- scvcral of the ecological seasons wcrc not
ing (artificial tidal inundation) before and rcplicatcd, but a seasonal pattern similar
after harvest showed no significant diffcrto that of phosphorus uptake appeared.
ence in the concentrations of dissolved in- The nutrient flux from sediment to plant
610
RORERT
J. REIMOLD
can then be considered as a continual
event with a seasonal maximum during
the acstival and serotinal seasons and a
minimum during the hibernal. This pattern is considerably different from that of
mineral accumulation in freshwater marsh
species ( Boyd 1969, 1970).
The washing of “2P-labeled Spartinn
leaves with membrane-filtered
estuarine
water showed that the plants continued to
release the phosphorus label for at least
2 weeks, after which the amount of label that could be washed from them dccreased. If the label had begun to bc
absorbed by this time into the clay lattice
of the sediments, this would account for
the decrease in activity of the wash water.
The end of the serotinal season and its
associated decrease in productivity
could
also account for the decrease in amount
of label present. These experiments demonstrate that Spartina can serve as a nutricnt pump; that is, it can translocate significant quantities of phosphorus from the
salt marsh sediment to the leaves and,
with tidal inundation,
into the estuarine
water, as can eelgrass (McRoy and Rarsdate 1970). Although this would result in
a seasonal cycle with maximum release
into the water during summer months, it
agrees with the findings of Pomeroy (1970)
and others concerning “atypical” phosphorus in the water when primary productivity may be at a maximum.
The average apparent loss of phosphorus from Spartina to water is 12.3 pug-atom
DIP/g wet wt of Spartina. With two tidal
cycles per day, this would result in 24.6
pg-atom P/g Spartina. The Duplin watershed consists of 1,142 ha of salt marsh
plants (nearly monospecific stand of Spartina: Reimold and Gallagher 1972). If
there is an average of 8-lo6 g (wet wt )
of Spartina/ha, the Spartina in this watershed could contribute 6,857 kg P/day to
the adjoining coastal waters.
This figure represents a maximum contribution
of phosphorus since not every
tidal cycle completely inundates the plants
in the marsh, During winter the contribution of phosphorus to the estuarine waters
would be considerably
lower (Reimold
and Daiber 1970). This is also related
to the seasonal changes in productivity
of the Spartina (Reimold and Gallagher
1972; Odum and Smalley 1959). Complete seasonal data on the specific activities of phosphorus in the Spartina and in
the flux of radionuclides through the salt
marsh ecosystem arc ncedcd for further
quantification.
This study has documented a pathway
whereby phosphorus moves from the sediment through Spartina into the estuarine
waters. The fate of the released phosphorus once it cntcrs the estuarine waters
remains to be identified.
This pathway
will be important in revision of mathcmatical models of the flux of phosphorus
through the salt marsh ecosystem (Pomeroy ct al. 1972). In opposition to recent
accelerated activities to destroy many salt
marshes, the role of Spartina in contributing inorganic phosphorus to the cstuarine and coastal waters points out the
need for cord grass to remain undisturbcd in the highly productive salt marsh
ecosystem.
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