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The effect of tidally induced changes in the creekbank water table on pore water chemistry
Estuarine,
Coastal
and Shelf
Science
(1985)
21,389-400
The Effect of Tidally Induced Changes in the Creekbank Water Table on Pore Water Chemistrya
Kathleen
Agosta
Marine Science Program and BelEe W. Baruch Institute for Marine and Coastal Research, University of South Carolina, Columbia, South Carolina 29208, U.S.A. Received
3 May
1984 and in revisedform
18January
Keywords: water movement; interstitial Spartina; South Carolina Coast
Biology
1985
water; water properties;
salt marshes;
Water moved into the creekbank sediments in direct response to the changing levels of the water table caused by the tides. The net water loss of the sediments was 3-30yb on each low tide and this loss was confined to within 4 m (horizontal) of the creek. The replacement of this water by incoming tidal water could not supply sufficient nutrients for the growth of creekbank Spartina. However, during ebb tide there was a replacement of water in the creekbanks with nutrient-rich water from the marsh interior as demonstrated by the large changes in pore water chemistry over a tidal cycle. The concentration and the range of a chemical parameter depended upon the stage of the tide, the tidal range, the time of year and (for salinity) the rainfall patterns of the month preceding sampling. Over a single tidal cycle the maximum ranges were: salinity %o, 26-33; alkalinity, 25-13.6 meq l-i, ammonia, 2-400 pM, sulfate, 23.5-29 mm01 l- ‘. Measurable concentrations of sulfide were only found in a few samples. This high nutrient water can supply nitrogen and probably other nutrients to Spartina. Introduction is the dominant plant in south-eastern salt marshes and is and coastal ecosystems.Two different height forms are generally observed-a tall form which grows along the creekbank and a medium to short form which grows elsewherein the marsh. The two forms are genetically the same(Mooring et al., 1971; Shea et al., 1975) and both transplantation studies (Shea et uZ., 1975) and Spartina
Loisel
aZterniflora
important
fertilization have shown
to estuarine
experiments (Sullivan & Daiber, that one form can be transformed
1971; Valiela to another.
et
al., 1978; Haines,
1979)
Suggested factors responsible for higher growth along the creekbank include an increased input of nutrients from tidal waters (Gallagher, 1975; Valiela et al., 1978), a tidal energy subsidy (Odurn & Fanning, 1973; Steever et ul., 1976), higher iron concentrations (King et al., 1982), more oxidized sediments (Linthurst, 1979; Howes et al., “Contribution
number
586 from
me Belle W. Baruch Institute for Marine Biology and
Coastal Research. 389 0272-7714/85/090389+
12 803.0010
1985 Academic
Press Inc. (London)
Limited
390
Ii. Anosta
1981), and increased drainage relative to interior sediments (Mendelssohn & Seneca, 1980; King et al., 1982). The key variable appears to be drainage and infiltration rates which in turn control the concentration of nutrients, trace metals and oxygen. Most studies on the effects of drainage have experimentally altered the drainage pattern in the root zone (Mendelssohn & Senaca, 1980; King et al., 1982) and observed the increase or decreaseof plant productivity relative to changes in drainage. The few studies that have examined natural marsh sediments have shown that over most of the marsh, seepageand infiltration are low (Gardner, 1973, 1975; Hemond & Burke, 1981; King et al., 1982). Significant flow of water through the sediment occurs only on creekbanks where the rise and fall of the tides produces coincident water table fluctuation (Gardner, 1973, 1975). Elsewhere the water table remains essentially horizontal throughout the tidal cycle and generally lies within 1 cm of the marsh surface even at low tide. Thus water movement, away from creekbanks, is largely limited to the infiltration required to balance evapotranspiration losses. Hemond and Fifield (1982) have examined the hydrological regime in a peaty, New England marsh away from creekbanks. They find that seepageis quite low and sediments remain saturated throughout the tidal cycle. This study focuseson water movements and pore water chemistry in the creekbank of a silty south-eastern marsh where water moves throughout the tidal cycle.
Site description The study site (Figure 1) was a high-salinity, Spartina marsh on the Baruch Plantation near Georgetown, SC. The actual sampling site was adjacent to Bread and Butter Creek-a well developed tidal creek which has a tidal range of about 1.4m. It is not completely drained at low tide. A profile of the creekbank and the sampling locations are given in Figure 2. The creekbank itself is covered to a mean depth of 70 cm at high tide. Spartina plants are well developed in the area (fall standing crop l-2 kg dry wt me2, Agosta, 198%) and there is evidence of crab burrowing in the sediments. This study was conducted from late fall, 1980through spring, 1982.
Methods Physical
In order to follow the fluctuations in the water table a seriesof small wells was dug into the sediment (Figure 2). Each well was lined with sand and a 2.5cm i.d. PVC pipe, with holes drilled along its length and covered with a nylon mesh, was placed in each well. Tests with the well nearest the creekbank (Wl) showed that it had a nearly instantaneous responseto the falling and rising tide down to 35 cm below the marsh surface. A similar well in the interior marsh (W6) showed no drop in water level below the marsh surface. To determine how much water could be exchanged on each tidal cycle, a set of cores was taken at each well site, one set on a falling tide just before the marsh surface was exposed (high tide sample),and another on a low tide just before flooding began (low tide sample). These sampleswere taken on spring tides to get maximum differences. The cores were weighed, then dried at 95°C to a constant weight. The percentage of water is: Obwater= 100 (wet weight-dry
weight)/(wet weight)
Tidal
effects
EASTERN
Figure
on pore
water
chemistry
391
U.S.
1. Location
map of study
area. t, indicates
sampling
site in salt marsh.
The percentage of water lost each tidal cycle is calculated from the differences between high tide and low tide water content (Appendix I). An apparent residence time (t) for water was calculated from: Residence time = (100/O,:, loss cycle- I)/( l-92 cycles day- ‘) Chemical
To sample interstitial waters, glass tubes with perforations at the closed end were inserted into the sediments and water was withdrawn by means of a manual vacuum pump. The water was passed through a 25mm in-line filter holder (containing both coarse and fine glass fiber filters) into sample bottles sealed with serum caps. Each sample was identified with a letter indicating sample site and a number indicating mean depth of sample (relative to the marsh surface) at that site.
392
K. Agosta
w4
(a)
P 09.30
WI
(b) 19.50
n
w2
18.10
W5
II
w3 0
W5 P-
WI
17.20
q
Standard
Figure 2. Schematic illustration of sampling area on Bread and Butter Creek with the location of the observation wells (Wl-WS) and sampling sites (A-C). Also shown are the levels of the water table through one tidal cycle, (a) falling or ebb tide and (b) rising or flood tide. Not shown is W6, a well 2 m to the interior of W5 which had no measurable drop in water level once the tide fell below the marsh surface. The numbers on the lines are the time of day in hours. At 10.00 and 17.20 h the water table coincided with the reference or standard surface (the average surface of the marsh interior). Note that from 16.15 to 16.55 the water table sloped in a direction opposite to that of the marsh surface.
Immediately after filtration the sample was subsampled with a syringe. Concentrated nitric acid (0.142 ml) was added to 5-10 ml of pore water for later cation analysis. Zinc acetate (1 ml of 294 solution) was added to 3-6 ml of pore water to precipitate and preserve sulfide for later analysis (Fonselius, 1976). The remainder of the sample was stored in a sealed, disposable syringe. All samples were kept on ice or at 04°C until analysed. Ammonia was determined calorimetrically using Presley’s (1972) modification for sediment samples. Analyses were completed within 24-48 h of sample collection. Alkalinity was determined potentiometrically (Gieskes & Rogers, 1973). The acidified
Tidal
effects on pore
water
chemistry
393
(pH=2.4) alkalinity samples were then used for gravimetric sulfate analyses. Salinity was calculated from chlorosity as determined by coulometric titration using a Radiometer CM10 Chloride Titrator. Precision (based on duplicate field samples= 1 std. dev./mean) was 10yO for ammonia, 694 for alkalinity, 1% for chloride and 0.5 mm01 1-i for sulfate. All chemical parameters (X) were converted to their equivalent value at a salinity of 35%0 (X35) by: X35 = (35) (concentration of X)/(salinity)
Results and discussion Water level
The fluctuations of water in the marsh sediments are shown in Figures 2 and 3. The water level is given relative to a standard surface where zero is the level of the marsh surface behind the creekbank (W6). Once the tide fell below the marsh surface the water table generally followed the profile of the marsh surface [Figure 2(a)] down to 20-30 cm below that surface. This movement of water in the sediment is faster than could be predicted from sediment permeability (Gardner, 1975) and hydraulic gradients. The depth of the rapidly falling water table corresponds to the depth of Spartina root massin the sediment (Howes et al., 1981; Agosta, unpublished data) and suggeststhat water flows along channels created by the plant roots. The water level in well Wl rose immediately with the incoming tide, but for a short time (lo-30 min) it remained the sameor even fell in the other wells. Thus, there was a temporary development of a water table sloping in a direction opposite to the marsh surface [Figure 2(b)]. In addition, from W3 and upslope, the water table had not risen to the marsh surface before the tidal waters flowed over the top of the marsh. This general flow pattern was observed in each tidal cycle although the details varied depending on the daily tidal range and height of the seasurface.
Figure 3. Height of the water table in the wells shown in Figure 2. The weil numbers on the y-axis indicate the height of the marsh surface at that well. All levels are given relative to a standard surface.
394
K. Agosta
TABLE
Distance from creek (cm,Wl=O)
1. Water
content
of the sediments
Length of core (cm)
(wt %I
20 15 15 12
695(0.6)* 66.6(0.5) 66.0 68.7
25 75 150 400
High
“See Appendix I for method of calculation. bNumbers in parentheses are the standard
Low
0” loss
(wt “Cl) 60.6(1.1) 64.7(0.8) 65.1 68.0
deviation
of three
T
per cycle”
(days)
32.5 8.1 3.4 3.2
1.6 6.4 15.3 16.3
measurements.
Water content
Although there was a measurable drop in the water table as far back as 400 cm from the creek, a significant difference in the water content of the sediments at high and at low tides only occurred within 1OOcm of the creek (Table 1). Elsewhere sediment permeability was so low that water did not drain from the sediments even though the water table dropped 10-20 cm below the marsh surface. These water lossesare comparable to those found by Valiela and co-workers (1978), although in their sampling area water losseswere significant only to 5 cm below the surface while in this area water losseswere significant down the length of the core (15-20 cm). Basedon the amount of water lost per tidal cycle, the residencetime of the water in the top 20 cm of the creek-bank sediment is 1.6-16 days during spring tides. On neap tides the water table drops only 10-15 cm below the marsh surface and the marsh is exposed for a shorter time. This would increase the mean residence time of the water up to one month. This latter figure is more in keeping with the chemistry and salinities observed, especially in the deeper, more interior sites. Calculations (Appendix 1) show that, even at the site closest to the creek, not enough creek water is exchanged over a year to supply the annual nitrogen requirements of the plants. Thus, whatever the tidal ‘ subsidy ’ available to the plants, it is not the input of nutrients from creek waters. Physical
effects
Though the bulk sediment in the creekbank environment remains saturated, the drop in the water table may serve to change the sediment chemistry in two ways. First, a negative pore pressure develops and the macrospacescreated by plant roots or burrowing animals allow for the passageof air into the sediments. That this indeed occurs is supported by the fact that once the water table dropped below the opening of the sample tubes it was usually impossible to collect a pore water sampleby suction. Secondly, once a sloping water table develops water will flow in the direction of the decreasing head. The effect of a falling water table is to cause movement of water in the sedimentsand a partial replacement of the water on the creekbanks by water from the marsh interior. Water chemistry
The effect of this water movement on pore water chemistry can be large and rapid and is illustrated by the results of four sampling periods given in Figures 4-7. Two sets
Tidal
effects on pore
water
chemistry
395
(January and May 1981) were sampled from high water to high water on a spring tide with a mean tidal range of 1.6 m. Two sets (September 1981 and May 1982) were collected from low water to low water on a neap tide with a mean tidal range of 1 m. Despite the variation in seasons and tidal regime some general patterns can be seen. Salinity The salinity patterns [Figures 4-7 and 8(a)] of the pore waters depend on the level of the water table and, even more so, on the amount of rain in the 20-25 days preceding sampling time. A heavy rainfall (more than 10 cm per 5 days) during that time produced a low salinity spike in the pore waters over a tidal cycle (Figure 8). In general, when salinity can be used as a tracer, it is evident that water is moving within the creekbanks before the marsh surface is exposed (bar on the X-axis of Figures 4-7). These movements correspond directly to the drop in the water table relative to the back marsh which is used as the standard or reference surface (height is 0 on the X-axis of Figures 4-7). In January (Figure 4) the deepest (A30) and most interior (ClO) sites have salinities similar to the creek water in December (S=352%0). These are less than the salinities found either in the January creek water (S= 36.3%0) or in the other sites. The resulting density instability must be temporary but could lead to vertical mixing of the pore waters at high tide. Such mixing has been reported in fresh water sediments where temperature-induced density inversions occur (Musgrave & Reeburg, 1982). Alkalinity The normalized alkalinity (see Figures 4-7) increased as the water table fell and decreased as the water table rose. In general, the deeper the water sample the greater both the absolute concentration of alkalinity and the measured change over a tidal cycle. Ammonia Normalized ammonia concentrations (Figures 4-7) also tended table fell and decrease as the water table rose. Unlike alkalinity, have its maximum concentration at the lowest level of the water samples necessarily have the highest concentrations. In these not coupled to alkalinity production or to sulfate reduction.
to increase as the water ammonia did not always table nor did the deepest sediments ammonia was
Sulfate Normalized sulfate concentrations tend to decrease as the water table fell and increase as the water table rose. However, most samples taken on the spring tide (January and May 1981) have sulfate concentrations within one mm01 1-i of creek water values. See below for further discussion of seasonal and tidal range effects. Sulfide The sulfide-containing samples appeared to follow no pattern with site, tidal cycle or season. Sulfide values have not been plotted since contain measurable dissolved sulfide (O-2-9 PM), even when sulfate 4.5 mmol l- ’ were measured. King and co-workers (1982) report trations in creekbanks and this could keep the pore water sulfide low.
regards to sample only a few samples depletions of up to high iron concen-
396
K. Agosta
We
A
Height
Figure
4. Chemistry Site
-241,
, -60-40-x
Figure
5. Chemistry
Site
of water
of creekbank
table
Site
C
km)
interstitial Site
A
B
B
water Site
for 9 January
1981.
C
r, 1 / 0 20 40-4020 0 20-*0%40 Height of water table (cm)
of creekbank
interstitial
water
for 17 and 18 May
1981
Figures 47. Chemistry of creekbank interstitial water during four sampling periods at (0,m) 10, (0,O) 20 and (fI,A) 30 cm. Open symbols are flood tide samples, filled are from ebb. The height of the water table is given relative to the standard surface. The bar along the x-axis indicates the height at which the marsh is exposed at that site. On
Tidal
effects on pore
Site
-40
Figure
water
A
Site
-20 0 20 40-40 Height of water
6. Chemistry
Height
of creekbank
of water
table
Figure 7. Chemistry of creekbank in creek water was 23.6%. the y-axis the concentration L (low tide) and H (high normalized to 35% salinity.
397
chemistry
-20 table
B
: d 0 20 km1
interstitial
. .: 40
water
for 7 and 8 September
water
for 15 May
1981.
(cm 1
interstitial
1982. Low
of the chemical parameter in creek tide). Ammonia, alkalinity and sulfate
tide salinity
water is shown concentrations
by are
398
K. Agosta
19 Jan 1981 (a) 36 --L
---
- ____.
17-18 _-
_. I: ;ki
7 0\o 32-
May 1981
__ _ -
7-8
Sep 1981
15 May 1982
_-
i
1
L
Figure 8. Rainfall influence on salinity. (a) Salinity at A10 (. .), A20 i------) and A30 (---) on sampling day(s). On they-axis the salinity in the creek water is shown by L (low tide) and H (high tide). the ebb and flood tides are marked on the x-axis. (b) Rainfall for the 30 days preceding the sampling date given above.
Seasonal
effects
There is a seasonaleffect in the data which is indicative of biological activity in the sediment. In winter, when biological processare at a minimum, there was little evidence of decomposition and respiration (Figure 4), except at the deepest (A30) and most interior (ClO) sites-where the salinity indicated that the water was oldest. Respiration and decomposition by-products reach a maximum in late summer to fall and this can be seenin the September data (Figure 6) where significant depletion of sulfate occurs and where ammonia values are lo-fold higher than in the spring. Tidal
range effects
The effects of tidal range may be seenby comparing a spring tide (Figure 5) and a neap tide (Figure 7) during the spring season.Alkalinity and ammonia values are higher on the spring tide than on the neap tide. The spring tide concentrations do not return to background (creek water) values, suggesting that water from far back in the marsh is moving to the creek banks where it is diluted but not entirely replaced by incoming creek water. On the neap tide in both May and September (Figures 6-7) the 10-20 cm pore water at high tide was similar to creek water. The sedimentsappear more oxidizing on the spring tide when there was only a slight depletion in sulfate concentration. This depletion accounted for, at most, 25% of the observed alkalinity increase. In contrast, sulfate depletion on the neap tide (in May) accounted for half of the increased alkalinity [seeAgosta (19856) for further details]. At any one site the water at different depths in the top 20 cm was chemically similar (note especially Figures 6-7). The 30-cm samplesare probably older and exchange less
Tidal
effects
on pore water
chemistry
399
often than the shallower samples. Indeed on the September (Figure 6) neap tide, when the water table did not drop below 15 cm at site B, there was little evidence of chemical fluctuations at B30. This lends further support to the conclusion that fluctuations in the water table are immediately responsible for the major chemical changes observed on the creekbank. In May 1982 water samples were collected at about the same level of the water table on both the flooding and the ebbing tides. On the ebb tide water passing points A or B at 10-20 cm contain more alkalinity, ammonia and salt than water flowing past on the flood tide. The drainage of pore water from the creekbanks during ebb tide could be responsible for the higher concentrations of alkalinity and ammonia usually found in the creek waters at low tide (note position of L and H in Figures 5-7). Conclusions The chemistry exhibited by the pore waters of creekbank sediments is a result of a complex set of interactions involving the biological and chemical reactions occurring in the sediments, physical factors such as tides and animal burrowing, and the climatic factors which can influence the preceding. (1) Only small amounts of water drain from creekbank sediments during ebb tide. The replacement of this water by incoming tidal water is not a sufficient mechanism to supply creekbank Spartina with the nutrients needed for growth. (2) Although the net water loss on each tidal cycle is small there is a significant flux of nutrient-rich pore water into the creekbanks from regions behind the bank. This water can supply nitrogen and probably other nutrients to Spartina. (3) Once the water table drops below the marsh surface, pore water in the sediments above the water table is held under negative pressure and air may flow into the sediments via crab burrows or root spaces, thus promoting aeration of the sediments. This aeration, along with the increase in nutrients, may be responsible for the increased growth of Spartina along the creekbanks. (4) The residence time of water in the top 30 cm of the sediments is on the order of three weeks. This water returns to the creek via seepage and may serve as a source of nutrients for the tidal creeks. References Agosta, K. 1985a Biogeochemistry of salt marsh sediments and the growth of Spartina alterni’ora in North Inlet, South Carolina. Ph.D. thesis. University of South Carolina. 128 pp. Agosta, K. 19856 Excess alkalinity in salt marsh sediments: Patterns and implications (submitted). Fonselius, S. 1976 Determination of hydrogen sulfide. In Methods of Seawater Analysis (Grasshoff, K., ed.1. Verlag Chemie, New York. pp. 71-77. Gallagher, J. L. 1975 Effect of an ammonium nitrate pulse on the growth and elemental composition of natural stands of Spar&a alternipora and 3uncus roemerianus. American Journal of Botan-v, 62, 644-648. Gardner, L. R. 1973 The effects of hydrologic factors on the pore water chemistry of interstitial marsh sediments. Southeastern Geology, 15, 17-28. Gardner, L. R. 1975 Runoff from an intertidal marsh during tidal exposureRecession curves and chemical characteristics. Limnology and Oceanography, 20,81-89. Gieskes, J. M. & Rogers, W. C. 1973 Alkalinity determination in interstitial waters of marine sediments. Journal of Sedimentary Petrology, 43,272-277. Haines, E. B. 1979 Growth dynamics of cordgrass, Spartina alternif2ora Loisel, on control and sewage sludge fertilized plots in a Georgia salt marsh. Estuaries, 2,50-53.
400
K. Agosta
Hemond, H. F. & Burke, R. 1981 A device for the measurement of infiltration in intermittently flooded wetlands. Limnology and Oceanography, 26,795-800. Hemond, H. F. & Fifield, J. L. 1982 Subsurface flow in salt marsh peat: A model and field study. Limnology and Oceanography, 27,126-136. Howes, B. L., Howarth, R. W., Teal, J. M. & Valiela, I. 1981 Oxidation-reduction potentials in a salt marsh: Spatial patterns and interactions with primary production. Limnology and Oceanography, 26,350-360. King, G. M., Klug, M. J., Wiegert, R. W. & Chalmers, A. G. 1982 Relation of soil water movement and sulfide concentration Spartina alterniflora production in a Georgia salt marsh. Science, 218,61-63. Linthurst, R. A. 1979 The effect of aeration on the growth of Spartina alterni’ora Loisel. AmericanJournal of Botany, 66,685-691. Mendelssohn, I. A. & Seneca, E. D. 1980 The influence of soil drainage on the growth of salt marsh cordgrass Spar&a alterniflora in North Carolina. Ecology, l&27-40. Mooring, M. T., Cooper, A. W. & Seneca, E. D. 1971 Seed germination response and evidence for height ecophenes in Spartina alterniflora from North Carolina. AmericanJournal of Botany, .58,48-55. Musgrave, D. L. & Reeburgh, W. S. 1982 Density driven interstitial water motion in sediments. Nature, 299,331-333. Odurn, E. l’. & Farming, M. E. 1973 Comparison of the productivity of Spartina alterni$ora and Spartina cynosuroides in Georgia coastal marshes. Bulletin of the Georgia Academy of Science, 31, l-12. Patrick, W. H., Jr. & Delaune, R. D. 1976 Nitrogen and phosphorus utilization by Spartina alterniflora in a salt marsh in Barataria Bay, Louisiana. Estuarine and Coastal Marine Science, 4,59-64. Presley, B. J. 1971 Techniques for analyzing interstitial water samples. Part I. Determination of selected minor and major inorganic constituents. Initial Report of the Deep Sea Drilling Project, 7, 1749-1755. Shea, M. L., Warren, R. S. & Niering, W. A. 1975 Biochemical and transplantation studies of the growth forms of Spartina alterni’ora on Connecticut salt marshes. Ecology, 56,461-466. Steever, E. S., Warren, R. S. & Niering, W. A. 1976 Tidal energy subsidy and standing crop production of Spartina alterniflora. Estuarine and Coastal Marine Science, 4,473478. Sullivan, M. J. & Daiber, F. C. 1974 Response in production of cord grass, Spartina alterniflora, to inorganic nitrogen and phosphorus fertilizer. Chesapeake Science, 15,121-123. Valiela, I., Teal, J. M. & Deusser, W. G. 1978 The nature of growth forms in the salt marsh grass Sparrina alternifiora. American Naturalist, 112,461-470.
Appendix
I: Spartina
nitrogen
requirement
Creekside Spartina plants have been found to contain 08-14, (dry weight) of nitrogen in the aboveground portion (Patrick & Delaune, 1976). Using 1000 g m-’ year- i for Spartina production in South Carolina marshes (a low estimate for creekbanks), requires 8 g or 571 mm01 N mm2 year-‘. Creekbank sediments contain: 80-85O, water by volume, 3-33% of that water exchanged per tidal cycle, 20-30 cm is the depth of maximum drainage. Using 85O, water volume and a 30-cm depth, 1 m2 contains 255 1 of water, of which 76-84 1 are exchanged each cycle, or 5400-59500 1 mm2 year - ’ Creek water contains about 5 pM of NH,-N. (NO, + NO,)-N is negligible most of the year and is always less than NH, (Whiting, personal communication). Therefore, 27-297 mmol of nitrogen are potentially available to the plants annually. To sustain a year’s growth of Spartina requires 1.9-21 years worth of creekwater nitrogen. Note that these are minimum times based on 1000,; efficiency in extracting available N, a maximum drop in the water table, and ignoring any belowground production.
“At high tide sediments contain from 66 to 69”, water by weight. As an example, consider a core which is 69% water by weight at high tide and 60% water by weight at low-tide. Since this core is saturated at high tide a 100-g core contains 69 g water and 31 g solids. At low tide, assuming no compaction of the sediments and that the weight of air is negligible, this same volume of sediment still contains 31 g of solids. However, this 31 g now makes up 40s; of the total weight. Thus the volume that weighed 100 g at high tide weighs 77.5 g at low tide (77.5= 31/0.4). The water which remains at low tide weighs 47.5 g (77.5-30), and 21.5 g (69-47.5) or 31°, (21.5/69) of the water has been lost.
Coastal
and Shelf
Science
(1985)
21,389-400
The Effect of Tidally Induced Changes in the Creekbank Water Table on Pore Water Chemistrya
Kathleen
Agosta
Marine Science Program and BelEe W. Baruch Institute for Marine and Coastal Research, University of South Carolina, Columbia, South Carolina 29208, U.S.A. Received
3 May
1984 and in revisedform
18January
Keywords: water movement; interstitial Spartina; South Carolina Coast
Biology
1985
water; water properties;
salt marshes;
Water moved into the creekbank sediments in direct response to the changing levels of the water table caused by the tides. The net water loss of the sediments was 3-30yb on each low tide and this loss was confined to within 4 m (horizontal) of the creek. The replacement of this water by incoming tidal water could not supply sufficient nutrients for the growth of creekbank Spartina. However, during ebb tide there was a replacement of water in the creekbanks with nutrient-rich water from the marsh interior as demonstrated by the large changes in pore water chemistry over a tidal cycle. The concentration and the range of a chemical parameter depended upon the stage of the tide, the tidal range, the time of year and (for salinity) the rainfall patterns of the month preceding sampling. Over a single tidal cycle the maximum ranges were: salinity %o, 26-33; alkalinity, 25-13.6 meq l-i, ammonia, 2-400 pM, sulfate, 23.5-29 mm01 l- ‘. Measurable concentrations of sulfide were only found in a few samples. This high nutrient water can supply nitrogen and probably other nutrients to Spartina. Introduction is the dominant plant in south-eastern salt marshes and is and coastal ecosystems.Two different height forms are generally observed-a tall form which grows along the creekbank and a medium to short form which grows elsewherein the marsh. The two forms are genetically the same(Mooring et al., 1971; Shea et al., 1975) and both transplantation studies (Shea et uZ., 1975) and Spartina
Loisel
aZterniflora
important
fertilization have shown
to estuarine
experiments (Sullivan & Daiber, that one form can be transformed
1971; Valiela to another.
et
al., 1978; Haines,
1979)
Suggested factors responsible for higher growth along the creekbank include an increased input of nutrients from tidal waters (Gallagher, 1975; Valiela et al., 1978), a tidal energy subsidy (Odurn & Fanning, 1973; Steever et ul., 1976), higher iron concentrations (King et al., 1982), more oxidized sediments (Linthurst, 1979; Howes et al., “Contribution
number
586 from
me Belle W. Baruch Institute for Marine Biology and
Coastal Research. 389 0272-7714/85/090389+
12 803.0010
1985 Academic
Press Inc. (London)
Limited
390
Ii. Anosta
1981), and increased drainage relative to interior sediments (Mendelssohn & Seneca, 1980; King et al., 1982). The key variable appears to be drainage and infiltration rates which in turn control the concentration of nutrients, trace metals and oxygen. Most studies on the effects of drainage have experimentally altered the drainage pattern in the root zone (Mendelssohn & Senaca, 1980; King et al., 1982) and observed the increase or decreaseof plant productivity relative to changes in drainage. The few studies that have examined natural marsh sediments have shown that over most of the marsh, seepageand infiltration are low (Gardner, 1973, 1975; Hemond & Burke, 1981; King et al., 1982). Significant flow of water through the sediment occurs only on creekbanks where the rise and fall of the tides produces coincident water table fluctuation (Gardner, 1973, 1975). Elsewhere the water table remains essentially horizontal throughout the tidal cycle and generally lies within 1 cm of the marsh surface even at low tide. Thus water movement, away from creekbanks, is largely limited to the infiltration required to balance evapotranspiration losses. Hemond and Fifield (1982) have examined the hydrological regime in a peaty, New England marsh away from creekbanks. They find that seepageis quite low and sediments remain saturated throughout the tidal cycle. This study focuseson water movements and pore water chemistry in the creekbank of a silty south-eastern marsh where water moves throughout the tidal cycle.
Site description The study site (Figure 1) was a high-salinity, Spartina marsh on the Baruch Plantation near Georgetown, SC. The actual sampling site was adjacent to Bread and Butter Creek-a well developed tidal creek which has a tidal range of about 1.4m. It is not completely drained at low tide. A profile of the creekbank and the sampling locations are given in Figure 2. The creekbank itself is covered to a mean depth of 70 cm at high tide. Spartina plants are well developed in the area (fall standing crop l-2 kg dry wt me2, Agosta, 198%) and there is evidence of crab burrowing in the sediments. This study was conducted from late fall, 1980through spring, 1982.
Methods Physical
In order to follow the fluctuations in the water table a seriesof small wells was dug into the sediment (Figure 2). Each well was lined with sand and a 2.5cm i.d. PVC pipe, with holes drilled along its length and covered with a nylon mesh, was placed in each well. Tests with the well nearest the creekbank (Wl) showed that it had a nearly instantaneous responseto the falling and rising tide down to 35 cm below the marsh surface. A similar well in the interior marsh (W6) showed no drop in water level below the marsh surface. To determine how much water could be exchanged on each tidal cycle, a set of cores was taken at each well site, one set on a falling tide just before the marsh surface was exposed (high tide sample),and another on a low tide just before flooding began (low tide sample). These sampleswere taken on spring tides to get maximum differences. The cores were weighed, then dried at 95°C to a constant weight. The percentage of water is: Obwater= 100 (wet weight-dry
weight)/(wet weight)
Tidal
effects
EASTERN
Figure
on pore
water
chemistry
391
U.S.
1. Location
map of study
area. t, indicates
sampling
site in salt marsh.
The percentage of water lost each tidal cycle is calculated from the differences between high tide and low tide water content (Appendix I). An apparent residence time (t) for water was calculated from: Residence time = (100/O,:, loss cycle- I)/( l-92 cycles day- ‘) Chemical
To sample interstitial waters, glass tubes with perforations at the closed end were inserted into the sediments and water was withdrawn by means of a manual vacuum pump. The water was passed through a 25mm in-line filter holder (containing both coarse and fine glass fiber filters) into sample bottles sealed with serum caps. Each sample was identified with a letter indicating sample site and a number indicating mean depth of sample (relative to the marsh surface) at that site.
392
K. Agosta
w4
(a)
P 09.30
WI
(b) 19.50
n
w2
18.10
W5
II
w3 0
W5 P-
WI
17.20
q
Standard
Figure 2. Schematic illustration of sampling area on Bread and Butter Creek with the location of the observation wells (Wl-WS) and sampling sites (A-C). Also shown are the levels of the water table through one tidal cycle, (a) falling or ebb tide and (b) rising or flood tide. Not shown is W6, a well 2 m to the interior of W5 which had no measurable drop in water level once the tide fell below the marsh surface. The numbers on the lines are the time of day in hours. At 10.00 and 17.20 h the water table coincided with the reference or standard surface (the average surface of the marsh interior). Note that from 16.15 to 16.55 the water table sloped in a direction opposite to that of the marsh surface.
Immediately after filtration the sample was subsampled with a syringe. Concentrated nitric acid (0.142 ml) was added to 5-10 ml of pore water for later cation analysis. Zinc acetate (1 ml of 294 solution) was added to 3-6 ml of pore water to precipitate and preserve sulfide for later analysis (Fonselius, 1976). The remainder of the sample was stored in a sealed, disposable syringe. All samples were kept on ice or at 04°C until analysed. Ammonia was determined calorimetrically using Presley’s (1972) modification for sediment samples. Analyses were completed within 24-48 h of sample collection. Alkalinity was determined potentiometrically (Gieskes & Rogers, 1973). The acidified
Tidal
effects on pore
water
chemistry
393
(pH=2.4) alkalinity samples were then used for gravimetric sulfate analyses. Salinity was calculated from chlorosity as determined by coulometric titration using a Radiometer CM10 Chloride Titrator. Precision (based on duplicate field samples= 1 std. dev./mean) was 10yO for ammonia, 694 for alkalinity, 1% for chloride and 0.5 mm01 1-i for sulfate. All chemical parameters (X) were converted to their equivalent value at a salinity of 35%0 (X35) by: X35 = (35) (concentration of X)/(salinity)
Results and discussion Water level
The fluctuations of water in the marsh sediments are shown in Figures 2 and 3. The water level is given relative to a standard surface where zero is the level of the marsh surface behind the creekbank (W6). Once the tide fell below the marsh surface the water table generally followed the profile of the marsh surface [Figure 2(a)] down to 20-30 cm below that surface. This movement of water in the sediment is faster than could be predicted from sediment permeability (Gardner, 1975) and hydraulic gradients. The depth of the rapidly falling water table corresponds to the depth of Spartina root massin the sediment (Howes et al., 1981; Agosta, unpublished data) and suggeststhat water flows along channels created by the plant roots. The water level in well Wl rose immediately with the incoming tide, but for a short time (lo-30 min) it remained the sameor even fell in the other wells. Thus, there was a temporary development of a water table sloping in a direction opposite to the marsh surface [Figure 2(b)]. In addition, from W3 and upslope, the water table had not risen to the marsh surface before the tidal waters flowed over the top of the marsh. This general flow pattern was observed in each tidal cycle although the details varied depending on the daily tidal range and height of the seasurface.
Figure 3. Height of the water table in the wells shown in Figure 2. The weil numbers on the y-axis indicate the height of the marsh surface at that well. All levels are given relative to a standard surface.
394
K. Agosta
TABLE
Distance from creek (cm,Wl=O)
1. Water
content
of the sediments
Length of core (cm)
(wt %I
20 15 15 12
695(0.6)* 66.6(0.5) 66.0 68.7
25 75 150 400
High
“See Appendix I for method of calculation. bNumbers in parentheses are the standard
Low
0” loss
(wt “Cl) 60.6(1.1) 64.7(0.8) 65.1 68.0
deviation
of three
T
per cycle”
(days)
32.5 8.1 3.4 3.2
1.6 6.4 15.3 16.3
measurements.
Water content
Although there was a measurable drop in the water table as far back as 400 cm from the creek, a significant difference in the water content of the sediments at high and at low tides only occurred within 1OOcm of the creek (Table 1). Elsewhere sediment permeability was so low that water did not drain from the sediments even though the water table dropped 10-20 cm below the marsh surface. These water lossesare comparable to those found by Valiela and co-workers (1978), although in their sampling area water losseswere significant only to 5 cm below the surface while in this area water losseswere significant down the length of the core (15-20 cm). Basedon the amount of water lost per tidal cycle, the residencetime of the water in the top 20 cm of the creek-bank sediment is 1.6-16 days during spring tides. On neap tides the water table drops only 10-15 cm below the marsh surface and the marsh is exposed for a shorter time. This would increase the mean residence time of the water up to one month. This latter figure is more in keeping with the chemistry and salinities observed, especially in the deeper, more interior sites. Calculations (Appendix 1) show that, even at the site closest to the creek, not enough creek water is exchanged over a year to supply the annual nitrogen requirements of the plants. Thus, whatever the tidal ‘ subsidy ’ available to the plants, it is not the input of nutrients from creek waters. Physical
effects
Though the bulk sediment in the creekbank environment remains saturated, the drop in the water table may serve to change the sediment chemistry in two ways. First, a negative pore pressure develops and the macrospacescreated by plant roots or burrowing animals allow for the passageof air into the sediments. That this indeed occurs is supported by the fact that once the water table dropped below the opening of the sample tubes it was usually impossible to collect a pore water sampleby suction. Secondly, once a sloping water table develops water will flow in the direction of the decreasing head. The effect of a falling water table is to cause movement of water in the sedimentsand a partial replacement of the water on the creekbanks by water from the marsh interior. Water chemistry
The effect of this water movement on pore water chemistry can be large and rapid and is illustrated by the results of four sampling periods given in Figures 4-7. Two sets
Tidal
effects on pore
water
chemistry
395
(January and May 1981) were sampled from high water to high water on a spring tide with a mean tidal range of 1.6 m. Two sets (September 1981 and May 1982) were collected from low water to low water on a neap tide with a mean tidal range of 1 m. Despite the variation in seasons and tidal regime some general patterns can be seen. Salinity The salinity patterns [Figures 4-7 and 8(a)] of the pore waters depend on the level of the water table and, even more so, on the amount of rain in the 20-25 days preceding sampling time. A heavy rainfall (more than 10 cm per 5 days) during that time produced a low salinity spike in the pore waters over a tidal cycle (Figure 8). In general, when salinity can be used as a tracer, it is evident that water is moving within the creekbanks before the marsh surface is exposed (bar on the X-axis of Figures 4-7). These movements correspond directly to the drop in the water table relative to the back marsh which is used as the standard or reference surface (height is 0 on the X-axis of Figures 4-7). In January (Figure 4) the deepest (A30) and most interior (ClO) sites have salinities similar to the creek water in December (S=352%0). These are less than the salinities found either in the January creek water (S= 36.3%0) or in the other sites. The resulting density instability must be temporary but could lead to vertical mixing of the pore waters at high tide. Such mixing has been reported in fresh water sediments where temperature-induced density inversions occur (Musgrave & Reeburg, 1982). Alkalinity The normalized alkalinity (see Figures 4-7) increased as the water table fell and decreased as the water table rose. In general, the deeper the water sample the greater both the absolute concentration of alkalinity and the measured change over a tidal cycle. Ammonia Normalized ammonia concentrations (Figures 4-7) also tended table fell and decrease as the water table rose. Unlike alkalinity, have its maximum concentration at the lowest level of the water samples necessarily have the highest concentrations. In these not coupled to alkalinity production or to sulfate reduction.
to increase as the water ammonia did not always table nor did the deepest sediments ammonia was
Sulfate Normalized sulfate concentrations tend to decrease as the water table fell and increase as the water table rose. However, most samples taken on the spring tide (January and May 1981) have sulfate concentrations within one mm01 1-i of creek water values. See below for further discussion of seasonal and tidal range effects. Sulfide The sulfide-containing samples appeared to follow no pattern with site, tidal cycle or season. Sulfide values have not been plotted since contain measurable dissolved sulfide (O-2-9 PM), even when sulfate 4.5 mmol l- ’ were measured. King and co-workers (1982) report trations in creekbanks and this could keep the pore water sulfide low.
regards to sample only a few samples depletions of up to high iron concen-
396
K. Agosta
We
A
Height
Figure
4. Chemistry Site
-241,
, -60-40-x
Figure
5. Chemistry
Site
of water
of creekbank
table
Site
C
km)
interstitial Site
A
B
B
water Site
for 9 January
1981.
C
r, 1 / 0 20 40-4020 0 20-*0%40 Height of water table (cm)
of creekbank
interstitial
water
for 17 and 18 May
1981
Figures 47. Chemistry of creekbank interstitial water during four sampling periods at (0,m) 10, (0,O) 20 and (fI,A) 30 cm. Open symbols are flood tide samples, filled are from ebb. The height of the water table is given relative to the standard surface. The bar along the x-axis indicates the height at which the marsh is exposed at that site. On
Tidal
effects on pore
Site
-40
Figure
water
A
Site
-20 0 20 40-40 Height of water
6. Chemistry
Height
of creekbank
of water
table
Figure 7. Chemistry of creekbank in creek water was 23.6%. the y-axis the concentration L (low tide) and H (high normalized to 35% salinity.
397
chemistry
-20 table
B
: d 0 20 km1
interstitial
. .: 40
water
for 7 and 8 September
water
for 15 May
1981.
(cm 1
interstitial
1982. Low
of the chemical parameter in creek tide). Ammonia, alkalinity and sulfate
tide salinity
water is shown concentrations
by are
398
K. Agosta
19 Jan 1981 (a) 36 --L
---
- ____.
17-18 _-
_. I: ;ki
7 0\o 32-
May 1981
__ _ -
7-8
Sep 1981
15 May 1982
_-
i
1
L
Figure 8. Rainfall influence on salinity. (a) Salinity at A10 (. .), A20 i------) and A30 (---) on sampling day(s). On they-axis the salinity in the creek water is shown by L (low tide) and H (high tide). the ebb and flood tides are marked on the x-axis. (b) Rainfall for the 30 days preceding the sampling date given above.
Seasonal
effects
There is a seasonaleffect in the data which is indicative of biological activity in the sediment. In winter, when biological processare at a minimum, there was little evidence of decomposition and respiration (Figure 4), except at the deepest (A30) and most interior (ClO) sites-where the salinity indicated that the water was oldest. Respiration and decomposition by-products reach a maximum in late summer to fall and this can be seenin the September data (Figure 6) where significant depletion of sulfate occurs and where ammonia values are lo-fold higher than in the spring. Tidal
range effects
The effects of tidal range may be seenby comparing a spring tide (Figure 5) and a neap tide (Figure 7) during the spring season.Alkalinity and ammonia values are higher on the spring tide than on the neap tide. The spring tide concentrations do not return to background (creek water) values, suggesting that water from far back in the marsh is moving to the creek banks where it is diluted but not entirely replaced by incoming creek water. On the neap tide in both May and September (Figures 6-7) the 10-20 cm pore water at high tide was similar to creek water. The sedimentsappear more oxidizing on the spring tide when there was only a slight depletion in sulfate concentration. This depletion accounted for, at most, 25% of the observed alkalinity increase. In contrast, sulfate depletion on the neap tide (in May) accounted for half of the increased alkalinity [seeAgosta (19856) for further details]. At any one site the water at different depths in the top 20 cm was chemically similar (note especially Figures 6-7). The 30-cm samplesare probably older and exchange less
Tidal
effects
on pore water
chemistry
399
often than the shallower samples. Indeed on the September (Figure 6) neap tide, when the water table did not drop below 15 cm at site B, there was little evidence of chemical fluctuations at B30. This lends further support to the conclusion that fluctuations in the water table are immediately responsible for the major chemical changes observed on the creekbank. In May 1982 water samples were collected at about the same level of the water table on both the flooding and the ebbing tides. On the ebb tide water passing points A or B at 10-20 cm contain more alkalinity, ammonia and salt than water flowing past on the flood tide. The drainage of pore water from the creekbanks during ebb tide could be responsible for the higher concentrations of alkalinity and ammonia usually found in the creek waters at low tide (note position of L and H in Figures 5-7). Conclusions The chemistry exhibited by the pore waters of creekbank sediments is a result of a complex set of interactions involving the biological and chemical reactions occurring in the sediments, physical factors such as tides and animal burrowing, and the climatic factors which can influence the preceding. (1) Only small amounts of water drain from creekbank sediments during ebb tide. The replacement of this water by incoming tidal water is not a sufficient mechanism to supply creekbank Spartina with the nutrients needed for growth. (2) Although the net water loss on each tidal cycle is small there is a significant flux of nutrient-rich pore water into the creekbanks from regions behind the bank. This water can supply nitrogen and probably other nutrients to Spartina. (3) Once the water table drops below the marsh surface, pore water in the sediments above the water table is held under negative pressure and air may flow into the sediments via crab burrows or root spaces, thus promoting aeration of the sediments. This aeration, along with the increase in nutrients, may be responsible for the increased growth of Spartina along the creekbanks. (4) The residence time of water in the top 30 cm of the sediments is on the order of three weeks. This water returns to the creek via seepage and may serve as a source of nutrients for the tidal creeks. References Agosta, K. 1985a Biogeochemistry of salt marsh sediments and the growth of Spartina alterni’ora in North Inlet, South Carolina. Ph.D. thesis. University of South Carolina. 128 pp. Agosta, K. 19856 Excess alkalinity in salt marsh sediments: Patterns and implications (submitted). Fonselius, S. 1976 Determination of hydrogen sulfide. In Methods of Seawater Analysis (Grasshoff, K., ed.1. Verlag Chemie, New York. pp. 71-77. Gallagher, J. L. 1975 Effect of an ammonium nitrate pulse on the growth and elemental composition of natural stands of Spar&a alternipora and 3uncus roemerianus. American Journal of Botan-v, 62, 644-648. Gardner, L. R. 1973 The effects of hydrologic factors on the pore water chemistry of interstitial marsh sediments. Southeastern Geology, 15, 17-28. Gardner, L. R. 1975 Runoff from an intertidal marsh during tidal exposureRecession curves and chemical characteristics. Limnology and Oceanography, 20,81-89. Gieskes, J. M. & Rogers, W. C. 1973 Alkalinity determination in interstitial waters of marine sediments. Journal of Sedimentary Petrology, 43,272-277. Haines, E. B. 1979 Growth dynamics of cordgrass, Spartina alternif2ora Loisel, on control and sewage sludge fertilized plots in a Georgia salt marsh. Estuaries, 2,50-53.
400
K. Agosta
Hemond, H. F. & Burke, R. 1981 A device for the measurement of infiltration in intermittently flooded wetlands. Limnology and Oceanography, 26,795-800. Hemond, H. F. & Fifield, J. L. 1982 Subsurface flow in salt marsh peat: A model and field study. Limnology and Oceanography, 27,126-136. Howes, B. L., Howarth, R. W., Teal, J. M. & Valiela, I. 1981 Oxidation-reduction potentials in a salt marsh: Spatial patterns and interactions with primary production. Limnology and Oceanography, 26,350-360. King, G. M., Klug, M. J., Wiegert, R. W. & Chalmers, A. G. 1982 Relation of soil water movement and sulfide concentration Spartina alterniflora production in a Georgia salt marsh. Science, 218,61-63. Linthurst, R. A. 1979 The effect of aeration on the growth of Spartina alterni’ora Loisel. AmericanJournal of Botany, 66,685-691. Mendelssohn, I. A. & Seneca, E. D. 1980 The influence of soil drainage on the growth of salt marsh cordgrass Spar&a alterniflora in North Carolina. Ecology, l&27-40. Mooring, M. T., Cooper, A. W. & Seneca, E. D. 1971 Seed germination response and evidence for height ecophenes in Spartina alterniflora from North Carolina. AmericanJournal of Botany, .58,48-55. Musgrave, D. L. & Reeburgh, W. S. 1982 Density driven interstitial water motion in sediments. Nature, 299,331-333. Odurn, E. l’. & Farming, M. E. 1973 Comparison of the productivity of Spartina alterni$ora and Spartina cynosuroides in Georgia coastal marshes. Bulletin of the Georgia Academy of Science, 31, l-12. Patrick, W. H., Jr. & Delaune, R. D. 1976 Nitrogen and phosphorus utilization by Spartina alterniflora in a salt marsh in Barataria Bay, Louisiana. Estuarine and Coastal Marine Science, 4,59-64. Presley, B. J. 1971 Techniques for analyzing interstitial water samples. Part I. Determination of selected minor and major inorganic constituents. Initial Report of the Deep Sea Drilling Project, 7, 1749-1755. Shea, M. L., Warren, R. S. & Niering, W. A. 1975 Biochemical and transplantation studies of the growth forms of Spartina alterni’ora on Connecticut salt marshes. Ecology, 56,461-466. Steever, E. S., Warren, R. S. & Niering, W. A. 1976 Tidal energy subsidy and standing crop production of Spartina alterniflora. Estuarine and Coastal Marine Science, 4,473478. Sullivan, M. J. & Daiber, F. C. 1974 Response in production of cord grass, Spartina alterniflora, to inorganic nitrogen and phosphorus fertilizer. Chesapeake Science, 15,121-123. Valiela, I., Teal, J. M. & Deusser, W. G. 1978 The nature of growth forms in the salt marsh grass Sparrina alternifiora. American Naturalist, 112,461-470.
Appendix
I: Spartina
nitrogen
requirement
Creekside Spartina plants have been found to contain 08-14, (dry weight) of nitrogen in the aboveground portion (Patrick & Delaune, 1976). Using 1000 g m-’ year- i for Spartina production in South Carolina marshes (a low estimate for creekbanks), requires 8 g or 571 mm01 N mm2 year-‘. Creekbank sediments contain: 80-85O, water by volume, 3-33% of that water exchanged per tidal cycle, 20-30 cm is the depth of maximum drainage. Using 85O, water volume and a 30-cm depth, 1 m2 contains 255 1 of water, of which 76-84 1 are exchanged each cycle, or 5400-59500 1 mm2 year - ’ Creek water contains about 5 pM of NH,-N. (NO, + NO,)-N is negligible most of the year and is always less than NH, (Whiting, personal communication). Therefore, 27-297 mmol of nitrogen are potentially available to the plants annually. To sustain a year’s growth of Spartina requires 1.9-21 years worth of creekwater nitrogen. Note that these are minimum times based on 1000,; efficiency in extracting available N, a maximum drop in the water table, and ignoring any belowground production.
“At high tide sediments contain from 66 to 69”, water by weight. As an example, consider a core which is 69% water by weight at high tide and 60% water by weight at low-tide. Since this core is saturated at high tide a 100-g core contains 69 g water and 31 g solids. At low tide, assuming no compaction of the sediments and that the weight of air is negligible, this same volume of sediment still contains 31 g of solids. However, this 31 g now makes up 40s; of the total weight. Thus the volume that weighed 100 g at high tide weighs 77.5 g at low tide (77.5= 31/0.4). The water which remains at low tide weighs 47.5 g (77.5-30), and 21.5 g (69-47.5) or 31°, (21.5/69) of the water has been lost.