Marine Chemistry, 22 (1987) 7143 71 Elsevier Science Publishers B.V., Amsterdam - - Printed in The Netherlands

THE I N F L U E N C E OF S E A G R A S S B E D S ON CARBONATE S E D I M E N T S IN THE B A H A M A S

JOHN W. MORSE I, JAMES J. ZULLIG I, RICHARD L. IVERSON 2a, GREGORY R. CHOPPIN 2b, ALFONSO MUCCI 3, and FRANK J. MILLERO 4

IDepartment of Oceanography, Texas A&M University, College Station, TX 77843 (U.S.A.) ~Department of Oceanography, 2bDepartment of Chemistry, Florida State University, TaUahassee, FL 32306 (U.S.A.) 3Department of Geologic Sciences, McGill University, 3450 University Street, Montreal H3A 2A7 (Canada) 4Division of Marine and Atmospheric Chemistry, Rosenstiel School of Marine and Atmospheric Science, University of Miami, 4600 Rickenbacker Causeway, Miami, FL 33149 (U.S.A.)

(Received July 1, 1986; revision accepted May 27, 1987)

ABSTRACT

Morse, J.W., Zullig, J.J., Iverson, R.L., Choppin, G.R., Mucci, A. and Millero, F.J., 1987. The influence of seagrass beds on carbonate sediments in the Bahamas. Mar. Chem., 22: 7143.

Chemical variables were measured in calcium-carbonate-rich sediments inhabited by the domi- nant tropical seagrass, Thalassia testudinum, and in adjacent seagrass-free sediments at several locations in the Bahamas Islands. Pore-water alkalinity and Pco2 were consistently greater, while pH was consistently lower in sediment-pore waters within seagrass beds. The ammonium and molybdate-reactive phosphate concentrations in sediment-pore water were variable for vegetated, compared with unvegetated, sample locations.

Thalassia testudinum can generate very large amounts of organic matter within calcium- carbonate-rich sediments. However, little of the organic matter is retained in the sediment and the effect of that organic matter on pore water chemical factors appears to he surprisingly small. These observations are markedly different from those for seagrass beds in high latitude clastic sediments and in Syringodium filiforme seagrass beds near San Salvador Island, where major influences of the seagrass beds on sediment chemistry have been observed. The generally coarser grain size of the carbonate sediments may be a primary factor contributing to these differences.

INTRODUCTION

S e a g r a s s e s a r e m a r i n e v a s c u l a r p l a n t s t h a t a r e w i d e l y d i s t r i b u t e d a r o u n d t h e m a r g i n s o f c o n t i n e n t s a n d i s l a n d s (den H a r t o g , 1970). C a l c i u m - c a r b o n a t e - r i c h s e d i m e n t s in m a n y t r o p i c a l a n d s u b t r o p i c a l l o c a t i o n s c o n t a i n s e a g r a s s beds , t h e e p i p h y t e s o f w h i c h p r o v i d e a m a j o r p o r t i o n o f t h e c a l c i u m c a r b o n a t e t h a t a c c u m u l a t e s o n s h a l l o w w a t e r b a n k s ( M i l l i m a n , 1974). T h e s e l a r g e e x p a n - ses o f v e g e t a t i o n p r o v i d e t h e m a j o r b i o l o g i c a l h a b i t a t in m a n y s h a l l o w - w a t e r c a r b o n a t e s e d i m e n t s l o c a t e d in t h e G u l f o f M e x i c o , C a r i b b e a n Sea , a n d B a h a - m a I s l a n d s w h e r e t h e y h a v e b e e n m a p p e d a n d i n v e s t i g a t e d ( N e w e l l e t a l . , 1959;

0304-4203/87/$03.50 © 1987 Elsevier Science Publishers B.V.

72

Buesa, 1974; Phillips et al., 1982; Iverson and Bittaker, 1986). Thalassia tes- tudinum is the most abundant seagrass species, in terms of biomass, in the Gulf of Mexico (Buesa, 1974; Iverson and Bittaker, 1986) and the Caribbean Sea (Phillips et al., 1982). In addition to providing habitat and food for fishes (Randall, 1967) and invertebrates (Hooks et al., 1976), seagrass beds form the basis for extensive detrital food webs in tropical environments (Thayer et al., 1978; Zieman et al., 1979).

Many of these seagrass beds, containing large stands of Thatassia tes- tudinum, grow in carbonate sediments that are removed considerable distances from land and from the influences of continental river discharge and ter- rigenous runoff. These large seagrass beds of the new world tropics and sub- tropics are located in areas where seawater is very oligotrophic. In such environments, interactions between the seagrasses and sediments become par- ticularly important for nutrient cycling (e.g. Patriquin, 1972). The Bahamas seagrass beds are among the best examples of this environment type.

While the significance of seagrass systems for carbonate sediment deposi- tion has long been recognized (Ginsburg and Lowenstam, 1958; Land, 1970; Scoffin, 1970; Milliman, 1974; Petta and Gerhard, 1977), the effects of seagrass organic matter production on the chemistry of carbonate sediments are not well understood. Living organic material associated with seagrass root and rhizome can reach 4.0kg dry weight m 2 within Florida Bay seagrass beds dominated by Thalassia testudinum (Iverson and Bittaker, 1986). Part of the leaf organic matter produced within seagrass beds is exported and part remains in the surficial sediments where it is regenerated (Zieman et al., 1979; Zieman, 1984), Although the source of the ammonium within sediments in which sea- grasses grow appears to be the regeneration of organic matter, the ultimate source of nitrogen is fixation by epiphytes and by bacteria associated with roots, which appears to provide most of the nitrogen required for seagrass growth in subtropical carbonate sediment environments (Capone et al., 1979).

Preliminary investigations suggested that the values of some chemical variables in carbonate sediments overlain by seagrass beds were not markedly different from the corresponding values in adjacent sediments that contained no seagrasses, and that molybdate-reactive phosphate concentrations in pore water were always < 0.5p (Morse et al., 1985).

We report data obtained from the Bahamas Islands which address the signifi- cance of Thalassia testudinum for several chemical variables in carbonate-rich sediments.

METHODS

Sites and sampling

Three study sites (see Fig. 1) were chosen in the Berry Islands of the Baha- mas based on our previous work in this area (Morse et al., 1985). The first site was 30 m west of Whale Cay. The sediment here consisted of moderate-to-

73

o~ b \ ~ 1 o~A ~ 1

Little Harbor Cay Sites/--.~\~ \ ~ T - - - - : /

I ~ 1

BERRY ISLANDS ~ [ ~

J i (. I - -

. . . . l(',-:F~-'~: A T L A N T I C ~ - ,~/" whale uay f / t ~ . ~ ~ / Site

, , - - - - ~ O C E A N ~ -

Fig. 1. Sampling sites.

coarse-grained oolitic sand (see Table I) overlain by ~ 0.5m of water at low tide. This area is relatively sheltered from the open ocean by the surrounding cays and shallows. The seagrass bed studied was dominated by Thalassia and was ~ 30 m wide. Seagrass growth was not lush, but it was dense enough to completely obscure the bottom in the central parts of the grassbed. The two other study sites chosen were near Little Harbor Cay of which the first site studied had an extensive grassbed extending 60 m from a mangrove-lined shore. It ended abruptly and was bordered by white carbonate sand, exhibiting large ripples (~ 0.3 m high) and showing extensive signs of benthic biologic activity. The water depth at low tide was ~ 1.5 m and the site lay on the edge of a channel through which water was rapidly exchanged with the nearby open ocean by tidal flushing. The second site studied near Little Harbor Cay lay in the middle of the tidal channel ~ 100 m from both bordering shores and was 0.6 km more distant from the open ocean. It had numerous seagrass beds which also had sharp borders with the surrounding carbonate sand. This sand was bioturbated with high relief (~ 0.7 m) resulting from numerous shrimp mounds. The water was shallower (~ 0.4 m at low tide) than at the other Little Harbor Cay site. The width of the grass bed studied was ~ 12 m, which made feasible a transect completely through the grass bed. Both seagrass beds at Little Harbor Cay were luxuriant and composed predominantly of Thalassia testudinum.

74

T A B L E l

Bahama seagrass sediment size distribution

S i t e T r a n s e c t ~ 52 W. o in s ize (gm) f r a c t i o n s

(m) 52 125 125 250 2 5 0 7 0 7 > 707

Whale C a y 0 ' 9.6 l.,i 6 .2 62.4 20.4

5 ~ 8.6 6.4 4.8 47.2 33.1

10" 12.6 2.3 3.3 18.4 63.3

15 k~ 2.3 0.6 4.4 45.0 47.7

20' 2.1 0.6 2.1 70.7 24.5

25 ~ 3.8 0.6 1.5 26.!) 67.2

30 ' 3.2 0,6 1.9 52.5 41.9

Little Harbor C a y I 0" 6.1 0.2 1.8 53.5 38.5

5 ~ 4.0 0.4 5.8 30.4 59.4

10 :~ 2.8 0.4 0.8 17.3 78.7

15 ~' 5.1 0.4 7.2 ~4.1 3.2

20" 8.5 1.1 7.0 i 2 . 3 41.0

25 ~ 6.8 3.0 5.9 50.4 33.8

30 c 6.7 2.0 5,2 20.6 65.6

Little Harbor C a y II 0 ' 4.1 2,6 4.3 62.0 27.0

0 ' 3.5 0.6 1.5 62.8 32.4

0 ' 7.6 2.4 6.5 53.7 29.8

5 ~ 4.7 2.2 5.1 32.6 55.3

10 ~' 10.6 5.2 8.6 51.3 24.3

15 ~ 9.3 7.4 11.0 5&5 I3.7

20" 7.6 4.2 9.0 51.4 27.9

25 ~' 3.4 1.6 4.7 52.8 37.6

30 ~, 5.0 2.0 4.2 44.8 43.9

Samples taken from: "the seagrass bed; h the edge of the seagrass bed: ~the surrounding sand.

Samples were collected in straight-line transects perpendicular to the edge of the grassbeds at 5-m intervals. The fact that the sediments were predomi- nantty composed of medium-to-coarse-grained carbonate (see Table I) made it impractical to collect pore-water by coring and squeezing sections of core. Instead an in situ harpoon sampler (dubbed the 'sand sucker') was used to extract pore water samples. This device and its operation were previously described by Morse et al. (1985). One limitation of this mode of sampling is that detailed pore water depth profiles are not possible. A uniform sampling depth of ~ 40 cm was used in this study. It was chosen because it generally coincided with the depth at which the most dense seagrass root zones ended, and there was significant decrease in the rate of pH change with depth (Fig, 2). The distribution of pH values with depth in the sediment was determined by taking

75

pH (Hansson Scale)

7.0 7.2 7.4 7.6 7.8 8.0 8.2

.ES l_--,s, 2O wc__~ / - I \ I

£ ~1~ ~ Sample

o , i . .o , . 40 ~1 S (g) " ~

50

60

Fig. 2. pH values of sediments determined by the 'punch-in' electrode method on cores from Whale Cay (WC) and Little Harbor Cay Site II (LHC II): (g) from the seagrass bed; (e) from the edge of the seagrass bed; (s) from the surrounding sand.

a core and measuring pH by punching an electrode into the sediment through holes in the side of the core barrel.

Analytic methods

Sediment grain size distribution was determined by wet sieving. This was not done in great detail, but only to establish whether finer size fractions were significantly enriched in the seagrass bed sediments, pH was measured on the Hansson (1973) scale. Total carbon dioxide and ti tration alkalinity (AT) of pore waters were measured by computer controlled titration. The partial pressure of CO2 (Pco2) and the saturation state of the pore waters with respect to aragonite were calculated as described by Millero (1979), using the solubility for ar- agonite of Mucci (1984). Sulfate concentration was measured using a Dionex ion chromatograph.

Ammonium and phosphate (total and reactive) were determined by the methods described by Grasshoff (1976). A GCA McPherson EU-700 spectro- photometer with 10 cm cells was used in the ammonia and phosphate analyses. Solid associated phosphate was determined by the total phosphate method after acid digestion of the sediment. The lower limit for phosphate detection with this system is 0.05 #M. Dissolved organic carbon was measured as CO2 by

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10 Sand ~, P ~ Grass

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lO

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( m e q / k g )

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8.0

p H 76

7.2

30 20 10 0

T r a n s e c t D i s t a n c e (m)

Fig. 3. Pore water analytic results for the transect at Whale Cay.

infrared analysis after the removal of inorganic CO2 and photo-oxidation of the sample as described by Collins and Williams (1977).

Adsorption-desorption experiment

In an effort to understand the interaction of inorganic phosphate with the surface of the sediment, a simple adsorption-desorption experiment was con- ducted: 50 g of solid sediment from the Whale Cay seagrass bed, collected at a depth of 32-37 cm below the sediment-water interface, was added to 1.51 of seawater that had been spiked with NaH2PO 4 to produce a phosphate con- centration of 85 #M. The uptake of phosphate by the sediment was followed with time. After < 0.5 gM phosphate remained in solution the sediment was allowed to settle to the bottom of the reaction vessel then, with rapid stirring, enough acid was added to the solution to dissolve 5% of the calcium carbonate. The concentration of dissolved phosphate was then followed with time.

RESULTS

The results of the sediment grain size analyses are presented in Table I. All sediments are dominated by the sand size fraction, with at least 70 wt % of the sediment being in the > 250 #m size fraction. The fine fraction (defined here as < 52 #m) was always < 13 wt. %. Scoffin (1970) pointed out that seagrass beds

78

. . . . q C " . /

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(10 ~ a t m / "-e ~ - . - ~ . . . _ . /

( m e q / k g ) / - /

d

i

T r a n s e c t D i s t a n c e i m l

Fig. 4. Pore water analytic results for the transect at Little Harbor Cay Site I.

can act as sediment traps by baffling near-bottom currents. This effect should be most dramatic in the fine fraction of the sediment. An examination of the data from the three grass beds investigated in this study indicates that there is about three times as much fine material in the grassbed sediments as in the surrounding barren sediment at the Whale Cay Site, twice as much as at Little Harbor Cay Site II and a similar amount to that at Little Harbor Cay Site I. Although it is not possible to make a strong generalization from only three study sites, it appears that the sediment trapping capacity of the seagrass beds studied decreases markedly in more hydrodynamically energetic environ- ments. This possibility needs to be more systematically investigated to esta- blish its universality and to quantify the relation.

The analytical data for the sediment pore waters are given in Table II. They are presented graphically for the three study sites in Figs. 3, 4 and 5. No direct evidence for sulfate reduction was found in any of the pore waters. The results are close to those found by Morse et al. (1985) for similar environments in the Bahamas.

At the Whale Cay site, ammonium exhibited the only major differences in concentration between the sand and grassbed samples. A slight rise inpco2 and alkalinity and a decrease in pH was found for the sample taken the furthest

79

015 ~ Sand = I ~ Grass ~ I ~ Sand

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lo t I

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o I

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74 I I I 0 10 20 30

Transec t D is tance (rn)

Fig. 5. Pore wa te r ana ly t i c r e su l t s for t he t r a n s e c t a t Li t t le Ha rbo r Cay Site II.

(15 m) into the grassbed. Reactive phosphate was below our detection limit of 0.05 ~M in all pore waters. A general observation at this site was the lack of a major impact by the seagrass on pore water chemistry.

Results from the first Little Harbor Cay study site (Fig. 4) indicate a stronger influence of the seagrass bed on pore water chemistry. Major elevations of Pco2 and alkalinity occurred in the sediments under the seagrass bed, while pH decreased by ~0.5. Unlike the Whale Cay site, ammonium concentration decreased in the grassbed sediments relative to the surrounding barren sand. At this site phosphate was detectable, and both reactive and total phosphate were determined. The reactive phosphate concentration decreased in the sea- grass bed sediments, while organic phosphate, calculated as the difference between total and reactive phosphate, was markedly increased. All phosphate concentrations were low, with total phosphate never exceeding 0.6 #M. Solid- associated phosphate concentrations were quite variable, and averaged 6.5 and 5.4 nmol g- 1 in the seagrass covered and barren sediments respectively. Dissol- ved organic carbon was also higher in the grassbed pore waters and roughly paralleled organic phosphate.

At the second Little Harbor Cay site (Fig. 5) it was possible to make a transect completely through the grassbed. Reactive phosphate was detectable but at a low concentration which had a range of only 0.10~.13~M. No signifi-

TABLE lII

Results of the phosphate adsorption-desorption experiment

Adsorption Desorption

Time PO~ Phosphate Time PO 4 Phosphate (min) (gM) adsorbed (rain} (/~M) desorbed

(%) (%)

Initial 85 0 Initial < 0.5 0 1 58 32 10 24 28 5 45 47 80 20 24

10 32 62 380 1.2 1 30 12 86 1500 < 0.5 (I

100 7 92 360 < 0.5 100

can t d i f ferences in c o n c e n t r a t i o n be tween the ba r r en sand and grassbed sedi- ments were observed. A m m o n i u m was ex t remely va r i ab le and exhib i ted no dis t inc t c o n c e n t r a t i o n pa t t e rn . This may have been the resu l t of the h igh in fauna l ac t iv i ty no ted a t this site. Pco2 and a lka l in i ty were e leva ted in the g rassbed sediments , while pH was m a r k e d l y depressed.

Resul ts of the expe r imen t conduc ted to de te rmine the abi l i ty of the car- bona te sed iments to adsorb p h o s p h a t e and the abi l i ty of mode ra t e acid t reat - men t to cause p h o s p h a t e desorp t ion are p resen ted in Table III. The two ma jo r object ives of th is expe r imen t were: (1) to ver i fy t h a t these sed iments could r emove subs t an t i a l a m o u n t s of p h o s p h a t e f rom s e a w a t e r to an ex ten t t ha t would resu l t in low concen t ra t ions ; and (2) to see if acid, such as t h a t gene ra t ed by seagrass roots , could cause re lease of phospha te . The resu l t s conf i rmed t h a t adsorp t ion of p h o s p h a t e by the c a r b o n a t e sed iment could be the r ea son for the low p h o s p h a t e c o n c e n t r a t i o n s observed in the sed iment pore waters . These resu l t s are in a g r e e m e n t wi th the more deta i led s tudy (de Kane l and Morse, 1979) of p h o s p h a t e i n t e r ac t i on wi th c a r b o n a t e mine ra l surfaces . The experi- men t also ind ica ted t h a t acid a t t a c k on the c a r b o n a t e sed iment sur faces could cause the re lease of phospha te , bu t t h a t it would subsequen t ly rap id ly read- sorb.

DISCUSSION

The inf luence of the th ree seag ras s beds s tudied on c a r b o n a t e sed iments was re la t ive ly small . Whi le f ine-grained m a t e r i a l was more a b u n d a n t in two of the th ree seagrass beds, the change was only a few percent , and the sed imen t s f rom all of the si tes were domina t ed by the > 250 ~m size f ract ion. The mos t consis- t en t inf luences of the seagrass beds on pore -wate r chemis t ry were assoc ia ted wi th the ca rbonic acid system. Pco2 and to ta l a lka l in i ty were e leva ted in the seagrass beds whi le the pH va lues were subs t an t i a l l y lower. This is p robab ly

81

the result of carbon dioxide production in the root zone and subsequent dissolu- tion of carbonate sediment. The fact that even in the seagrass beds the sedi- ments must be relatively well irrigated by infaunal organisms is reflected by the lack of any detectable sulfate reduction.

The influences of seagrass beds on pore-water nutrient chemistry were variable. The three possible different influences of seagrass beds on dissolved ammonia concentrations--increase, decrease and no change were observed. The first Little Harbor Cay site was the only one that had a marked variability in dissolved phosphate concentrations. Reactive phosphate decreased, while organic phosphorus increased in the grassbed sediments. Solid-associated pho- sphorus exhibited considerable variability but no substantial average dif- ference was found between the seagrass and barren sediments.

Our observations of relative minima in ammonium concentrations within seagrass beds at two of the three sites sampled in the Bahamas are contrary to observations of increased ammonium concentrations in temperate seagrass beds. Regeneration of sediment organic material provides ammonium used in growth of Zostera marina in high latitude clastic sediments (Iizumi et al., 1982; Short, 1983). The sediments sampled in this investigation were sulfide-free, which suggests that organic carbon regeneration rates were very low, or, more probably, exchange with overlying water was rapid. The variability in sedi- ment ammonium concentration between sediments with and without vegeta- tion is not surprising, since pore-water ammonium concentration represents a balance between the regenerative input of nitrogen and plant ammonium uptake. However, relative ammonium minima within seagrass beds are unex- pected based on the temperate seagrass bed pattern and may represent an imbalance between uptake and regeneration rates within seagrass beds on carbonate sediments. The pore-water alkalinity change between vegetated and unvegetated areas was less for the Whale Cay seagrass bed, which exhibited increased sediment-ammonium concentration within the bed (Fig. 3), suggest- ing that metabolic activity was lower within the Whale Cay site than the Little Harbor Cay site (Fig. 4 and 5).

A hypothesis has been proposed that phosphorus availability limits organic carbon production by benthic plants in an enclosed sal twater basin in Austra- lia (Smith and Atkinson, 1984) and in an open site in the Bahamas Islands (Short et al., 1985). We observed reactive phosphate concentrations of the order of 0.02 pM in pore water samples collected within the seagrass bed at the two Little Harbor Cay sites. While we cannot determine whether the low observed phosphate concentration was caused by plant withdrawal of phbsphate from sediment pore waters or by a reduction of the dissolved-reactive-phosphate concentration in pore water by carbonate surface-adsorption processes, it is clear that carbonate sediment pore water is unlikely to be a significant dissol- ved-molybdate-reactive phosphate pool. The strong adsorptive affinity of phos- phate for calcium carbonate indicates that it is very unlikely that carbonate sediments can serve as a readily available phosphorus reservoir, in contrast to the hypothesis of Patriquin (1972). Dissolved organic phosphorus concentra-

~2

tions were significantly greater within some of the seagrass bed sediment pore waters at the Little Harbor Cay Site I (Fig. 4), but there was no clear difference in total dissolved-organic-carbon concentrat ion in pore waters obtained from sediment samples taken across the seagrass bed.

The dissolved reactive phosphate and dissolved ammonium concentrat ions observed in pore waters obtained from both the Whale Cay and the Little Harbor Cay sites were an order of magnitude lower than the concentrat ions of both of those compounds reported for a site near San Salvador Island occupied by Syringodium filiforme (Short et al., 1985). Whether this represents a signifi- cant difference between the environments occupied by the two different sea- grass species, or is a consequence of random heterogeneity in sediment biogeochemical processes, is not clear. It is interesting to note that at the first Little Harbor Cay site, where the seagrasses had the least influence on sedi- ment grain size, the largest relative influence on pore water chemistry was observed. This may simply reflect the fact that the influence of Thalassia testudinum on the chemical properties of these coarse-grained sediments was relatively small.

CONCLUSIONS

The influence of seagrass beds dominated by Thatassia testudinum on coarse-grained calcium-carbonate-rich sediments in the Berry Islands of the Bahamas is distinctly observable, but substantially less than found in other environments, where more fine-grained sediments are generally present. Both chemical and sediment grain size variability between the seagrass beds and adjacent grass-free sediments appears to be influenced by the hydrodynamics of the overlying water. While the seagrasses can generate large amounts of organic matter, little is retained in coarse-grained carbonate sediments. This is probably due to rapid oxidation and exchange of pore water with overlying water as a result of bioturbation and the energetic hydrodynamic environ- ments occupied by these sediments. Our results bring into question the signifi- cance of seagrass beds in common carbonate environments as sources of sedimentary organic matter and the diagenesis of these sediments.

ACKNOWLEDGMENTS

This research was supported by the Marine Chemistry Program of the Ocean Sciences Section of the National Science Foundat ion (Grants OCE79-19242 and OCE82-41947, J.W. Morse, P.I.; OCE81-20659, F.J. Millero, P.I.) and the Nation- al Sciences and Engineering Research Council of Canada (Grant A2647, A. Mucci, P.I.).

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