organic matter, nutrients and major ions in the sediments of coral reefs...

Indian Journal of Marine Sciences Vol. 28, December 1999, pp. 383-393

Organic matter, nutrients and major ions in the sediments of coral reefs and seagrass beds of Gulf of Mannar biosphere reserve,

southeast coast of India

N V Vinithkurnar, S Kurnaresan, M Manjusha & T Balasubrarnanian*

Centre of Advanced Study in Marine Biology, Annamalai University, Parangipettai-608 502, Tamil Nadu , India

Received 30 March 1998, revised 30 September 1999

Comparative investigations have been made to study the distribution pattern of organic matter, nutrients and major ions in the Gulf of Mannar biosphere reserve ecosystem. Sediment samples have been collected during January 1996 from six islands (Shingle, Kurusadai, Kori, Pumarichan, Manauli and Hare) of this region covering coral reef zone, seagrass bed and adjacent areas. Although textural composition of sediment samples showed predominantly sandy, coral bed areas were found to have greater percentage of coral stones and shell fragments and with very low silt and clay fractions. About 0.5 to 40 % by dry weight of the sediments were made up of Ca derived from the coral and molluscan organisms, due to weathering processes. The coral reef sediments recorded low salinity and higher pH compared to seagrass bed and adjacent areas. Total organic carbon (TOC) content (1.38 to 9.11 mg/g) of the reef sediments were low when compared to the seagrass bed and adjacent areas. Higher concentrations of total nitrogen and total phosphorus were found in the coral reef zone and seagrass bed revealed that these areas may be viewed as a sink for nutrients, whereas the adjacent areas act as reservoir for nutrients. The contribution of seagrass and reef associated organisms play an important role in the recycling of nutrients in these environments. The seagrass bed sediments recorded higher Na and K concentrations than the coral reef zone and adjacent areas could be due to the utilization and trapping of these ions by seagrass and associated organisms.

Coral reefs are unique for tropical environment and are mostly associated with seagrass beds and mangroves with a perfect interaction between them. Although they are oligotrophic in nature, production of organic matter and turnover of nutrients results in high productivity and biodiversity. Especially the contribution from sediment seems to play a greater role in the recycling and regeneration of organic matter and nutrients. Gulf of Mannar (GOM) is an established "National Marine Park" harbouring rich corals with diverse fauna and flora. However, baseline data on water and sediment quality and their related biological processes are very meagre for this distinct biotope. Studies on primary production, and secondary production in relation to hydrography are available as a scant report for Gulf of Mannar, nearshore waters of the Palk Bay and area around Tuticorin l

-6

. An attempt is being made in the present study by collecting sediments from a depth range of 0.25 to 4 m at 57 sampling sites covering different strategic locations such as coral reef zone, seagrass beds and adjacent areas of Gulf of Mannar.

*For correspondence

Materials and Methods The sampling stations were located in and around

six major islands namely Shingle, Kurusadai, Kori , Pumarichan, Manauli and Hare (Fig. I) . The coral reef zones, seagrass beds and adjacent areas are located alternatively and adjacently and the stations 8, to, 15,16,18,19,23,24,25,26,37,43,46,47,50, 51, 52, 53, 54 are coral reef zones, the stations I, 2, 3, 4, 12, 13, 14, 17,20,21 , 22, 34, 35, 36, 39,40,42, 48,49 are seagrass beds and the stations 5, 6, 7, 9, II , 27,28,29,30,31,32,33,38,41,44,45,55,56,57 are adjacent coastal areas.

Sediment samples were collected once during January 1996, using a Peterson Grab, transferred to clean polybags, transported to the laboratory and air dried. Duplicate samples were well mixed and subsampled. Duplicate analysis of textural composition, pH and salinity were made from the air dried sediment samples, whereas rest of the analyses were made by using standardized procedures, after grounding the air dried sediment samples to fine powder and oven drying at 1 ID°e. Average values calculated were processed statistically. As the sediments were predominantly sandy in nature, dry

384 INDIAN 1. MAR SCI., VOL. 28, DECEMBER 1999

79 5 E 79° 15'

20'

PALK BAY

Ma~pam~.~~~~

31 44 Island 30 32 45

33 29 34 39 6 9°

28 57 IA),Q 35 3~ 47 48

7

S

6 27 S6 '?~~tI~Jl 5~ 1S

26 S5 S3 ®W. 52 49D

23 2425

20~~ 19 ®

18

GULF OF MANNAR

1-57. Sampling stations A. Pumarichan island B. Kori island C. Kurusadai island D. shingle island E. Manauli island F. Hare island

Fig. I-Sampling stations around six islands in Gulf of Mannar

sit~ving was made, using a Ro-Tap mechanical sieve shaker for 15 minutes7

. The different size fractions such as stone and gravels (>2 mm), sand (>0.025 mm), coarse silt (>0.031 mm), medium, fine silt and clay «0.031 mm) obtained were weighed and expressed as percentage basis. Salinity and pH of the sediment samples were measured by employing a calibrated water analysis kit (Century-CTK 711) after the suitable dilution with waters. Total organic carbon (TOC) was estimated by chromic acid digestion method9

• The total nitrogen (TN) was determined by adopting a slightly modified method developed by Libby & Wheeler lO

• The sediment samples were digested with acid persulphate, filtered and the filtrate was used subsequently following the cadmium reduction method" for the TN determination. The

estimation of total phosphorus (TP) was made from oven dried samples extracted'2 with IN HCI and then digested with potassium persulphate. The supernatant was filtered (Whatman GF/C filters) and analysed by following molybdate ascorbic acid method' ~ . The calculated values of TN and TP were expressed in J1 mol/g. For the eitimation of sodium, potassium and calcium, samples were digested with HCI and HNOr HCI04 mixture'4 at 300°C. Few drops of HF acid was added for complete digestion . The filtered digests were aspirated in digital flame photometer (Model No. CL 22D, Elico Pvt. Ltd., India), analysed and expressed in mg/g.

Results and Discussion In general, the present study area ,s shallow and

VINITHKUMAR et al.: GULF OF MANNAR SEDIMENT CONTENTS 385

Stations Adj 5 6 7 9 11 27 28 29 30 31 32 33 38 41 44 45 55 56 57 Sgr 1 2 3 4 12 13 14 17 20 21 22 34 35 36 39·40 42 48 49 Cor 8 10 15 16 18 19 23 24 25 26 37 43 46 47 50 51 52 53 54

0 . 0.5

'l 'l 'l 'l 'l 'l '~ '~

I 1.5

2 :5

2.5 a. al 0 3

3.5

4 OCoral reef zone (Cor) .Seagrass beds (Sgr)

4.5 8 AdJacent areas (AdJ)

Fig. 2-Depth of water column in sampling stations (in meters)

especially the areas lying nearer to the mainland and islands were very shallow (Fig. 2). The ranges in depth of water column recorded in the coral reef zones, seagrass beds and adjacent areas were 0.25 to 3 m, 0.75 to 3 m and 1.2 to 4 m respectively. Adjacent areas were found to be slightly deeper than the coral and seagrass beds. Both coral reef zone and seagrass meadows were mostly found in the shallow areas around the islands and mainland.

The sediment fraction was found to be mostly composed of sand in all the 57 stations with a mean value of 87.55 ± 11.71 % (Fig. 3). The mean stone and gravel fractions of sediment was 3.60 ± 3.59%, coarse silt content was 6.23 ± 8.64% and the mean medium, fine silt with clay content of the sediment was 1.75 ± 3.96% (Table 1). Coral reef and seagrass bed sediments recorded higher percentage of stone and gravel and sand fractions due to relatively shallow depth in the island margins and washing away of finer sediment fractions by wave action. The stone and gravel and sand fractions mostly represent the dead remains of coral and molluscan shells with higher percentage of calcium. Relatively higher percentage of silt and clay fractions due to settling of these finer particles in the absence of strong waves was recorded in the slightly deeper adjacent area sediments. The sediments of coral reef zone stations 43 and 46 were having higher percentage of stones and gravel fractions. Both stations are subjected to high water current. In general, stations located in the southern side of islands were mostly composed of sand, as they were influenced by higher wave action from open sea.

The sediment salinity values (Fig. 4) fluctuated between a minimum of 9 and a maximum of 42 %0

with a mean value of 31.21±7.64%0. The level of salinity concentrations were in the order of seagrass bed> adjacent area> coral reef zone. Low salinity value was recorded from the co'ral reef and highest value was recorded from the seagrass beds (Table I). The sediments showed a lower salinity at sts., 30, 31 and 43 in the absence of freshwater input. The sediment particles of coral reef zone had higher amount of coral stone and gravel fraction with coarser sand and thus having less salt retention capacity when compared to the fine textured sediment of seagrass bed area, which can retain more salt as reported earlierl5

• 16. The high metabolic activity of flora and fauna could also contribute to higher percentage of salt in the seagrass bed areas I?

The pH of sediment samples (Fig. 4) ranged between 7.85 (st.!) and 8.71 (st. 36) with a mean value of 8.27 ± 0.15 . The levels of pH was found to be higher in coral reef zone when compared to other areas and were in the order of coral reef zone>seagrass bed>adjacent areas (Table I). This may be due to large amount of calcium carbonate present in the coral sediments. The pH of sediments at seagrass beds sts. 1-3 exhibit lower vall?es might be due to lower calcium concentrations in the sediment. The deposition of inorganic carbon as calcium carbonate was reported to shift the pH of water column by 0.15 units in Shark Bay of Western Australia l8

. This could be true in the case of sediments in Gulf of Mannar, during the present study.

Organic matter and nutrients

The total organic carbon (TOC) content of


• Stones and gravels B Sand 0 Coarse silt II Medium, Fine silt and clay

60% .

60%

40%

20%

6 10 15 16 16 19 23 24 25 26 37 43 46 47 50 51 52 53 54 Coral reef zone stations

60% CD

g' 60% "E ~ 40% CD a..

20%

2 3 4 12 13 14 17 20 21 22 34 35 36 39· 40 42 48 49

Seagrass bed stations

5 6 7 9 11 27 26 29 30 31 32 33 38 41 44 45 55 56 57

Adjacent area stations

Fig. 3-Textural composition of sediment in coral reef zones (A), seagrass beds (B) and adjacent areas (t)

sediment samples varied between 1.38 (st. 37) to 9.11 mglg (st. 38) with a mean value of 3.61 ± l.46 mglg (Fig. 5). TOe content between three biotopes exhibited wide fluctuations in their concentration and this has also been evidenced by the ANOV A (F=4.96; P<O.05; F crit=3.17). The seagrass beds recorded higher TOe concentration, when compared to coral reef stations and the concentrations were decreased in the order of seagrass bed>adjacent area>coral reef zone (Table 1). This may be due to contribution from high benthic biodiversity, besides death and decay of seagrass resulting higher release of organic materials and nutrients, which are retained according to the

nature of sediment texture. Detritus formation and release during death and decay from the macrophytes have been viewed as a major source of TOe l2 for the reef areas adjacent to grass meadows. Further, aggregation of migrating fishes , crustaceans and molluscs and their excretory products could also seem to serve as another source of Toe in the reef areas, as the Gulf of Mannar has rich representation of these organisms I9

-21

. The adjacent area (st. 38) and seagrass bed (st. 39) located in lower depth near the Kurusadai island showed highest concentration of TOe. This is mainly because of the sediment texture, as the sediment of these stations contain higher percentage

J.


Table I-Textural composition, minimum and maximum values observed in sedimentological parameters of coral reefs. sea grass beds and adjacent areas (mean and standard deviation in parentheses)

SI. no. Parameters All 57 stations

Textural composition (%)

(Mean values)

a) Stones and gravel (> 2 mm) 3.60 ± 3.59

b) Sand (> 0.025 mm) 87.55 ± 11.71

c) Coarse silt (> 0.31 mm) 6.23 ±8.64

d) Medium, fine silt and Clay 1.75 ± 3.96

«0.031 mm)

2 Depth (m) 0.25-4.0

3 Salinity (%0) 9-42 •

(31.21 ±7.64)

4 pH 7.85-8 .71

(8.27 ± 0.15)

5 Total organic carbon (TOC) 1.38-9.11

(mg/g) (3.61 ± 1.46)

6 Total nitrogen (TN) 19.5-266

(J1 mol/g) (139.47 ± 63.07)

7 Total phosphorus (TP) 3.66-23.64

(Ji mol/g) (7.23 ± 5.22)

8 C:N ratio 0.14:1-4.11:1

9 C:P ratio 1.20:1-18:1

10 N:P ratio 2: 1-20:1

II Sodium (Na) 13.75-43.76

(mg/g) (24.75 ± 5.30)

12 Potassium (K) 2.5-27

(mg/g) (l2±5.24)

13 Calcium (Ca) 57-400

(mg/g) (180±79.90)

of finer silt and clay fractions. Further, water with higher silt, clay content and organic matter derived from the Palk Bay to Gulf of Mannar, through Pamban pass opens in this region, leading to the settlement of particles and resulting higher TOe. The correlation coefficient values of TOe showed significant positive correlation between salinity (r=0.57; P<O.OI) in the seagrass bed, whereas the coral reef zone and adjacent areas did not show significant relationship between . these parameters, suggesting a positive influence and the interaction of biological activities and salinity.

Total nitrogen (TN) concentration in the sediments showed wide fluctuation and ranged from 19.5 (st. 40) to 266 J..l mol/g (st. 22) with a mean value of 139.47 ± 63.07 J..l mollg (Fig. 5). The concentrations

Coral reef zone Seagrass beds Adjacent areas

5.69 ±4.33 2. 36±3.18 2.74 ± 2.02

90.70 ± 40.29 89.88 ± 10.34 82.10±15.22

2.40±3.17 5.69 ± 8.88 10.60 ± 10.35

0.41 ±0.69 1.17 ± 1.49 3.65 ± 6.35

0.25-3 .0 0.75-3 .0 1.2-4.0

9-36 18-42 9-42

(28.58 ± 6.58) (33.15 ± 6.96) (31 .89 ± 8.84)

8.08-8.48 7.85-8.06 8.06-8.53

(8.29 ± 0.12) (8.27 ± 0.21 ) (8.25 ± 0.12)

1.38-4.35 1.86-6.83 1.86-9.11

(2.82 ± 0.76) (4.14± 1.47) (3.88 ± 1.69)

38.56-256.3 19.5-226.5 27.5-195.35

(157 .5±53.41) (148.59 ± 77.08) (112 .27 ± 48 .(6)

3.66-18.68 3.99-23.64 4.09-14.47

(11.48 ± 4.33) (11.38±6.53) (6.55 ± 2.53)

0.14:1-0.95:1 0.15 :1-4. 11 : I 0.18:1-3.12: 1

(0.34: 1 ± 0.20: I) (0.8 1:1 ± 1.14;1) (0.76 ± 0.66 : I)

1.59: 1-7.94: I 1.20:1-18:1 1.85: 1-13.68: I

(3:1 ± 1.66:1) (5.79 ± 4.65: I) (6.67: I ± 3.39 : I)

6: 1-13.1 2: 1-18: I 4: 1-20: I

(9:1 ± 2:1) (8 :1 ±3:1) (II.I ±4: I)

17.25-43.75 17.75-31.5 13.75-33 .25

(25 .79 ± 5.98) (25.62 ± 3.76) (24.08 ± 5.98)

2.5-18 6.25-27 6.5-21

(7.92 ± 4.28) (12.97 ± 5.47) (14.41 ±3 .58)

78-400 57-280 60± 260

(232.32 ± 78.13) (164.89 ± 66.36) (127.37±58.17)

of TN decreased in the order of coral reef zone>seagrass beds>adjacent areas (Table I). Even though seagrass bed stands in the second order, the sts. 39, 40 and 42 were found to be lowest in TN concentrations. This may be due to utilization of nitrogenous compounds and lower nitrogen fixation , particularly in this stations . In general, the major source of nitrogen in sediment was attributed to the nitrogen fixation by benthic algal communities, which are strongly influencing the nitrogen budget of coral reefs22

• 23. It was also reported that transformation and fixation of nitrogen and effective utilization of phosphorus occurs efficiently in algal as well as in seagrass bed areas24

. Further, influence of plant roots on nitrification in anaerobic sediments was also evidenced in the seagrass beds25

. It was also pointed out that nitrate would have produced by bacterial


45 o Coral reef zone (Cor) • Sea grass beds (Sgr) 8 Adjacent areas (Adj)

A · 40

35

t 30

~ 25

:£ 20 ro 15 en

10 5 0 ~, .r

8.8 8 8.7 8.6 8.5 8.4

:z: 8.3 Q..

8.2 8.1

8 7.9 7.8 ~

Adj 5 Sgr 1 Cor 8

6 7 9 11 27 28 29 30 31 32 33 38 41 44 45 55 56 57 2 3 4 12 13 14 17 20 21 22 34 35 36 39 40 42 48 49 10 15 16 18 19 23 24 25 26 37 43 46 47 50 51 52 53 54

Stations

Fig. 4-Distribution of salinity (A) and pH (8)

nitrification in sediment using the available ammonia and oxygen released from mangrove roots or diffusion from sediment surface26. Similar role of seagrass and mangrove plants towards contribution to nitrogen species was reported earlier27

. Coral reef flats are known to export DON23.28.29. The released nitrogen, may be, permanently lost from the system and exchange among the systems could be the reasons for the changes observed in concentrations between these three biotopes. However, in the present study coral reef sediments were found to record higher concentrations of TN may be due to the following reasons. The coral reef zones were found to be shallower than the seagrass beds. The space and surface available in coral reefs for the nitrogen fixing organisms would be much higher than the seagrass beds. Also, the nitrogenous waste material produced in the coral reefs could be higher. The settlement and aggregation of organic matter through migratory fishes may contribute certain amount of TN21 . Thus in the present study, nitrogen concentrations observed in sediments of seagrass beds and coral reef zones would reflect the efficient exchange and cycling of nitrogen among these biotopes.

The TN showed negative correlation with salinity (r=-0.42; P<0.05) and TOC (r=-0.53; P<O.O I) only in the seagrass beds. Whereas the coral reef zones and adjacent areas did not show significant relationship between these parameters and this could be due to the utilization of salt minerals such as sodium, potassium and other organic compounds from sediments by the seagrass communities during their growth. This was also evidenced from the fact that seagrass beds recorded lower sodium concentrations than the coral reefs and is discussed latter.

The concentration of total phosphorus (TP) ranged from 3.66 (st. 22) to 23.64 (st. 43) f.1 mol/g, with a mean · value of 7.23±5.22 f.1 mol/g (Fig. 5) . TP concentrations of sediments exhibited a wide range among the three areas as evidenced from ANOV A (F=6.69; P<0.05 ; F crit=3.17) . The level of concentrations were decreased in the order of coral reef>seagrass bed>adjacent area (Table I). The fluctuation in phosphorus concentration noticed in the present study might be due to exchange between the components of reef bottom and water column·'o. Apart from this, the release from benthic fauna, which is

300

_ 250 g ~ 200 -

.3 a.. 150 I-"0 ffi 100

~ 50

300

- 250 J!.l ~ 200

.3 f= 150 "0

ffi 100 z I-

50

o

300

250 -g ~ 200

.3 a. 150 I-"0

f6 100

~ 50

o

VINITHKUMAR el al.: GULF OF MANNAR SEDIMENT CONTENTS

A. Coral reef zones f'1'.mlTotal Nitrogen (u mol/g)

_Total Phosphate (u mol/g) -0- Total Organic Carbon (mg/g)

8 10 15 16 18 19 23 24 25 26 37 43 46 47 50 51 52 ·53 54

Coral reef stations

B. Seagrass beds

2 3 4 12 13 14 17 20 21 22 34 35 36 39 40 42 48 49 Seagrass bed stations

C. Adjacent Areas

5 6 7 9 11 27 28 29 30 31 32 33 38 41 44 45 55 56 57 Adjacent area stations

10

9

8

7

6~ 55 4g 31-

2

1

o

10

9 8

7

6 ~ 5 5 4 g 3 I-

2 1

o

10

9

8

7 ....... 6J!.l

Ol

55 4g 3 I-

2

1

o

389

Fig. 5-Distribution of total organic carbon (mg/g), total nitrogen (J1 mol/g) and total phosphorus (J1 mol g) in coral reef zone (A),

seagrass beds (B), adjacent areas (C)


rich in P composition and the detritus formed from seagrass and other macrophytes would be the other major contributions of organic carbon and phosphorus in sediments. It was also reported that most of the phosphorus in sediments may be bound with shells and bones of invertebrate animals l8 and when the shells brake, the P releases back into water column or to sediment itself. In the present investigation also, sediments of coral reef zones were found to be composed of bones and shells of corals and molluscs, which traps the P as bound inorganic fraction and acts as a prominent source of 'P' in this area. Thus, the P immediately available to the corals may be lower. TP of coral reef zone exhibited· a significant positive correlation with TOC (r=0.53; P<O.Ol). Whereas in the seagrass bed, TP showed significant negative correlation with TOC (r=-0.46; P<0.02). However, no significant correlation was observed from the adjacent areas . This indicates that, P may be deposited with organic matter possibly with the shells and bones of coral s and molluscs 18.

Whereas in the seagrass beds, utilization of P from the sediment by seagrass community and subsequent release into the water would have resulted in the inverse relationship. The TP showed a highly significant (P<O.OOl) positive correlation with TN in all the stations of coral reef zones (r=0.76), seagrass beds (r=0.91) and adjacent area (r=0.71). This may represent the increased nitrogen fixing activity from increased availability of phosphate as earlier reported 31

• It was also reported that, increase in concentration of P would increase the growth rate of certain algae32, particularly those that could retain or fix sufficient amounts of N.

Interrelations between organic matter

The C:N ratio (Fig. 6) in sediments ranged between 0.14: 1 to 4.11: 1 and the seagrass bed sediment showed higher mean value than the coral reef zone and this may be due to lower concentration of organic matter as reported earlier from the Minicoy island lagoon27. The C:N ratios were decreasing from seagrass bed>adjacent area>coral reefs (Table 1) and were influenced more by the concentrations of organic carbon. The C:P ratios (Fig. 6) fluctuated between 1.20: 1 and 18: 1 and were in the decreasing order from adjacent areas>seagrass beds>coral reef zones. The lower mean C:P ratio observed in coral reef zones in the present study suggests that most of the P in sediment was present as inorganic P. Slightly higher mean value of C:P ratio observed in seagrass

beds could be due to the increase in organic matter with a better mineralization. The N:P ratio (Fig. 6) ranged from 2: 1 to 20: I. The N:P ratios were decreasing in the order of adjacent areas>coral reefs zones>seagrass beds (Table I) . It seemed that C:N and N:P ratios observed in sediment exhibited phosphorus limitation in this environment. It may be due to low nitrogen fixation and low utilization of phosphorus by benthic biotic components in the adjacent areas. Further, higher accumulation of phosphorus fractions in shells and JJones of animals may lead to the lower N:P as well as C:P ratios. Similar observation was also reported from the Shark Bay of Western Australia l 2

.

Major ions

Sodium (Na) concentrations (Fig. 7) varied between 13.75 at st. 7 to 43.76 mg/g at st. 50 with a mean value of 24.75 ± 5.30 mg/g. The potassium (K) concentration (Fig. 7) varied from 2.5 to 27 mg/g with a mean value of 12 ± 5.24 mg/g. Sodium content was decreasing in the order of coral reef zone>seagrass bed>adjacent area. Potassium content was decreasing in the order of adjacent area>seagrass bed>coral reef zone. The higher Na concentrations in st. 50 might have resulted from carbonate and calcium bound fractions of the sediment. Seagrass bed sediments recorded higher K levels, which may be due to the presence of K rich seagrass in these beds as recorded in the Minicoy island lagoons27. This was also evident in the present study by a significant positi ve correlation between K and TOC (r=0.55 ; p<o.o I) only iri seagrass bed.

Calcium (Ca) concentrations (Fig. 7) fluctuated between 57 (St.-3) to 400 (st. 8) mg/g with a mean value of 180 ± 79.90 mg/g. The Ca concentration was higher in coral reef zone and lower in adjacent areas and was present in the order of coral reef>seagrass bed>adjacent area (Table 1). This can be attributed to the fact that sediments of coral reef stations contain a higher percentage of coral fragments , which are mainly composed of calcium carbonate. Hi gher percentage (about 85-90 % dry weight of sediment) of calcium carbonate in the coral sediments was also reported for the Shark Bay sediments of Western Australia12

. The wide difference in concentrati ons of Ca in sediments was also reflected in the ANOV A, which showed significant variation among the three areas (F=I1.60; P<0.05; F crit=3.17). Calcium showed a significant positive correlation with TOC (r= 0.41; P < 0.05) in coral reef zone, whereas in

,.

.i..

VINITHKUMAR et at.: GULF OF MANNAR SEDIMENT CONTENTS 391

4.5 4

3.5 3

A o Coral reef zone (Cor)

• Seagrass beds (Sgr)

B Adjacent areas (AdD ,g e! 2.5 z 2 (j 1.5

1 0.5

0

20 B 18 16 14

0 12 ~ ... 10 a.. 8 (j 6 4 2 0

20 18

C

16 14

0 12 ~ 10 a.. :Z 8

6 4 2 0 ~

Adj 5 6 7 9 11 27 28 29 30 31 32 33 38 41 44 45 55 56 57 Sgr 1 2 3 4 12 13 14 17 20 21 22 3435 36 39 40 42 48 49 Cor 8 10 15 16 18 19 23 24 25 26 37 43 46 47 50 51 52 53 54

StaUons

Fig. 6-Distribution ofC:N (A), C:P (8), N:P (C) ratios

seagrass beds, it showed a significant negative correlation with TOC (r=0.45; P<0.02). This reveals the deposition of Ca with organic carbon in the coral reef stations. Moreover in the coral reef zones, Ca significantly and positively correlated with TP. Whereas, in seagrass beds and adjacent areas no such correlation was observed. This could be one of the evidence for the co-precipitation of CaC03 and CaP04 and trapping of P in the shells and bones of corals and associated organisms as bound inorganic fractions in the study area of Gulf of Mannar. Similar phenomenon was reported earlier for the Shark Bay sediments of Western Australia12

• There was a significant correlation observed between Ca and TN

(r=0.53; P<O.Ol) only in the coral reef zone, may be due to release of nitrogenous material by the coral reef organisms.

Overall, the present study revealed that sediments of coral reef contain low nutrients compared to the seagrass bed and adjacent areas. Therefore, it is speculated that, apart from the oceanic input, essential nutrients required by the coral bed associated organisms are supplied from nearby seagrass bed and adjacent areas. The seagrass beds with higher amount of silt in the sediment revealed the trapping ability of suspended organic matter from the water. With respect to the soil texture, the si ft mixed with clay and organic matter in the sediment of


45 40

35

:g; 30 0> 25

~ 20 z 15

10 5

A o Cora reef zone (Cor)

• Seagrass beds (Sgr) 8 Adjacent areas (Adj)

o ~~~.u~~~~EW~~~~EW~~W.~~~~~~~~ 30 B

25

-- 20 .g:

15 .s ~ 10

5

0 .r .r ~. .r .r I,~ .r .r .r ,~ ,~ 450 c 400 350

__ 300 C)

0,250 E -; 200 u 150

100

50

0 ~ ~ .r ~ ~ m.r ~ ~. m Adj 5 Sgr 1 Cor 8

6 7 9 11 27 28 29 30 31 32 3338 41 44 45 55 56 57 2 3 4 12 13 14 17 20 21 22 34 35 36 39 40 42 48 49 10 15 16 18 19 23 24 25 26 37 43 46 47 50 51 52 53 54

Stations

Fig. 7-Distribution major ions Na (A), K (B), Ca (C) concentrations (mg/g)

seagrass beds and adjacent areas may be acting as a reservoir for the essential nutrients required by coral

. reef and associated organisms.

Acknowledgement The authors are thankful to Ministry of

Environment and Forests (MoEn&F), Government of India, for sponsoring the project and to the Director, CAS in Marine Biology for extending necessary facilities to carryout the work.

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Insl Oceanogr, 7 (1974) 157.

4 Balasubramanian T & Wafar MY M, Mahasa f.(ar-Bull NaIL Insl Oceanogr, 8 (1975) 87.

5 Marichamy R & Pon Siraimeetan, J Mar Bioi Ass India, 21 (1979) 67.

6 Marichamy R, Gopinathan C P & Pon Siraimeetan , J Mar Bio Ass India , 27 (I985) 129.

7 Folk R L, Sedimentology, 6 ( 1966) 73.

8 Jackson M L, Soil chemical analysis, (Prentice Hall of Indi a, Pvt Ltd, New Delhi), 1967, pp. 583.

9 Wakeel S K & Riley 1 P, J du COllseil22 ( 1957) 180. 10 Libby P S & Wheeler P A, Mar Cltem, 48 ( 1994) 31 .

II Strickland J D H & Parson T R, Bull Fish Res Bd Canada, 167 (1972).

12 Atkinson M J, Estuar Coast Shelf Sci, 24 ( 19117) 335.

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organic matter, nutrients and major ions in the sediments of coral reefs...

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