Changes in the macrobenthos community of a sand flat after erosion

Estuarine, Coastal and Shelf Science (1995) 40, 21-33

C h a n g e s in t h e M a c r o b e n t h o s C o m m u n i t y of a Sand Flat After Erosion

B e s s i e O n g and Sekaran K r i s h n a n Centre for Marine and Coastal Studies, Universid Sains Malaysia, 11800 Penang, Malaysia Received 22 February 1993 and in revised form 14 ffanuary 1994

Keywords: macrobenthos; erosion; sand flat Following a severe erosion of the sand fiat at the Telok Aling beach, Penang, the macrobenthos composition changed from being dominated"by the gastropod Umbonium vestiarium Lamarck to a predominantly polychaete/bivalve community. Species richness increased to a total of 47 species per 0-25 m 2 compared to eight pre-erosion, but the mean population density decreased 10 times to 158-330 individuals per 0-25m 2. All the ambient species, except Echinodiscus bisperforatus Leske, were replaced numerically by the polychaete Diopatra neapolitana Delle Chiaje, Prionospio malmgreni Clarap~de and the bivalve Tivela sp. Recovery of U. vestiariurn began in May 1989 and attained relatively high densities 6 months later, i.e. about 2 years after their disappearance from the beach.

Introduction Community structure of marine soft-bottom habitats can be shaped by natural physical and biological disturbances which affect the substratum stability. For instance, storminduced waves have caused erosion of sand bottoms and changes in the abundance and composition of the dominant organisms CYeo & Risk, 1979). Reworking of sediments by deposit feeders has resulted in an unstable bottom and water turbulence which is unsuitable for suspension feeders (Eagle, 1975; Brenchley, 1981; Probert, 1984; Brey, 1991), while tube-building polychaetes stabilized the sediment and offered refuges to others (Fager, 1964; Young & Rhoads, 1971; Woodin, 1981). In the intertidal sandy beach, community structure is better correlated with the overall morphodynamic state, or the way in which wave energy is dissipated over the entire beach face, and less with biological interactions (Brown & McLachlan, 1990). In general, species density, diversity and number of species increase as exposure to wave action declines (Dexter, 1992). Seasonal changes of intertidal sand fauna in the tropics are often associated with the monsoons which cause severe erosion of the substratum and reduced salinity from torrential rainfall (Ansell et al., 1972; McLusky et al., 1975). The purpose of this study was to describe the effect of a tropical storm, which caused excessive freshwater runoff and erosion of the sand flat, on the macrobenthos composition, density and zonation, and to examine fluctuations in community structure over a 22-month period. 0272-7714/95/010021 + 13 $08.00/0

© 1995 Academic Press Limited

B. Ong & S. Krishnan

22

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Figure I. Map showing Telok Aling beach. Inset showing Penang Island.

Materials and method

Site T h e study area, T e l o k Aling, P e n a n g (100°1 I ' E , 5028'N, F i g u r e 1), consists o f a small stretch of sand flat b a c k e d by a hill (average slope of 30 °) of Dipterocarpus forest. T h e r e is no freshwater input except t h r o u g h seepage a n d surface runoff. T h e tidal cycle here is semi-diurnal with extreme ranges o f 0 . 2 - 2 . 9 m above chart d a t u m . T h e sandy d o w n s h o r e flats, which are the focus of this study, e x t e n d over 60 m from 1.5 to 0-2 m above chart d a t u m . T h e m a c r o b e n t h o s c o m m u n i t y consisted o f eight species, namely the g a s t r o p o d s Natica maculosa L a m a r c k , Natica antionii PhiUippi, Nassarius stotatus G m e l i n , Polinices pyriformis Recluz, Polinices didyma Bolton, Umbonium vestiarium L a m a r c k , and the e c h i n o d e r m s Astropecten vappa M u e l l e r a n d T r e s c h e l a n d Echinodiscus bisperforatus; there were virtually no bivalves or polychaetes (Berry, 1984). Umbonium vestiarium c o m p r i s e d m o r e than 99% o f the total n u m b e r s a n d dry tissue o f the benthic fauna. A l t h o u g h annual fluctuations in the n u m b e r o f Umbonium averaging from over 6000 to m o r e t h a n I 0 000 p e r m 2 have b e e n r e c o r d e d (Berry & Z a m r i 1983; Berry, 1984, 1987), these eight species c o n t i n u e d to d o m i n a t e the b e a c h up to 1987, after which w o r m tubes b e c a m e visible on the flat a n d this s t u d y was initiated.

Sampling F o u r transect lines (A, B, C, D) a b o u t 50 m apart, were established along the b e a c h spanning 1 . S m ( M H W N ) to 0 - 2 m ( E L W S ) above chart d a t u m . Five s a m p l i n g stations were m a r k e d out for each transect, n a m e l y station 1 b e i n g 15 m f r o m the

Changes in a macrobenthos community

23

level and others at 10-m intervals up to the ELWS level. Stations 4 and 5 (45 and 55 m from ~ were exposed during extreme low tides, while station 1 was submerged only during high tides. Stations 2 and 3, however, were not entirely drained as a little seawater was retained during ebb tide by a slightly raised sand bar at the lower stations. Sampling was done between 09.00 and 11.00 h during every low water spring tide over a period of 22 months. Samples were collected with a hand-held corer with 'a diameter of 10 cm and length 30 cm. A long handle was soldered to the corer so that sampling was possible when the beach was slightly submerged. The corer was inserted to a depth of about 30 cm, slowly slanted, then lifted with the sand into a sieving box with mesh size of 1 mm. Sieving was done on site and the sediment retained was packed with polyethylene bags, preserved with 5% formalin and stained with Rose Bengal. Subsequent sorting and identification was done in the laboratory under the microscope. At each station four cores were collected and pooled, giving a total area sampled of 0.0314 m 2.

Sediment analysis A separate core of sediment was collected for particle size analysis by dry sieving (Holme & McIntyre, 1971). N o treatment with acid to dissolve calcium carbonate was done, but sediment was pre-treated with 6% hydrogen peroxide (H202) to oxidize organic matter. Complete oxidation was ensured by boiling the sediment, previously soaked overnight in H202, until no more reaction had occurred, adding more HzO 2 during heating if necessary. The sediment was dried in a hot-air oven at 100 *C and the loss in weight equated to organic content. Analysis of sediment was done for the first sampling month only. Median particle size and quartile deviation were derived directly from the cumulative size-frequency curve. Beach profiles were measured with ropes and marked poles with reference to tidal heights during the calm period in June and during the monsoonal m o n t h in January throughout the study period. Data analysis All data on biological characteristics were pooled across transects and within months for the respective stations to give a total area per m o n t h as 0.25 m 2 (0.0314 per station x 4 transects x 2 samples per month). For each monthly total, species diversity was calculated using the Shannon-Wiener index (Shannon & Weaver, 1948) and the distribution of abundance among the species by the evenness index (Pielou, 1977). The variation among stations and among months was determined by one-way A N O V A (Zar, 1984), tested a posteriori. Results The density of macrobenthos at the five stations ranged from a low of about 135 to over 300 individuals per 0.25 m 2 (Table 1). A distinct seasonal variation in total density occurred at all the stations, with significantly (P<0.05) lower numbers during the monsoonal months of December-February, which was characterized by increased wind force but no rain (Figure 2). Higher numbers were always recorded during the calm change-over period of September-October.

24

B. Ong Cv"S. Krishnan

TABLE 1. Biological characteristics of the five stations at Telok Aling F (ANOVA)

Distance downshore (m) 15

25

35

No. of individuals 135+70 232+55 268+170 per 0.25 m e No. of species 135:5 11+2 11+2 per 0.25 rn2 Shannon Wiener 1.69 5:0-43 1.33 + 0.29 1"285:0-34 index, H Evenness 0-585:0.11 0.58+0.07 0.605:0-14 index, ff

Station Month

45

55

309:t:236

281+111

5-24

9-21

9+2

10:t:2

0.24

0"22

1-295:0.28

1'02 5:0-11

0.06

0-58

0.62+0-12

0.63+0.13

0"15

0-78

C o m m u n i t y structure, such as species diversity, total n u m b e r of species a n d evenness were n o t affected ( / ' > 0 . 0 5 ) by the m o n s o o n s , although the evenness index was relatively low due to the a b u n d a n c e of a few species (Figures 3 and 4). Station 1 s u p p o r t e d the largest n u m b e r of species and the highest diversity (H), particularly d u r i n g the calm period, which exceeded 20 and 2-5, respectively, b u t the lowest density. Conversely n u m b e r of species and diversity h a r d l y c h a n g e d t h r o u g h o u t the s t u d y p e r i o d for station 5. T h e initial m a c r o b e n t h o s , except Echinodiscus bisperforatus, was r e p l a c e d p r e d o m i nantly by polychaetes a n d bivalves, b o t h of which previously were virtually absent. Polychaetes c o m p r i s e d m o r e than 70% o f the p o p u l a t i o n for the first year o f study a n d were m u c h affected by the m o n s o o n s which r e d u c e d their n u m b e r s c o n s i d e r a b l y (Figure 5). Molluscs were the next a b u n d a n t b u t were also less affected seasonally. T h e r e f o r e , percentage c o m p o s i t i o n of molluscs always surpassed that o f polychaetes d u r i n g the m o n s o o n a l m o n t h s . T h e sharp increase in m o l l u s c a n p o p u l a t i o n after June 1989 was attributed to r e c r u i t m e n t of U. vestiarium, while that d u r i n g 1988 to Tivela. The r e c r u i t m e n t of Umbonium b e g a n in M a y 1989, just after the few m o n t h s o f r e d u c t i o n in the p o p u l a t i o n of Tivela. Subsequently, the Tivela p o p u l a t i o n never a t t a i n e d the density of 1988, b u t the Umbonium p o p u l a t i o n increased rapidly, despite the m o n s o o n s . P r e d a t o r y naticinid and polinicid g a s t r o p o d s together with the starfish Astropecten vappa were also observed to return to the b e a c h shortly after the r e c r u i t m e n t o f Urnbonium. T h e m o s t frequently collected species were the polychaetes, Diopatra neapolitana, Prionospio malmgreni C l a r a p t d e , Lanice socialis Willey, Parasclerocheilus brianchiatus Fauvel, Poecilochaetus serpens Allen, Nainereis kalkudaensis de Silva, Glycera spp., Notomastus spp., Thalenessa djiboutiensis Gravier, the bivalve Tivela sp. a n d the e c h i n o d e r m Echinodiscus bisperforatus (Figure 6). F o u r o f these species (D. neapolitana, P. malmgreni, Tivela sp. a n d U. vestiariurn) were n u m e r i c a l l y d o m i n a n t a n d t o g e t h e r constituted m o r e than 78% to the total m a c r o b e n t h o s a b u n d a n c e ( T a b l e 2). T h e first three species also exhibited m a j o r seasonal fluctuations that c o i n c i d e d with the m o n s o o n (Figure 6). T h e other species were less affected, b u t their counts were too low to have significant effect on the total density. T h e species c o m p o s i t i o n at station 1 was almost exclusively m a d e u p o f polychaetes ( > 9 0 % ) , with very few bivalves a n d crustacea (Figure 7). T h e p o l y c h a e t e distribution was mainly due to the distribution of Diopatra which were m o s t a b u n d a n t at station 1 (Figure 8). Polychaete c o m p o s i t i o n declined t o w a r d s the lower tidal stations 2, 3 a n d 4,

Changes in a macrobenthos comrnuniry

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Figure 2. Abundance of macrobenthos from March 1988 to December 1989.

which b e c a m e n u m e r i c a l l y d o m i n a t e d b y the bivalve Tivela. E c h i n o d e r m d i s t r i b u t i o n p e a k e d at station 4, while station 5 was again d o m i n a t e d b y p o l y c h a e t e s , p a r t i c u l a r l y P. rnalrngreni, a n d also s u p p o r t e d a fair d e n s i t y ( > 2 0 % ) o f the g a s t r o p o d U. vestiarium. T h e s e d i m e n t d i s t r i b u t i o n s h o w e d a d e c r e a s i n g m e a n grain size d o w n s h o r e w h i c h also b e c a m e b e t t e r s o r t e d ( T a b l e 3). T h e m o s t h e t e r o g e n e o u s s e d i m e n t was always e n c o u n t e r e d at station 1 while s e d i m e n t organic c o n t e n t was high at all stations.

Discussion After the erosion there was an increase in the n u m b e r o f species, diversity a n d a c h a n g e in the species c o m p o s i t i o n o f the T e l o k A l i n g s a n d flat. T h i s k i n d o f p h e n o m e n o n was

26

B. Ong & S. Krishnan

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Figure 3. Number of species of macrobenthos from March 1988 to December 1989.

reported by Grassle and Sanders (1973) to be due to a biological unsaturation which allowed more species to occupy the habitat, mainly by immigration of other species from the vicinity. Recolonization of a defaunated area usually involved different species composition with opportunistic or fugitive species becoming abundant in the early stages (Johnson, 1974; Arbugov, 1982). The impact of storms has been reported to cause defaunation or reduction of species due to mortality, removal by erosion (Yeo & Risk, 1979) or mortality by reduced salinity from rainfall (Ansell et al., 1972). Conversely, Dobbs and Vozarik (1983) did not observe any differences in density in pre- and post-storm collections, but reported large post-storm increases in the number of infaunal species and individuals in the water column. They suggested that the water turbulence suspended benthic infauna and helped in the widespread dispersal of both larvae and adults.

Changes in a macrobenthos community

27

1.4

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6~

2 Station 3

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2

o,~

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I

1

o I I I I I I I I I I I . I I I I I I I I I I II MAM J JAS OND J FMAMJ JA SOND 1988 1989 Figure 4. Changes in diversity (H: - - D - - ) and evenness (J: - - t - - ) from March 1988 to December 1989.

It is n o t k n o w n whether Umbonium individuals were killed or redistributed to subtidal stations, as suggested by Berry a n d Z a m r i (1983), w h e n n u m b e r s apparently reduce d u r i n g the m o n s o o n a l m o n t h s . W e are of the o p i n i o n that their disappearance could be due partly to redistribution by erosion a n d partly to death from r e d u c t i o n in salinity caused by u n i m p e d e d r u n o f f after removal of the vegetation. F o r instance, d u r i n g the rainy season in 1988, we observed gullies cut by fxeshwater r u n o f f from the hill all along the u p p e r zone of the beach. R e - e s t a b l i s h m e n t of Umbonium b e g a n only in M a y of the second year, which coincided with their d o c u m e n t e d a n n u a l s p a w n i n g cycle (Berry, 1987), suggesting that r e c r u i t m e n t was by larval settlement. T h e r e could be several reasons for the late recovery. D u r i n g

28

B. Ong C_~S. Krishnan

100

80

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Figure 5. Distribution of rnacrobenthos by taxa from March 1988 to December 1989. (--II--) Mollusca; (--Lq--) Polychaeta. the first year, the substratum may still be too unstable for the larvae spawned in March-May 1988 to settle. Second, there could have been some competitive interaction or amensalism between Tivela and Umboniurn, both of which occupy similar zones. Although this can only be confirmed by experimental manipulation, it is noteworthy that recruitment of the latter began just after the Tivela population dropped significantly. The life-cycle of Urnboniurn, which showed a brief pelagic phase of less than 48 h and a spawning season in March-May each year during still water at neap tides (Berry, 1986), is not favourable for dispersal of larvae, especially when the adults have been redistributed elsewhere. By the second year, the mobility of sand may be abated sufficiently, possibly from a regrowth of vegetative cover on the sand dune and partly from the stabilizing effect of the tube-building behaviour of Diopatra (Fagar, 1964; Young & Rhoads, 1974), for the settlement of Umbonium larvae. Typical of sandy substrate, fluctuations in community level in density, evenness and diversity were clearly influenced by the population dynamics of the numerically abundant species (Holland & Polgar, 1976; Dexter, 1984). Similarly, polychaetes were not represented in collections made during the monsoon (Ansell et al., 1972), while Dexter (1992) generalized that as exposure to wave action decreased, relative abundance of polychaetes increased. Zonation of the animals also appeared to be related to the gradients of bottom stability, besides the degree of exposure of the beach and sediment (Gray, 1974). This gradient can be defined as the changes in the sand level of the beach profile as well as grain size and sorting coefficients. The most unstable zones were therefore at stations 1 and 2, where the change in sand level was more than 0"3 m for June 1988, with reference to the previous profile at about the same time of the year (Berry, 1982). A crude index of beach instability, generated by Allen and Moore (1987) from values of gain/loss in beach sand level, showed that some of the most unstable beaches undergo changes exceeding 0.3-0.5 m. These two stations were numerically dominated by the polychaete lh'opatra neapolitana which build reinforced tubes protruding several centimetres above the

0

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Nainereis laevigata

Poecilochaetusserpens

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Lanice socialis

Thalenessadjiboutiensis

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Figure 6. Monthly distribution of the most frequently collected macrobenthos species at Telok Aling from March 1988 to December 1989.

MAMJJA SONDJFMAMJJASONDJF 1988 1989

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Notomastus spp.

Glycera sp. I

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30

B. Ong & S. Krishnan

TABLE 2. Mean number of individuals per m z collected from all stations. Percentages are shown in parentheses Mollusca Bivalvia

Gastropoda

Atn'na vexillum Born Gefarium diverticarum Gmelin Merethrix merethrix L. Solen sp. Tivela sp. Natica maculosa Lamarck Nassan'us stotatus Gmelin Polinices pryiforrnis Recluz Turricula jurana L. Umbonium vestian'um Lamarck

3 6 6 15 582 4 6 8 3 403

(0-14) (0-28) (0"28) (0"73) (27"49) (0.19) (0.28) (0-38) (0" 14) (18"90)

Aglaurides fulgida Savigny Arabella iricolor Montague Axiothella sp. Brianchiomma nigromaculata Baird Cirratulus sp. Diopatra cuprea Bose Diopatra neapolitana Delle Chiaje Eteone sp. Exogone sp. Clycera sp. I Glycera sp. II Lanice socialis Willey Marphysa macintoshi Crossland Nainereis kalkudaensis de Silva Nereis sp. Notomastus sp. I Notomastus sp. II Owenia fusiformis Delle Chiaje Parascleroclerocheilus brianchiatus Fauvel Paralepidenotus sp. Phyllodoce sp. Poecilochaetus serpens Allen fh'sta sp. Potomilla leptochaeta Southern l~'onospio malmgreni Clarap6de Scolopolos chevalieri Fauvel Scolelepis sp. Syllis sp. Thalenessa djiboutiensis Gravier

4 5 28 7 5 3 317 5 25 30 12 25 5 22 10 20 16 4 19 5 2 37 6 16 320 7 10 15 11

(0.19) (0.24) (1.32) (0"33) (0.24) (0" 14) (14.90) (0.24) ( 1"18) (1 "41) (0-56) (1-18) (0.24) (1-03) (0.47) (0"94) (0"75) (0.19) (0"90) (0-24) (0"09) (1.70) (0-28) (0.75) (15" 15) (0.19) (0"47) (0-71) (0-51)

Astropecten vappa Mueller and Troschel Echinodiscus bisperforatus Leske

2 44

(0"01) (2.07)

Apseudes sp. Monoculodes sp. Platyishnopus sp. Sphaeroma sp.

7 22 2 1

(0.33) (1 "03) (0.09) (0"05)

4 8

(0-19) (0-38)

Polychaeta

Echinodermata

Crustacea

Others

Aspidosiphon sp. Sipunculus nudus L.

s u b s t r a t u m . T h e d e n s i t y o f Diopatra h a s b e e n f o u n d to b e p o o r l y c o r r e l a t e d w i t h p a r t i c l e size d i s t r i b u t i o n or w i t h o r g a n i c m a t t e r , b u t d i r e c t l y w i t h c u r r e n t v e l o c i t y a n d t h e p r e s e n c e o f t u b e - b u i l d i n g m a t e r i a l s s u c h as shell f r a g m e n t s ( M a n g u m et al., 1968;

Changes in a macrobenthos community

31

100

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Figure 7. Percent contribution of macrobenthos by taxa at the five sampling stations. I , Crustacea; [~, Echinodermata; ~, Bivalvia; ~, Gastropoda; [], Polychaeta.

~ 1-5~,

Distancedownshore (rn) /

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250 7~'Vela sp.

Figure 8. Habitat zonation of the most abundant macrobenthos and beach profiles of Telok Aling. Dotted line indicates profile before erosion, ' a ' and ' b ' indicate profiles in June and January, respectively.

W o l f s o n et al., 1978). Diopatra have also b e e n s h o w n to exhibit r e m a r k a b l e b e h a v i o u r a l adaptations to unstable b o t t o m s . F o r instance, My er s (1972) d e m o n s t r a t e d D. cuprea co u l d i m m e d i a t e l y r e b u r r o w a n d r e b u il d its tube to the surface w h e n c o m p l e t e l y w a s h e d o u t by erosion or b u r i e d by sediments. S u c h adaptations t o g e t h e r with the p r o t e c t i o n offered by the tubes w o u l d allow Diopatra to successfully exploit the m o s t unstable zones o f the T e l o k Aling b e a c h at 1 5 - 2 5 m.

32

B. Ong & S. Krishnan

TABLE 3. Sediment characteristics of the five stations at Telok Aling Distance downshore (m)

% Organic matter % Silt/clay Quartile deviation (mm)

15

25

35

45

55

3.00 0.70 0-56

3.20 0.25 0-09

3.20 0-25 0"11

3.52 0.21 0-07

3.15 0.18 0"04

C o n v e r s e l y , t h e m o s t stable z o n e was at t h e l o w e s t i n t e r t i d a l stations (55 m ) w h e r e t h e s e a s o n a l s a n d - l e v e l c h a n g e s w e r e slight a n d t h e rigours o f e x p o s u r e d u r i n g l o w tides w e r e less. S a n d o f m e d i a n g r a i n size o f 0" 18 m m is c o n s i d e r e d to b e m o s t easily m o v e d b u t b e t t e r sorted, since the r o u g h n e s s , s e t t l i n g a n d t h r e s h o l d v e l o c i t i e s are all e q u a l ( S a n d e r s , 1958). H e t h e n d e f i n e d an e n v i r o n m e n t c o n s i s t i n g o f a large c o n c e n t r a t i o n o f this g r a i n size, as at station 5, to be i n d i c a t i v e o f little active s e d i m e n t t r a n s p o r t a n d is t h e r e f o r e stable a n d c o n d u c i v e for b e n t h o s c o l o n i z a t i o n . T h i s z o n e t h e r e f o r e c o n t a i n e d the h i g h e s t m a c r o b e n t h o s d e n s i t y a n d was m o s t p r e f e r r e d b y t h e s m a l l s p i o n i d p o l y c h a e t e , PrT"onospio malmgrenL T h e r e are c o n t r a s t i n g o p i n i o n s as to w h e t h e r r e c o l o n i z a t i o n o f a d e f a u n a t e d p a t c h is c a u s e d by a release o f c o m p e t i t i o n , w h i c h frees r e s o u r c e s for o t h e r species, o r t h a t e x p l o i t a t i o n is a r e s p o n s e to r e s o u r c e s c r e a t e d by t h e d i s t u r b a n c e ( T h i s t l e , 1981). T h e s c o p e o f this s t u d y d o e s n o t p e r m i t us to d e t e r m i n e the c a u s e o f r e c o v e r y , b u t t h e d i s a p p e a r a n c e o f Umbonium m a y , to s o m e extent, ease the c o m p e t i t i o n for s p a c e in the b e a c h . It r e m a i n s to b e s e e n w h e t h e r w i t h the r e c o v e r y o f U m b o n i u m to p r e v i o u s densities, the m a c r o b e n t h i c c o m m u n i t y will r e v e r t b a c k to t h e initial o n e or w h e t h e r t h e p r e s e n t p o l y c h a e t e / b i v a l v e - d o m i n a t e d c o m m u n i t y will persist.

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