Detrital pathways in a coral reef lagoon - Paul K. Daytondaytonlab.ucsd.edu/Publications/Riddleetal90.pdf2 Marine Biology Research Division, Scripps Institution of Oceanography, A-002

Marine Biology 104, 109-118 (1990) Marine ==,BiOlOgy

@ Springer-Verlag 1990

Detrital pathways in a coral reef lagoon I. Macrofaunal biomass and estimates of production *

M . J . Riddle 1, D . M . Alongi 1, p . K . D a y t o n 2, j . A . Hansen 3 and D . W . Klumpp 1

1 Australian Institute of Marine Science, P.M.B. No. 3, Townsville M.C., Queensland 4810, Australia 2 Marine Biology Research Division, Scripps Institution of Oceanography, A-002 La Jolla, California 92099-0227, USA a School of Biological Sciences, A12, University of Sydney, Sydney, New South Wales 2006, Australia

Abstract

Coral reef lagoons are generally regarded as zones of net heterotrophy reliant on organic detritus generated in more productive parts of the reef system, such as the seaward reef flat. The abundance and biomass of sediment infauna were measured seasonally for one year (1986) within the lagoon of Davies Reef, central Great Barrier Reef, to test the hypothesis that macrofaunal biomass and production of coral reef lagoons would decrease with distance from the reef flat and would change seasonally. In general, there were no simple relationships between infaunal standing stock or production and distance from the reef flat or season. Bioturba- tion by callianassid shrimps negatively affected the abundance of smaller in fauna, suggesting a community limited by biogenic disturbance rather than by supply of organic material. Polychaetes and crustaceans were dominant amongst the smaller infauna (0.5 t o 2 m m ) while larger animals ( > 2 mm) were mostly polychaetes and molluscs. Mean biomass of infauna at both sites and all seasons was 3 181 mg C m -2. The smaller animals (0.5 to 2 mm) contributed about 40% of total macrofaunal respiration and production although they represented only 15% of the total macrofaunal biomass. The biomass of macrofauna was about equal to that of the bacteria and meiofauna, while respiration represented 10 to 20% of total community respiration. Consumption by macrofauna accounts for only 3 to 11% of total organic inputs to sediment, with a further 14 to 17% being lost by macrofaunal respiration.

Introduction

It has been suggested that coral reef lagoons are zones of net heterotrophy requiring energy inputs from more productive

* Contribution No. 488 from the Australian Institute of Marine Science

parts of the reef system (Odum and Odum 1955, Lewis 1980), but there are virtually no data to substantiate this assumption (Hatcher 1983, Kinsey 1985 a, Alongi 1988). On the contrary, Kinsey (1985 b) suggests that organic detritus from the seaward reef flats flushes through the lagoon without settling and that in situ production by benthic algal mats is the dominant source of organic material in reef lagoons. Wilkinson (1987) summarised known estimates of standing stock and carbon flow rates within the lagoon of Davies Reef, central Great Barrier Reef, and highlighted some of the major gaps which still exist; these include the role of sediment macrofauna as secondary producers and the effects of seasonal variation of detrital inputs.

This contribution is the first of a series arising from a multi-disciplinary study of sediment community energetics conducted at Davies Reef lagoon. The study involved seasonal measurements of total community respiration and production, estimation of production by microbial, meio- faunal and macrofaunal communities, and measurements of rates of detrital inputs to the lagoon. The overall objective of the series was to test the hypothesis that coral reef lagoon-communities are detritus-based and that the main sources of detritus are the reef front and flat.

Reported here are seasonal counts and biomass measurements of sediment macrofauna from two zones in the reef lagoon. These measurements were used to estimate production and consumption rates which are compared with published rates for other components of the lagoon community.

The objectives of this study were to determine whether biomass and production of the major groups of sediment- living animals would decrease with distance from the reef flat and would change seasonally. Large (up to 50 cm high), contiguous mounds resulting from the sediment reworking activity of callianassid shrimp are ubiquitous on the lagoon floors of reefs in this region. The correlation between height on mound and abundance of infauna was measured to determine whether disturbance by sediment reworking limits infaunal abundance.

110 M.J. Riddle et al. : Macrofaunal biomass and production in coral reef lagoon

Materials and methods

Davies Reef (18°50'S; 47°39'E) is a middle shelf reef in the central section of the Great Barrier Reef (GBR; Fig. 1) The lagoon comprises approximately 50% of the total reef area and is largely open to the west and bounded to the south- east (the direction of the prevailing trade winds) by a reef flat of about 300 m width. It has an average depth of 16 m (Pickard 1986) and ranges from 5 to 27 m. The reef is in an area with net currents predominantly southward to south- eastward. However, on the reef, flow is in the direction of the wind and results in a flushing time for the lagoon estimated at 1 to 2 d during strong winds and 4 to 6 d in lighter winds (Pickard 1986).

Two sampling stations were established in each of two zones within the lagoon (Fig. 1). The shallow zone was im- mediately behind the reef flat, the deeper zone in the centre of the lagoon. Samples were taken on four occasions during 1986 (February, May, August and November) representing the austral summer, autumn, winter and spring, respectively.

All samples were taken with cylindrical PVC corers pushed manually to a depth of 200 to 250 mm into the sediment. To ensure that the biomass of large animals did not mask changes in the small fauna, two size-components of the macrofauna were sampled. Four sets of five 55 mm- diam cores were taken to quantify the smaller animals, each set was from an area about I m 2, and the sets were spaced about 4 m apart. The five cores in each set were bulked to form the basic sampling unit (119 cm 2) and were washed on nested 2 mm and 0.5 mm square-meshed sieves. The material passing the 2mm mesh but retained by the 0.5 mm screen was preserved in 10% formalin solution in seawater and stained with Rose Bengal to aid sorting. To quantify the larger animals, four sets of six 143 mm-diam cores were taken in an array similar to that of the smaller cores. The six cores in each set were bulked to form the basic sample unit (964 cm2), washed on a 2 mm sieve and preserved as before.

Animals were separated from the sediment by flotation during repeated washings. The sediment residue was checked under a dissecting microscope. Animals were identi- fied to family level, counted and subsequently grouped into six categories; macro- and microphagous polychaetes (as defined by Fauchald and Jumars 1979), crustaceans, bivalve and gastropod molluscs, and all other taxa. Groups with calcareous skeletons or shells were decalcified in dilute phos- phoric acid. Each group was freeze-dried to constant weight, burnt at 550 °C for 6 h, reweighed, and ash-free dry weight (AFDW) was calculated by difference.

Large mounds (up to 30 cm high) produced by the feeding activity of callianassid shrimp are the most prominent feature of lagoons from reefs of the central GBR. Eight contiguous series of samples, each with 7 to 16 cores of 55 mm-diam, were taken at one of the deeper sites to test whether the mounds influence macrofaunal abundance or biomass. Each series crossed through the lowest part of the valley between adjacent peaks (Fig. 2). Total abundance and biomass of all animals retained by a 0.5 mm mesh were measured for individual samples as above.

Davies 4 ~ I~ + .,. - Reef ~ ~ .,~.

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# :m'.. > .. :. , 2 2

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Fig. 1. Location of Davies Reef on Australian coastline and posi- tion of sampling sites within lagoon

Fig. 2. Contiguous series of cores traversing adjacent callianassid mounds in the highly bioturbated lagoon sediments of Davies Reef

Data analysis

Analysis of variance was used to test the effect of season and depth on the abundance, biomass, and production of the major faunal groups and of the total fauna in each size class. Season and depth were treated as fixed factors and the repli- cate sites within each depth were treated as a random factor nested within depth. Seasons were orthogonal to depth.

M.J. Riddle et al. : Macrofaunal biomass and production in coral reef lagoon 111

There were four replicates within each combination of the three factors.

Homogeneity of variance for all analyses was deter- mined used Cochran's test (Winer 1971). For those cases in which Cochran's test was significant (P<0.05), data were transformed (log x + 1) and the variances were retested for homogeneity.

In cases where the nested factor, sites, was not significant at p > 0.25, sites were pooled to increase the test's power to detect the effects of season and depth (Underwood 1981), and the data were retested.

Allometric relationships

ship between P:B and individual body mass (Banse and Mosher 1980) was used to calculate a second estimate, here after referred to as P:B(anometric):

P: B(allometric) = 0.6457 M - o. 37, (4)

where M = individual body mass in kcal. All estimates of rates are reported in kJ m - 2 yr- 1. Con-

version factors used were as follows:

1 g dry wt=0.9 g ash-free dry wt (Waters 1977) 1 g organic matter (ash-free)= 5 kcal (Crisp 1971) 1 g carbon = 10 kcal (Crisp 1971) 1 litre 02 = 4.83 kcal (Miller et al. 1971) I ca1=4.2 J

A series of published allometric relationships (McNeill and Lawton 1970, Miller et al. 1971, McMahon 1973) were used to estimate rates of respiration, secondary production and consumption of organic material. The allometric estimates of production together with measured levels of biomass were used to derive production:biomass ratios (P:B) and these were compared with literature values. Esti- mates of the P: B ratio were also obtained from a published allometric equation relating P:B to biomass (Banse and Mosher 1980).

Respiration rate (R) was estimated from:

R=7.0 W °'v5 , (1)

where R=respiration rate, nl 02 (individual h) -1, and W= individual body weight,/~g dry wt:

The value 7 for the proportionality coefficient was derived by Ivleva (1980) from a collection of crustaceans from tropical waters (29 °C). A value of 0.75 for the expo- nent is justified on theoretical grounds by McMahon (1973), Blum (1977) and Platt and Silvert (1981) and is supported by the empirical findings of Banse (1982).

Production (P) was calculated from the estimate of respiration using:

P=0.8039 R 0'8262, (2)

where P = production in kcal (m 2 yr-1) and R = respiration in kcal (m 2 yr 1).

The constants were calculated by McNeill and Lawton (1970) from a data set of short-lived poikilotherms.

Consumption of food energy (C) was calculated from the estimates of respiration and production as:

C = R + P (3) 0.6 '

where C--energy consumed as food, kJ m -2 yr -1, R = energy used in respiration, kJ m -2 yr-1, and p=energy used in production, kJ m - z yr - 1.

The assimilation efficiency of 60% is that estimated by Miller et al. (1971) as an average for herbivores.

Production:biomass (P: B) ratios were estimated by two methods. The values of production calculated from Eqs. (1) and (2) together with the measured level of biomass were used to provide a ratio (P:B). A direct allometric relation-

Results

Faunal composition

The relative contributions of the major categories of infauna to the total abundance were very variable with both season and depth (Fig. 3.). However, some consistent patterns are apparent: the polychaetes and crustaceans were numerically dominant in the smaller fauna while the polychaetes and molluscs dominated the larger animals.

The relative contributions of each faunal group to total biomass were more consistent across both zones and seasons (Fig. 3). The polychaetes, particularly microphages, were the major contributor to the biomass of smaller infauna, crustaceans ranked second, and molluscs were a relatively minor component. In contrast, molluscs contributed the greatest part of the biomass of the larger animals, overshad- owing all other groups. The major effects of depth on biomass were that small microphagous polychaetes and the larger bivalves were a relatively greater component of the fauna at the deeper site than they were at the shallow site, while large gastropods were more important at the shallow site.

Abundance, biomass and production

The smaller infauna were more than an order of magnitude more abundant (3 241 to 10 775 m 2) than the larger animals (171 to 755 m 2) (Fig. 4a). Effects of season and depth were complex (Table 1), with significant second-order interactions for most faunal groups in the smaller size class and significant effects of the nested factor, site, for some groups of larger animals. The large crustaceans were the only group to show a significant main effect (season) without significant interactions.

The total biomasses (measured as ash-free dry weight) of the larger animals (2 to 15 g m 2) were an order of magnitude greater than those of the smaller animals (0.6 to 1.3 g m 2) (Fig. 4 b). In general neither season nor depth had a significant effect on the biomass of the sediment fauna in the two size classes studied (Table 2). Only the smaller macro-

112 M J. Riddle et al. : Macrofaunal biomass and production in coral reef lagoon

Lar.ge infauna abundance 100% , , I |

' - ' q - H

N 50- ~

N ,-,-

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Small infauna abundance

:.:,: :.:.: ,:,:,: . . . . . .

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Microphagous p o l y c h a e t e s

Maerophaoous

iiiiiiii~i~ ~ polychaetes

i!iiiii::ii:: Gastropods - - ~ - / B i v a l v e s

Large infauna biomass 1 0 0 % - ~

/ . 4 7 2 . , ::iiN::i /`4~/// . . / . 4 / / / . 4 / / / . 4 ~ / / / . 4 / / / . 4 / / / ~ / /

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February May

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Small infauna biomass ... ....

iiii : ' : Z £ .

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August November

Fig. 3. Composition(percent abundance and biomass) ofthe large and smallinfauna in shallow and deep lagoon of Davies Reef during 1986

Large Smal l infauna infauna

(>2 m m ) (0.5 - 2 ram)

1000-

E

100" E #

1 0

Deep lagoon Sha l l ow lagoon

1 6 .

( a )

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Fig. 4. Abundance (a), biomass (g ash-free dry wt) (b) and estimated production (c) of large and small infauna in the

[ two deep lagoon sites of Davies Reef as a o n

- ,

400

E 300-

g 200-

n

100- j f

(c )

0 Feb May Aug Nov 0 Feb May Aug Nov Feb May Aug Nov


Table 1. Summary of three-factor analysis of variance involving abundance of sediment infauna separated into major taxonomic divisions and into two size classes. F: degrees of freedom of F ratio; np: not pooled

Source of variation F Macrophagous Microphagous Crustaceans Bivalves Gastropods Other Total polychaetes polychaetes taxa fauna

Larger infauna ( > 2 mm) Season 3,6 Depth 1,2 Season x depth 3,6 Site (depth) 2,48 Season x site (depth) 6,48 Season (sites pooled) 3,56 Depth (sites pooled) 1,56 Season x depth (sites pooled) 3,56

ns ns * ns ns ns ** ns ns ns ns ns ns ns ns ns ns * ns ns * • * ns ns ns ns ** ns ns ns ns ns ns ns

ns np np np np np np ns np

ns np

Smaller infauna (0 .5-2 ram) Season 3,6 ns Depth 1,2 ns season x depth 3,6 ns Site (depth) 2,48 ns Season x site (depth) 6,48 **

n s n s n s n s n s n s

n s * n s n s n s n s



• * * * * * * * * n s * * * * *

Significance levels: * 0.05 > p > 0.01 ; ** 0.01 > p > 0.001 ; *** p < 0.001 ; ns: p > 0.05

Table 2. Summary of three-factor analysis of variance involving biomass of sediment infauna separated into major taxonomic divisions and into two size classes. F: degrees of freedom; np: not pooled


Larger infauna ( > 2 mm) Season 3,6 ns ns ns ns ns ns ns Depth 1,2 ns ns ns ns ns ** ns Season x depth 3,6 ns ** ns ns ns ns ns Site (depth) 2,48 ns *** ns ** ns ns ns Season x site (depth) 6,48 ns ns ** ns ns ns ns Season (sites pooled) 3,56 ] ns np np Depth (sites pooled) 3,56 / np np np np ns np np Season x depth (sites pooled) 3,56 ns np np

Smaller infauna (0.5 2 mm) Total molluscs

Season 3,6 ns ns ns ns ns ns Depth 1,2 ns ns ns * ns ns Season x depth 3,6 ns * ns ns ns ns Site (depth) 2,48 ns ns ns ns ns ns Season x site (depth) 6,48 ns ns ns * ns ns Season (sites pooled) 3,56 ns ns ns np ns ns Depth (sites pooled) 1,56 * ns ns np ns * Season x d e p t h (sites pooled) 3,56 ns * ns np ns *

Significance levels: * 0 . 05>p>0 .01 ; ** 0 .01>p > 0.001; *** p<0 .001; n s : p > 0 . 0 5

p h a g o u s p o l y c h a e t e s a n d the l a rger " o t h e r t a x a " s h o w e d a

s ign i f i can t m a i n effect (dep th ) w i t h o u t s ign i f i can t in t e rac -

t ions .

P r o d u c t i o n o f the l a rge r a n i m a l s d id n o t c h a n g e cons is -

t en t ly w i th e i the r s e a s o n or d e p t h (Table 3). P r o d u c t i o n by

all c o m p o n e n t s o f the smal le r f a u n a excep t the g r o u p " o t h e r

t a x a " were s ign i f ican t ly a f fec ted by d i s t ance f r o m the reef

(dep th) . P r o d u c t i o n by m a c r o p h a g o u s p o l y c h a e t e s a n d

c r u s t a c e a n s was g rea t e r n e a r e r the reef flat, p r o d u c t i o n by

m i c r o p h a g o u s p o l y c h a e t e s was g r ea t e r in the deepe r p a r t o f

the l agoon , whi le p r o d u c t i o n by the smal l mo l lu scs was

s o m e t i m e s g rea t e r n e a r to the reef f la t a n d a t o t h e r t imes

g rea t e r in the deepe r l agoon . H o w e v e r , n e i t h e r s eason n o r

d e p t h s ign i f ican t ly c h a n g e d the t o t a l p r o d u c t i o n o f e i the r

size class. B i o m a s s o f to t a l i n f a u n a ( b o t h size classes c o m b i n e d ) a t

the deep site was cons i s t en t ly g rea t e r t h a n a t the sha l low site

(Table 4). E s t i m a t e s o f ra tes o f t o t a l energe t ic p rocesses were

s l ight ly g rea t e r in the deep site o n all occas ions except d u r i n g

N o v e m b e r .

E s t i m a t e s o f p rocess ra tes ca l cu la t ed f r o m the a l lome t r i c

r e l a t i o n s h i p s i nd i ca t e t h a t the smal l e r a n i m a l s were o f

114 M.J. Riddle et al.: Macrofaunal biomass and production in coral reef lagoon

Table 3. Summary of three-factor analysis of variance involving estimated production of sediment infauna separated into major taxonomic divisions and into two size classes. F: degrees of freedom; np: not pooled


Larger infauna (> 2 mm) Season 3,6 ns ns ns ns ns ns ns Depth 1,2 ns ns ns ns ns * ns Season x depth 3,6 ns ns ns ns ns ns ns Site (depth) 2,48 ns * ns ns ns ns ns Season x site (depth) 6,48 ns * * ns ns ns * Season (sites pooled) 3,56 ns np np np np * np Depth (sites pooled) 3,56 ns np np np np ns np Season x depth (sites pooled) 3,56 ns np np np np ns np

Smaller infauna (0.5-2 mm) Total molluscs

Season 3,6 ns ns ns ns ns ns Depth 1,2 * ns * * ns ns Season x depth 3,6 ns ns ns ns ns ns Site (depth) 2,48 ns ns ns ns ns ns Season x site (depth) 6,48 ns ns ns *** ns ns Season (sites pooled) 3,56 ns ns np np np np Depth (sites pooled) 1,56 ** * np np np np Season x depth (sites pooled) 3,56 ns ns np np np np

Significance levels: * 0 .05>p>0.01; ** 0.01>p>0.001; *** p<0.001; ns: p>0.05

Table 4. Biomass and rates of energetic processes of sediment infauna from shallow and deep lagoon on Davies Reef on four occasions throughout a year. Values are means -+95% confidence limits, n = 8

Season and zone Biomass Respiration rate Production rate Consumption rate (mg C m - 2) (kJ m - 2 yr- 1) (kJ m - z yr i) (kJ m - 2 yr- 1)

February (summer) shallow 3 405_+1 333 1 014_+319 422+_111 2 394_+ 717 deep 5 071 _+ 3 167 1 074_+477 435_+ 155 2 513_+ 1 052

May shallow 2762___1 333 767_+239 340_+ 86 1 845_+ 544 deep 3 429_+ 1 619 907_+ 294 388 _+ 103 2 160 + 660

August (winter) shallow 2 048 +_ 1 262 802_+ 187 367 -+ 67 1 947 -+ 422 deep 3 048_+1 048 913-+270 396_+ 95 2 183-+ 607

November shallow 2 810-+ 1 976 902_+ 357 388 _+ 122 2 149_+ 799 deep 2881-+1095 858_+268 368_+ 92 2044_+ 601

greater significance than suggested by their b iomass a lone

(Table 5). They represented only 15% of the s tanding s tock

b iomass bu t con t r ibu ted a b o u t 40% to es t imated levels o f total m a c r o f a u n a l respira t ion, p roduc t i on and consump-

tion. P roduc t ion by the smaller m a c r o f a u n a was in the range 103 to 216 kJ m 2 y r - 1 while tha t o f animals larger than

2 m m was 130 to 436 kJ m -2 yr -1 (Fig. 4c).

Effect o f b io tu rba t ion on abundance and b iomass

There was a highly significant negative correlat ion (p < 0.001, Spearman ' s rank correlat ion coefficient) between total abun-

dance o f in fauna and relat ive height on cal l ianassid m o u n d s

(Fig. 5), indica t ing that abundance decreases f rom the val-

leys to the peaks o f mounds . There was no signif icant corre-

la t ion be tween to ta l b iomass and m o u n d height (p > 0.05). A b u n d a n c e was d o m i n a t e d by smal ler animals (Fig. 4a) ,

while b iomass was d o m i n a t e d by the larger animals (Fig. 4 b). The results suggest tha t small an imals have a

pa tchy d is t r ibut ion related to cal l ianassid activity, while the

larger animals do not .

Discussion

The bu r rowing m a c r o f a u n a o f Davies R e e f l agoon is a sig-

nif icant c o m p o n e n t o f the sediment c o m m u n i t y b o t h in


190 Biomass per core ( mg ) 60

Biomass

20

0 l 1 peak valley

107

peak

i

126 Number per core

6O

40

20

Fig. 5. Abundance and biomass (mg ash-free dry wt) of infauna at different heights on mounds produced by callianassid shrimps

Table 5. Distribution of biomass and rates of energetic processes amongst different size classes and taxonomic divisions of sediment infauna of Davies Reef Lagoon. Values are means _+ 95% confidence limits, n = 64. Respiration, production and consumption rates taken from Eqs. (1), (2) and (3), respectively; allometric P:B from Eq. (4)

Av body wt Biomass Respiration Production Consumption P : B P: B (allo- (mg individ- (mg C m -a) rate rate rate (yr-1) metric ual 1) (kJ m -2 yr -1) (kJ m -2 yr -1) (kJ m -2 yr -1) (yr - I )

Larger infauna (> 2.0 mm) Macrophagous polychaetes 12.61 _4- 5.62 Microphagous polychaetes 4.94 4- 1.56 Crustaceans 5.32 __ 2.32 Bivalves 29.76 ± 13.10 Gastropods 50.49 4-_ 16.30 Other taxa 8.15 _+ 3.08

Total

Small infauna (0.5-2 mm) Macrophagous polychaetes 0.188 ± 0.046 Microphagous polychaetes 0.239 4- 0.038 Crustaceans 0.072 ± 0.008 Molluscs 0.184_+ 0.038 Other taxa 0.064_+ 0.058

Total Total (both size classes)

124_+ 55 29+ 8 15_+ 3.6 73_+ 19.2 5.0 2.6 217_+ 80 65_+21.6 29_+ 8.0 157_____ 49.2 5.4 3.3

86_+ 27 27_ 6.2 15-t- 2.8 69_ 15.2 5.9 3.5 1 443_+418 288__+58.0 106__+17.8 657-t-126.0 2.6 1.9

748-+267 119-+37.6 48_+13.2 277_+ 84.0 2.5 1.9 98_+ 34 24__+27.0 13-+ 3.4 60-t- 18.0 5.0 5.4

2714-+505 552-t-74.4 226-+24.4 1 293-t-164.4

71__+ 16 50+ 9.0 25__+ 3.8 126__+ 21.2 10.9 10.3 224+ 24 157__+ 15.0 67__+ 5.2 373-+ 33.6 7.5 8.5 112-+ 20 103+__ 15.6 46-+ 5.8 249-+ 35.8 10.9 12.8 52__+ 12 40_+ 8.4 21_+ 3.6 10t___ 20.0 10.9 9.7 7+ 4 5-+ 2.0 4-+ 1.2 12+ 3.2 16.5 14.3

467_+ 45 335_+ 30.2 163_+11.6 861_+ 69.6 3 181 _+550 907_+ 105 389_+36 2 154_+234

terms of biomass and est imated contr ibut ion to communi ty energetics (Table 6). Winter levels of s tanding stock biomass are equal to, or exceed those of both bacteria and meiofauna (Wilkinson 1987). The biomass of animals larger than 2 mm (2 to 15 g ash-free dry wt m - 2 ) exceeds values repor ted for the sediments of Muru roa lagoon (0.35 to 2.05 g ash-free dry wt m -2) for all animals larger than 1.0 m m and exceeds values repor ted from Tikehau lagoon (0.01 to 3.48 g ash-free dry wt m -2) (Villiers et al. 1987).

The rates of energetic processes est imated here appear to be reasonable even though the allometric relationships used were derived in other ecosystems. I t is very difficult to assess the validi ty of these estimates because there are few studies of t ropical infauna that have measured product ion or other process rates from first principles. Longhurs t and Pauly (1987), reviewing the ecology of t ropical oceans, were unable to locate any such studies. In a recent review of the role of tropical sof t -bot tom communities, Alongi (1989) cites only

three studies of benthic product ion. Growth rates of two species of bivalves from a tropical estuary are repor ted as 10 to 12 times the growth rates of similar temperate species (Parulekar 1984); however the method used to measure growth rates of the tropical species is not clear. The biomass (g dry wt m - 2 ) and annual product ion (kcal m - 2 ) of four

species of mangrove crab are repor ted by Mac in tosh (1984) and P: B ratios, calculated using the conversion factors listed earlier, are in the range 0.6 to 1.8. Product ion was est imated by measuring growth increments and adding the measured energy content of moults and reproductive output . Ansell et al. (1978) used two methods to calculate the product ion of

two intert idal species of the bivalve Donax from beaches in India and concluded that a similar biomass in the tropical beaches produced a rate of turnover of biomass some ten times that of temperate beaches. Analysis of biomass changes within age classes through time suggested P:B ratios of 5.9 and 43.1 for populat ions of D. incarnatus and D.


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spiculum, respectively. P:B ratios of 13.0 and 135.3, respectively, were estimated for the same populations using allometric relationships to calculate respiration from weight and then production from respiration. Annual production of 135 times the standing-stock biomass (equivalent to a biomass turnover every 2.7 d) for a moderate sized bivalve such as Donax spp. appears improbably high. Using the same relationships, Ansell et al. (1978) calculated P:B ratios in the range 5.5 to 54.1 for other molluscs and for selected polychaetes and crustaceans. Perhaps the most reliable estimates of P:B ratios for a tropical marine benthic population yet published are those reported by Maslin and Pattee (1989) for the bivalve Corbula trigona from a brackish lagoon in West Africa. They used two methods to calculate production from direct measurements of growth, and calculated P: B ratios in the range 1.9 to 3.8. This is within the range of P:B ratios (0.3 to 4.1) calculated for other small burrowing marine bivalves from temperate regions. Thus, the best available evidence suggests that P: B ratios of tropical marine benthic animals are much the same as those for temperate populations (although this is a question that clearly requires further work) and the estimates of P: B reported here (1.9 to 16.5) do not appear unreasonable.

Macrofaunal respiration rates at the shallow (871 kJ m 2 yr 1)and deep (938 kJ m - 2 yr- 1) sites calculated from Eq. (1) account for 25 and 7%, respectively, of the community respiration measured by Hansen et al. (1987).

Consumption by macrofauna accounts for only 3 to 11% of total organic inputs to the sediment (Table 6: benthic production plus detrital material), another 14 to 17% is lost to community respiration, leaving a significant amount still unaccounted for. Part of this deficit may be explained by the exclusion of two prominent members of the sediment macrofauna from this study (holothurians and callianassid shrimp). The aspidochirotid holothurians are the largest and most easily seen of the coral reef deposit-feeders. Although individual rates of sediment processing are high (25 to 94 g d a, Hammond 1982), holothurian densities within coral reef lagoons are generally low (2.8 to 70.8 x 10 .4 m - Z ;

Harriot 1984, equivalent to 0.028 to 0.708 g dry wt m -z , assuming an average dry weight of 100 g per individual), suggesting that they are relatively unimportant in terms of carbon consumed per square metre. The callianassids are amongst the most elusive of coral reef animals. Their high density (indicated by the frequency of their feeding mounds) and high sediment-turnover rates [up to 2 500 cm 3 m - 2 d - i (Tudhope 1983), equivalent to the top 20 cm recycled 4V2 times per year] suggest they are major consumers. No satisfactory technique has yet been developed to quantify these animals or their contribution to total community metabolism; however, their influence on other components of the sediment fauna has been demonstrated here and in other studies (Aller and Dodge 1974, Hansen et al. 1987).

The original hypothesis that the biomass and production of lagoon macrofauna would decrease with distance from the reef flat and change seasonally was not supported. There are a number of reasonable explanations for the observed results; e.g. (1) food supply does not vary with either distance


from the reef flat or with season, (2) sediment fauna is not food resource-limited.

I t is quite feasible that food supply does not decrease across the lagoon; such a gradient would only be expected if the reef flat were the main source of detritus and water move- ment over the reef was as a slow, unidirectional flow from the reef flat and through the lagoon. There is now considerable evidence to refute the simple view of coral reef lagoons as sinks for detri tal mater ial produced on the reef flat (Kin- sey 1985 b). The windward margin of the semi-enclosed reefs of the central Grea t Barrier Reef reefs (such as Davies Reef) are usually entire and offer considerable resistance to t idal flow; conversely, the leeward margins are open, allowing free exchange of water between the lagoon and the open sea. This generates a reverse tide flow through the leeward back- reef irrespective of wind velocity (Kinsey 1985 b). Al terna- tively, if the principal origin of organic detritus is a source other than the reef flat, possibly in situ product ion by benthic algal mats (Kinsey 1985 b) or from lagoon phytoplankton, and if these do not decrease across the lagoon, then a reduction in macrofaunal product ion with distance from the reef flat is not likely. Gross pr imary product ion measured at three sites across the lagoon of Davies Reef was greatest at the deepest lagoon site (Hansen et al. 1987).The absence of an effect of season on macrofaunal biomass is not so easily explained. Significant increases in summer rates of pr imary product ion have been recorded on Davies Reef for the lagoon benthic algal mat (Kinsey 1985 a,b), lagoon phyto- p lankton (Furnas and Mitchell 1988), total reef flat (Barnes 1988) and reef flat epilithic algal communit ies (Klumpp and McKinnon in press). I f any of these are a limiting food source this seasonali ty should be reflected in the biomass of the smaller macrofauna at least.

I f the sediment fauna are l imited by some factor other than food supply, then a change in rate of detri tal input may not necessarily cause a corresponding change in biomass. Evidence from other studies of coral reef sediment-communities suggests that organic mater ial is not limiting. No correlat ion was found between the amount of living organisms in the sediment and the amount of detri tal mat te r in reef sediments from the Gul f of A q a b a (Vaugelas and Na im 1981) and in French Polynesia (Vaugelas 1981). These au- thors conclude that this indicates that only a small propor- t ion of detri tal mater ial is reused. The reduction of infaunal abundance towards the peaks of callianassid mounds repor ted in the present study supports the suggestion of Aller and Dodge (1974) that infauna of highly b io turba ted lagoon floors are disturbance-l imited. This agrees with the conclu- sions of Hansen et al. (1987) that organic carbon and nitro- gen concentrations, and bacterial and ciliate densities are negatively effected by call ianassid b ioturbat ion, suggesting that macro-infauna, together with other components of the reef-lagoon sediment-communities, are limited by biogenic disturbance rather than by food supply.

Acknowledgements. C.R. Wilkinson read an earlier draft and his many helpful comments are appreciated. G. Russ provided advice on statistical analyses.

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Date of final manuscript acceptance: September 22, 1989. Communicated by G. F. Humphrey, Sydney

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