Abundance, biomass and estimated production rate of
net-zooplankton community in the tropical coral-reef waters at
Tioman Island, Peninsular Malaysia
マレー半島・ティオマン島の熱帯サンゴ礁海域における
ネット動物プランクトン群集の個体数・生物量・生産量について
06D5503 中嶋 亮太
指導教員
戸田 龍樹
SYNOPSIS
サンゴ礁生態系において、動物プランクトンは１次生産物をサンゴや魚などの高次栄養段階者へ伝達する栄養力学リンクである。そのため、動物
プランクトン群集の組成や生物量並びに生産速度について調べることは、サンゴ礁生態系の栄養構造を理解する上で極めて重要である。しかし、
現在までサンゴ礁動物プランクトン個体群の生産速度について論じた研究例は少なく、個体群を形成する群集の生産量についてはほとんど知ら
れていない。本研究ではマレー半島・ティオマン島のサンゴ礁において、プランクトンネットによって採集されるネット動物プランクトン群集を昼夜連
続して採集し、これをサイズ分画（100-200, 200-335, >335 µm）して、その個体数・生物量・組成を調べ、生産速度の推定を行った。また動物プラン
クトンの餌環境について調べるために、サイズ 100 µm 以下の粒状態有機物（POM）の量・組成について調べた。調査期間を通じて、ネット動物プ
ランクトン群集の生物量は平均 3.03 (± 0.45) mg C m-3 であり、生物量が夜間に高くなる日周性が見られた。夜間の生物量の増大は主に大きなサイ
ズの分画(>335 µm)によって引き起こされていた。植物プランクトンが POM に占める割合は 6.5％と非常に低く、POM の大部分はデトリタスで構成
されていた。ネット動物プランクトン群集の生産速度は平均 1.39 (± 0.34) mg C m-3 d-1、あるいはオタマボヤのハウス生産速度も考慮した場合 2.08
(± 1.00) mg C m-3 d-1 と推定された。日間餌要求量について解析を行ったところ、植物プランクトンだけでは植食性動物プランクトンの餌要求量を十
分に満たすことできず、豊富に存在するデトリタスが重要な餌供給源の 1 つとなっている可能性が示された。POM の CN 比(4.48)はサンゴ粘液の
CN 比(4.87)と類似しており、デトリタスは主にサンゴ粘液に由来すると推定された。以上のことから、本調査域に生息する動物プランクトン群集の生
産は、植物プランクトンと主にサンゴ粘液に由来するデトリタスによって支えられていると思われ、これらを基点に本調査域サンゴ礁の漂泳区生態
系は成り立っていると考えられた。
Keywords: coral mucus, diel variation, detritus, food requirements, trophic structure
qualitative aspects of material flow at lower trophic levels.
Production estimates of community or populations of certain
net-zooplankton have been made in several coral reefs, such as
in Tikehau Atoll [2], Takapoto Atoll [3], Uvea Atoll [4], Great
Barrier Reef [5], and Palau [6]. However, the studies on the
production estimates of reef net-zooplankton are still few
compared to those in temperate waters or other sub- and
tropical waters, and thus I have attempted to estimate it in this
study.
The primary purpose of this study was (1) to examine the
abundance, biomass and composition of coral reef
net-zooplankton community on a diel basis, (2) to examine
quality and quantity of particulate organic matter to estimate
the food environment for the net-zooplankton community, and
(3) to estimate the production rate of the net-zooplankton
community in order to better understand the net-zooplankton
ecology in coral reef ecosystem.
Materials and Methods
Study site
This study was carried out at Tioman Island off the east coast of
Peninsular Malaysia (Fig. 1). The island forms a typical fringing coral
reef. Zooplankton and water sampling was conducted at a jetty near
Mango Reef of Tioman Island (2º50΄00˝N; 104º09΄40˝E) during 20-22
October in 2003, 22-24 August and 1-3 October in 2004 and 25-27
February and 2-4 June in 2005. Acropora formosa, one of the dominant
corals in Mango Reef, were taken on 13-24 March 2006 for chemical
analysis of coral mucus.
104º 09’ E
South China Sea
Laboratory
Marine park office
Jetty
Peninsular
Malaysia
Tioman
Island
Zooplankton and
water sampling
Depth:. 8 m
Coral sampling
Depth:. 3 m
Reef
100 m
Mango
its
tra ca
e S lac
Th Ma
of
Introduction
Net-zooplankton are one of the integral components of coral
reef ecosystems, for they are one of the energy sources to many
reef inhabitants including scleractinian corals and fishes.
Zooplankton abundance in the water column increases at night
over coral-reefs. This increase is caused by onshore advection
of pelagic zooplankton which have diel migration offshore
and/or migration of demersal zooplankton which stay during
daytime in or on the substratum or near the bottom, and migrate
into the water column at night. Daytime collections only are,
therefore, not suitable for providing a real picture. Coral reef
zooplankton often produces density peaks at various times
throughout the night, e.g. soon after sunset or before sunrise,
and thus investigation on zooplankton density with short time
intervals would provide fundamental information on their diel
behavior and the representative values of zooplankton density.
To understand the net-zooplankton ecology, it would be
important to quantify the amount and composition of their food
sources. The food sources consist of living organic particles
including pico-, nano-, and microplankton, and non-living
organic particles, i.e. detritus. The latter, detritus is considered
as one of the important food sources for net-zooplankton in
oligotrophic environments including coral reefs. For instance, it
is considered that net-zooplankton utilize detritus as a
significant food source to compensate for the low available
phytoplankton stocks in coral reef lagoon of Great Barrier Reef
(GBR) [1].
To understand the ecological dynamics of marine ecosystem
including coral reef system, it is important to know the
qualitative and quantitative aspects of material or energy flow
from lower to higher trophic levels. It requires the estimation of
the production rates of populations at different trophic levels
and the transfer efficiency between adjacent trophic levels.
Because net-zooplankton are one of the important trophic link
between primary producers and higher trophic levels, the
importance of studies on production rates of net-zooplankton
community has been recognized to assess the quantitative and
Indonesia
Fig. 1. Map of the sampling sites in Tioman Island, Peninsular Malaysia.
04º 49’ N
Sampling
Net-zooplankton was collected every 3-h for 48-hs by five gentle
vertical tows of a plankton net (mesh size, 100-µm) with a flowmeter
(Rigosha) from the water column of 1 m above the sea bottom to the
surface. The collected samples were pooled and immediately brought
back to the laboratory. Prior to the net-zooplankton collection, water
was sampled with a 10 L Niskin bottle at 1 m depth below the surface
and 1 m above the sea bottom for measurements of POM and
chlorophyll-a (chl-a) concentrations, inorganic nutrient concentrations
(PO4, NO2, NO3 and SiO3), and community composition of pico-, nano-,
and microplankton. The water from the two depths were pre-filtered
through a 100 µm mesh screen to remove net-zooplankton and later
combined. The combined water (ca. 20 L) was brought back to the
laboratory along with the net-zooplankton sample.
Coral mucus was collected directly by air-exposure of Acropora
formosa for measurements of particulate organic C and N. The
collected corals were exposed to air, inverted and hung under sun light
for 5 to 10 min. The coral specimens rapidly released gel-like mucus,
which was immediately collected in sterilized 50 ml Corning® tube.
Ambient seawater was collected simultaneously with coral mucus
collection for comparison using a 5 L Niskin bottle at the jetty.
Sample analysis
The net-collected samples (>100 µm) were size-fractionated into
three size-classes (100-200 µm, 200-335 µm and >335 µm) by mesh
screens of 200 µm and 335 µm, and fixed with 5 % buffered formalin
seawater for microscopic analysis. Large zooplankton or rare species
(e.g. mysids, larval decapods, fish larvae, etc) were first counted and
sorted out, then the remaining was split (1/1-1/32), from which all
zooplankton were characterized and enumerated under a dissecting
microscope. Copepods were identified to species, stage (adult or
copepodites) and sex whenever possible. The length of an appropriate
body portion was measured using an eyepiece micrometer. At least 300
zooplankton were measured in each sample. The length estimates were
converted to carbon biomass using the previously reported
length-weight regression equations such as by Uye [7].
Subsamples for inorganic nutrients analysis were filtered through a
0.45 µm filter and analyzed using Bran+Lubbe AASC II Autoanalyzer.
A subsample (2000 ml) of the pre-filtered seawater was used for POC
analysis and filtered onto a 2 inch GF/F filter (Whatman) which was
pre-combusted (500 ºC, 4h) and pre-weighted. The GF/F filters were
rinsed with distilled water and placed over fuming HCl in a closed
glass container to remove carbonates for 24-h. The filters were ground
using mortar and pestle, from which subsamples were put into a CN
analyzer (Fisons model EA 1108 CHNS/O) for POC measurements
(PON was not detected due to the inadequate amount of the subsamples
that were put into the CN analyzer.
To measure the size structure of plankton, 18 L of the pre-filtered
seawater (<100 µm) were gently filtered through a 35 µm mesh screen
to obtain two size-fractions, i.e. <35µm and 35-100 µm. Analysis of the
filtrate (<35 µm) and the particle retained on the 35 µm mesh screen
(35-100 µm) were handled as follows. Chl-a concentrations were
determined on a subsample (2000 ml) of the filtrate (<35 µm) that were
successively filtered through a 3 µm pore-size membrane filter
(Millipore) and a GF/F filter. Nominal opening size of GF/F filter is
defined as 0.7-0.8 µm; I used 0.8 µm for the opening size expediently
in this study. The filter was placed in N,N-dimethylformamide (DMF)
and stored at -20 ºC until analysis. Chl-a concentrations were
determined using a fluorometer (Turner Designs 10-AU). A subsample
(100 ml) and a subsample (50 ml) of the filtrate (<35 µm) were taken
for microscopic analysis and fixed in 1% glutaraldehyde seawater and
in 2% buffered formalin seawater, respectively. The preserved samples
were collected in Corning® tube and stored in darkness at -20 ºC until
observation. The observation was carried out within one month for the
formalin samples and one year for the glutaraldehyde samples. The
particles retained on the 35 µm mesh screen (35-100 µm) were divided
into two aliquots with a Folsom plankton splitter. One aliquot was used
for Chl-a measurement and the other for microscopic analysis. The
aliquot for Chl-a measurement was filtered onto a GF/F filter, which
were placed in DMF for pigment extraction and stored at -20 ºC until
analysis as described above. The aliquot for microscopic analysis was
fixed with buffered formalin to a final concentration of 2% and stored
at 5 ºC until observation. Duplicate subsamples (5 ml) of the collected
mucus for POC and PON measurements and duplicate subsamples
(1000 ml) of the ambient seawater for comparison were filtered onto
pre-combusted GF/F filter and analyzed as described above.
To enumerate heterotrophic bacteria (hereafter bacteria),
cyanobacteria, and flagellates, the glutaraldehyde fixed sample was
filtered onto a 0.8 µm black polycarbonate filter and stained with
primulin. Cyanobacteria, bacteria, flagellates were counted with an
epifluorescence microscope using blue and green excitation at a
magnification of 1000 ×. For cyanobacteria and bacteria, at least 400
cells were counted per slide. For flagellates, at least 100 microscope
fields per slide were scanned. Cell volumes of bacteria and flagellates
were calculated from the length and width measured. The bacterial cell
volumes were converted to carbon units using a conversion factor of
0.209 pg C µm-3 [8]. The flagellate cell volumes were converted to
carbon units using a conversion factor of 183 fg C µm-3 [9]. Shrinkage
of cell volume with preservation was taken into account [10].
Cyanobacterial cell numbers were converted to carbon units using a
conversion factor of 200 fg C cell-1 [9]. The microscopic sample of the
fraction 35-100 µm was characterized into different taxonomic groups
such as ciliate, copepod nauplii, copepodites, and other metazoans and
counted under a stereomicroscope. Their body length was measured to
determine lorica volume of tintinnids, cell volume of naked ciliates and
carbon weight of metazoans. The lorica volume of tintinnids was
converted to carbon weight using the regression equation: CW (pg) =
444.5 + 0.053 LV [11], and the cell volume of naked ciliate was
converted to carbon unit using a factor of 0.14 pg C µm-3 [12].
Shrinkage of cell volume with preservation was taken into account [10].
The carbon weight of metazoans was calculated from their body length
using the previous regression equations as described earlier.
Production rate and food requirement estimates
The production rate (P, mg C m-3 d-1) of a given taxonomic group
was estimated based on its biomass (B, mg C m-3) and specific growth
rate (G, d-1): P = B × G. Shrinkage with formalin preservation were
taken into account for the biomass. The specific growth rate of
copepods, other crustaceans, chaetognaths, cnidarians, larvaceans and
polychaete larvae was estimated from regression equations proposed by
Hirst et al. [13]. For the remaining taxa (e.g. bivalve larvae), the
production rate (P, mg C m-3 d-1) was estimated by the equation given
by Ikeda & Motoda [14]. Although this study focuses chiefly on
production rates of net-zooplankton community, microzooplankton
production rates were also estimated. The growth rate of ciliates was
derived from the multiple regression equation given by [15]. Secondary
and tertiary productions were calculated separately on the basis of the
feeding habits (i.e. herbivorous, omnivorous and carnivorous) of each
group based on literatures [16]. The production by typical herbivores
and carnivores was assigned to secondary and tertiary production,
respectively. The production by omnivores was halved and added to
each production. To determine the potential carbon flow from prey
organisms to predators, the amount of carbon required by the
consumers to support their estimated production rate was calculated
using a gross growth efficiency of 0.3 for metazoans [14] and 0.4 for
protozoan microzooplankton [17].
Results and Discussion
Diel variations of net-zooplankton community
Mean abundance (inds. m-3) of total zooplankton (>100 µm)
was 7,061 (± 1,836) inds. m-3 ranging from 3,890 (± 1,690)
inds. m-3 in October 2003 to 8,339 (± 3,245) inds. m-3 in
February 2005. Mean biomass (mg C m-3) was 3.03 (± 0.45)
mg C m-3 ranging from 2.29 (± 1.34) mg C m-3 in June 2005 to
3.39 (± 1.85) mg C m-3 in October 2004. The examination of
the diel variation of different size-fractions revealed that the
night increase occurred most strongly in the large fraction
(>335 µm) (Figs. 2, 3). This result is similar to that in a report
from the Gulf of Aqaba, Red Sea, that a night increase in
zooplankton biomass was generally due to an increase in
larger-sized zooplankton (>200 µm) [18]. Intense daytime
zooplanktivory by fish may be one of the major factors
determining diel variation in the abundance of coral-reef
zooplankton. Some larger individuals experience a greater
susceptibility to visual predators and hence they need to
descend and spend the daytime near the bottom or in the
crevices of the coral substratum. This behavior in the large
sized zooplankton may have caused the strong day/night
100-200 µm
(a)
(d)
(j)
1.2
2.5
1.2
1
2
1
0.8
1.5
0.8
0.6
0.6
1
0.4
0.4
0.5
0.2
(b)
(e)
6
5
1.5
4
1
3
2
0.5
1
0
0
(k)
Bivalve larvae
Polychate larvae
Copepods
Decapods
Other crustaceans
Chaetognaths
Appendicularians
Others
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
7
2
0
(h)
8
2.5
0.2
0
0
200-335 µm
Biomass (mg C m-3)
(g)
1.4
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
>335 µm
(c)
(f)
(i)
2
1.8
1.6
1.4
1.2
1
0.8
0.6
0.4
0.2
0
5
4.5
4
3.5
3
2.5
2
1.5
1
0.5
0
12
18
22 Aug
0
6
12
23 Aug
18
0
6
24 Aug
(l)
2.5
3.5
2
2.5
3
2
1.5
1.5
1
1
0.5
0.5
0
18
0
1 Oct
6
12
2 Oct
18
0
6
3 Oct
12
12
18
25 Feb
0
6
12
26 Feb
18
0
6
27 Feb
0
12
18
2 Jun
0
6
12
3 Jun
18
0
6
4 Jun
Fig. 2. Diel variation in biomass of size-fractionated zooplankton composition.
Black bars indicate hours of night.
100-200 µm
0.5
0.45
0.4
0.35
0.3
0.25
0.2
0.15
0.1
0.05
0
(a)
1.4
(d)
1.4
1.2
1.2
1
1
0.8
0.8
0.6
0.6
0.4
0.4
0.2
0.2
(g)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
0
(j)
200-335 µm
Biomass (mg C m-3)
difference [19].
Temporal variations in zooplankton concentration in
shallow water depend generally on two factors: (1) transport of
pelagic species by horizontal currents and (2) vertical
migrations of demersal zooplankton. Demersal zooplankton on
reefs is mainly comprised swarmers and epibenthic forms. The
swarmers, active aggregations of individuals, maintain position
near the bottom or around coral formations without settling on
the substratum during the day and disperse at night, while the
epibenthic species reside on and/or within the bottom substrate
or coral formations during the day and some migrate into the
water column at night. Among the zooplankton taxa that
significantly increased during the night, Pseudodiaptoms spp.,
benthic harpacticoids, cumaceans, ostracods, amphiopods,
polychaetes and many decapods are categorized as epibenthic
species, while Acartia erythraea and Centropages orsinii are
categorized as swarmers. Some Oithona species are known to
be swarmers at various coral reefs, but it is obscure that
whether the Oithona in the present study are swarmers since
they were not identified into species level and thus pelagic
species originating from offshore might be included. During
ebb tide, when the external influence is minimal, zooplankton
samples collected in reefs contain almost exclusively demersal
zooplankton. However, the density of many pelagic species
such as Paracalanidae increased coinciding with both minimum
and maximum tide levels throughout the study periods. Typical
pelagic zooplankton undergo diel migration offshore and advect
inshore with the current or tide at night. Some species
traditionally characterized as pelagic forms behave like typical
demersal zooplankton when they inhabit a coral reef
environment to prevent being swept off the reef by surface
currents or to avoid heavy predation by abundant visual
predators such as fish [20]. It is also possible that pelagic
species, which have a diel migration offshore, maintain vertical
migration when advected into the reef but cannot reach their
normal maximum depth [20]. Therefore, the increase during the
night in this study may be caused by both demersal
zooplankton and the pelagic species [21].
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(b)
1
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
(e)
0.9
(h)
1.8
0.8
1.6
0.7
1.4
0.6
1.2
0.5
1
0.4
0.8
0.3
0.6
0.2
0.4
0.1
0.2
Acartia
Centropages
Subeucalanus
Paracalanus+Acrocalanus
+Bestiolina
(k)
Calanopia
Pseudodiaptomus
Temora
0
0
>335 µm
3
(c)
1.4
2.5
2
1.5
1
0.5
(f)
1.4
1.2
1
1
0.8
0.8
1.5
0.6
0.6
1
0.4
0.4
18
22 Aug
0
6
12
23 Aug
18
0
6
24 Aug
6
12
2 Oct
18
0
6
3 Oct
12
Macrosetella
Euterpina
Corycaeidae
Oncaea
Nauplii
Others
0.5
0
18
0
1 Oct
Oithona
Microsetella
(l)
2
0.2
0
12
2.5
1.2
0.2
0
(i)
0
12
18
25 Feb
0
6
12
26 Feb
18
0
6
27 Feb
12
18
2 Jun
0
6
12
3 Jun
18
0
6
4 Jun
Fig. 3. Diel variation in biomass of size-fractionated copepod composition. Black
bars indicate hours of night.
Composition of particulate organic matter
POC concentration in the study area was 160-178 mg C m-3.
The mean total chl-a concentration was 0.22±0.07 mg m-3.
The phytoplankton assemblage was dominated by pico- and
nanoplankton (85-90% of the total chl-a), especially by
picophytoplankton cells (>50%), which may reflects the
nutrient-poor environment (PO4, 0.07±0.04 µM; NO2, 0.02±
0.01 µM; NO3, 0.14±0.04 µM), though the concentration of
silicate was high (7.52±0.98 µM). A C:Chl-a ratio of 50 [22]
was used for calculating phytoplankton carbon biomass. The
phytoplankton C biomass contributed 3.8-9.8% of the total
POC concentrations (mean: 6.5%). Detritus mass (mg C m-3)
can be estimated by subtracting the value of living organic
carbon from that of POC. The detrital contribution to the total
POC varied from 89-96% (mean: 93%). This high proportion
confirmed previous measurements performed in other coral
reefs; 77% detritus of POC in Davies reef, GBR [1], 84% in
Tikehau atoll, French Polynesia [23], and 93% in Enewetak
atoll in winter [24]. Since the detritus was the major component
of POC in this study, most of the diet of particle-feeders or
suspension feeders would consist in detritus.
Detrital particles may be derived from dead turf algal
community, coral mucus, larvacean’s house and other materials
(e.g. moults and feces). As the detrital particles were the major
component of POM, their origin may be able to estimate by the
C:N ratio of POM. The C:N ratios of POM in this study was
4.48±0.17, which was very similar to those of coral mucus
produced by Acropora formosa (4.87±0.13). It was supposed
that the detritus in the present study mainly originates from
mucus produced by corals [25]. Corals exude as mucus ca. half
of the carbon assimilated by their zooxanthellae into the water
column, of which ca. 30-50% was released as particulate forms
[26]. The coral mucus is expected to contribute to the main diet
of the suspended particle-feeders or detritivores in the study
area.
Production estimates
The daily production rate of net-zooplankton was on
average 1.39 (± 0.34) mg C m-3 d-1. The size fraction which
contributes most to the total production rate varied depending
on the month. The large fraction (>335 µm) was the major
contributor to the total production rate in August and October
2004 and June 2005, while the small fraction (100-200 µm)
was the major contributor in February 2005 (Fig. 4). Copepods
were one of the most important groups in total net-zooplankton
production rate. The total production rate of copepods (>100
µm) was 0.69 (± 0.09) mg C m-3 d-1. Paracaranidae
(Paracalanus + Bestiolina + Acrocalanus), Oithona and
copepod nauplii were the most important components among
the copepod community production rates. Larvaceans were
secondary dominant except in October 2004.
Aug 2004
Oct 2004
Feb 2005
Jun 2005
Copepods
1
Other zooplankton
0.8
0.6
0.4
>335
200-335
100-200
200-335
>335
100-200
>335
200-335
100-200
200-335
>335
0.2
100-200
Production rate (mg C m-3 d-1)
1.2
Size class (µm)
Fig. 4. Estimated production rate of net-zooplankton in different size-classes
(100-200 µm, 200-335 µm, >335 µm).
In larvaceans, house production can be an important
contribution to total growth. However, as the equation for
larvaceans production rate I used in this study is not exactly
reflected in the house production data, the production rate for
larvaceans may be underestimated. The larvaceans production
would be increased if I account for the production of houses,
which are discarded and re-secreted frequently. The carbon
content of newly secreted houses has been estimated as 23% of
body carbon for Oikopleura species [27]. The rate of house
production depends on ambient temperature and it has been
estimated to 15 houses per day at 28 ºC [28]. Considering the
average temperature was 28.4 ºC in this study, at 15 houses per
day, this would be equivalent to an additional 0.25-1.70 mg C
m-3 day-1. This yields a total net-zooplankton production
estimate of 1.33-3.53 mg C m-3 day-1 (overall mean: 2.08±1.00
mg C m-3 day-1), which is 119-193% that of the previously
estimated net-zooplankton production.
To demonstrate the food chain structure, averages of
phytoplankton and zooplankton biomass and of zooplankton
production rate were compared. Net-zooplankton biomass was
divided into herbivorous net-zooplankton (i.e. typical
herbivores
and
1/2
omnivores)
and
carnivorous
net-zooplankton (i.e. typical carnivores and 1/2 omnivores).
The diagram of the food chain structure of the each study
period and overall mean are shown in Fig. 5.
(a)
(b)
CNZ
CNZ
CNZ
0.26 / 0.93
0.29 / 0.82
0.95
0.86
CNZ
0.25 / 0.80
CNZ
0.33 / 0.78
0.84
HNZ
MZ
1.58 / 2.45
1.01 / 2.47
MZ
0.40 / 0.65
4.64
7.86
MZ
0.30 / 0.65
0.77 / 1.59
3.55
Phytoplankton
Phytoplankton
Phytoplankton
10.14
10.44
Aug 2004
0.94
1.11
HNZ
HNZ
0.81 / 1.36
0.28 / 0.83
Oct 2004
11.67
Feb 2005
HNZ
HNZ
MZ
0.64 / 0.90
1.06 / 1.60
5.63
Phytoplankton
12.23
Jun 2005
MZ
1.10 / 2.03
0.54 / 0.89
5.42
Phytoplankton
11.12
Overall
Fig. 5. Schematic carbon flow diagram at Tioman Island. Values in boxes of
phytoplankton denote their biomass (mg C m-3) and of MZ, HNZ, CNZ are
production rate (mg C m-3 d-1) / biomass (mg C m-3). Biomass expressed in bold.
Values with arrows are daily requirements (mg C m-3 d-1) of the components
above them. MZ: microzooplankton, HNZ: herbivorous net-zooplankton, and
CNZ: carnivorous net-zooplankton.
Overall, the relative biomass of the primary, secondary and
tertiary producers was 74.8%, 19.6% and 5.6%, respectively.
The daily carbon requirement of the tertiary producers
corresponded to 32.3% of secondary producers. The transfer
efficiency from the secondary to tertiary production was 17.2%.
The daily carbon requirement of the secondary producers (5.42
mg C m-3 d-1) approximated to 48.8% of phytoplankton carbon
biomass and it is likely to be high. Although primary
production of phytoplankton was not measured in this study, it
can be roughly estimated from chlorophyll and light data. The
primary production of phytoplankton (P, g C m-2 day-1) can be
estimated using a regression proposed by Ryther & Yentsch
[29] as: P = R/k × C × 3.7, where R is relative photosynthesis
for the appropriate value of surface radiation (m-3 day-1), k is
the extinction coefficient (m-2), and C is phytoplankton biomass
(g Chl-a m-3). The average phytoplankton biomass was 0.22 mg
Chl-a m-3. The extinction coefficient (k) is calculated to 0.085
from the Secchi disc reading (SD), i.e. k = 1.7/SD [30],
considering the Secchi disc depth off the study site was 20.1 m.
The daily surface radiation near to the study site is 480.8 cal
cm-2 day-1 and thus R can be determined as 8.8 from the Figure
1 of [29]. If it can be assumed that the daily irradiance is
constant throughout the year because the latitude of the study
site (2º50΄00˝N) is close to the equator, the primary production
of phytoplankton would be estimated to 0.084 g C m -2 day-1 or
9.7 mg C m-3 day-1 considering an average depth of the study
site (8.7 m). Therefore, the transfer efficiency from the primary
to secondary production would be 16.9% or 24.1% taking the
house production by larvaceans. The carbon requirement of
secondary producer would be equal to 80.4% of the primary
production considering the larvaceans house production. In this
case, the remaining (19.6%) primary production is probably
barely adequate to support bacterial production, which
generally averages 20-60% of the primary production [31].
Hence, a certain amount of carbon required by the secondary
producer might be supplied with detritus which constituted the
major proportion of particulate organic carbon as in other coral
environment. Phytoplankton alone may not be sufficient to
sustain the secondary production. The carbon requirements of
herbivorous net-zooplankton can be compared with their
ingestion rates on detritus to know whether the detritus fulfill
the estimated zooplankton carbon requirement. A filtration rate
on heterotrophic particles including detritus has been estimated
as 2 liters mg C-1 h-1 for natural net-zooplankton assemblage in
Davies Reef, GBR [1]. By multiplying the filtration rate with
the average detritus mass (176.4 mg C m-3) and herbivorous
net-zooplankton biomass (2.03 mg C m-3), it was estimated that
the ingestion rate on detritus of 13.8 mg C m-3 d-1 for the
herbivorous net-zooplankton assuming all the herbivorous
net-zooplankton assimilate 80% of their ingested carbon [32], a
value is much higher than their estimated carbon demand (5.42
mg C m-3 d-1). Obviously not all the detritus is available to the
herbivorous net-zooplankton, but even if only half the detritus
particles could be captured by the net-zooplankton, the
ingestion of detrital carbon would be fulfill their metabolic
demands. Consequently, the detrital carbon can be considered
to be one of the important carbon sources in the study site.
Coral mucus probably contributes most to the detritus as
inferred by C:N ratio. Coral mucus detritus may provide one of
the significant pathways for the conversion of coral primary
productivity to higher trophic levels as laboratory experiments
using labeled coral mucus have shown that both reef copepods
and mysids can ingest and assimilate mucus [33, 34].
Investigations on availability of coral mucus by reef
zooplankton would, therefore, be important to better understand
the coral reef pelagic ecosystem.
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