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Chapter 28. The impact of organic matter on ostracods from an equatorial deltaic area, the Mahakam delta–southeasternu Kalimantan
P. CARBONEL AND T.HOIBIAN
a population transformation marked by the successive replacement of species or specific groups,
controlled by progressive mixing of waters. This water mixing/species replacement relationship
suggests the existence of factors such as tide and season (high and low water level) as determining
the positioning of the zone in which waters mix. Similarly, these factors condition the dynamic
intensity and mode of action and subsequently, the formation or extinction of biotopes. They also
affect the biological rhythm responsible for the productivity, i.e. supply on the bottom. This 'longitudial' evolution is overlain by a 'vertical' evolution dependent upon the environment's intertidal
quality (Carbonel, 1978, 1980, 1982). Such an evolution is typical of fresh/saltwater systems under
temperate climatic conditions. By contrast, in an equatorial climate, the smoothness of seasonal
effects and the reduced euryhaline impact produce a completely different situation, which we attempt at explaining here through the use of the Mahakam delta example in Indonesia.
The Climatic and Hydrosedimentary Context
The Mahakam delta covers an area of 5000 km2ranging between 0" 4's and 1" 10's and 117" W E
and 117"40'E. It presently represents a crisscross of marsh land and channels where sea and delta
are closely interrelated (Text-fig. 1). It is a transitional system of fresh/saltwater functionning under
equatorial climatic conditions. This system implies :
- constant humidity leading to diminished salt in intertidal environments. The salt deficiency
phenomena are linked to rainfall frequency and supply of fresh water by aquifers;
TEXT-FIG.l-Mahakam delta? a) Geographic area; b) General surface water circulation in the Makassar strait.
Impact of Organic Matter on Ostracoh 355
- high water temperature provoking an oxygen deficit at the waterlsediment interface;
-relatively low salinity (32 %o.) at least in the upper water section.
The drainage of the river basin brings water masses which have passed through areas of
exceedingly dense vegetation into the delta. The crossing of these areas induces:
-water bearing organic matter (O.M.)
with low pH, depleted even more by the rate of precipitation ;
-heavy solid charge.
Parameters, results of the marine influence, are active at several levels:
-the mean tidal range is 1.20 at 1.50 m with quite strong tidal currents of the order of 1 m/s;
-the low tidal range is due to a very high complex topography.
The delta itself is the result of sedimentary accumulation taking place when the river attains its
basal level. In the midst of this complex channel tangle, 2 large units may be observed (Text-fig.
2), characterised by specific hydrological regimes (Allen et al., 1979; Hoibian, 1984; Carbonel
et al., 1985; Gayet et Legigan, 1985).
The deltaic plain is subdivided into 2 groups: (a) the fluviatile deltaic plain grouping milieus in
the upstream limit of the salt intrusion and (b) the tidal deltaic plain delineating the milieus affected
by the saline increase.
Within this tidal complex (s.1.) a distinction may be made between channels of fluvial run-off
or distributaries, and tidal channels or interdistributaries (Text-fig. 2).
The delta front is characterised by environments of a quasi-permanent marine character. It occupies the area between the downstream limit of inlets and the prodelta edge. Its inner most part
may undergo slight desalination (Hoibian, 1984).
The prodelta area correspondsto the talus, limited at the top by the -5m isobath. It is a strictly
A-Fluvial delatic plain
Alluvial deltaic plain
B-Tidal deltaic plain
Amont: seasonal tides
Aval: diurnal tides
C D e l t a front
RRQP i1;12 tidal fiatdcl
I.. ., _ - 1.
1. .; . ' . I
2-Mahakam delta: General morphology (after Gayet 1985).
AND T. HOIBIAN
Inter to kfratidal
- = degraded,
v = very,
Cytherelloidea gr. atmai
H. retic. :20 % Alcopocythere
N. micro degrd. kendengensis
H . retic.:lO % Trachyleberis dacyi
N. micro agrad. Cythereis resmani
Keqia sp., Cythereis zenkeri
Argilloecia sp. div.
+ = aggraded.
The Present-day Characteristics of Ostracod Fauna
The Mahakam ostracod fauna may be qualified as:
Indopacific, for it is more similar to the Persian Gulf fauna than to the Birman coast fauna (Paik,
1976; Keij, 1964; Malz, 1980).
Small-sized, compared with the corresponding species of the continental shelf.
Mixed, a mixture created by their habitat (local currents, topography).
Because of the low pH in the river waters (pH<5) carbonates are absent. Calcareous fauna
appear to be limited in slightly desalinated waters. The hydrochemical factor plays a most significant part especially with regard to carbonate equilibrium. Five population communities have
been identified (Hoibian, 1984; Carbonel et al., 1985).
RESPONSIBLE FOR THE DISTRIBUTION
Distribution of ostracod assemblagesis very heterogeneous (Text-fig. 3) and influenced by three
- stable marine salinity;
- topography (channels, intertidal zones);
- location (inlets, interhediary zones i.e., areas free of deltaic water influence).
The occurrence of ostracods only in euhaline waters suggests intolerance of the dissolving riverborne effect of waters at pH 5. It is only when pH is sufficientlyhigh (i.e. in marine waters) that organisms can bioprecipitate their carbonate test; the development of these organisms is discontinuous.
Topography exerts its classical effect and also reveals the dynamics and emersion impact (Car-
Tern-no. 3-Ostracod distribution in the Mahakam delta area (after Carbonel et ul., 1985).
bonel, 1980). Ostracods proliferate in intertidal biotopes thanks to the enrichment in nutritive
(N, P) and constructiveelements (S) which is more intense in these oxygen-rich areas than in emerg-
358 P. CARBONEL
ed low-oxygenated environments. The intense water circulation in channels also prevents the
proliferation of ostracods.
The difference in composition and diversity of ostracods within the same assemblage depend-
milieux latCraux (b) (SituCs entre ou hors des embouchures)
(between or out of the inldr)
A :Abondant I Abundant
B : Commun I common
: Trbs divasifiC / very diversified
: diverdib I Diverdied
PD : peu diversif%I sbhtly diversified
M : rnonoap6cifiqueI monospecific
Tw-FIO. 44stracod faunatharacteristicsin the Mahakam delta area (after Carbonel et ul., 1985).
Impact of Organic Matter on Ostracods 359
ing on location as well as ornamentation differences in certain species, is more difficult to account
for. Indeed, it must be noted that quality of fauna, its quantitative characteristics (density and
diversity) and the polymorphism of certain species change according to the habitat zone and the
place where biotopes are found, whether in the inlet or intermediary zones of the delta front.
These faunas follow a faunal gap which prevailed in almost all the deltaic plains. With respect to
those in the intermediary zones, the inlet zone faunas are richer, less diversified and less ornamented
(Text-fig. 4). There is in all likelihood a ‘continent’phenomenon. This statement being vague, other
facors are necessary to reveal the effects of chemical equilibrium at the water/sediment interface,
namely the carbonate equilibrium. These factors must be able not only to stop bioprecipitation
of calcite during the moulting stage, but also to upset the osmotic processes, i.e. modify the pH
input and eventually, salinity. This context becomes the action field of organic matter.
The dominant characteristic in the Mahakam deltaic zone is the abundance in organic matter
issuing from both the mangrove swamps and vegetation throughout the river basin and from true
bacterial mixtures developing in the interdistributary zones.
The nature of the organic matter differs in accordance with the drainage of the deltaic plain
(distributary or tidal channels). Referring to the model proposed by Etcheber (1983) it may be
noted that this envisages:
- a distributary zone in which the dominant part of the organic matter is allochthonous and
highly degraded because of a long ‘journey’ before its arrival into the estuary. The organic matter
becomes polymeric with its most labile molecules entirely mineralized under the action of exoenzymes, light, etc. Furthermore, the remaining organic matter is likely to be closely associated
with a particularly abundant inorganic phase of clays and colloids. It is also probable that the
autochthonous organic matter, i.e. the matter in the estuarine waters, represents only a minor
fraction because of:
- abundance of suspended matter (S.M.) darkening the water layer and greatly restricting
phytoplankton and aquatic plant development (Head, 1974);
- the current intensity toward the lower part of the estuary preventing colonisation by zooplankton Castel, 1980);
- the occurrence of maximum turbidity prohibiting the development of benthos (Pujos, 1976).
- an interdistributary zone in which the organic matter is more abundant and diversified. It
is often allochthonous and at certain evolutionary stages it may be represented by a material very
little developed, made up of biopolymers and a complex original organic matter (cellulose, lignite,
resin . . . ) arriving in the estuary after a short transport period. These poorly transformed molecules become a very attractive pole for bacteria responsible for the proliferation observed, and the
autochthonous organic matter made up of an exceeding abundance of phyto and zooplankton,
the debris of which will increase the bacterial population.
On the bottom, the extremely variable organic matter sources are associated with the residence
time rate of organic elements determining oxygenation and pH conditions. The latter are particularly negative until levels outside the ‘inlet’ areas are reached (Allen et al., 1979).
The abundance of organic matter transported by water may be reflected in different ways:
- precipitation of iron in the inlets of the interdistributaries, a phenomenon not observed in the
distributary zones (Goldsmith, 1954; Dussart, 1960).
- low development of carbonates in the inlet areas (Allen et al, 1979, Text-fig. 5).
- eqyjlibrium of carbonates dissolved at the water/sediment interface with repect to bioprecipi-
360 P. CARBONEL
AND T. HOIBIAN
c I O/O
/ I ZONE1
5- -Percentage of carbonates in the sands (after Allen et al., 1979).
tation of the shelled benthonic organisms, this point being essential in the development of ostracods (Text-fig. 5).
The precocious degradation of organic matter on the bottom by bacteria increasing the pCOz
and provoking a drop in pH (Lalou, 1957) results in a dissolving environment for carbonates.
This result is herewith called ‘the organic matter effect’ having in our study a twofold impact:
organic matter intensity and dilution by marine waters.
Ostracods in channels, whether distributary or tidal, are practically non-existent. In distributary
channels, this absence is probably due to the transitory water mass which is highly turbide, preventing the development of a benthonic fauna. By contrast, the development of faunas may be encountered in intertidal zones.
In the tidal channel, maceration of the organicmatter leads to true bacterial culture mediums, the
high activity of which provokes quasi-anoxic conditions on the bottom (Allen et al., 1979; Etcheber, 1983). Anoxia is also an obstacle in the development of a benthonic fauna, hence, the lack of
fauna in the deltaic zme and in the intertidal zones of tidal channels.
Impact of Organic Matter on Ostracods 361
In the inlet areas of the delta front, i.e. in places where the organic matter impact remains
highest despite its diluted state, as regards the assemblagediversification there was (1) a considerable
decrease and (2) a change in the quality when certain species such as Hemicytheridea reticulata
Kingma became dominant after the elimination of many other species.
Polymorphism observations were also noted in the ornamentation of certain species due to the
'agradation/degradation' phenomenon (Peypouquet et al., 1980; Farmer and Carbonel, 1984).
This phenomenon is reflected in the reduction or increase of the valve sculpture; a variation associated with carbonate equilibrium at the water/sediment interface where ostracods live and
moult. Within a dissolving environment, i.e. where ostracods must use maximum energy to bioprecipitate the calcite of their test, organisms work 'economically'; their valves have very little
ornamentation. By contrast, within a precipitating environment, where ostracods can bioprecipitate their carapaces without great effort, ornamentation is much more elaborate.
To conclude, 'the organic matter effect' differs according to the biotope location induced by a
more or less intense dilution. This effect may be reflected in the following model (Text-fig. 6).
Zoo + phytoplonkton
Unminerolired 0 M
Minerolired 0 M
Low bacterial activity
loco1 0 M poor
No carboroh bonthor
MIXHG WITH MARINE WATER
Morine water 100%
Abundont ond vary
W>Dw M w a M w < D w
bt rlck an6
6-Modes of action of organic matter in the Mahakam delta area. Effect on the ornamentation of Neomo*oceratina.
362 P. CARBONEL
AND T. HOIBIAN
In the delta front inlet areas (mostly tidal channels) the organic matter impact is somewhat
slowed down whilst the specific diversity is reduced and the majority of polymorph individuals are
inclined to lose their ornamentation (degradation of the organic matter induces bottom dissolving
conditions). Correspondingly, oxygenation is low, accounting for the presence of ostracods such as
Parakrithe or Pseudopsammocythere with a large vestibule characteristic of low oxygen content
(Peypouquet, 1977) and Alocopocythere with a highly sinuous inner margin (Hoibian, 1984)
In the delta front intermediary zones the organic matter effect is practically nil and the specific
diversity at its maximum. Although at a deficit, oxygenation is normal for these types of equatorial
environmentswhere the milieu is precipitating and the polymorph individuals are generallyagraded.
The distribution of Neomonoceratina microreticulata morphotypes is highly explicit (Text-fig.
8); poorly ornamented individuals prevail in the zones subjected to the deltaic influence. Further
off or towards the north, ornamentation increases and becomes the dominant characteristic in the
The above phenomenon elicits the irregular distribution of ostracods within the different areas
of the delta front (Fig. 9):
- the Muara Badak northern zone corresponds to a small distributary channel in which the organic matter input is low, highly marine and reinforced by littoral outflow to the south (Allen et
al., 1979). It is in this area that ostracods reach the inner most part of the delta front (Hoibian,
- the Tanjung Bayor central zone showing a vast fauna-free area around and off the interdistributary channels (maximum organic matter effect).
- the Handil Dua southern zone, a major distributary, expelling the faunas to the outer part
of the delta front, but maintaining, however, a normal development of the intertidal faunas.
Finally, broadly speaking, the fauna is better ornamented in the north (low input, nearby
reefs supplying CaCO,) than elsewhere. This phenomenon was observed in the ostracod fauna of the
Parakrithe sp. A I
Zone marginale krge
Large marginal zone
Zone marginale ktroite
Small marginal zone
v e s t i w e of Parakrithe sp. and ventral margin of Alocopocythere kendengmsis.
Organic matter influence
Effet de rnatite organique
I Neomonoceratina cf N. mediterranea
2 Neomonoceratina microreticulafa deg.
3 Neomonoceratina microreticulafa
4 Neomonoceratina microreticulota agr.
5 Neomonocemrina entomon
6 n'romonocemtina sp. A
TEXFFIO.8-Distribution of some species and morphotypes of Neomonoeerutinumicroreticulutu;effect of organic
DensitC de la faune d'ostracode dans le delta d e la Mahakam
Pas d'ostracode I no ostracodr
Faune pauvre I poor
Faune abondantel abundant
Distribution des asmciations d'odracodes dans les zones Ctudihes.
u Faune des marais supratidaux I supratidal niarsh
Faune des slikkes/ intertidal
Faune inter a infratidale I inter to infratidal
I'aunc infratidalc c6ti8re
a : embouchures /inlets
b : zones intermediaires1 intcrnicdhry areas
distribution in areas of the Mahakam delta.
a : cniboucliures4inlets
b :zones internibdiairs/ intermediary areas