34 understand how and to what extent benthic communities respond to natural and man-made
environmental change and to assess their role for nutrient cycling in the mangrove forest. Data will also be used to evaluate the nursery function of the SA Lagoon for fish, shrimp
and mud crabs and thus its ecological and economic potential. Results
1. Diversity and spatio-temporal distribution of the benthic community
Diversity, abundance, biomass, and spatio-temporal distribution of benthic organisms were studied in the lagoon, its tidal flats and fringing mangroves. The
investigations were conducted bimonthly between May 2004 and May 2006 and included i quantitative sediment sampling using cores up to 20 cm depth; sieving with 250, 500,
and 1000 µm mesh sizes along two transects in the mangrove forest and tidal flats of the central and eastern part of the lagoon, respectively, and ii semi-quantitative sediment
sampling at 21 stations in the lagoon using an Ekman grab; 500 µm mesh size.
Overall, 161 taxa were recorded including Mollusca Gastropoda 57 taxa, Bivalvia 23 taxa, Scaphopoda 3 taxa, Brachyura 47 taxa, Polychaeta 23 taxa, Sipuncula,
Phoronidea, Oligochaeta, Isopoda, Amphipoda, Copepoda and mud skippers 3 taxa. The most important gastropod families are Thiaridae, Potamididae, and Neritidae. Gastropod
diversity was much higher in the mangroves of the central lagoon than in the eastern part. By contrast, polychaete diversity was higher in the eastern part, including the main taxa
Nereidae, Capitellidae, and Spionidae. Dominant mangrove crab families are Ocypodidae 20 species and Grapsidae 22 species. Whereas the biomass of crabs belonging to these
families was similar in the eastern lagoon, biomass of the Grapsidae was higher than that of the Ocypodidae in the central lagoon Hinrichs, 2006; Geist, 2007. The density and
composition of the mangrove vegetation was important for the distribution of litter- consuming grapsid crabs whereas the sediment structure was relevant for the distribution
of most ocypodid species.
A high spatial and seasonal variability of species number, abundance and biomass was found in all areas Figures 1 and 2. Species composition differed significantly
between mangrove transects in the central and eastern lagoon Figure 1. Sipuncula, rarely found in the central lagoon, accounted for one third of all individuals in the eastern lagoon
despite the potential pollution sources effluents from oil and cement industry, households. By contrast, the gastropod family Thiaridae showed high densities and
diversity in the central lagoon but were almost lacking in the eastern lagoon. In both areas, diversity was slightly higher in the dry than in the rainy season. Mean abundance of
mangrove macrobenthos was 2.5-fold higher in the central than in the eastern lagoon Figure 2. By contrast, mean biomass was slightly higher in the eastern lagoon 10.1 ± 7.4
g afdm m
-2
; central: 8.2 ± 9.6 g afdm m
-2
which was due to high numbers of sipunculid worms which show high individual biomass values compared to the tiny gastropods found
in the central part. In shallow lagoon waters, the number of benthos taxa increased from the western to the central and to the eastern part of the lagoon. Density and biomass of
benthic organisms were very low at most sampling stations. The overall low abundances of bivalves suggest that sedimentation rates are too high to allow the survival of these filter
feeders, particularly in the western part of the lagoon.
In comparison to former studies conducted in the SA Lagoon higher species numbers and abundances of benthic organisms were recorded during our work Yuwono et
al ., 2007. This is probably related to a better spatial coverage of the lagoon and the
35 consideration of the endobenthos. When compared to other mangrove forests and tidal flats
abundances of benthic organisms in the central lagoon were in the same range but they were low in the eastern lagoon Alongi, 1989; Dittmann, 1995; Pepping, 1999; De Boer
and Prins, 2002; Dittmann, 2002; Kober, 2004.
Figure 1. Mangrove macrobenthic community composition: Number of individuals of the main taxa of total at the central and eastern part of the Segara Anakan Lagoon during
the dry and rainy season 1 mm mesh size; 20 cm depth; May 2004 – January 2006. Figure 2. Density of mangrove macrobenthic fauna at the central Mean: 927 ± 1,350 ind.
m
-2
and eastern part of the Segara Anakan Lagoon Mean: 374 ± 240 ind. m
-2
; 1 mm mesh size; 20 cm depth; n = 8 at each station.
Sipuncula: 31.2
Mollusca: 41.9
Sipuncula: 0.6
Polychaeta: 14.9 others: 4.0
Mollusca: 78.8 Crustacea: 1.8
Crustacea: 6.2 Polychaeta:
20.4
Rainy season
Sipuncula: 36.3
Mollusca: 23.0
Polychaeta: 25.0
Crustacea: 15.8
Polychaeta: others: 2.6
Mollusca: 49.8
Crustacea: 2.3 45.2
Dry season Eastern lagoon
Central lagoon
Ma y
Ju ly
Se p
No v
Jan Ma
rc h
Ma y
Ju ly
Se p
No v
De c
Jan
May 2004 - January 2006
500 1000
1500 2000
2500 3000
3500
In d
iv id
u a
ls m
-2 Eastern lagoon
Central lagoon
Median 25-75
Non-outlier range
36
2. Feeding ecology of mangrove crabs
