Snellius-II Expedition Research Articles

Distribution and malformation of extant calcareous nannoplankton in the Indonesian Seas

The calcareous nannoplankton distribution has been determined from surface and subsurface water samples, and floating and moored sediment trap samples, taken during the Indonesian-Dutch Snellius-II Expedition in July 1984–May 1985 from the Banda Sea and adjacent seas, an area with seasonally changing upwelling and downwelling conditions. The surface water (0–5 m) coccolithophorid standing crop and species diversity were very low to zero, in the subsurface water (50–300 m) coccolithophorids were more abundant. A total of 36 living species were recorded. Seasonal changes in coccolithophorid abundance in the surface water and previous pigment data suggest that coccolithophorids are present but devoid of coccoliths during the northwest-monsoon in January–February. It seems that low salinity and nutrient depletion of the surface waters during the northwest-monsoon (January–February) restrict coccolith formation, since normal coccoliths do develop during the southeast-monsoon (August) when upwelling causes nutrient enrichment and normal salinity of the surface layer. During the change of the northwest- to the southeast-monsoon (May) conditions for calcification become more favourable. During the initial phase of this change the coccolithophorids produce malformed coccoliths. In the Flores and Java Seas further west the influence of the Banda Sea vertical mixing is limited and malformation persists in the surface and subsurface water during the southeast-monsoon. Two types of malformation as a result of incomplete calcification are found in placolith bearing species: (1) coccoliths with a weakly developed central tube that easily disintegrates and (2) placoliths with incomplete shields, but with a solid central tube.

Monsoonal effects on the hydrography of the upper waters (

In the past, upwelling related to the southeast monsoon has been postulated around the Outer Banda Arc and from the Aru Basin onto the Sahul Shelf. These areas were surveyed during the Snellius-II Expedition, in August 1984 (southeast monsoon) and in February/March 1985 (northwest monsoon). During both cruises, a well-developed mixed layer was generally found, with a thickness of on average 40 to 50 m but dependent on ambient wind conditions and salinity profile. The salinity of the surface waters of the Arafura Sea was below 34, due to the rivers of Irian Jaya. In August, the westward surface current appeared accompanied by a counterdirected bottom current along the eastern slope of the Aru Basin, as water originating from 100 to 150 m was found on the Sahul Shelf up to depths of 40 to 60 m. The intrusion covered hundreds of kilometres both north and south of the Aru Islands, and probably gave significant enrichment along the coast of Irian Jaya by mixing with the less saline upper water. Repeated sampling of stations at one to three week intervals showed that upwelling also occurred in the Aru Basin and, weakly, in the eastern Banda Sea. Density planes at depths of 50 to 150 m moved upwards at speeds between 0.3 and 3 m·d −1, presumably causing large-scale entrainment of waters from the thermocline into the mixed layer. Surfacing of ‘cold’ water was not observed, but at some sites the top of the thermocline was within 25 m of the surface. In the Aru Basin, the mean depth of σ t (∼25°C) increased from 60 m to more than 110 m between August 1984 and February 1985. In the eastern Banda Sea this density plane only dropped from 80 to 90 m depth, but there downwelling still proceeded at a rate of 1 m·d −1. Analysis of data of earlier cruises (9 in total) supports the alternation of up- and downwelling, with highest amplitude in the Aru Basin. Large interannual variations occur up to 300 m, in probable relation to El Niño. In February 1985, the T-S values in the 150 to 300 m layer were somewhat higher than in August 1984, indicating influx of Indian Ocean water during the northwest monsoon.

Acoustic surveys of pelagic fish resources in the Banda Sea during August 1984 and February–March 1985

Acoustic surveys with a 38 kHz echosounder were carried out during the Snellius-II Expedition by R.V. ‘Tenggiri’ in the eastern Banda Sea and northwestern Arafura Sea. The area of approx. 360 000 km 2 was surveyed both during the southeast and the northwest monsoon season. The objective was to make a first rough assessment of the distribution and abundance of the pelagic fish stock in the area, using a digital echointegration system. Significant differences in depth and volume of backscattering were observed between night and day, possibly related to diel vertical migration and avoidance of the vessel. Using the target strength of Decapterus of 14 cm length, the mean densities of the pelagic fish stock in the upper 100 m varied between 5.38 (day) and 8.82 (night) tonnes per square nautical mile in August and between 1.41 and 2.46 in February. The seasonal difference corresponded to biomass differences detected at lower trophic levels. Highest concentrations of backscattering were found in the eastern part of the area in both seasons, near the Outer Banda Arc and the Arafura Shelf. Total fish biomass estimates, based on the recording by daylight in the upper 100 m for the area surveyed, were 570 000 tonnes for August and 150 000 for February. No identification of fish species was accomplished. If daylight recordings mainly involve small pelagic fish, the total stock in the eastern Banda Sea is very large compared with local annual landings of non-tuna fish.

Abundance of planktonic crustacean larvae, especially decapods, in the northern Arafura Sea in relation to the monsoons

During the Snellius-II Expedition, vertical hauls with a 150 μm plankton net were made by R.V. ‘Samudera’ around the Aru Islands during August 1984 and February 1985. Subsamples were analysed on cirriped, stomatopod and decapod larvae. Mean numbers per m 2 were highest at stations at sites of <50 m depth and lowest at stations in the Aru Basin. During both cruises large concentrations of brachyuran and caridean larvae occurred at stations southeast of Aru. In the upwelling season (August) high numbers were also found in the zone with increased chlorophyll along the coast of Irian Jaya. Acetes larvae reached densities of up to 20–40 per m 3 at some shallow sites, but were not found in the February catches. Larvae of Penaeus sp. occurred in both monsoons, but densities were higher in August than in February.

Spatial and seasonal differences in acoustic recordings of the Banda Sea (Indonesia), obtained with a 30 kHz echosounder

Within the framework of the Indonesian-Dutch Snellius-II Expedition, acoustic surveys with a 30 kHz echosounder were made in the eastern Banda Sea during the southeast monsoon (August 1984) and northwest monsoon (February 1985). Occurrence, patterns, and migratory behaviour of the sound scattering layers (SSL) and of distinct echopatches (EP) were studied. No significant seasonal differences in number and pattern of SSL's occurred. SSL patterns in the Banda Sea area were typified by the absence of a deep layer at night, which is different from the (eastern) Pacific Ocean but similar to the pattern described for the equatorial Indian Ocean, suggesting a relationship with this area. The thickness of the SSL's was, in accordance with higher biomasses of the micronekton, on average larger in August than in February. However, no clear relation between net catches of mesopelagic fishes and crustaceans and night-time thickness of SSL was found, suggesting that larger organisms than those captured were responsible for the observed backscattering. 90% of the EP's occurred during the day and showed diel vertical migration down to day-time depths of 300 to 400 m. The number of EP's observed was twice as high in August as in February, with a clear concentration in the Aru Basin, the area with the most pronounced upwelling in August 1984. As EP's are generally attributed to schools of pelagic fish, the data suggest a seasonal increase and decrease in the fish stock corresponding with biomass fluctuations of other trophic levels.

Grazing in tropical copepods, measured by gut fluorescence, in relation to seasonal upwelling in the Banda Sea (Indonesia)

The amount of chlorophyll pigments in the gut was measured fluorometrically in adult females of 27 species of copepods sampled in the eastern Banda Sea during the Indonesian-Dutch Snellius-II Expedition. During the southeast monsoon (August 1984) and the northwest monsoon (February–March 1985) vertical net hauls were made near a subsurface drifter at four stations, covering several day and night periods and the depth strata 0 to 50, 50 to 150 and 150 to 250 m, with 250 to 500 m added at some stations. Diurnal feeding rhythms were frequently observed. At all stations, significantly higher values of gut fluorescence occurred at night in about 60% of the species, except for 22% at the August station in the Aru Basin, where upwelling was clearest. Reverse rhythms were not detected. Intraspecific differences reflected the vertical heterogeniety between stations: at the upwelling site 67% of the species had higher fluorescence levels in the upper 50 m; at the oligotrophic February stations, with a deep chlorophyll maximum at 50 to 70 m, this was only 5%. In many species the increase in gut fluorescence by night was not limited to the upper layer but extended to 150 to 250 m, which suggests rapid sinking of satiated animals. Carbon body weights were measured to determine the weight-specific gut fluorescence. This was at a maximum at the upwelling station, and there highest in Eucalanus and Temora, and only moderate in the upwelling species Calanoides. At the other stations no clear differences in weight-specific gut fluorescence occurred between species differing in feeding behaviour. In Cosmocalanus, Undinula, Euchaeta and Candacia the carbon weight of adult females was twice as high in the upwelling season as in the other season. Daily rations, estimated using a mean gut passage time of 30 min, were on average only 11% of body carbon. But at the upwelling station about 70%. Estimates of daily mesozooplankton grazing varied between 2 and 6% of the standing stock of chlorophyll, and between 5 and 26% of the primary production. Highest figures concern the upwelling site, where ambient HPLC-determined phaeophorbide concentrations were also at a maximum.

Nutrient distribution in the upper 300 m of the eastern banda sea and northern Arafura sea during and after the upwelling season, August 1984 and February 1985

During the Indonesian-Dutch Snellius-II Expedition monsoonal differences in nutrient profiles were studied around the Outer Banda Arc (R.V. ‘Tyro’) and the Aru Islands (R.V. ‘Samudera’). During the cruises in the southeast monsoon (August 1984) coastal upwelling on the Arafura Shelf was most conspicuous. Aru Basin water from a depth of 100 to 150 m intruded eastwards over the bottom of the shelf up to depths of 40 to 50 m, both north and south of Aru. High chlorophyll concentrations in the Arafura Sea were evidently not due to river runoff but to enrichment of the upper, less saline layer by vertical mixing with the nutrient-rich deeper water. In the eastern Banda Sea and in the Aru Basin, patchy nutrient enrichment of the mixed layer occurred by doming and entrainment. At stations northwest of the Aru Islands and southwest of the Kai Islands nutrient profiles were shifted upwards into the photic zone. A minimum estimate of the mean daily new production based on vertical NO 3 and PO 4 fluxes during August 1984 came to 0.5 g C·m −2, compared to a net daily primary production of 1.9 g C·m −2. In the northwest monsoon (February 1985), nutrient concentrations in the upper 100 m were much lower. The mixed layer was nitrate-depleted with very low NO 2 and NH 4 values. Nutrient concentrations at isothermal surfaces of 18, 15 and 12°C (depth range 150 to 300 m) did not markedly differ between the upwelling and the oligotrophic season. During both cruises especially H 4SiO 4 concentrations were significantly higher in the Aru Basin than west of the Outer Banda Arc. Using silicate as tracer, it was calculated that the enrichment by Aru Basin water of some stations visited during the Indopac Expedition (1976) in the Molucca Sea also involves the 150 to 300 m water layer.

Zooplankton abundance in the eastern Banda Sea and northern Arafura Sea during and after the upwelling season, August 1984 and February 1985

During the Indonesian-Dutch Snellius-II Expedition, zooplankton sampling was done in the area of the deep basins around the Outer Banda Arc and in the shallow waters (50 m) in the northernmost part of the Arafura Shelf. Cruises were in the southeast monsoon (August 1984) and in the northwest monsoon (February 1985), both by R.V. ‘Tyro’ and R.V. ‘Samuder’. Surveys involved in total 70 stations, with hauls from a depth of ca 150 m or, on the shelf, from near-bottom to the surface. The area of the Aru Basin was sampled by both ships and mean catches (mesh size 200 versus 300 μm) did not differ significantly. Variation in catch size due to patchiness and to diel vertical migration was large, up to 200% in the 0–150 m layer. All catches were measured on displacement volume and analysed on zooplankton groups. Part of the samples were freeze-dried and carbon/ nitrogen contents determined. For August 1984, in the upwelling season, pooled data gave a mean displacement volume of 25 cm 3·m −2 (1.1 gC·m −2) around the arc and of 32 cm 3·m −2 on the shelf. During February 1985, in the more oligotrophic season, values had dropped to 9 cm 3·m −2 (0.4 g C·m −2) and 6 cm 3·m −2. Mean zooplankton density in the 0–50 m layer during August was significantly lower in the basins than on the shelf (0.41 versus 0.94 cm 3·m −3) but during February about equal (0.16 versus 0.19 cm 3·m −3). All major zooplankton groups decreased in abundance by a factor of 2 to 4 from August to February, numbers of Luciferidae even by a factor of more than 10. In both seasons, copepods numerically predominated, constituting on average 80% of the catch in the basins and 64% on the shelf. In the 0–150 m layer during August, the copepods Calanoides philippinensis and Rhincalanus nasutus reached numbers of ca 4000·m −2 west of the Aru Basin/Arafura Shelf, i.e. downstream of the main upwelling area with surface chlorophyll concentrations of 3 to 4 mg·m −3. On the shelf and in February these species were virtually absent in the layer sampled. Guesses on secondary production in the eastern Banda Sea and Aru Basin come to an annual value of ca 24 g C·m −2, about 5% of the primary production in this area.

On the hydrology and productivity of the northern Arafura Sea

In the context of the Snellius-II Expedition data, on temperature, salinity, dissolved oxygen, various nutrients and some biological variables were collected in the northern part of the Arafura Sea during two cruises of R.V. ‘Samudera’, carried out in August 1984 (southeast monsoon) and February 1985 (northwest monsoon). Evidence was found for upwelling during the August survey, in particular along the south coast of Irian Jaya, the upwelled water being probably derived from the western slope of the Arafura Sea. The area was clearly stratified in August, as evidenced by differences in several variables in surface and near-bottom water. Stratification was weaker or absent in February, probably due to a higher vertical mixing. Such a higher mixing could also explain the relatively high nutrient concentration found in surface waters in February. Mean nutrient concentrations over the water column were higher in August than in February, which was attributed to different circulation patterns in the two seasons, leading to nutrient enrichment by slope upwelling in August, whereas nutrient supply in February is mainly dependent on riverine input.

Coccolithophores in the upper quaternary of some southeast Indonesian basins

Well-preserved, late Quaternary calcareous nannofossil floras were found in three piston cores collected during the Snellius-II Expedition (1984–1985) in Indonesian waters. In particular, the high numbers of unusual species like Reticulofenestra parvula and Dictyococcites sessilis is conspicious. In all the cores Florisphaera profunda, a deep-living species, is dominant. According to oxygen isotope analysis of the nannoplankton carbonate the oldest core has an age of 300 ka approximately, whilst the other two cores reach an age of ca 80 ka; 14C dating and biostratigraphical results confirm these ages. Gephyrocapsa sp. aff. G. ericsonii vanishes between 17 and 14 ka BP in all the three cores. Only in one core, some species frequencies seem to reflect Pleistocene climatic cycles. Principal components analysis indicates a correlation the the group of species consisting of Gephyrocapsa oceanica, Florisphaera profunda and Reticulofenestra parvula and the δ 13C record. This correlation is tentatively interpreted in terms of fertility. The results of this study suggest that only weak changes in the palaeoenvironmental conditions occurred during the time span studied. Those changes which seem to be significant are not related to temperature, but probably to fertility or other environmental factors.

Actuomicropalaeontology and sediment distribution of three transects across the Banda Arc, Indonesia (Snellius-II Expedition, Cruise G5)

Systematic sampling along three traverses across the Banda Arc has resulted in the recognition of distinct litho- and biofacies for each of the structural units and their corresponding watermasses. Position of the lysocline and CCD have been determined at 1700 and 3300 m respectively. The distribution of planktonic organisms (foraminifera, radiolaria and calcareous nannoplankton) is related to the prevailing monsoon direction. Bathymetric trends are revealed by benthic foraminifera and ostracods.

Late quaternary palaeoceanography of the Banda Sea, eastern Indonesian piston cores (Snellius-II Expedition, cruise G5)

Two Banda Sea piston cores were analysed for CaCO 3 content, foraminiferal, radiolarian and spore-pollen distribution, and for oxygen and carbon isotopes. Late Quaternary palaeoclimatology and -oceanography are deduced. In this tropical area the two-step deglaciation is seen in the oxygen isotope profile. It did not lead to higher surface water temperatures but to a wetter climate as recorded in concurrent changes of the palynofacies. The core record further shows effects of an increasing monsoon regime on ocean circulation and land climate around 10 ka. At about 10.5 ka climate got wetter, while upwelling intensity increased around 9.2 ka and monsoonal intensity decreased again at about 2.7 ka.

Planktonic foraminifera in recent bottom sediments of the Flores, Lombok and Savu Basins, eastern Indonesia

The distribution of Recent planktonic Foraminifera in the surface sediments of the Flores, Lombok and Savu Basins has been studied, based on box-core samples obtained during the Snellius-II Expedition. The planktonic foraminiferal assemblages are different in the Flores Basin and the Lombok and Savu Basins, and the distribution of the most abundant species is discussed. In the Flores Basin Neogloboquadrina dutertrei is dominant followed by Globorotalia menardii, Pulleniatina obliquiloculata s.l., Hastigerina siphonifera and Globigerina bulloides. The Lombok and Savu Basins are characterized by the dominant species Globorotalia menardii, which is associated with Pulleniatina obliquiloculata, Globigerinoides ruber and Globorotalia tumida. These different distribution patterns and abundances of planktonic foraminiferal species are discussed in the light of differences in oceanography and dissolution effects (lysocline/CCD).

Early diagenesis and sediment-water exchange in the Savu Basin (Eastern Indonesia)

The Savu Basin (max. depth 3400 m) was visited during the Snellius-II Expedition in March 1985 and May 1985. The watercolumn and the sediments were sampled at twelve stations. O 2 profiles were measured in subcores with microelectrodes, and the pore waters were analysed for NO 3 −, NO 2 −, NH 4 +, dissolved Si, dissolved Mn, and dissolved Fe. The sediment was analysed for organic C, organic N, and for Mn- and Fe-oxides. The maximum penetration of O 2 into sediment was restricted to 1.3 cm. Fluxes calculated from pore-water gradients showed that O 2 is responsible for about 60% of the total organic C oxidation, NO 3 − for about 25%, and Mn-oxide and So 4 accounted each for about 7%. This contrasts with a station in the nearby Indian Ocean. With a 2.5 fold lower sediment accumulation rate, the contribution of O 2 to organic C oxidation in the sediment was 86%, and NO 3 − reduction accounted for 13%. Pore-water profiles of dissolved Si showed large increases from the sediment-water interface to 1.5 cm depth. Below this depth, concentrations stabilized, reaching values of 550–850 μM at different stations. The diffusive flux of dissolved Si creates an excess of dissolved Si in the overlying water. From the total integrated excess of dissolved Si in the water below the sill depth of the basin and the diffusive flux of dissolved Si from the sediment, a transit time of 7 to 13 years for this water mass was estimated.

Geotechnical property variability over an active plate margin in the eastern Banda Sea

Water content, bulk density and shear strength were measured on the sediments of box cores collected on the Snellius-II Expedition along three traverses crossing active plate margins in the eastern Banda Sea. A single geotechnical property value for each core has been calculated. These data for each traverse were plotted in profile against water depth and a summary of sedimentological and physiographical information was added to the profile. Average values by traverse are tabulated. A map is presented showing the areal distribution of each geotechnical property along the three traverses in the eastern Banda Sea. Water content tends to be high in areas of pelagic clay in the deeps and troughs and low in other areas. Shear strength tends to be high in the volcanic sediments and where the water depth is least. Sediments of the backarc basin have low shear strengths. No clear evidence was found that the shear strength of the sediments in the subduction zone shows any recognizable relationship to tectonism for the near surface sediments studied. However, the relatively higher water contents in the surficial sediments of the actively subducting Seram and Timor Troughs, compared to the lower water contents in the inactively subducting Aru Trough, may be related to tectonism.

Submarine slope instability, Eastern Banda Sea

A submarine slope instability analysis of the eastern Banda Sea is presented based on geological, seismological, geophysical, and geotechnological information. The analysis resulted in the production of various overlays which were placed on top of a bathymetrical base map. Geotechnical data acquired during the Snellius-II Expedition were statistically analysed and formed, together with the information on the constructed overlays, the input for the geotechnical stability calculations. As a result, stability numbers were found and by plotting them on an overlay, a submarine slope instability hazard map of the eastern Banda Sea was produced on which potentially unstable regions were indicated. Since evidence for actual slumping and sliding is present in seismic profiles and by examination of piston-core material, it is concluded that large regions along the eastern Banda Sea area are potentially unstable due to high seismicity, steep slopes and soft sediments.

Slumping, sliding and the occurrence of acoustic voids in recent and subrecent sediments of the Savu Forearc Basin, Indonesia

High resolution seismic and acoustic profiles were acquired during legs G2 and G6 of the Snellius-II Expedition in the Savu Basin. They show that acoustic voids occur widespread in this basin. The acoustic voids are found along the periphery of an area of over 3900 km 2 that has been strongly influenced by recent and/or subrecent mass movements. The acoustic voids occur as bright spots with a transparent acoustic character. They incidentally have a relief of up to 100 m above the surrounding seafloor. Horizontal dimensions are up to 3 km width. Within the voids bedding planes locally are well preserved and continuous, although the bedding planes may appear weaker and sometimes are completely absent. Methane formation because of rapid sedimentation of organic-rich sediment may have caused the voids. However, the characteristic acoustic masking effect occurring normally on top of such deposits has nowhere been found in the Savu Basin and the distribution of the voids does not support this hypothesis. Thus, we suggest that the acoustic voids are formed as a result of local expulsion of pore-waters, caused by sediment mass movements down the uplifted ridge between the Islands of Sumba and Savu/Roti, rather than by methane formation. Southwest of Timor shale diapirism, probably associated with thrust tectonics is suggested as agent for the formation of the voids.

Executive summary of theme I snellius-II expedition ‘geology and geophysics of the banda arc and adjacent areas’

The El Niño–Southern Oscillation (ENSO) is a climatic phenomenon that affects socio-economical welfare in vast areas in the world. A continuous record of Holocene ENSO related climate variability of the Indo-Pacific Warm pool (IPWP) is constructed on the basis of stable oxygen isotopes in shells of planktic foraminifera from a sediment core in the western Pacific Ocean. At the centennial scale, variations in the stable oxygen isotope signal (δ18O) are thought be a representation of ENSO variability, although an imprint of local conditions cannot be entirely excluded. The record for the early Holocene (10.3–6 ka BP) shows, in comparison with the mid- to late Holocene, small amplitude variations in the δ18O record of up to 0.3‰ indicating relatively stable and warm sea surface conditions. The mid- to late Holocene (6–2 ka BP) exemplified higher variability in δ18O and thus in oceanic IPWP conditions. Climatically, we interpret this change (5.5–4.2 ka BP) as a phenomenon induced by variability in frequency and/or intensity changes of El Niño. In the period 4.2–2 ka BP we identified several periods, centred on 1.9, 2.1, 2.7, 3.3, 3.7 and 4.2 ka BP, with in general heavy δ18O values. During these periods, the IPWP was relocated to a more eastward position, enhancing the susceptibility for El Niño-like conditions at the core site. Over the last 2000 yr precipitation increased in the area as a response to an increase in Asian monsoon strength, resulting in a freshening of the surface waters. This study corroborates previous findings that the present-day ENSO activity started around 5.5 ka BP.

Late cenozoic vertical movements of non-volcanic islands in the Banda Arc area

During onshore campaigns of the Snellius-II Expedition late Cenozoic sections were recorded and systematically sampled on the non-volcanic outer Banda Arc Islands of Timor, Buton, Buru, Seram and Kai. Microfaunal studies provided age and palaeobathymetric data to construct geohistory diagrams. Geohistory analysis of field and laboratory data allows to calculate rates of vertical movements of the island basements.The vertical movements were intermittent and differed widely from place to place in the arc; short periods of uplift alternated with longer periods of tectonic rest or subsidence and lateral variations in timing and magnitude seem to be more the rule than the exception. Movements affected larger segments of the arc at about the same time, especially since the late Pliocene, when widespread vertical movements started, which led to the present configuration of the arc. Rates of uplift or subsidence differed within each segment. On an intermediate scale, deformation has the character of tilting or doming of whole islands or parts of islands. On a local scale, various types of deformation occur.Calculated duration of uplift pulses is in the order of a million years where less than 50 cm·ka−1 of vertical movements are involved. Sections, however, with a high time stratigraphic resolutions show pulses of uplift with a duration of only some hundreds of thousands of years and rates of more than 500 cm·ka−1. The duration of such pulses therefore is comparable to that of eustatic third order sea level changes. But because their amplitude is an order of magnitude larger, this implies that in tectonically active areas eustatic signals, preserved in the sedimentary record, will be overprinted by tectonics, i.e. will be difficult to disentangle from the tectonic signal.

Soft-bottom benthic community in the estuarine waters of East Java

During the Snellius-II Expedition, both in the period July–August and in the period November–December 1984, the structure of the macrobenthic communities in the estuaries of the rivers Porong, Wonokromo and Solo was investigated. The numerical densities of the benthic organisms (>0.5 mm) from the Porong, Wonokromo, and Bengawan Solo estuaries during the Snellius-II Expedition varied between 87 and 5787 ind·m −2; biomasses between 0.4 and 602 g DW·m −2. The high numerical density (1573 ind·m −2) found at st.2 of the Porong estuary in period I (July–August 1984) was attributed to the dense population of the bivalve Varicorbula rotalis. Consequently a very high biomass (602 g DW·m −2) was measured at this station. The mollusc Theora lata and the polychaetes Ancistrosyllis parva, Nepthys dibranchis, Paraprionospio pinnata, and Sternaspis laevis occurred at almost every station during the investigation periods. Paraprionospio pinnata and Theora lata, which are considered indicator species of disharmonic environments, were found at almost every station of the study areas in both periods. Species diversity at all the stations was low, ranging form 0.1 to 1.6.