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Reef islands accumulate as shallow landforms on or adjacent to reef complexes and are mainly composed of reef-derived carbonate sediment. Due to their unconsolidated and low-lying nature, reef islands are exposed to hydrodynamic processes, whereby they naturally experience erosion and accumulation. In order to compensate deficits of sediments lost during erosion, a sufficient supply of suitable carbonate sediment is required for accumulation. Under changing environmental conditions, the reefs are exposed to multiple stress factors that, among other things, can alter carbonate production and thus also threaten the stability of the reef islands. Hence, it is of importance to understand how past changes in the production of carbonate sediment have affected the evolution of reef islands systems, in order to assess potential future scenarios. Here we reconstruct the Holocene and recent sedimentological dynamics from one inhabited Indonesian reef island on the outer shelf of the Spermonde Archipelago, directly bordering the Strait of Makassar. We investigate the carbonate facies from sediment obtained from deep push cores up to 8 meters below surface and from shallow, hand-drilled boreholes. Targeted radiocarbon dating helps to reveal the temporal understanding of facies evolution and formation dynamics. Our data show that the island complex started to form in the mid-Holocene, around 6,400 years cal BP. The carbonate sediment from this time is almost exclusively composed of sand-sized coral fragments. Over time, the sediment composition becomes more diverse, with increased abundances of mollusks and algae, likely reflecting the ecological evolution of the reef. The shallow samples close to mean relative sea-level show radiocarbon ages of 4,000 to 3,000 years cal BP, suggesting that the initial island formed around this time and accreted in the following millennia. Of note, the proportions of the green algae Halimeda is even further increased in the youngest sediments. Overall, this suggests that the island has been able to continue growing despite changes in the ecosystems that provide the sediment, and that the increased production of green algae may have even promoted island accretion in general. Our study provides valuable insights into the dynamics of reef island development in the context of evolving reef systems and extends the understanding of Holocene island formation in the study area.

Reef islands in monsoonal regions undergo constant erosion and accumulation, making seasonally shifting morphologies part of their nature. Additionally, sea-level rise alongside climate change is thought to be incisive and challenge coastal communities. With multiple pressures acting on sediment-generating coral reef ecosystems, changes in sediment supply may force further response of these dynamic landforms. Here we present new sedimentological data from a larger reef island in the Spermonde Archipelago, Indonesia. By evaluating the subsurface data in the context of the island’s morphological behavior, we reconstruct its agile past and present. Based on remote sensing data complemented by reports of local citizens, we find the inhabited island to have tripled its surface area in the past century, however also losing more than 10% of surface area in the recent decades. The deeper sediments of the island are dominated by coral fragments, the youngest and uppermost sediments indicate the green algae Halimeda as dominating material contributor. Our study thereby (1) underlines the highly and far-reaching dynamics of reef islands and (2) suggests their adaptive potential to altering material budgets.

Indonesian seagrass communities are among the most diverse compared with those of other tropical or temperate regions. In this chapter, we describe some of the results of our research on seagrass beds in Sulawesi during the German–Indonesian Research Project “Science for the Protection of Indonesian Coastal Ecosystems” (SPICE) from 2004 to 2016. We studied aspects of the distribution and characteristics of these ecosystems within the Spermonde Archipelago, the role of keystone species and eco-engineers, their function as a habitat for fishes, their impact on carbon flow and storage as well as the threat they face due to anthropogenic activities. Our results contributed to these topics either by confirming known data or by originating new ideas on the interactions of seagrasses with animals and physical drivers. The alarming loss of seagrass beds globally is a serious threat for the function of our oceans as carbon sink. To save the seagrass beds, we suggest immediate measures at a regional level for the Spermonde Archipelago. We further recommend detailed research on the role seagrass ecosystems play within the complex interactions between land use and coastal changes. Abstrak Komunitas Lamun Indonesia termasuk salah satu yang paling beragam dibandingkan dengan wilayah tropis dan subtropis lainnya. Pada bab ini, kami menguraikan beberapa hasil penelitian yang dilakukan di padang lamun Sulawesi selama kegiatan riset bersama Jerman-Indonesia bertajuk “Sains untuk Perlindungan Ekosistem Pantai Indonesia” dari 2004 hingga 2016. Kami mengkaji aspek distribusi dan karakteristik dari ekosistem padang lamun yang ada di Kepulauan Spermonde, peran spesies kunci dan rekayasa ekologi, fungsi padang lamun sebagai habitat bagi ikan, dampak padang lamun terhadap aliran karbon dan penyimpanannya, termasuk ancaman yang dihadapi padang lamun akibat aktifitas manusia. Peringatan tentang berkurangnya luasan padang lamun di seluruh dunia adalah ancaman serius terhadap fungsi laut kita sebagai penyimpan karbon. Untuk menyelamatkan padang lamun ini, kami menyarankan untuk secepatnya melakukan penelitian dalam skala regional. Kami juga merekomendasikan dilakukannya riset yang lebih detail terkait peran ekosistem lamun dalam interaksi kompleks antara penggunaan lahan (land use) dan perubahan pantai.

We surveyed the elevation and age (14C) of paleo sea-level indicators in five islands of the Spermonde Archipelago. We describe 24 new sea-level index points from fossil microatolls, and we compare our dataset with both previously published proxies and sea-level predictions from a set of 54 Glacial Isostatic Adjustment (GIA) models, using different assumptions on both ice melting histories and mantle structure and viscosity. We then investigate the implications of our data and models in terms of vertical land movements in the study area, with two main results.First, data from the heavily populated island of Barrang Lompo are significantly lower (ca. 80 cm) than those at all the other islands. In absence of instrumental data (e.g., GPS or tide gauges) in any of the islands, we advance the hypothesis that this difference may be due to groundwater extraction and loading of buildings on Barrang Lompo, that would cause this island to subside at rates in the order of ~3-11 mm/a.Second, Common Era data (0-400 a BP) seem to indicate that the islands in the archipelago may be affected by tectonically-driven vertical land motions in the order of -0.88±0.61 mm/a (1-sigma), albeit slight uplift cannot be excluded. Different assumptions on vertical land motions affect, in turn, the assessment of which GIA model shows the best match with Late Holocene (ca. 4-5 ka) sea level data. Tectonic stability or slight uplift would favor iterations of ANICESELEN (De Boer et al., 2014), while subsidence would cause the sea level data to fit better with iterations of ICE-6G (Peltier et al., 2015).ReferencesDe Boer, Bas, Paolo Stocchi, and Roderik Van De Wal. A fully coupled 3-D ice-sheet-sea-level model: algorithm and applications." Geoscientific Model Development 7.5 (2014): 2141-2156.Peltier, W. R., D. F. Argus, and R. Drummond. Space geodesy constrains ice age terminal deglaciation: The global ICE‐6G_C (VM5a) model. Journal of Geophysical Research: Solid Earth 120.1 (2015): 450-487.AcknowledgmentsThis project is funded by SEASCHANGE (RO-5245/1-1) and HAnsea (MA-6967/2-1) from the Deutsche Forschungsgemeinschaft (DFG), part of the Special Priority Program (SPP)-1889 "Regional Sea Level Change and Society". Parts of this study are under review in Climate of the Past (https://www.clim-past-discuss.net/cp-2019-63/)

Indonesia is a country composed of several thousand islands, many of them small, low-lying and densely inhabited. These are, in particular, subject to high risk of inundation due to future relative sea level changes. The Spermonde Archipelago, off the coast of Southwest Sulawesi, consists of more than 100 small islands. This study presents a dataset of 24 sea-level index points from fossil microatolls, surveyed on five islands in the Spermonde Archipelago and compares these new results with published data from the same region and with relative sea level predictions from different Glacial Isostatic Adjustment (GIA) models. The newly surveyed fossil microatolls are located around the islands of Tambakulu, Suranti (both ~ 60 km offshore of Makassar city), Bone Batang and Kodingareng Keke (both located in the center of the Archipelago) and Sanrobengi (located ~ 20 km south-southwest of Makassar). Results from the near- and mid-shelf islands indicate that relative sea level between 4 to 6 ka BP was less than one meter above present sea level. The only exception to this pattern is the heavily populated island of Barrang Lompo, where we record a significant subsidence when compared to the other islands. These new results support the conclusions from a previous dataset and are relevant to constrain late Holocene ice melting scenarios. Samples from the two outer islands (Tambakulu and Suranti) yielded ages spanning the Common Era that represent, to our knowledge, the first reported for the entire Southeast Asian region.

It has been postulated that mound-building callianassid shrimp and seagrass have mutually negative effects on each other. On reef flats in the Spermonde Archipelago, Indonesia, sediment mounds of callianassid shrimp are rare in wave exposed seagrass meadows, occur frequently in sheltered seagrass meadows, and reach their highest densities (4.2 mounds m− 2) in unvegetated subtidal areas. There sediment turnover is high at an estimated 3408 g DW m− 2 ∗ d− 1. Based on collected specimens and burrow casts, the most important bioturbators are Glypturus armatus within seagrass beds and Neocallichirus vigilax in unvegetated subtidal areas. Six shrimp exclusion, six control and six zero treatment plots were set up in an unvegetated subtidal N. vigilax bed. Half of the plots for each treatment were ca. 1.5 m (“shallow”) and the other half ca. 2 m or slightly more (“deep”) below spring low tide level. The survival of transplanted shoots of the six seagrass species Enhalus acoroides, Thalassia hemprichii, Cymodocea rotundata, Halodule uninervis, Syringodium isoetifolium and Halophila ovalis was monitored over 27 months. At the end, E. acoroides had survived in about half of the plots with no obvious relation to either treatment or water depth. C. rotundata and H. uninervis had successfully established themselves on the shallowest exclusion plot only, and had disappeared from all other plots. T. hemprichii and S. isoetifolium had disappeared from all plots. H. ovalis expanded rapidly in the dry seasons, but wet seasons caused a temporary decline in shoot numbers. At the end, the highest shoot numbers were counted in shallow exclusion plots, some shoots survived in deep exclusion plots and the species disappeared from all deep control and zero treatment plots. H. ovalis shoot numbers were significantly influenced by water depth (ANOVA p < 0.01) and, to a lesser extent, by treatment (ANOVA p < 0.05) but there was no interaction between the two factors. It is concluded that seagrasses are only controlled by shrimp bioturbation at the lower limit of their distribution in sheltered subtidal areas. Here they are already under considerable stress from a combination of factors such as light limitation and sedimentation. However, the distribution pattern of seagrasses vs. shrimp elsewhere on the islands suggests that seagrasses are equal competitors in sheltered intertidal areas, where they may even profit from the abundant small tidal pools between mounds created by the shrimp, and dominant competitors in exposed intertidal and subtidal areas.

A new tunicate, Ascidia subterranea sp. nov., was found in burrows of the axiid crustacean Axiopsis serratifrons on De-rawan Island, Indonesia. It differs from other ascidians in its habitat as well as numerous morphological peculiarities which are described in detail. The shrimp Rostronia stylirostris Holthuis, 1952 was found inside A. subterranea sp. nov., and 4 species of bivalves, 3 species of polychaetes, 1 gastropod, 1 polyplacophoran and 1 sponge species were found as burrow associates besides the ascidian.

Seagrass meadows support abundant and diverse fish assemblages, but there are very few studies on the relation between seagrass beds with distinctly different plant canopies and their associated fish fauna. In the present study, fish assemblages were investigated by underwater visual census at intertidal and subtidal sites with varying seagrass species composition, shoot density, biomass, and leaf area index (LAI) on two small coral islands in the Spermonde Archipelago, Indonesia. We investigated (1) whether fish assemblages in distinctly different seagrass beds differ regarding community parameters, and (2) whether seagrass parameters affect fish abundances. Overall, more than 120 fish taxa were found. Bray–Curtis cluster analysis and non-Metric Multidimensional Scaling ordination (nMDS) showed site-specific similarities for fish assemblage structure with a distinct separation into subtidal and intertidal sites. Species accumulation curves for gamma diversity, single study sites, and the two most diverse fish families (Labridae, Pomacentridae), suggesting that species numbers are likely to increase with diel sampling. Total fish abundance and abundance for six out of the nine most common species varied distinctly among the study sites. The study indicates that seagrass beds with differing canopy parameters support distinct fish assemblages that differ with respect to species richness, dominant species, and the abundance of total and most common fish species. These differences are likely due to different canopy structures and water depth.

The feeding habit of Austrolethops wardi (Gobiidae) in the seagrass beds of Barrang Lompo and Bone Batang Island in the Spermonde Archipelago, South West Sulawesi, Indonesia, was investigated through gut content analysis. The feeding preferences of this species are very similar on both islands: A. wardi, a burrow associate of Neaxius acanthus, was found to feed almost exclusively on seagrass (which was found in 100% of the investigated stomachs and made up >94% of food items). However, seagrass epiphytes (

Burrows of the thalassinidean shrimps Neaxius acanthus and Corallianassa coutierei are striking aspects in tropical seagrass beds of the Spermonde Archipelago, Indonesia. Burrow construction, behaviour, burrow type and associated commensal community were investigated to clarify the ecological role and food requirements of these shrimps and their commensals. Gut content analysis and stable-isotope data were used to unravel the food sources and the trophic interactions among the commensal community. Individuals of Neaxius acanthus were caught on Bone Batang Island. In narrow aquaria filled with sediment they constructed burrows resembling those found in the field. During burrow construction and maintenance only little sediment was brought to the surface, most was sorted and compacted to create a distinct lining. Maintenance work by single shrimps typically took about 5 min, after which the shrimp walked up to the entrance and rested for a similar period of time. There were no differences in behaviour between day and night. Intrasexual encounters inside the burrow were characterised by a high level of aggression and all resulted in one participant being driven out of the burrow. Intersexual encounters led to coexistence with both animals taking turns in burrow maintenance and guarding the entrance. Offered seagrass leaves were pulled underground, cut into pieces and eventually integrated into the lining. Burrows of Corallianassa coutierei resembled a deep U-shape. Chambers branching off halfway down and at the deepest point contained seagrass fragments. All steep parts of the burrow were lined similar to burrows of N. acanthus. No commensals were found associated with Corallianassa coutierei. However, burrows of Neaxius acanthus in the field typically contained a pair of shrimps, up to 8 individuals of the commensal bivalve Barrimysia cumingii and large numbers of gammarid amphipods. Other animals found associated with the burrow were the goby Austrolethops wardi, a palaemonid shrimp species and two species of tube-building polychaetes, one of which was also found as an epibiont on N. acanthus. Stable-isotope and gut content analyses indicate that the diet of Neaxius acanthus, its commensal Austrolethops wardi, and Corallianassa coutierei is mainly derived from detrital seagrass leaves, with a potential contribution of sediment organic matter and seagrass epiphytes. In contrast the isotopic signature of Barrimysia cumingii suggests the presence of symbiotic sulphide metabolism bacteria. This study underlines that, besides their interactions with the surrounding ecosystem, thalassinid shrimp burrows play an important role as a sub-habitat with a unique associated fauna.

The abundance, burrow characteristics, and in situ behaviour of the burrowing shrimps Neaxius acanthus (Decapoda: Strahlaxiidae) and Alpheus macellarius (Decapoda: Alpheidae) were studied to quantify the collection of seagrass material, to identify the fate of this collected material, and to determine the importance of these burrowing crustaceans in the nutrient (nitrogen and phosphorus) cycling of two tropical seagrass meadows on Bone Batang, South Sulawesi, Indonesia. Alpheus macellarius harvested 0.70 g dry weight (DW) burrow −1 d −1 seagrass material, dominantly by active cutting of fresh seagrass leaves. Neaxius acanthus collected 1.66 g DW burrow −1 d −1 , mainly detached leaves which floated past the burrow opening. The A. macellarius and N. acanthus communities together collected in their burrows an amount of seagrass leaf material corresponding to more than 50% of the leaf production in the meadows studied. The crustacean species studied might therefore fulfil an important function in the nutrient cycling of tropical meadows. In the burrow most of the collected material is shredded into pieces. The burrows of both species had special chambers which serve as a storage for seagrass leaf material. Neaxius acanthus incorporated most of the material into the burrow wall lining, which is made of small sediment particles and macerated seagrass leaves. Phosphate concentrations measured in N. acanthus burrows compared with pore-water and water-column concentrations suggests that a substantial amount of the seagrass material undergoes decomposition in the burrows. Oxygen levels measured in these water bodies are indicative for a possible exchange of water between the burrow and its surroundings, most likely supported by the shrimps irrigating their burrows. By collecting leaf material in their burrows, nutrients that are otherwise lost from the seagrass meadow associated with detached leaves and leaf fragments carried away in the water column, are maintained in the meadow and may form an important source of recycled nutrients.

The occurrence of the gobiid Austrolethops wardi in seagrass beds at Bone Batang, a small island of the Spermonde Archipelago, south-west Sulawesi, Indonesia, is reported; the first record of A. wardi in Indonesia. All fish were found to be associated with burrows of the thalassinid shrimp Neaxius acanthus, where they were caught either as single specimens or as pairs. The main diagnostic characters of this rarely found gobiid are presented.