Tropical Volcanic Island Research Articles

Trait-independent habitat associations explain low co-occurrence in native and exotic birds on a tropical volcanic island.

On oceanic islands, strong human impacts on habitats, combined with introductions of exotic species, modify the composition of terrestrial bird assemblages and threaten their ecological functions. In La Réunion, an oceanic island located in the Madagascan region, a national park was established in 2007 to counter the ecosystem-level effects of three centuries of habitat conversion, native species destruction and exotic species introductions. Here, we investigated how bird assemblages were structured in these human-modified landscapes, 10 years before the national park set out its first conservation measures. We used a combination of multivariate statistics and generalized additive models to describe variations in the taxonomic and functional composition and diversity of 372 local bird assemblages, encompassing 20 species, along gradients of habitat composition and configuration. We found that native species were tied to native habitats while exotic species were associated with urban areas and man-modified landscape mosaics, with some overlap at mid-elevations. Species' trophic preferences were segregated along habitat gradients, but ecological traits had an overall weak role in explaining the composition of species assemblages. Hence, at the time of the survey, native and exotic species in La Réunion formed two spatially distinct species assemblages with contrasting ecological trait suites that benefited from antagonistic habitat compositions and dynamics. We conclude that our results support the analysis of historical data sets to establish reference points to monitor human impacts on insular ecosystems.

Trace metals and nutrients in tropical volcanic island rivers: Insights on chemical weathering and anthropogenic influences of Pohnpei, Micronesia

River systems in tropical islands play a vital role in maintaining the ecosystem function of surrounding oceans by delivering land-derived materials. Researches on tropical river systems frequently focused on continents and peninsulas, while little research has been conducted in tropical islands. In the present study, we conducted field surveys in a Pacific volcanic island, Pohnpei, Micronesia, and determined the substance composition of river waters (major ions, trace metals, nutrients, etc.) to quantify terrestrial material loading and biogeochemical influence on the surrounding oceans. In Pohnpei, total dissolved solutes were observed to be 71.4 mg/L; the NH4+, NO3−, PO4−, and SiO32- concentrations were 0.46, 3.9, 0.52, 221.7 μmol/L, respectively; and NO2− was lower than the detection limit (0.14 μmol/L). The trace metals concentrations of rivers in Pohnpei Island were much lower than records in tropical rivers of the world. The variation in concentration levels of major ions and nutrients among rivers was relatively limited. A similar pattern was observed in the distribution of Al and V. Trace elements, including As, Cd, Co, Cr, Cu, Fe, Mn, Ni, Pb, Se, Sr, U and Zn in rivers were most enriched in Urban Area (human activity dominated region), compared with the Mixing Area (Multiple effect region) to Natural Area (natural weathering region). Exchangeable trace metals concentrations from suspended particulate matter (SPM) were lower than in dissolved state in river waters, while water-soluble, while acid-soluble metals concentrations in the sediment were much higher than in dissolved state in rivers. In our results, trace metals in Pohnpei rivers may mainly be influenced by domestic sewage and agricultural activity—these data can be used in biogeochemical models to estimate the contribution of domestic sewage. Furthermore, we have quantified the land-derived input of trace metals from tropical volcanic islands in Pacific (TVIP) into the surrounding oceans. Though the land area of TVIP only accounts for 0.1 % of global land area, the terrigenous inputs of some trace metals from TVIP account for approximately 1 %—and even more than 2 % of those in global riverine flux. Therefore, the nutrients and trace metals inputs from TVIP would be significant for the cycle of biogenic elements in the receiving ocean.

Relationships between erosion rates and physiographic factors of drainage basins through a regression analysis: The case of Tahiti Nui Island

Tahiti Nui is a quiescent tropical volcanic island (French Polynesia) characterized by an intense erosion (up to 0.25 km3·kyr−1 over the last 1 Ma) that resulted in 27 main drainage basins radially distributed around a central depression. Those basins are characterized by different erosion rates (from 0.07 to 24.6 10−2 km3·kyr−1), ages (from 469 to 892 ka), micro-climate set-ups and other landform factors. In this study, we assess, in terms of explicative power, possible links between the erosion rates and relevant physiographic factors, through a regression analysis obeying the parsimony principle. The best regression models include, in decreasing order of importance, the following factors: the size (Planar Area or Maximal River Length) and/or the height (Maximal Altitude); the planar shape (Shape Factor or Relative Width); the inclination (Main Longitudinal Slope, Mean Slope, Maximum Altitude or Depth) and the mean precipitation (Average Annual Rainfall). The Erosion Duration and the positions along the North-South and East-West directions, which were used to highlight a possible effect of the winds, did not show significant effect on the erosion rates. In all the best fitting models, the greatest weight was associated with the size or the height factors, revealing that the highest erosion rates are the ones measured on the largest and/or highest basins. The best model is based on five physiographic factors per basin, in order of importance: Planar Area, Relative Width, Maximum Altitude, Main Longitudinal Slope and Average Annual Rainfall. The Planar Area factor seems to be the most relevant parameter since it explains by itself the largest part of the erosion rates. Finally, the role of average annual rainfall is not consistent between the regression models. This suggests that a more suitable physiographic factor to constrain the effect of precipitations on erosion rates must be considered, probably linked to the temporal climate variability as well as the frequency of cyclone-driven rainfall. Besides, the weight of the Average Annual Rainfall factor was generally positive and small in most regression models, synonymous of a minor role.

Sequestration of P fractions in the soils of an incipient ferralisation chronosequence on a humid tropical volcanic island

BackgroundPhosphorus (P) is the limiting nutrient in many mature tropical forests. The ecological significance of declining P stocks as soils age is exacerbated by much of the remaining P being progressively sequestered. However, the details of how and where P is sequestered during the ageing in tropical forest soils remains unclear.ResultsWe examined the relationships between various forms of the Fe and Al sesquioxides and the Hedley fractions of P in soils of an incipient ferralitic chronosequence on an altitudinal series of gently sloping benches on Green Island, off the southeastern coast of Taiwan. These soils contain limited amounts of easily exchangeable P. Of the sesquioxide variables, only Fe and Al crystallinities increased significantly with bench altitude/soil age, indicating that the ferralisation trend is weak. The bulk of the soil P was in the NaOH and residual extractable fractions, and of low lability. The P fractions that correlated best with the sesquioxides were the organic components of the NaHCO3 and NaOH extracts.ConclusionsThe amorphous sesquioxides, Feo and Alo, were the forms that correlated best with the P fractions. A substantial proportion of the labile P appears to be organic and to be associated with Alo in organic-aluminium complexes. The progression of P sequestration appears to be slightly slower than the chemical and mineralogical indicators of ferralisation.

Commensal Rodent Habitat Expansion Enhances Arthropod Disease Vectors on a Tropical Volcanic Island.

On volcanic islands, the release of animals from predators and competitors can lead to increased body size and population density as well as the expanded habitat use of introduced animals relative to their mainland counterparts. Such alterations might facilitate the spread of diseases on islands when these exotic animals also carry pathogenic agents; however, this has rarely been investigated. The commensal Asian house rat (Rattus tanezumi) is confined to human residential surroundings in mainland Taiwan but can be observed in the forests of nearby Orchid Island, which is a tropical volcanic island. Orchid Island is also a hot spot for scrub typhus, a lethal febrile disease transmitted by larval trombiculid mites (chiggers) that are infected primarily with the rickettsia Orientia tsutsugamushi (OT). We predicted an increase in chigger abundance when rodents (the primary host of chiggers) invade forests from human settlements since soils are largely absent in the latter habitat but necessary for the survival of nymphal and adult mites. A trimonthly rodent survey at 10 sites in three habitats (human residential, grassland, and forest) found only R. tanezumi and showed more R. tanezumi and chiggers in forests than in human residential sites. There was a positive association between rodent and chigger abundance, as well as between rodent body weight and chigger load. Lastly, >95% of chiggers were Leptotrombidium deliense and their OT infection rates were similar among all habitats. Our study demonstrated potentially elevated risks of scrub typhus when this commensal rat species is allowed to invade natural habitats on islands. Additionally, while the success of invasive species can be ascribed to their parasites being left behind, island invaders might instead obtain more parasites if the parasite requires only a single host (e.g., trombiculid mite), is a host generalist (e.g., L. deliense), and is transferred from unsuitable to suitable habitats (i.e., human settlements on the mainland to forests on an island).

Water quality challenges associated with industrial logging of a karst landscape: Guadalcanal, Solomon Islands

Human disturbance of karst landscapes in tropical volcanic islands present a unique challenge for understanding sediment transport to the coastal zone. Here we present the first evidence of urban drinking water quality impacts from industrial logging in the Solomon Islands. Despite only 6% of the Honiara's drinking water catchment being disturbed by logging, rhodamine dye tracers demonstrated complex karst sinkholes that led to high suspended sediment concentrations being transported from neighbouring Kovi catchment into the Kongulai water supply offtake point for Honiara. This has resulted in the exceedance of practical treatment thresholds of 20 NTU 9.5% of the time, leading to water supply for the majority of Honiara's residents being unavailable for 58 days in 2019. This work highlights the cost-benefit disparity between industrial logging yielding minimal short-term economic yields in comparison to on-going broader impacts of increased coastal sediment transport while restricting water supply to a developing nation's capital.

Imprint of chemical weathering and hydrothermalism on the Ge/Si ratio and Si isotope composition of rivers in a volcanic tropical island, Basse-Terre, Guadeloupe (French West Indies)

A significant portion of the disproportionally high chemical weathering flux in volcanic island arcs may originate from hydrothermal fluid-rock interaction, thereby compromising the accurate estimate of atmospheric CO2 consumption rates. The objective of this study is to evaluate how the riverine Ge/Si ratio and Si isotopes, two well-established tracers of weathering, respond to hydrothermal inputs. The work took place in Basse-Terre, Guadeloupe, a tropical volcanic island with a dense river network, high chemical weathering fluxes and various hydrothermal surface manifestations. We characterized the Ge/Si ratio and δ30Si of 15 thermal springs, nine non-impacted (NI) rivers and 13 hydrothermally-impacted (HI) rivers. The soil solution from a highly weathered soil profile (Ferralsol) and a clayey-rock corresponding to the material exposed in an extinct hydrothermal system were also measured. A new purification method was successfully developed in order to allow the reliable measurement of Si isotopes in SO42−- and Cl−-rich thermal spring and HI river waters by mass spectrometry. Basse-Terre's thermal springs have variable Ge/Si ratios (0.05–21.03 μmol.mol−1) and δ30Si (0.71–1.50‰), but with no apparent relationship to the water compositional type. The Ge/Si ratio (0.15–2.57 μmol.mol−1) and Si isotope composition (0.26–1.21‰) values of the NI rivers reveal differences in the watersheds' weathering degree. Dissolution of Ge- and 28Si-rich secondary minerals explains the high Ge/Si and isotopically light composition of the northern NI rivers draining strongly weathered terranes. The Ge/Si ratio and δ30Si values measured for the NI and HI rivers overlap, implying that they cannot be used to diagnose hydrothermal contributions to river basins unambiguously. However, when combined with the Cl− and SO42− concentrations, the analysis of Ge and Si in the HI rivers suggests that water seeping through an extinct hydrothermal system produces SO4-rich drainages with distinctively lower Ge/Si ratios than those inferred for watersheds receiving thermal spring discharges associated with an active hydrothermal system. Overall, our results provide new constraints for applying and interpreting Ge/Si and Si isotope measurements to study weathering in volcanic environments.

Biogeographic Changes in Forest Soil Microbial Communities of Offshore Islands—A Case Study of Remote Islands in Taiwan

Biogeographic separation has been an important cause of faunal and floral distribution; however, little is known about the differences in soil microbial communities across islands. In this study, we determined the structure of soil microbial communities by analyzing phospholipid fatty acid (PLFA) profiles and comparing enzymatic activities as well as soil physio-chemical properties across five subtropical granite-derived and two tropical volcanic (andesite-derived) islands in Taiwan. Among these islands, soil organic matter, pH, urease, and PLFA biomass were higher in the tropical andesite-derived than subtropical granite-derived islands. Principal component analysis of PLFAs separated these islands into three groups. The activities of soil enzymes such as phosphatase, β-glucosidase, and β-glucosaminidase were positively correlated with soil organic matter and total nitrogen. Redundancy analysis of microbial communities and environmental factors showed that soil parent materials and the climatic difference are critical factors affecting soil organic matter and pH, and consequently the microbial community structure.

Feeding Ecology of Sicydium bustamantei (Greeff 1884, Gobiidae) Post-Larvae: The “Little Fish” of São Tomé Island

The rivers of São Tomé Island are colonized by Sicydium bustamantei (Greeff 1882), an amphidromous fish that spawns in those areas. After hatching, larvae drift to the ocean with the river flow. In the marine realm, the planktonic larvae develop and migrate to freshwater as post-larvae. The migrations of post-larvae support important local fisheries at the mouth of rivers in tropical volcanic islands. Amphidromous post-larvae rely on plankton as their main source of organic matter. However, the biology and ecology of S. bustamantei in the West African islands are understudied, despite its importance for local fisheries. Thus, this study aimed to start bridging this gap by studying its feeding ecology. Our objectives were to identify the main prey of S. bustamantei post-larvae, combining gut content with stable isotope analyses. The gut contents included zooplankton (Chaetognatha, Ostracoda, and unidentified crustaceans), debris from plant and/or macroalgae-derived material, and microplastics (including microfibers). The stable isotopes analysis indicated that zooplankton and macroalgae detritus were the main sources of organic matter assimilated by this species. We also demonstrated that S. bustamantei post-larvae are omnivorous and secondary consumers. These data provide pioneering information that can be used in management plans that still need to be developed.

Physical properties of epilithic river biofilm as a new lead to perform pollution bioassessments in overseas territories

Chlordecone (CLD) levels measured in the rivers of the French West Indies were among the highest values detected worldwide in freshwater ecosystems, and its contamination is recognised as a severe health, environmental, agricultural, economic, and social issue. In these tropical volcanic islands, rivers show strong originalities as simplified food webs, or numerous amphidromous migrating species, making the bioindication of contaminations a difficult issue. The objective of this study was to search for biological responses to CLD pollution in a spatially fixed and long-lasting component of the rivers in the West Indies: the epilithic biofilm. Physical properties were investigated through complementary analyses: friction, viscosity as well as surface adhesion were analyzed and coupled with measures of biofilm carbon content and exopolymeric substance (EPS) production. Our results have pointed out a mesoscale chemical and physical reactivity of the biofilm that can be correlated with CLD contamination. We were able to demonstrate that epilithic biofilm physical properties can effectively be used to infer freshwater environmental quality of French Antilles rivers. The friction coefficient is reactive to contamination and well correlated to carbon content and EPS production. Monitoring biofilm physical properties could offer many advantages to potential users in terms of effectiveness and ease of use, rather than more complex or time-consuming analyses.

Evidence of freshened groundwater below a tropical fringing reef

Submarine groundwater discharge (SGD) is widely acknowledged as a key driver of environmental change in tropical island coral reefs. Previous work has addressed SGD and groundwater-reef interactions at isolated submarine springs; however, there are still many outstanding questions about the mechanisms and distribution of groundwater discharge to reefs. To understand how groundwater migrates to reefs, a series of offshore 222Rn (radon) and submarine electrical resistivity (ER) surveys were performed on the tropical volcanic island of Mo’orea, French Polynesia. These surveys suggest that fresher water underlies the fringing reef, apparently confined by a <1-m-thick low-permeability layer referred to as a reef flat plate. Reef flat plates have been documented elsewhere in tropical reefs as thin, laterally continuous limestone units that form through the super-saturation of calcium carbonate in the overlying marine waters. In other tropical reefs, the reef flat plate is underlain by a highly permeable karstic limestone formation, but the submarine reef geology on Mo’orea is still uncertain. Numerical modeling of two-dimensional reef transects and SGD quantifications, based on water budget and radon/salinity mass balance, support the confining nature of the reef flat plates and indicate important implications for SGD impacts to tropical reefs. Except where incised by streams or local springs, reef flat plates may route SGD to lagoons or to the reef crest 100s of meters offshore. Because groundwater can transport pollutants, nutrients, and low pH waters, the reef flat plate may play an important role in the spatial patterns of reef ecology and coastal acidification.

New Species of Turbo (Marmarostoma) (Gastropoda, Turbinidae) from the Miocene Limestone in Central Japan: A Window into the Miocene Marine Biodiversity in the Northeastern Philippine Sea

The late early to middle Miocene limestone bodies in the Sagara and Izu Peninsula areas of central Japan are reefal deposits that were deposited under a tropical climate and provide a window into the paleodiversity of tropical volcanic islands in the northeastern Philippine Sea Plate. Specimens from the early middle Miocene Megami Limestone in the Sagara area and from the middle Miocene limestone blocks within the Yugashima Group on the Izu Peninsula are described as Turbo (Marmarostoma) histrioides sp. nov. and Turbo (Marmarostoma) izuensis sp. nov., respectively. Discovery of these two new species raises the number of Marmarostoma species from four to six in the Sagara and Izu Peninsula areas, demonstrating that the subgenus was more diverse in the northeastern Philippines Sea Plate during the middle Miocene than it is today. In particular, there are double the number of Marmarostoma species (four species) from the middle Miocene in the Izu Peninsula compared with the modern fauna on the tropical islands of the Izu-Ogasawara Arc. The highest richness of modern species of Marmarostoma is found in the central Indo-West Pacific, where molecular phylogenetic studies have suggested that the subgenus rapidly diversified in the late Oligocene or early Miocene. Nevertheless, Miocene species of this subgenus are sparse in the central IWP, being incongruent to the diversity pattern expected from the molecular studies. The findings reported herein provide an insight into the origin of the central IPW biodiversity hotspot.

Reconciling chemical weathering rates across scales: Application of uranium-series isotope systematics in volcanic weathering clasts from Basse-Terre Island (French Guadeloupe)

To elucidate the factors that contribute to the orders of magnitude difference generally observed in apparent weathering rates across scales, we utilized a U-series isotopic technique to directly determine the duration and rates of chemical weathering recorded in weathering clasts. In this study, we systematically compiled 15 individual volcanic weathering clasts collected from eight andesitic watersheds along a gradient of mean annual precipitation ranging from ∼1600 mm to 6400 mm on the tropical volcanic Basse-Terre Island of French Guadeloupe. We measured U-series isotope compositions to quantify weathering advance rates along the core-rind transects in these weathering clasts. Total rind formation ages, the length of time it took for these weathering clasts to form from andesitic rock fragments or volcanic clasts in the soil zones, range from 60 kyr to 300 kyr. As expected for each individual core-rind weathering transect, the rind ages generally increase almost linearly with distance away from the core-rind boundary. The derived clast weathering rates range from 0.08 ± 0.04 to 0.34 ± 0.01 mm kyr−1, and the rates exhibit a strong positive correlation (R2=0.74) with the annual runoff values of these watersheds. This correlation documents the first direct evidence that weathering over geological timescales on the island is controlled by the amount of precipitation and corroborates the relationship that was previously reported for the riverine weathering fluxes at Basse-Terre Island in Guadeloupe, but measured with much shorter time scales. The clast-scale rates (inferred for the characteristic length scale of mm) are compared to the watershed-scale rates of Basse-Terre Island (length scale of km): constantly higher rates are observed at the watershed scales than the clast scale and the discrepancy equals ∼1800 ± 400 times, despite changes in watershed characteristics such as watershed size, relief, and runoff values. The discrepancy is attributed to the fractal nature of the roughness and surface area across scales. For example, the presence of undulations and fractures (e.g. roughness) at the bedrock-saprolite contact of the watershed, which is generally not assessed in the watershed surface area estimates, is most likely the main contributor to the constant rate difference of ∼1800 times.

Nutrient dynamics in submarine groundwater discharge through a coral reef (western Lombok, Indonesia)

AbstractSubmarine groundwater discharge (SGD) from tropical volcanic islands locally transports significant amounts of terrestrial nutrients into the coastal ocean, which can affect sensitive coastal ecosystems such as coral reefs. In northwestern Lombok, terrestrial submarine groundwater discharges through several submarine springs at the seafloor into a coral reef. In order to understand the transport mechanisms of nutrients from land into the reef, we investigated nutrient fluxes via SGD within the area using a combination of different methods, including recharge estimates in the coastal catchment area, echosounder profiling of the seafloor, discharge measurements, and hydrochemical analyses (stable isotopes of water, radon, and nutrients). Different types of springs can be distinguished based on their morphology. One spring showed a “crater”‐shaped structure while other springs consisted of fissures. Recharge rates in the coastal catchment amounted to 5 L s−1, while discharge rates from the largest spring were 5 L s−1 for total SGD and 2.5 L s−1 for fresh SGD, which is similar to discharge rates from smaller onshore springs at the slopes of a volcano. SGD was in general a source of nutrients such as dissolved silicon (~ 226 mol d−1), phosphate (0.61 mol d−1), or total dissolved nitrogen (13.4 mol d−1). Nitrate concentrations varied among the different springs, probably due to different sources in the hinterland, transformations (e.g., denitrification), or biological uptake during transport from land into the ocean. As nitrate can considerably affect coral reef health, our findings suggest that especially nitrate should be monitored around submarine springs in Indonesia.

Subaerial disconformities, microkarst and paleosols in Ordovician limestones at Bowan Park and Cliefden Caves, New South Wales, and their geoheritage significance

The Ordovician Daylesford Limestone at Bowan Park and the Fossil Hill Limestone at Cliefden Caves have diagenetic and pedogenic features of microkarst, paleosols and calcrete associated with subaerial disconformities in their stratigraphic sequences, all of which, as an ensemble, have global geoheritage significance. The original shelly limestones, lime mudstones, and coralline limestones have selectively dissolved to form vugular limestone whose cavities have filled with sparry calcite and/or crystal silt. The limestones also have been calcretised to develop massive and laminar calcrete and calcrete ooids. Below disconformity surfaces are bleached limestone, crystal-silt and spar-filled fossil moulds and enlarged moulds, micro-breccia-filled moulds and fissures filled with crystal silt, calcrete pellets and calcrete ooids. The disconformity surfaces are irregular or undulating interfaces between lithologies, fissures and fissure-fills, and calcrete. Above disconformities there are limestone lithoclasts, remanié fossils, calcreted limestone, veined limestone, calcrete ooids, laminated calcrete, lithoclast grainstone, or calcrete-ooid grainstone, and lithoclasts with fossils moulds filled with crystal silt and/or spar. The lithological, stratigraphic and possibly landscape differences, make the subaerial diagenesis/pedogenesis in the Daylesford Limestone subtly different to that of the Fossil Hill Limestone. Subaerial disconformities and associated diagenesis/pedogenesis, as recorded in these formations, are not widely reported globally nor well represented in Ordovician limestones. The microkarst features provide insights into the types of subaerial diagenesis/pedogenesis during the Ordovician and into climate, landscape setting, paleohydrology, and groundwater/rainwater alkalinity. Consequently, the story of the Ordovician microkarst, paleosols and calcrete ooids is unique and globally of geoheritage significance as examples of subaerial alteration in an ancient high-rainfall, tropical climate volcanic island environment in a tectonically active region.

From unweathered core to regolith in a single weathering andesitic clast: Rates and trends of in situ chemical weathering on a tropical volcanic island (Basse Terre Island, French Guadeloupe)

We conduct X-ray microprobe, chemical and U-series isotope analyses on an oriented weathering clast collected from the regolith of a weathered Quaternary volcanoclastic debris flow on Basse Terre Island, French Guadeloupe. The sample consists of an unweathered basaltic andesite core surrounded by a weathering rind, and an indurated crust that separates the rind from the overlying soil matrix. U/Th disequilibria dating indicates that rind age increases away from the core-rind boundary to a maximum of 66 ka. This translates to a rind-advance rate of ~0.2 mm kyr−1, broadly consistent with rind advance rates calculated elsewhere on Basse Terre Island. The overlying indurated crust is 72 ka, indicating a possible minimum duration of the rind formation. Elemental variations are constrained by a bulk chemical analysis along a vertical transects from the core to the overlying soil matrix and parallel electron microprobe analyses. The hierarchy of elemental loss across the core-rind boundary varies in the order Ca > Na ≈ Mg > K > Mn > Si > Al > Ti = 0 > P > Fe, consistent with the relative loss of phases in the clast from plagioclase ≈ glass ≈ pyroxene > apatite > ilmenite. The abrupt, <900 μm wide, Ca, Na and porosity reaction fronts at the core-rind boundary approximately equal the length of the long dimension of plagioclase phenocrysts observed in the unweathered core. The <1000 μm wide reaction front at the rind-soil interface is marked by an indurated horizon with Fe and Mn enrichment that spans into enrichment of Mn, Ba, Al, Mg and K in the soil matrix. Unlike previously studied clasts, the preservation of the rind-soil interface permits characterization of weathering reactions and material exchanges between the weathering core, the rind, and the surrounding soil matrix, shedding insights into communication between the enveloping weathering rind and host regolith. The lack of an enrichment signal of Mn within the weathered rind suggests that weathering processes active within clasts are distinct from surrounding soil formation processes.

Carbon dioxide dynamics in a lake and a reservoir on a tropical island (Bali, Indonesia).

Water-to-air carbon dioxide fluxes from tropical lakes and reservoirs (artificial lakes) may be an important but understudied component of global carbon fluxes. Here, we investigate the seasonal dissolved carbon dioxide (CO2) dynamics in a lake and a reservoir on a tropical volcanic island (Bali, Indonesia). Observations were performed over four seasonal surveys in Bali’s largest natural lake (Lake Batur) and largest reservoir (Palasari Reservoir). Average CO2 partial pressures in the natural lake and reservoir were 263.7±12.2 μatm and 785.0±283.6 μatm respectively, with the highest area-weighted partial pressures in the wet season for both systems. The strong correlations between seasonal mean values of dissolved oxygen (DO) and pCO2 in the natural lake (r2 = 0.92) suggest that surface water metabolism was an important driver of CO2 dynamics in this deep system. Radon (222Rn, a natural groundwater discharge tracer) explained up to 77% of the variability in pCO2 in the shallow reservoir, suggesting that groundwater seepage was the major CO2 driver in the reservoir. Overall, the natural lake was a sink of atmospheric CO2 (average fluxes of -2.8 mmol m-2 d-1) while the reservoir was a source of CO2 to the atmosphere (average fluxes of 7.3 mmol m-2 d-1). Reservoirs are replacing river valleys and terrestrial ecosystems, particularly throughout developing tropical regions. While the net effect of this conversion on atmospheric CO2 fluxes remains to be resolved, we speculate that reservoir construction will partially offset the CO2 sink provided by deep, volcanic, natural lakes and terrestrial environments.

Source partitioning of anthropogenic groundwater nitrogen in a mixed-use landscape, Tutuila, American Samoa

This study presents a modeling framework for quantifying human impacts and for partitioning the sources of contamination related to water quality in the mixed-use landscape of a small tropical volcanic island. On Tutuila, the main island of American Samoa, production wells in the most populated region (the Tafuna-Leone Plain) produce most of the island’s drinking water. However, much of this water has been deemed unsafe to drink since 2009. Tutuila has three predominant anthropogenic non-point-groundwater-pollution sources of concern: on-site disposal systems (OSDS), agricultural chemicals, and pig manure. These sources are broadly distributed throughout the landscape and are located near many drinking-water wells. Water quality analyses show a link between elevated levels of total dissolved groundwater nitrogen (TN) and areas with high non-point-source pollution density, suggesting that TN can be used as a tracer of groundwater contamination from these sources. The modeling framework used in this study integrates land-use information, hydrological data, and water quality analyses with nitrogen loading and transport models. The approach utilizes a numerical groundwater flow model, a nitrogen-loading model, and a multi-species contaminant transport model. Nitrogen from each source is modeled as an independent component in order to trace the impact from individual land-use activities. Model results are calibrated and validated with dissolved groundwater TN concentrations and inorganic δ15N values, respectively. Results indicate that OSDS contribute significantly more TN to Tutuila’s aquifers than other sources, and thus should be prioritized in future water-quality management efforts.

Quantifying chemical weathering rates along a precipitation gradient on Basse-Terre Island, French Guadeloupe: New insight from U-series isotopes in weathering rinds

Inside soil and saprolite, rock fragments can form weathering clasts (alteration rinds surrounding an unweathered core) and these weathering rinds provide an excellent field system for investigating the initiation of weathering and long term weathering rates. Recently, uranium-series (U-series) disequilibria have shown great potential for determining rind formation rates and quantifying factors controlling weathering advance rates in weathering rinds. To further investigate whether the U-series isotope technique can document differences in long term weathering rates as a function of precipitation, we conducted a new weathering rind study on tropical volcanic Basse-Terre Island in the Lesser Antilles Archipelago. In this study, for the first time we characterized weathering reactions and quantified weathering advance rates in multiple weathering rinds across a steep precipitation gradient. Electron microprobe (EMP) point measurements, bulk major element contents, and U-series isotope compositions were determined in two weathering clasts from the Deshaies watershed with mean annual precipitation (MAP)=1800mm and temperature (MAT)=23°C. On these clasts, five core-rind transects were measured for locations with different curvature (high, medium, and low) of the rind-core boundary. Results reveal that during rind formation the fraction of elemental loss decreases in the order: Ca≈Na>K≈Mg>Si≈Al>Zr≈Ti≈Fe. Such observations are consistent with the sequence of reactions after the initiation of weathering: specifically, glass matrix and primary minerals (plagioclase, pyroxene) weather to produce Fe oxyhydroxides, gibbsite and minor kaolinite.Uranium shows addition profiles in the rind due to the infiltration of U-containing soil pore water into the rind as dissolved U phases. U is then incorporated into the rind as Fe-Al oxides precipitate. Such processes lead to significant U-series isotope disequilibria in the rinds. This is the first time that multiple weathering clasts from the same watershed were analyzed for U-series isotope disequlibrian and show consistent results. The U-series disequilibria allowed for the determination of rind formation ages and weathering advance rates with a U-series mass balance model. The weathering advance rates generally decreased with decreasing curvature: ∼0.17±0.10mm/kyr for high curvature, ∼0.12±0.05mm/kyr for medium curvature, and ∼0.11±0.04, 0.08±0.03, 0.06±0.03mm/kyr for low curvature locations. The observed positive correlation between the curvature and the weathering rates is well supported by predictions of weathering models, i.e., that the curvature of the rind-core boundary controls the porosity creation and weathering advance rates at the clast scale.At the watershed scale, the new weathering advance rates derived on the low curvature transects for the relatively dry Deshaies watershed (average rate of 0.08mm/kyr; MAP=1800mm and MAT=23°C) are ∼60% slower than the rind formation rates previously determined in the much wetter Bras David watershed (∼0.18mm/kyr, low curvature transect; MAP=3400mm and MAT=23°C) also on Basse-Terre Island. Thus, a doubling of MAP roughly correlates with a doubling of weathering advance rate. The new rind study highlights the effect of precipitation on weathering rates over a time scale of ∼100kyr. Weathering rinds are thus a suitable system for investigating long-term chemical weathering across environmental gradients, complementing short-term riverine solute fluxes.

Fern species richness and abundance are indicators of climate change on high-elevation islands: evidence from an elevational gradient on Tahiti (French Polynesia)

Inherent characteristics of island species make them particularly susceptible to anthropogenic changes and need to be assessed to implement appropriate conservation strategies. The impacts of climate change are increasingly being investigated along elevational gradients since they provide natural laboratories to study how species respond to climatic variation. Ferns are particularly sensitive to air humidity and temperature and are therefore potentially useful as bio-indicators. This study addresses the question of whether the distributions of fern species richness and abundance have climatic correlates along an elevational gradient on the tropical volcanic island of Tahiti (French Polynesia). Analyses were conducted on two datasets: island-wide richness was estimated using published data on species elevational ranges, and local richness and abundance were addressed through a transect survey. Correlations with water availability, temperature, area availability, and a randomly-generated species richness pattern were investigated. Results showed that both diversity and abundance varied in association with climate. Rainfall was collinear with diversity in the lower half of the elevational gradient (R2 = 0.97), while temperature was the most important climatic correlate for diversity in the upper half (R2 = 0.98). The number of terrestrial fern individuals and epiphytic fern cover were both correlated with temperature (R2 = 0.86 and 0.81, respectively). Our results imply that impacts of climate change on ferns on Tahiti might include change in diversity and abundance, and increased extinction risk due to low overlap between current and projected species distributions. Ferns represent important indicator organisms that can be used to study species distributional responses to climate change in island ecosystems.