Groundwater dynamics on small carbonate islands: Insights from radiocarbon and stable isotopes in Kikai Island, Southwest Japan.

Groundwater, the world's largest freshwater resource, faces significant challenges due to the overexploitation and depletion of aquifers in the 21st century. Small island groundwater aquifers are particularly valuable, and a scientific understanding of the behavior of subsurface water systems is vital. A comprehensive study using radiocarbon, stable oxygen isotopes, stable hydrogen isotopes, and hardness analysis (Δ14C, δ18O, δD, Ca, Mg) of groundwater was conducted in Kikai Island, a southern island in the Amami archipelago, Japan. The geological features and small size of the island make it an ideal location for assessing groundwater recharge and discharge relationships. Groundwater dynamics were investigated using samples collected seasonally from 15 points around the island (wells, springs, and an underground dam). Δ14C results indicated that despite considerable differences in precipitation, spatial variations were more prominent than seasonal variations, suggesting the presence of a large groundwater reservoir. The stable isotopes and hardness values, commonly used to detect groundwater dynamics, did not provide clear evidence to support this trend for the study site, a low-lying small island. However, the combination of deuterium excess (d-excess) values with radiocarbon analysis has the potential to provide a better understanding of groundwater flow. This study further illustrates that a combined approach utilizing Δ14C, δ18O, δD, and hardness levels can yield invaluable insights into groundwater dynamics. Considering geomorphic and geological features, groundwater in Kikai Island was categorized into five groups, providing insights into spatial groundwater flow. Results of this study indicate that the use of 14C allows the detection of groundwater movement with a high dynamic range and increased sensitivity, deepening our understanding of the diverse carbon sources that influence the groundwater system. Insights from this study are especially important for the efficient water management in comparable small carbonate islands and for tackling issues associated to overexploitation, pollution, and water scarcity.

Groundwater oxygen anomaly related to the 2016 Kumamoto earthquake in Southwest Japan.

Here, we report the groundwater oxygen isotope anomalies caused by the 2016 Kumamoto earthquake (MJMA7.3) that occurred in Southwest Japan on April 16, 2016. One hundred and seventeen groundwater samples were collected from a deep well located 3 km to the southeast of the epicenter in Mifune Town, Kumamoto Prefecture; they were drinking water packed in PET bottles and distributed in the area between April 2015 and March 2018. Further, the oxygen and hydrogen isotopes were evaluated via cavity ring-down spectroscopy without performing any pretreatment. An anomalous increase was observed with respect to the δ18O value (up to 0.51‰) soon after the earthquake along with a precursory increase of 0.38‰ in January 2016 before the earthquake. During these periods, there was no noticeable change in the hydrogen isotopic ratios. Rapid crustal deformation related to the earthquake may have enhanced the microfracturing of the aquifer rocks and the production of new surfaces, inducing δ18O enrichment via oxygen isotopic exchange between rock and porewater without changing δ2H.

Organic Carbon and Nitrogen Isoscapes of Reef Corals and Algal Symbionts: Relative Influences of Environmental Gradients and Heterotrophy.

The elemental (C/N) and stable isotopic (δ13C, δ15N) compositions and compound-specific δ15N values of amino acids (δ15NAA) were evaluated for coral holobionts as diagnostic tools to detect spatiotemporal environmental heterogeneity and its effects on coral health. Hermatypic coral samples of eight species were collected at 12 reef sites with differing levels of pollution stress. The C/N ratios, δ13C values, and δ15N values of coral tissues and endosymbiotic algae were determined for 193 coral holobionts, and the amino acid composition and δ15NAA values of selected samples were analyzed. δ15N values were influenced most by pollution stress, while C/N ratios and δ13C values depended most strongly on species. The results imply that δ13C and δ15N values are useful indicators for distinguishing the ecological niches of sympatric coral species based on microhabitat preference and resource selectivity. Using δ15NAA values, the trophic level (TL) of the examined coral samples was estimated to be 0.71 to 1.53, i.e., purely autotrophic to partially heterotrophic. Significant portions of the variation in bulk δ15N and δ13C values could be explained by the influence of heterotrophy. The TL of symbionts covaried with that of their hosts, implying that amino acids acquired through host heterotrophy are translocated to symbionts. Dependence on heterotrophy was stronger at polluted sites, indicating that the ecological role of corals changes in response to eutrophication.

Retrospective monitoring of salinity in coastal waters with mussel shells.

Sea surface salinity (SSS) is a key parameter to understand and predict many physical, chemical and biological processes in dynamic coastal environments. Yet, in many regions, instrumental measurements are spatially sparse and insufficiently long, hindering our ability to document changes, causes, and consequences of SSS across different time scales. Therefore, there is an need to develop a robust proxy to extend SSS records back in time. Here, we test whether SSS can be reconstructed reliably and quantitatively from shell oxygen isotopic ratios (δ18Oshell) of the mussel Mytilus galloprovincialis (Lamarck, 1819) in Otsuchi Bay, Northern Japan. δ18Oshell ratios vary spatially and temporally and exhibit strong linear correlations with both sea surface temperature (SST) and SSS measurements, indicating that the composite signal recorded by δ18Oshell measurably responds to variations in both parameters. By combining contemporaneous variations of SST and δ18Oshell, SSS records encoded into mussel shells are deconvolved that significantly correlate with in situ SSS values. To further validate the robustness of δ18Oshell as a quantitative SSS proxy, high-resolution and temporally aligned time-series of δ18Oshell-derived SSS are reconstructed that are highly synchronous with the instrumental records. In particular, two lowered SSS scenarios occur concomitantly with periods of the summer monsoon and typhoon events. δ18Oshell-derived SSS time-series are also comparable to those from numerical modeling. In conclusion, our findings demonstrate that mussel δ18Oshell signatures can be used as a useful tool to construct high-resolution records of SSS in the coastal regions.

Assessment of coastal turbidity improvement potential by terrigenous sediment load reduction and its implications on seagrass inhabitable area in Banate Bay, central Philippines.

This paper demonstrates the effects of terrigenous sediment load reduction by watershed managements on coastal turbidity in Banate Bay, Iloilo located in central Philippines, using field observations and numerical simulations. Measurements of the total suspended solid and particulate organic carbon indicated that the bulk component of the coastal turbidity comprised terrigenous mineral particles rather than phytoplankton at the rise of the river after heavy rain. The suspended sediment concentration and underwater light intensity were simulated by an atmosphere-watershed-coastal ocean model to investigate the contribution of the terrigenous sediment load to the coastal turbidity in rainy season. The coastal sediment simulation indicated that the turbidity in Banate Bay is highly impacted by terrigenous sediment inputs from distant watersheds, which are transported to the bay by coastal currents. In contrast, the contributions of sediment loads from the adjacent watersheds to the bay turbidity were limited. The simulation also indicated that the majority of the bay is not inhabitable for seagrasses due to limited light availability caused by the high turbidity. Scenario analysis of the sediment load reduction demonstrated that significant reduction of turbidity and improvement of light penetration are conditionally expected only when the remediation is implemented with cooperative management of a series of neighboring watersheds because of the significant contributions of sediment loads from multiple basins.

Groundwater oxygen isotope anomaly before the M6.6 Tottori earthquake in Southwest Japan.

Geochemical monitoring of groundwater in seismically-active regions has been carried out since 1970s. Precursors were well documented, but often criticized for anecdotal or fragmentary signals, and for lacking a clear physico-chemical explanation for these anomalies. Here we report - as potential seismic precursor - oxygen isotopic ratio anomalies of +0.24‰ relative to the local background measured in groundwater, a few months before the Tottori earthquake (M 6.6) in Southwest Japan. Samples were deep groundwater located 5 km west of the epicenter, packed in bottles and distributed as drinking water between September 2015 and July 2017, a time frame which covers the pre- and post-event. Small but substantial increase of 0.07‰ was observed soon after the earthquake. Laboratory crushing experiments of aquifer rock aimed to simulating rock deformation under strain and tensile stresses were carried out. Measured helium degassing from the rock and 18O-shift suggest that the co-seismic oxygen anomalies are directly related to volumetric strain changes. The findings provide a plausible physico-chemical basis to explain geochemical anomalies in water and may be useful in future earthquake prediction research.

Phosphorus as a driver of nitrogen limitation and sustained eutrophic conditions in Bolinao and Anda, Philippines, a mariculture-impacted tropical coastal area.

The dynamics of nitrogen (N) and phosphorus (P) was studied in mariculture areas around Bolinao and Anda, Philippines to examine its possible link to recurring algal blooms, hypoxia and fish kills. They occur despite regulation on number of fish farm structures in Bolinao to improve water quality after 2002, following a massive fish kill in the area. Based on spatiotemporal surveys, coastal waters remained eutrophic a decade after imposing regulation, primarily due to decomposition of uneaten and undigested feeds, and fish excretions. Relative to Redfield ratio (16), these materials are enriched in P, resulting in low N/P ratios (~6.6) of regenerated nutrients. Dissolved inorganic P (DIP) in the water reached 4µM during the dry season, likely exacerbated by increase in fish farm structures in Anda. DIP enrichment created an N-limited condition that is highly susceptible to sporadic algal blooms whenever N is supplied from freshwater during the wet season.

17-year change in species composition of mixed seagrass beds around Santiago Island, Bolinao, the northwestern Philippines.

Effects of fish culture can alter the adjacent ecosystems. This study compared seagrass species compositions in 2012 with those in 1995, when fish culture was less intensive compared to 2012 in the region. Observations were conducted at the same four sites around Santiago Island, Bolinao: (1) Silaqui Island, (2) Binaballian Loob, (3) Pislatan and (4) Santa Barbara, and by using the same methods as those of Bach et al. (1998). These sites were originally selected along a siltation gradient, ranging from Site 1, the most pristine, to Site 4, a heavily silted site. By 2012, fish culture had expanded around Sites 2, 3 and 4, where chlorophyll a (Chl a) was greater in 2012 than in 1995 by one order of magnitude. Enhalus acoroides and Cymodocea serrulata, which were recorded in 1995, were no longer present at Site 4, where both siltation and nutrient load are heavy.

Human impacts on large benthic foraminifers near a densely populated area of Majuro Atoll, Marshall Islands.

Human impacts on sand-producing, large benthic foraminifers were investigated on ocean reef flats at the northeast Majuro Atoll, Marshall Islands, along a human population gradient. The densities of dominant foraminifers Calcarina and Amphistegina declined with distance from densely populated islands. Macrophyte composition on ocean reef flats differed between locations near sparsely or densely populated islands. Nutrient concentrations in reef-flat seawater and groundwater were high near or on densely populated islands. delta(15)N values in macroalgal tissues indicated that macroalgae in nearshore lagoons assimilate wastewater-derived nitrogen, whereas those on nearshore ocean reef flats assimilate nitrogen from other sources. These results suggest that increases in the human population result in high nutrient loading in groundwater and possibly into nearshore waters. High nutrient inputs into ambient seawater may have both direct and indirect negative effects on sand-producing foraminifers through habitat changes and/or the collapse of algal symbiosis.

Use of stable nitrogen isotope signatures of riparian macrophytes as an indicator of anthropogenic N inputs to river ecosystems.

Deterioration of aquatic ecosystems resulting from enhanced anthropogenic N loading has become an issue of increasing concern worldwide, and methods are needed to trace sources of N in rivers. Because nitrate from sewage is enriched in 15N relative to nitrate from natural soils, delta(15)N values of stream nitrate (delta(15)Nnitrate) should be an appropriate index of anthropogenic N loading to rivers, as should the delta(15)N values of riparian plants (delta(15)Nplant) because they are consumers of nitrate. We determined the delta(15)N values of stream nitrate and six species of riparian macrophytes in 31 rivers in the Lake Biwa Basin in Japan. We then tested the correlation between these values and various land-use parameters, including the percentage of land used for residential and agricultural purposes as well as for natural areas. These delta(15)N values were significantly positively correlated with land use (%) that had a high N load (i.e., residential or agricultural use) and significantly negatively correlated with forest (%). These findings indicate that delta(15)N values of stream nitrate and riparian plants might be good indicators of anthropogenic inputs of nitrogen.

Dynamics of 15N natural abundance in wood-decomposing fungi and their ecophysiological implications.

Nine species of basidiomycota and one species of ascomycota were grown in an ammonium sulphate media and on beech wood; and the general (15)N dynamic patterns of the hyphae were examined. The fungal body initially became depleted in (15)N in both the types of incubation. However, the underlying mechanisms were quite different, that is, significant fungal (15)N drop on the beech wood is associated with the fungal N reallocation and the uptake of atmospheric ammonia and/or NO(x), in addition to isotope fractionation during assimilation. Although the (15)N values of the wood-decomposing basidiocarps were generally close to the (15)N values of the wood, it does not always indicate that the wood derived N was the sole N source for the fungi throughout the growth periods as shown in our wood-decomposing experiment.