Hydrological environment affects the nutrient retention and runoff function of naturally re-wetted agricultural peatland in lowland river floodplain.

Re-wetting of agricultural areas reclaimed by draining peatlands reportedly entails risks of nutrient loads downstream because of leaching of dissolved nutrients from pools in the soil. On floodplain fens, nutrient retention and runoff function have been recognized as dependent upon the hydrological environments of re-wetted agricultural peatland (RAP). Although many studies have been conducted for artificially re-wetted agricultural peatlands (artificial RAPs), knowledge on naturally re-wetted agricultural peatlands (natural RAPs) has been lacking. This study assessed the natural re-wetting of agricultural areas in floodplain fens in terms of risks of nutrient loading in the basin of Kushiro Mire, northern Japan. Flooding of the adjacent river caused by heavy rainfall remarkably increased the water flow, and the inflow and outflow fluxes of nitrogen (N) and phosphorus (P) of a test plot in the natural RAP. Flood waters supplied mainly inorganic nutrients to the test plot, including NO3-N and PO4-P. Larger amounts of dissolved organic N and P, NH4-N, and PO4-P that had accumulated in surface water and surface groundwater in the plot flowed out. Consequently, the test plot represented net runoff of 3 and 0.4 mg m-2 day-1 as total N and total P, respectively, for the average of the whole observation period. The test plot was a source of N loading downstream, which was contrary to results obtained for artificial RAPs in many studies. However, the test plot showed a smaller amount of net phosphorus runoff. Our findings suggest that water level fluctuation and river flood water inflow affect the nutrient retention and runoff functions of RAPs. Repeated inundated and dried conditions, with no continuous inflow of river water, explain the nutrient runoff in the test plot.

Effects of forest cover on richness of threatened fish species in Japan.

Estuaries--one of the most vulnerable ecosystems globally--face anthropogenic threats, including biodiversity loss and the collapse of sustainable fisheries. Determining the factors contributing to the maintenance of estuarine biodiversity, especially that of fish, is vital for promoting estuarine conservation and sustainability. We used environmental DNA metabarcoding analysis to determine fish species composition in 22 estuaries around Japan and measured watershed-scale land-use factors (e.g., population size, urban area percentage, and forest area percentage). We sought to test the hypothesis that the richness of the most vulnerable estuarine fish species (i.e., registered by the Japanese Ministry of the Environment in the national species red-list) is determined by watershed-scale land-use factors. The richness of such species was greater, where forest cover was highest; thus, forest cover contributes to their conservation. The proportion of agriculture cover was associated with low species richness of red-listed fishes (redundancy analysis, adjusted R2 = 43.9% of total variance, df = 5, F = 5.3843, p = 0.0001). The number of red-listed species increased from 3 to 11 along a watershed land-use gradient ranging from a high proportion of agriculture cover to a large proportion of forest cover. Furthermore, the results showed that throughout Japan all the examined watersheds that were covered by >74.8% forest had more than the average (6.7 species per site) richness of red-listed fish species. This result can be attributed to the already high average forest cover in Japan of 67.2%. Our results demonstrate how the land use of watersheds can affect the coastal sea environment and its biodiversity and suggest that proper forest management in conjunction with land-use management may be of prime importance for threatened fish species and coastal ecosystems in general.

Efectos de la Cobertura Forestal sobre la Riqueza de Especies Amenazadas de Peces en Japón Resumen Los estuarios-uno de los ecosistemas más vulnerables a nivel mundial-enfrentan amenazas causadas por el hombre, incluyendo la pérdida de biodiversidad y el colapso de las pesquerías sustentables. La determinación de los factores que contribuyen al mantenimiento de la biodiversidad estuarina, especialmente la de los peces, es vital para fomentar la conservación y sustentabilidad estuarinas. Usamos un análisis de metasecuenciación de ADN ambiental para determinar la composición de especies de peces en 22 estuarios de Japón y medimos los factores del uso de suelo a nivel de cuenca (p. ej.: tamaño poblacional, porcentaje de área urbana y porcentaje de área forestal). Buscamos probar la hipótesis de que la riqueza de las especies de peces más vulnerables (es decir, aquellas registradas por el Ministerio Japonés del Ambiente en la lista roja de especies a nivel nacional) está determinada por los factores de uso de suelo a nivel de cuenca. La riqueza de dichas especies fue mayor en donde la cobertura forestal era la más alta; por lo tanto, la cobertura forestal contribuye a la conservación de estas especies. La proporción de la cobertura agrícola estuvo asociada con una baja riqueza de especies de peces en la lista roja (análisis de redundancia, R2 ajustada = 43.9% de la varianza total, gl = 5, F = 5.3843, p = 0.0001). El número de especies en la lista roja incrementó de 3 a 11 a lo largo del gradiente de uso de suelo de la cuenca, yendo desde una proporción alta de cobertura agrícola a una proporción alta de cobertura forestal. Además, los resultados mostraron que, en Japón, todas las cuencas analizadas que contaban con una cobertura forestal >74.8% tenían mayor riqueza de especies de peces en la lista roja que el promedio (6.7 especies por sitio). Este resultado puede atribuirse a la ya de por sí elevada cobertura forestal de Japón (67.2%). Nuestros resultados demuestran cómo el uso de las cuencas puede afectar al ambiente costero marino y a su biodiversidad y sugiere que la gestión adecuada de los bosques en conjunto con el manejo del uso de suelo puede ser de suma importancia para las especies amenazadas de peces y los ecosistemas costeros en general.

Long-term nationwide spatiotemporal changes of freshwater temperature in Japan during 1982-2016.

We analyzed freshwater temperature data from 1982 to 2016 throughout Japan to better understand how waters are warming in Japan. We used linear regression to determine the temperature change rate and Mann-Kendall tests to identify significant temporal trends in the annual maximum and mean temperatures. Among 11,240 monitoring sites screened, 159 with fewer missing values were selected for analysis. On the basis of this analysis, we identified and ranked the sites showing significant temporal increasing or decreasing trends for future management. At nearly half (42%) of the analyzed sites, the annual mean freshwater temperature was increasing; thus, in the future, adverse impacts from warm temperatures may increase in those aquatic ecosystems. The temperature change rate of fresh water was higher than that of air, indicating that the observed increases in freshwater temperature were not due to atmospheric warming only. Among individual sites, the annual maximum freshwater temperature change rate ranged from -1.27 to 1.91 °C/decade, and the annual mean rate ranged from -1.13 to 1.28 °C/decade. Few other studies have reported decreasing temperatures for fresh water. We expect our results will improve understanding of how freshwater temperatures are changing at a large scale, enhance understanding of human impacts on the aquatic environment, support effective management of ecosystems experiencing temperature changes, and help to minimize the loss of biodiversity over the next half century.

Evaluation of fish biodiversity in estuaries using environmental DNA metabarcoding.

Biodiversity is an important parameter for the evaluation of the extant environmental conditions. Here, we used environmental DNA (eDNA) metabarcoding to investigate fish biodiversity in five different estuaries in Japan. Water samples for eDNA were collected from river mouths and adjacent coastal areas of two estuaries with high degrees of development (the Tama and Miya Rivers) and three estuaries with relatively low degrees of development (the Aka, Takatsu, and Sendai Rivers). A total of 182 fish species across 67 families were detected. Among them, 11 species occurred in all the rivers studied. Rare fishes including endangered species were successfully detected in rich natural rivers. Biodiversity was the highest in the Sendai River and lowest in the Tama River, reflecting the degree of human development along each river. Even though nutrient concentration was low in both the Aka and Sendai Rivers, the latter exhibited greater diversity, including many tropical or subtropical species, owing to its more southern location. Species composition detected by eDNA varied among rivers, reflecting the distribution and migration of fishes. Our results are in accordance with the ecology of each fish species and environmental conditions of each river.

Long-term spatiotemporal changes of 15 water-quality parameters in Japan: An exploratory analysis of countrywide data during 1982-2016.

We conducted an exploratory analysis of 15 water-quality parameters collected countrywide during 1982-2016 to better understand human impacts on aquatic ecosystems in Japan. We used the Mann-Kendall test to identify temporal trends. On the basis of this analysis, we identified and ranked the sites for future management where there were trends toward lower water quality. The study showed general improvement of dissolved oxygen, biochemical oxygen demand, chemical oxygen demand, suspended solids, Escherichia coli counts, n-hexane extracts, total nitrogen, total phosphorus, and total zinc. We concluded that management of wastewater has been effective throughout Japan, but with the caveat that conditions have deteriorated at some sites, which should be the focus of studies aimed at identifying the causes of the lower water quality. Concentrations of chloroform, formaldehyde, nonylphenol, and linear alkylbenzene sulfonates, which are toxic environmental pollutants that can have adverse effects on human and ecosystem health, showed significant increasing or decreasing temporal trends at only a few monitoring sites. Sites where concentrations of these toxicants increased should be targeted for further study to determine whether remedial actions are needed.