Biogeochemical processes controlling the dynamics of dissolved organic matter in streams in the Shirakami Mountains, Japan.

The Shirakami Mountain range, including the largest primeval beech forest in East-Asia, is undergoing ecological change. Dissolved organic matter (DOM) plays an important role in nutrient and material cycling in forest ecosystems. Because the quality of DOM varies based on its origin and diagenetic and runoff processes, changes in the environment surrounding DOM can be rapidly detected by monitoring its quality. Herein, concentrations and fluorescence composition of DOM at 14 sites in 13 streams in the Shirakami Mountain range were monitored monthly for over 2 years, excluding winter (December-March), to gain insight into the catchment hydrological and soil characteristics affecting DOM concentrations and composition in stream water. Based on the pattern of temporal changes in fluorescent component composition, monitoring sites were categorized into four groups (streams with small catchments, large catchments, catchments facing the Sea of Japan, and open waters in the catchment) with similar catchment characteristics affecting DOM dynamics. Multiple linear regression analysis showed that DOM concentrations in each group could be attributed to rainfall on the survey date, short-term (1-2 days) rainfall, midterm (~1 month) accumulated rainfall, midterm (7-11 days) accumulated temperature, and catchment characteristics as explanatory variables. The degree of influence of these variables differed among the four groups. The results of this study show that grouping streams according to catchment hydrological characteristics can help identify the impact of climate and environmental change on DOM dynamics in stream water.

Forecasting a 2-methylisoborneol outbreak in a brackish lake.

2-Methylisoborneol (2-MIB) is the primary cause of the earthy and musty odor produced by cyanobacteria, which deteriorates the quality of fishery products and tap water. Despite the need for controlling outbreaks, few studies have been conducted on 2-MIB in brackish lakes, where capture fisheries are active. This study examined the association between water quality and the outbreak of 2-MIB in a brackish lake using statistical analysis of long-term monitoring data and developed forecasting models for 2-MIB outbreaks. We investigated Lake Ogawara, which is a brackish lake with a cool-temperate climate in Japan, where 2-MIB outbreaks frequently occur between August and December. Logistic regression analyses were performed using the outbreak or non-outbreak of 2-MIB (2-MIB(+ / -)) as the dependent variable and water quality parameters as the independent variables. The results suggested that the density of 2-MIB-producing cyanobacteria was higher when (1) dissolved inorganic nitrogen concentrations were low under the relaxation of phosphorus limitation and/or (2) salinity or micronutrient concentrations were high. In addition, we successfully developed forecasting models with a high predictive power that determined 2-MIB(+ / -) in August-December using only two water quality parameters: dissolved inorganic phosphate and pH in April and total nitrogen/total phosphorous and salinity in May.

Spring to summer nitrogen level in a brackish lake is higher in abundant snowmelt years: Correlation and causation.

Eutrophication is an issue of concern in many brackish lakes with an agricultural watershed. The amount of snowfall in snowy areas is anticipated to decline because of global climate change. The aim of this study was to assess the impact of changes in the inflow of snowmelt on the nutrient concentrations of a downstream brackish lake. In Lake Ogawara, a brackish lake in a snow-covered agricultural area of Japan, we examined the relationships between inflowing river discharge (D/C) during spring and total nitrogen (TN) and total phosphorus (TP) concentrations in the mixolimnion of the lake ([TNmix ] and [TPmix ], respectively) using 29 yr of monitoring data. In addition, we assessed the causal relationship between the D/C and the lake nutrient concentrations. There was large year-to-year variation in D/C during April (D/CApr ), which accounted for 7-31% of the mixolimnion volume. Significant positive correlations were observed between D/CApr and [TNmix ] from the ensuing April to September. On an annual basis, 49% of the interannual variation of the mean [TNmix ] during the ensuing April to September was explained by the interannual variation of D/CApr . Therefore, D/CApr could be useful as a simple index to [TNmix ] in the ensuing spring to summer. It is notable that the relationships between D/CApr and [TNmix ] from April to September was indicated to be acausal by statistical causal inference. Common climate conditions that increase D/CApr (i.e., a cold winter with a high level of precipitation) were found to drive other biogeochemical processes that increased [TNmix ] during the ensuing spring to summer.