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

Abstract

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.