日本地球惑星科学連合2019年大会

講演情報

[E] ポスター発表

セッション記号 H (地球人間圏科学) » H-TT 計測技術・研究手法

[H-TT18] 環境トレーサビリティー手法の開発と適用

2019年5月28日(火) 15:30 〜 17:00 ポスター会場 (幕張メッセ国際展示場 8ホール)

コンビーナ:陀安 一郎(総合地球環境学研究所)、大手 信人(京都大学大学院情報学研究科社会情報学専攻)、Gabriel J Bowen(University of Utah)

[HTT18-P01] Isotopic study on seasonal nitrogen dynamics at Lake Biwa

*石橋 孝晃1尾坂 兼一1木庭 啓介2中村 高志3西田 継3 (1.滋賀県立大学 環境科学部、2.京都大学生態学研究センター、3.山梨大学大学院・国際流域環境研究センター)

キーワード:窒素動態、窒素安定同位体、季節変動

To clarify nitrogen dynamics in Lake Biwa, we measured the concentrations of nitrogen compounds and their nitrogen stable isotopic ratios in 14 depths in north basin of Lake Biwa. In the lake, particulate nitrogen (PN), such as phytoplankton, is generally decomposed to dissolved nitrogen (DN); in order of dissolved organic nitrogen (DON), ammonia nitrogen (NH4+) and nitrate nitrogen (NO3-). These dissolved nitrogen compounds are taken up by phytoplankton again, and the nitrogen circulation in the lake is established. To predict the water quality in future, it is necessary to understand the nitrogen cycle in the lake, as well as nutrient load to the lake.

In this study, we collected lake water sample at 14 depths in north basin of Lake Biwa (water depth is 90 m) in 1-2 months interval. The lake waster sample was analyzed for the concentration of PN, PC, DN, NO3-, NO2- and NH4+, d15N of PN, DN and NO3-.

NO3- concentration was lower in the surface layer (0-20m), and higher in the deep layer (20-90m) during the stratification period, whereas NO3- concentration was constant in all layers during the circulation period. PN concentration also increased in the surface layer from April to June, decreased sharply in July. These indicate that phytoplankton which grew on the surface layer from April to June used NO3-, thereafter, NO3- was depleted after June, then, phytoplankton decreased after July respectively. On the other hand, NO3- concentration is considered to be increased by decomposition of the precipitated PN from surface layer. It is considered that the NO3- concentration became constant in all layers because of mixing of deep layer and the surface layer.

The weighted average d15NDN, d15NNO3 in the unit water columns decreased about 3‰ in the circulation period, in the circulation period, then, increased during the stratification period. The decrease of d15NDN, d15NNO3 at the water column during circulation period can not be explained by mixing of deep layer and the surface layer. We will discuss the mechanisms of d15NDN, d15NNO3 decrease during circulation period in the presentation.