CHEMICAL STUDIES ON CARBON CYCLE IN LAKE SUWA BY DETERMINING PH AND TOTAL CARBON DIOXIDE

抄録

Carbon cycle in Lake Suwa (maximum depth : 6.7 m;area : 14.5 km²) which is a highly polluted and eutrophic lake in Nagano Prefecture, Japan, was studied by determining pH and total carbon dioxide as follows :
(1) Total carbon dioxide (Total CO₂) of the lake water was determined by KOYAMA's method. The same water sample was aerated in order to be equilibrated with air. Total CO₂ of the aerated water sample was determined as an original total carbon dioxide (Orig. CO₂).
In the stably stratified lake waters, the difference (ΔCO₂) between Orig. CO₂ and Total CO₂ corresponds to the net fixation of CO₂ (N. Fix. CO₂) in a case of positive value and the net formation of CO₂ (N. Form. CO₂) in a case of negative value.
In the case of Lake Suwa, however, it should be suitable to call the former as the preserved amount of net fixation of CO₂ (P. A. N. Fix. CO₂) and the latter as the preserved amount of net formation of CO₂ (P. A. N. Form. CO₂), because the lake water layers are easily mixed and some of the dissolved CO₂ may be exchanged for the atmospheric CO₂ through all seasons.
From a vertical distribution of ΔCO₂, P. A. N. Fix. CO₂ or P. A. N. Form. CO₂ in a water column was calculated on the basis of (g C/m²).
(2) Relation curves between pH and Total CO₂ of the lake water were constructed from pH and Total CO₂ values obtained from the water samples collected in different months.
(3) Greater part of the numerous data of Total CO₂ values used in this study were estimated from a large number of vertical distributions of pH values obtained at different times in different days while comparing with the relation curves between pH and Total CO₂.
(4) Average P. A. N. Fix. CO₂ (A. P. A. N. Fix. CO₂) and average P. A. N. Form. CO₂ (A. P. A. N. Form. CO₂) in a whole day were estimated from the values of P. A. N. Fix. CO₂ and P. A. N. Form. CO₂ in a water column, which were estimated at different times in the day.
(5) Numerous data of these A. P. A. N. Fix. CO₂ or A. P. A. N. Form. CO₂ in the water column were estimated on different days in different months.
(6) A histogram of A. P. A. N. Fix. CO₂ and A. P. A. N. Form. CO₂ was then constructed against months.
(7) This histogram shows the following facts : (a) A. P. A. N. Fix. CO₂ ranged between 3.8 and 6.6 g C/m² from April through August. (b) A. P. A. N. Form. CO₂ ranged between -8.2 and -12.9 g C/m² from September through December. (c) On the assumption that the average exchangeability of CO₂ between air and lake water in a month is similar to those of the adjacent months, the difference between A. P. A. N. Fix. CO₂ or A. P. A. N. Form. CO₂ of a month and that of an adjacent month corresponds to the average net fixation rate of CO₂ during the two months in a case of positive value and to the average net formation rate of CO₂ in a case of negative value. (d) The average net fixation rate of CO₂ and the average net formation rate of CO₂ correspond to the average net production rate and the average net decomposition rate of organic matter respectively.
(8) The maximum net production rate which was found between March and April amounted up to 4.5 g C/m². day and the maximum net decomposition rate which was found between August and September amounted up to-13.5 g C/m². day.