Abstract
AbstractA trend of shortening duration of ice coverage has been reported in many rivers, ponds and lakes around the world due to climatic warming. The shortening will have much influence on organic production due to the greater supply of light intensity and hence on water quality. Chemical Oxygen Demand (COD), a useful measure of water quality, has been monitored over the past thirty years in Saroma-ko Lagoon, a boreal lake in Japan characterized by wintertime ice coverage. Here the data set of water quality in the lagoon is analyzed to reveal long-term trends in COD and the influence of climatic warming on such trends. There has been a significant increasing trend, caused primarily by an abrupt increase in the early 1990s. However, the increase could not be explained by the increased load of organic matter on the lagoon from the river basin and organic production in the lagoon. On the other hand, periods of freezing of the entire surface have tended to be shorter with inevitable thinning of ice over the past forty years, probably due to climatic warming. Ice and planktonic algae are exposed to the low light intensity in situ, which is not optimum for the algal production of organic matter. The shortening and thinning should thus bring an improvement in ambient light conditions for algae, resulting in an increase of organic matter in winter. Most of the organic matter possibly remains even after spring because of the low water temperature. It is highly possible that climatic warming promotes deterioration in water quality in ice-covered lakes.
Highlights
Saroma-ko Lagoon, which is[150] km2 in area and 19.5 m in maximum depth, is located on the northeast coast of Hokkaido and is connected to the Sea of Okhotsk by two narrow channels
The high Chemical Oxygen Demand (COD) can be due to a large load of organic matter into the lagoon due to the large inflow of river water
The mean COD values shifted from 1977 to 1993 within a range of relatively low values, and neither a significantly increasing nor decreasing trend was found in the values in spite of year-to-year variation (P>0.05; Fig. 1b)
Summary
Saroma-ko Lagoon, which is[150] km[2] in area and 19.5 m in maximum depth, is located on the northeast coast of Hokkaido (ca. 44°N, 144°E) and is connected to the Sea of Okhotsk by two narrow channels. The high COD can be due to a large load of organic matter into the lagoon due to the large inflow of river water.
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