The influence of road salts on the salinity and the meromictic stability of Lake Svinsjøen,southeastern Norway

Primarily as a result of road salting, the water masses ofLake Svinsjøen, a small meromictic lake in southeasternNorway, have been subject to great changes in salinity duringthe period 1947–1995. The greatest change in saltconcentration has occurred in the upper part of themonimolimnion (depth 10–15 m) where mean conductivityincreased 104.2 per cent, from 143 to 292 S cm^–1. Inthe upper mixolimnion (depth 0-5 m), mean conductivity rosefrom 130 to 238 S cm^–1 during the same period. Theions responsible for the salinity changes were Na⁺ andCl^– from de-icing salts, and Ca²⁺ and Cl^– fromsalts used to keep down dust from roads. Further sources ofCa²⁺ are the road asphalt and increased weathering andleaching of the lime-rich rocks caused by acid precipitation,the main source of the additional inputs of SO tothe lake. The salinity changes caused major changes inmeromictic stability, S . In the period1947-1966, S increased by 24 g cm cm^–2,and the maximum level of meromictic stability, 125 gcm^–2, was found in 1966. As a result of higher rate ofsalt accumulation in the upper part of the monimolimnion andin the mixolimnion, S decreased by 30 g cmcm^–2 during the period 1966-1991, and a simultaneousrise in the chemocline took place. In the period 1991-1995 anadditional decrease of 26 g cm cm^–2 occurred. Continuedectogenic inputs of salts through processes typical of thetime period investigated will in future further weaken thelake's meromictic stability, and may cause the demise ofmeromixis in Lake Svinsjøen, a development which may haveimportant implications for primary productivity of thelake.     

Internal energy,the work of the wind,and the thermal stability in Lake Tyrifjord,southeastern Norway

Lake Tyrifjord consists of Holsfjord and Steinsfjord, two lakes of highly contrasting morphology, was used as the basis for a comparison of internal energy, the work of the wind, and thermal stability in two lakes under identical large-scale climatic conditions. Because of simultaneous temperature measurements available for the summer of 1973 from the two lakes, the present paper is based on these observations. The deep Holsfjord (z_max = 295 m) accumulates internal energy of an order representative for its geographical situation. Its annual heat budget in 1973 was 129 735 j cm^–2. Steinsfjord (z_max = 22 m) accumulated 60 per cent of the latter. At the stratifications representing maximum stability in 1973, the stability S in Holsfjord exceeded the corresponding value in Steinsfjord by a factor of 37, whereas the work of the wind B in Holsfjord was only 1.45 times the Steinsfjord B value. The much greater center of volume moment under homoiothermal conditions typical of Holsfjord was responsible for the great difference between energy conditions in the two lakes, as clearly demonstrated by the B + S values. The ratio (B + S)/B shows that the water masses of Steinsfjord would maintain homoiothermal conditions throughout the heating season with an increase in B (i.e. an increase in external energy supply) of ca 40 per cent. The additional energy supply needed to maintain homoiothermal conditions in Holsfjord was of the order of 1200 per cent of B for Holsfjord. Accordingly, the metalimnetic thermal discontinuity represents a far more effective mixing barrier in Holsfjord than in Steinsfjord. Calculations of the (B + S)/B ratios for other lakes show that this ratio is useful for a judgment of the degree of stagnancy of a lake's hypolimnetic water masses.