A lake s u r v e y c o n s i s t i n g of 987 r a n d o m l y s e l e c t e d lakes was c o n d u c t e d in F i n l a n d in a u t u m n 1987.
The survey c o v e r e d the w h o l e country, and the w a t e r q u a l i t y of the lakes can be c o n s i d e r e d as r e p r e s e n t a t i v e of the a p p r o x i m a t e l y 56 000 lakes larger than 0.01 km 2 in Finland.
The m e d i a n TOC c o n c e n t r a t i o n is 12 mg L -I and the m e d i a n pH 6.3. The p r o p o r t i o n of lakes w i t h TOC c o n c e n t r a t i o n s a 5 mg L -I in the w h o l e c o u n t r y is 9 1 % . O r g a n i c a n i o n is the m a i n anion in the full data set (median 89 peq L-I).
The high organic m a t t e r c o n c e n t r a t i o n s in F i n n i s h lakes are a s s o c i a t e d with c a t c h m e n t areas c o n t a i n i n g large p r o p o r t i o n s of p e a t l a n d s and acid o r g a n i c soils u n d e r c o n i f e r o u s forest.
The survey d e m o n s t r a t e d that o r g a n i c m a t t e r s t r o n g l y affects the acidity of lakes in Finland.
The d e c r e a s i n g e f f e c t of o r g a n i c m a t t e r on the pH v a l u e s was d e m o n s t r a t e d by both r e g r e s s i o n analysis and ion balances.
At c u r r e n t d e p o s i t i o n levels of *SO 4 the pH of h u m i c lakes in F i n l a n d is d e t e r m i n e d to a greater e x t e n t by high TOC c o n c e n t r a t i o n s than by *SO 4 in most areas. In lakes w i t h pH values lower than 5.5 the a v e r a g e organic a n i o n c o n t r i b u t i o n is 56 % and n o n -m a r i n e s u l f a t e c o n t r i b u t i o n 39 %.
However, in the s o u t h e r n parts of the country, where the a c i d i c d e p o s i t i o n is highest, the m i n e r o g e n i c a c i d i t y c o m m o n l y exceeds the c a t c h m e n t d e r i v e d o r g a n i c acidity.