Methods
Study Site
The data set used in this study consists of 390 streams
and 361 lakes sampled as part of the Swedish national
stream and lake survey in autumn 2000 (Johnson and
Goedkoop 2000; Wilander and others 2003) (Fig. 1). A
number of factors suggested that this dataset was sufficiently
robust for examining among-site similarities/dissimilarities
in surface water chemistry of boreal streams
and lakes. Firstly, streams and lakes were selected randomly;
thus, the samples should be representative of the
population of streams and lakes sampled. In selecting
lakes, only lakes with surface areas >4 ha were included,
and two size classes were used for stratifying stream sites
(catchment area classes of 15 to 50 and 50 to 250 km2).
Because we were interested in obtaining a depth-integrated
measure of surface water chemistry, lakes were
sampled during autumn turnover. Hence, sampling started
in the northernmost parts of the country and progressed
southwards. A more detailed description of stream and
lake selection is given in Wilander and others (2003). In
this study, we were interested in understanding the effects
of local and regional-scale variables on the expected
natural variability of selected water chemistry variables.
Thus, sites deemed to be affected by liming, acidification
(lakes: critical load exceedence of S and N > 0; Rapp and
others (2002)) and agriculture/silviculture (catchments
with more than 25% defined as arable and affected by
clear-cutting, respectively) were not included in this
dataset.