Statistical Analyses
First, detrended correspondence analyses were conducted to
obtain the gradient length of both the stream and lake
chemistry data. Because the gradient lengths were in both
cases £1.5 SD, the linear method redundancy analysis (RDA)
was used to study the effects of environmental variables
representing geographic position and regional- and localscale
factors on stream and lake water chemistry. Moreover,
preliminary analyses of water chemistry (total phosphorus
concentration) and catchment land use (% agriculture) did
not reveal any step changes between the northern and
southern regions. RDA was performed on a correlation matrix
and is a form of direct gradient analysis (like Principal
Components Analysis). In a first step in RDA, the entire set of
60 environmental variables was tested to determine the significance
of individual variables using a Monte Carlo permutation
test (with 999 unrestricted permutations).
Variables that were not significantly correlated with the three
water chemistry variables or that were found to co-vary with
other environmental variables (i.e., variance inflation factors
>100) were removed (n = 22) from the data set.