Species richness components and depth diversity gradient measures
As depth-independent component of species richness, we calculated gamma richness as the total number of species per lake and year.
As depth-dependent component of species richness, we determined for every lake and year an additive alpha richness as the number of species per depth range averaged across of transects (Fig. 1b). The gamma richness for every lake and year was defined as the total number of species per depth range (Fig. 1c). We then calculated an additive beta richness as gamma richness minus alpha richness (Tuomisto 2010) (Fig. 1d).
To further characterize the diversity depth gradient, we identified the peak of the richness depth curve (Fig. 1c). For each transect we filtered the depth with the maximal species number. Thereafter, we averaged this valued across transects per lake and year, from now on termed the depth with maximal alpha richness (D(α,max) ). The corresponding maximal species number averaged across transects is termed the maximal alpha richness (R(α,max) ). Similarly, the depth with maximal gamma richness (D(γ,max) ) is the depth of the maximal gamma richness (R(γ,max) ) along depth, as well as maximal beta depth (D(β,max) ) describes maximal beta richness (R(β,max) ) along depth.