Restoration responses of 19 lakes: are TP thresholds common?

A key outcome of this 19-lake analysis is that non-linear responses of phytoplankton biomass to declining TP concentrations are indeed common: 12 lakes showed pronounced non-linear responses, and among these, 5 stratified and 2 shallow lakes showed clear TP thresholds above which TP did not control seasonal means of Chl.a concentrations (Table 4). Others showed resilience against Chl.a-reduction in spite of a significant range of TP-reduction - e.g. Wahnbach Reservoir in the range of mean TP-concentrations declining from 35 to 20 μg/L and Lake Geneva as TP declined from 45 to 25 μg/L. Yet others have not moved through a sufficiently large span of TP-concentrations to show such thresholds (e.g. Lago Maggiore, Petersdorfer See, Melangsee and the Galten Basin of Lake Mälaren). Furthermore, some non-linear response patterns are also due to other factors such as changes in grazing pressure, both in deep stratifying lakes (e.g. in Lago Maggiore, Lake Geneva) and in shallow lakes (e.g. Veluvemeer). For the shallow lakes the plateaus of Chl.a over TP during their highly eutrophic phases are not as pronounced - rather, in the range of high TP concentrations their seasonal means of Chl.a against TP zig-zag or spiral between higher and somewhat reduced levels, with Chl.a following TP only in some years. As their mean TP-concentrations declined further through the lower concentration range, means of Chl.a follow TP more closely. Contrary to the early results published by Sas (1989), for deeper stratified lakes the longer time series now available show markedly less variability than those for the shallow lakes, both when comparing responses between lakes and when analysing time series for individual lakes.