Do phytoplankton communities correctly track trophic changes? An assessment using directly measured and palaeolimnological data

1. Measurements of total phosphorus (TP) concentrations since 1975 and a 50-year time series of phytoplankton biovolume and species composition from Lake Mondsee (Austria) were combined with palaeolimnological information on diatom composition and reconstructed TP-levels to describe the response of phytoplankton communities to changing nutrient conditions. 2. Four phases were identified in the long-term record. Phase I was the pre-eutrophication period characterised by TP-levels of about 6 mu g L-1 and diatom dominance. Phase II began in 1966 with an increase in TP concentration followed by the invasion of Planktothrix rubescens in 1968, characterising mesotrophic conditions. Phase III, from 1976 to 1979, had the highest annual mean TP concentrations (up to 36 mu g L-1) and phytoplankton biovolumes (3.57 mm(3) L-1), although reductions in external nutrient loading started in 1974. Phases II and III saw an expansion of species characteristic of higher nutrient levels as reflected in the diatom stratigraphy. Oligotrophication (phase IV) began in 1980 when annual average TP concentration, Secchi depth and algal biovolume began to decline, accompanied by increasing concentrations of soluble reactive silica. 3. The period from 1981 to 1986 was characterised by asynchronous trends. Annual mean and maximum total phytoplankton biovolume initially continued to increase after TP concentration began to decline. Reductions in phytoplankton biovolume were delayed by about 5 years. Several phytoplankton species differed in the timing of their responses to changing nutrient conditions. For example, while P. rubescens declined concomitantly with the decline in TP concentration, other species indicative of higher phosphorus concentrations, such as Tabellaria flocculosa var. asterionelloides, tended to increase further. 4. These data therefore do not support the hypotheses that a reduction in TP concentration is accompanied by (i) an immediate decline in total phytoplankton biovolume and (ii) persistence of the species composition characterising the phytoplankton community before nutrient reduction.