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We analysed long-term changes in
phytoplankton composition in relation to hydrological,
meteorological and nutrient loading data in the
large (270 km2) shallow (mean depth 2.8 m) Lake
Vo˜ rtsja¨rv. Nutrient loads to the lake were heavy in
the 1970s and 1980s and decreased considerably
thereafter. The average nutrient concentrations for
1985–2004 (1.6 mg l-1 of total nitrogen and
53 lg l-1 of total phosphorus) characterize the lake
as a eutrophic water body. All four calculated
taxonomic indices showed a unidirectional deterioration
of the lake’s ecological status, despite reduced
concentrations of nutrients. We focused our analysis
on the PTSI index, which revealed a stepwise change
between the years 1977 and 1979 that coincided with
a large increase in water level, but also with a change
of investigator. After correcting input data for
possible investigator-induced differences, the step
change remained because it was caused by major
changes in the whole phytoplankton community. The
previous dominant Planktolyngbya limnetica was
replaced by two species of seasonally altering
Limnothrix. Among phytoplankton functional groups,
there was a decrease in all groups comprising smallsized
phytoplankton species, such as X1, E, F, J, N
and an increase in S1 and H1, both represented by
filamentous cyanobacteria. Our results suggest a
non-linear response of phytoplankton to changing
nutrient loadings, and that the change observed
between 1977 and 1979 was a regime shift triggered
by water level change. High shade tolerance of the
new dominants, and their ability to create shade,
obviously stabilized the new status making it resistant
to restoration efforts.
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