| A dynamic, process-based acidification model, MAGIC7, has been applied to three
small, strongly acidified lakes in the Bohemian Forest, the Czech Republic. The model was
calibrated for a set of experimental records on lake water composition over the 1984–2000
period, and produced hindcast concentrations that compared well, even with older (40-year)
irregular determinations of nitrate, chloride and pH. Water and soil chemistry forecasts
up to 2050 were based on reductions in S and N emissions presupposed by the Gothenburg
Protocol. Modelled sulphate and chloride concentrations were predicted to decrease to the
levels at the beginning of the 20th century by 2050. The lake water carbonate buffering
system is predicted to be re-established in only two lakes (Cerné and Plešné), with
current soil base saturations of 12-15%. Concentrations of ionic aluminium species
decreased sharply, from 110 μeq l-1 in the mid-1980s to the current
~40 μeq l-1, and were
predicted to decrease below 10 μeq l-1 in the 2020s. Diatom-inferred
pH in pre-industrial
times was substantially lower than modelled pH. It is suggested that the diatom pH,
based almost entirely on non-planktonic species, is biased by inwash of diatoms from
more acidic tributaries into the sediment of these small lakes. Generally significant
results can be summarised as follows: (1) Simulated sulphate levels agree well with
observations during acidification progress and retreat only for values of soil
SO42- adsorption capacity three to six times
(20 to 40 μeq kg-1)
higher than those found experimentally. This implies a further mechanism of S retention
and release in addition to physical sulphate adsorption to Fe and Al oxides of soils.
(2) The catchments’ ability to retain deposited N appeared to decline after ~1950 but
this was not connected with a sufficient change in the C:N ratio of the soils. Agreement
between modelled and observed concentrations of nitrate was therefore achieved by
empirical restriction of N retention in the soils. Based on their current ability to
retain N, the catchments will remain N-saturated and could, temporarily, produce more
inorganic N than they receive due to additional nitrate production from soil N-organic
pools. This situation has occurred already in the Cerné Lake catchment. (3) Differences
in responses of individual lakes can be attributed to different land usages over the
past several centuries as well as to differences in geology and primary production.
Keywords: MAGIC, atmospheric deposition, N retention, diatom-inferred pH, sulphate,
nitrate, base cations, aluminium, Czech Republic |