Download Holocene development of lakes on the Great Alvar, Öland

The last 11 000 years history of the alvar lake Dröstorps mosse
Geoffrey Lemdahl and Börje Ekstam
Dept. of Biology and Environmental Science, University of Kalmar, 391 82 Kalmar
The lakes of the Great Alvar of Öland are very shallow and highly influenced by seasonal
fluctuations in the water-levels. A number of these lakes are characterised by “islands” with
reed dominated vegetation in the central parts of the basins. Results from earlier litho- and
pollenstratigraphical studies show that these wetlands were once open lakes, only with
submersed vegetation of Chara algae. Curiously, if compared with smaller lakes on the main
land of S. Sweden, the overgrowing successions seems to start in the deepest parts of the lake
basins instead of as expected in the shallow parts. Thus the question arise how these unique
wetlands have developed to their present state and when this process started. What were the
mechanisms for the changes in the limnic ecosystems and what consequences had that for the
biodiversity. We are convinced that this is very important problems to address for sustainable
nature managements of these wetlands.
In order to reconstruct the lake dynamics back in time, Dröstorp mosse was chosen as a first
object for a palaeolimnological pilot study. So far, we look at lithostratigraphical changes and
macroscopic remains of plants, insects, ostracods and molluscs, and changes in sediment
chemistry. The first radiocarbon dates show that the limnic sedimentation started around
11 100 years ago. The lake basin was thus isolated from the Baltic Sea. Until ca 6 000 years
ago calcareous gyttja and lake marl were deposited more or less continuously, reflecting open
water with Chara-dominated vegetation. Water plants such as Potamogeton filiformis,
Myriophyllum spicatum and Nymphaea alba were also present. The lake margin vegetation
was dominated by Juncus spp., Carex spp. and Cladium mariscus. At 55 cm sediment depth
(around 6 000 years ago), there is a distinct, very sharp change in the lithology (hiatus). The
light coloured lake marl change abruptly to a dark brown coarse detritus gyttja. Just above this
datum there is a marked drop in the number of Chara-remains. At the same time the diversity
of other water plants increase. New pondweed species appear such as P. natans, P.
praelongus, P. perfoliatus and P. pusillus. There are also frequent remains of Alisma
plantago-aquatica and Polygonum persicaria. There is a marked increase of fruits of sedges
and possible remains of Typha latifolia. The latter finds suggest that reed vegetation now
establish in the centre of the lake basin. Moreover, the change in vegetation is also coupled
with a striking increase in the number of insect species, both water living and carr species. At
around 42 cm (ca 4 600 year ago) there is a rather gradual change to a carr peat, which is
more compact between 42 to 18 cm, compared with above. The first results from chemical
analyses also show distinct changes. At around 55 cm depth the copper concentration
increases markedly. At ca 42 cm lead starts to increase, whereas cadmium and zinc shows
increasing concentrations particularly around 25 cm.
Though the study of Dröstorps mosse is still in an early stage, the first results are very
instructive. During the first five thousand years the lake was characterised by open water and
characean dominated vegetation with a low diverse limnic fauna, mainly consisting of a few
species of molluscs, ostracods and chironomids. Around 6 000 years ago there is an abrupt
change in the lake ecosystem, were reed vegetation establish in the central part. This time
coincides with a period of exceptional low lake-levels in southern Sweden. Thus climate
change may have triggered the system. The disturbance seems to have had the general effect
that the lake vegetation became more mosaic and the species diversity increased. According
to historical sources the “island” with reed vegetation was used for hay making at least from
the middle of the 17th to the end of the 19th century. Likely human impact, such as grazing and
mowing, was important disturbances during the last thousand years, probably also further
back in time. Vegetation maps from the 1960’s show that particularly tufted sedge (Carex
elata) has invaded earlier Chara-dominated areas during the last forty years.
Our conclusions for nature management of wetlands and alvar lakes in particular is that
historical knowledge of the lake development is very important in order to chose the best
management strategy for the object. In the case of Dröstorps mosse the reed-dominated
“island” has a long history and the mosaic environment host a relatively high species diversity
of plants and invertebrates. This area has to be preserved and managed with care. However,
areas that were rather recently invaded by tufted sedge would probably improve if they were
brought back to open water areas.