Aquatic time capsules

Sifting around the mud at the bottom of lakes might not initially sound very appealing. However, a group of scientists known as paleoecologists are able to reconstruct ecosystems that existed in past millennia by doing just that.

Taking a core through the sediment (mud) and examining what occurs at different depths can reveal what plants and animals lived both in the lake and its catchment. The age of the material is determined using isotopes of elements such as lead and carbon. The deeper into the lake mud you go, the older the material.

Paleoecologists also examine pollen deposited in the sediment.  These are the signatures left by plant communities of old. Pieces of charcoal indicate when fires occurred in the catchment while the remains of a number of aquatic organisms can also be stored to tell the story of past aquatic ecosystems.

Eunotia pectinalis - s species of diatom once common in Lake Euramoo.

Eunotia pectinalis – s species of diatom once common in Lake Euramoo.

One of the most popular indicators of past aquatic ecosystems are diatoms. Diatoms are microscopic plants (algae) that have distinctive silica shells. These shells are generally well preserved in sediments. They are useful indicators of environmental change because when water quality changes, so do the diatom species and we know what some of those relationships are.

One example of where lake sediments have been used to tell the story of ecosystems past was a study of Lake Euramoo* (Ngimun in the local language) in north Queensland. The lake is a double volcanic crater lying within an expanse of tropical rainforest in Danbulla National Park.

Analysis of the sediment at the bottom of Lake Euramoo indicated a stable rainforest environment between the 13th and 19th centuries with the dominant plant species found in the pollen record fluctuating only slightly. Periodic fires were evident up until the 16th century but the charcoal record indicated few fires between the 17th and 19th century.

Danbulla National Park: Photo by DERM

Danbulla National Park: Photo by DERM

The most significant changes occurred in the late 1800s with the reduction in abundance of key timber species such as Kauri in the pollen record. At the same time, exotic plants such as Lantana appeared and fires became more common.

In the lake itself the aquatic community showed long term (centennial) fluctuations in dominant diatom species. Then, at the same time as the terrestrial vegetation changed, the suite of species in the aquatic community shifted. The new community was indicative of more alkaline and higher nutrient waters.

Staurosirella pinnata - a diatom more common today. :Image by E A Morales

Staurosirella pinnata – a diatom more common in Lake Euramoo today. Image by E A Morales

These rapid and profound changes coincided with the onset of European occupation in the region. They were most likely the result of clearance and burning activities associated with the timber and farming industries.

A sobering discovery was that despite protection of the catchment for the last 50 years there was no evidence for even a partial return of ecosystems to the pre-European state. Loss of soil fertility and reduced resilience of soils to recover after deforestation may be one explanation for the poor recovery of plant, algal and insect communities in the Lake Euramoo catchment.

*The full article on which this piece is based is: The impact of European occupation on terrestrial and aquatic ecosystem dynamics in an Australian tropical rain forest. By: S.G. Haberle, J. Tibby, S. Dimitriadis and H. Heijnis. Journal of Ecology, 2006, 94, 987-1002.

Background information on diatoms was sourced from: http://www.ucmp.berkeley.edu/chromista/bacillariophyta.html

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