Doctoral studies

Doctoral thesis studied oribatid mites and diatoms as complementary environmental indicators

Tallinn University School of Natural Sciences and Health doctoral student Piret Vacht defended her thesis in which she studies diatoms and oribatid mites in soils under anthropogenic disturbances at riparian zones, three Estonian airfields and urban wastelands in Tallinn.


The studied areas are affected by very different anthropogenic disturbances. The soils in riparian zones are affected by air and water pollution, the airfields by runway maintenance methods, and air traffic pollution. The author observes the urban wastelands based on their age.

In her thesis, Piret Vacht identifies the composition of diatoms and oribatid mite populations at different locations and how large is the common share of the populations found at different sampling sites.

According to Vacht, identifying the common share of the populations helps to understand how similar the studied ecosystems are and how the populations are potentially affected by disturbances such as air pollution. “It was identified that the Tallinn, Tartu and Pärnu airfields only share about 20% of the same diatoms and oribatid mites species,” the author explains. She adds that finding the rest of the species is likely due to the characteristics of each location and the differences in using and administering the airfields.

The doctoral thesis reveals that compared to the wastelands, the diatoms and oribatid mites populations in airfield and riparian zone soils are more numerous and diverse. While in case of riparian zones, a growth in biodiversity was evident when level of disturbance declined, but no concrete tendency was identified in case of airfields.

However, according to Vacht, surprisingly high biodiversity was noted for example on the runway edges at Tallinn airfield, in case of diatoms also on the snow melting areas at Pärnu airfield.

Although using diatoms and oribatid mites as environmental indicators is quite common (in case of diatoms the common areas are bodies of water and wetlands), these two organism groups have rarely been used as complementary bioindicators until now. This is also the innovation in this doctoral thesis.

For example, the diatoms and oribatid mites can be well used together in case of riparian zone soils. “Both organism groups can also be used for gathering information on environmental conditions from airfield soil, but here expanding the sample sizes should be considered in future,” the author adds.

Tallinn wastelands proved to be the most challenging habitats for both organism groups according to Vacht. Without amending the methodology or involving an additional indicator group (e.g. an order of mites or springtails) applying these two groups, especially the diatoms, is not effective in these conditions, because the numbers of both groups were very low there.

Until now, only a few research papers have studied both diatoms and oribatid mites in Estonian soil ecosystems. Within the doctoral thesis research, the author among else identified 44 new oribatid mite species in Estonia.

Piret Vacht’s doctoral thesis provides new information about the fauna and flora of Estonian soils. Since the share of soils under anthropogenic disturbances is increasing both in Estonia and elsewhere, it is crucial to know about the biodiversity of soil and the indicators of possible anthropogenic disturbances. This allows us to identify the disturbances and potentially also assess the effectiveness of restoration methods.

The public defence of the Tallinn University School of Natural Sciences and Health doctoral student Piret Vacht’s doctoral thesis “Diatoms and oribatid mites as bioindicators under different anthropogenic disturbances” („Sarvlestad ja ränivetikad bioindikaatoritena erineva inimmõju tingimustes“) took place on Friday, 20 March at Tallinn University. 

The doctoral thesis supervisors were visiting professor and senior research fellow Tiiu Koff and Tallinn University of Technology Tartu College teaching track associate professor Annely Kuu. Opponents were Tallinn University of Technology Tartu College professor emeritus Mari Ivask and Giessen University researcher Andrey S. Zaytsev. The doctoral thesis is available at the TU Academic Library digital environment ETERA.