02: Bioindicators

“In nature nothing exists alone” – Rachel Carson

In the second episode of the Core Ideas podcast, we discuss bioindicators – species or communities that are so closely associated with particular environmental conditions that their presence can be used in paleolimnology to infer whether those conditions have occurred in the history of a lake.

Bioindicators are very useful to paleolimnologists, as environmental concerns are typically raised after recognition of a problem. As the fossil remains of organisms are constantly accumulating in the sediments (24 hours a day, 7 days a week, etc.), they can provide an archive of past environmental information.

Characteristics of useful bioindicators:

  • Wide geographic distribution
  • Rapid rates of reproduction
  • Taxonomically diagnostic remains that preserve well in sediments
  • Abundant remains
  • Well defined environmental preferences (i.e. optima and tolerances)* 

*If we consider these characteristics to be ‘rules’, the last one is the most easily bent, as even when the preferences of an individual organism are not well understood, the general environmental conditions that they are most likely to be found in high abundance are often known.

Many biological groups fulfill these criteria, so it is important to choose an appropriate indicator for the specific question of interest. Some of the many bioindicators used in paleolimnology [and the typical environmental variables they are used to infer ] include:

  • Diatoms (and Chrysophytes) [pH/nutrients/salinity/climate]
  • Cladocerans [predation/acidification]
  • Chironomids (and Chaoborids) [deepwater oxygen/fish presence]
  • Pollen [terrestrial vegetation]
  • Plant macrofossils [terrestrial vegetation (local)]
  • Phytoliths [terrestrial vegetation (grasses)]
  • Charcoal [fire history]
  • Testate amoebae [local hydrology]
  • Ostracods [isotopes/salinity/climate]

Some indicators bridge the biological and chemical: 

  • Sterols and Stanols [the sources of organic matter in the catchment]
  • Environmental DNA or eDNA [biodiversity]
  • Plant lignins and n-Alkanes [higher plants]

Next time we will begin to explore physical/chemical indicators by exploring how paleolimnologists develop a chronology for sediment cores, or in other words, determine how old sediments are at differing depths.

Episode 02 can be found here.