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Feature

And in the Bog there lived...

Protozoan bog dwellers can tell us a lot about past climates.

Dr Janet M. Wilmshurst

Not many people have squeezed a handful of fresh sphagnum moss from a peat bog and looked at the water under a microscope, but if you were to do so, you would see an entirely new world populated with thousands of tiny organisms. After this it would be impossible to walk on a bog again without thinking that under each footprint, you just stomped on the Protozoan equivalent of several large cities. More than 50,000 animals could easily be eking out a living in a mere gram of fresh moss.

The Protozoa include a variety of microorganisms, most of which are single-celled animals that come in a range of shapes and sizes. While some are sculptured into familiar shapes like spheres or rods, others assume such bizarre forms they could get together and form a mini freak show. Most Protozoa are between 5 and 250 microns in size, which is about the same size range as pollen grains or very fine dust. These animals must be magnified at least 200 times before you can see enough detail to identify a species.

One group of Protozoa, the testate amoebae, have been useful in climate change research because they can help to decipher past rainfall. These animals are single celled, cased in a tough little shell and creep about using pseudopodia which protrude from the body through one or two openings. The pseudopodia are the equivalent of temporary retractable limbs, their jelly-like protrusions allowing movement, capture of food and attachment to their surroundings.

The shells, or tests, are made up of organic layers secreted by the animals, and some testate amoebae will prefabricate siliceous scales inside their cytoplasm and add these to their shell, cementing them immaculately into place. The more bizarre shells have all sorts of unusual adornments embedded in them, and are often good clues to the identity of a species. Such accessories can include mineral particles, pollen grains, and the recycled plates and remains of microorganisms the testate may have eaten, such as other testates and whole diatoms.

Amoebae Signal Ancient Rainfall

The lilliputian amoeboid characters live in a variety of moist habitats including freshwater, forest litter, soils, and mossy bogs but it is in the mossy peat bogs that they have been used as indicator species to help reconstruct certain aspects of past climate. Previous studies of peat bogs have shown that the surface moisture content of a bog is controlled by the climate. This is particularly so for bogs that receive their water supply only from rainfall rather than from inflowing streams or runoff. Therefore, if the past surface moisture content of a bog can be reconstructed, then so too can past rainfall. Testate amoebae can be used by proxy to provide a measurement of the surface moisture of a bog. This is a really exciting prospect in palaeoclimate studies, because rainfall is one aspect of the climate that we know little about in the post-glacial history of New Zealand.

Studies of modern testate amoebae assemblages living in sphagnum bogs show individual species or groups of species have distinct moisture tolerances and requirements, particularly in terms of water table depth and surface moisture. Fortunately, the robust little shells the amoebae call their home when they are alive persist long after their death. Year after year, the remains of decaying bog plants and their contingent of burnt-out micro-wildlife accumulate and become permanently entombed in the waterlogged and acidic conditions persisting below the water table in a bog.

And in the Bog there lived... Figure B (25KB)
Amphitrema wrightianum inhabits pools on the surface of bogs, making it one of the wettest testae amoebae indicators.

And in the Bog there lived... Figure A (28KB)
Trigonopyxis arcula is one of the driest-loving testae amoebae.

The resulting peat that builds up therefore holds a permanent record of all the testate amoebae assemblages that have lived on the bog surface since peat bog growth began. If a core of peat can be collected from the bog and sampled at intervals down the length of the profile, then the testates, along with other fossil information derived from the peat such as pollen, charcoal and plant remains, can collectively reveal vital clues about past vegetation and climate.

Until now, analyses of fossil testate amoebae have only been used in palaeoclimate work in the Northern Hemisphere. Although taxonomic work on this group of animals to date has been fairly rudimentary in New Zealand, it is fortunate that most of the species encountered are cosmopolitan both in their distribution and habitat requirements. This means that some of the relationships already derived from studies of modern testate amoebae communities in Northern Hemisphere bogs holds true for New Zealand bogs too.

A colleague, Dan Charman from Plymouth University in Britain, has carried out a number of modern and fossil testate studies in Canada and Europe. Dan came to New Zealand in 1996 to undertake a survey of the modern testate amoebae assemblages in some New Zealand bogs, adding to the existing body of information about these diminutive bog dwellers. His work revealed strong links between key species and surface moisture, and set a promising pathway for the future use of these microfossils in palaeoclimate work in New Zealand.

Since the modern testate data was collected, I have identified and counted fossil testate amoebae assemblages in peat cores collected from some eastern South Island peat bogs. In some types of bogs, fossil testate amoebae were not abundant enough to count. This was because sphagnum, the preferred mossy habitat, was not as abundant in the past as it is today on many of our bogs. It appears that in most bogs sphagnum moss has only become a dominant species since human clearance and modification of the vegetation.

However, in one bog where sphagnum occurs throughout the peat profile, the testate work clearly indicates that there have been distinct periods in the past when surface moisture has fluctuated dramatically. At exactly the same time the testate amoebae assemblages change, the macrofossil record (preserved leaves, seeds etc) shows that the bog vegetation also changed accordingly. In addition, the pollen record from the same bog, independently established by palaeoecologist Matt McGlone at Landcare Research, indicates that certain trees and wetland plants also responded to the same changes in moisture regime.

While this is a very exciting discovery, further work is now underway to try and calibrate the moisture changes indicated by the fossil testates into rainfall estimates, by comparing the modern testate amoebae and surface moisture data with the fossil record. This work will then be extended to new sites in other regions to see if the fluctuations in surface moisture are more than a regional phenomenon.

And in the Bog there lived... Figure C (96KB)
A freshly collected core from an upland bog in Central Otago.

Dr Janet Wilmshurst is a palaeoecologist with Landcare Research in Lincoln.

Dr Janet Wilmshurst is a palaeoecologist with Landcare Research in Lincoln.