The effect of acetolysis on desmids

ABSTRACT

Acetolysis, an oxidising technique common in palynological preparation, is beneficial for pollen analysts who employ it to remove ‘unwanted’ organic matter from peat and lake samples. Since this technique was introduced by G. Erdtman in 1934, however, several researches have noted concerns, such as the destruction of thin-walled pollen grains in addition to non-sporopollenin pollen components, and selective destruction of protoperidinioid dinoflagellate cysts. Desmids are conjugate green algae with a wide range of environmental preferences whose half-cells are known from sediments dating back to at least the Neogene (possibly as far back as the Devonian), and they have proven useful in modern and palaeolimnological studies (e.g. as indicators of nutrient loading, anthropogenic impact and drought). Desmids are rarely mentioned in palynological studies, however, except to illustrate fluvial transport to nearshore marine settings. A diverse desmid flora was found in samples processed without acetolysis from Smith's Bay in Lake Simcoe, and desmid and thecamoebian (testate amoeba) assemblage changes record eutrophication up-core. Very low concentrations of both desmids and Pediastrum, another group of algal non-pollen palynomorphs (NPP), record siltation and inhibition of photosynthesis attributed to two phases of land-clearing and agriculture (Wendat/Huron and Euro-Canadian). After acetolysis, the desmid abundance in the same residues drops significantly (between 36 and 100%, mean = 87%) and the assemblage is skewed towards the most robust Cosmarium spp. However, other low-relief NPP, like Pediastrum, may be easier to observe after acetolysis as they are not obscured by amorphous organic matter. Because of the observed detrimental impact acetolysis has on the desmid assemblage, recommendations include: (i) not acetolysing, as pollen and Pediastrum can be observed either way, or (ii) using a two-pronged approach where susceptible microfossils are observed pre-acetolysis and resistant ones observed post-acetolysis.

Keywords:

• Non-pollen palynomorphs
• Lake Simcoe
• anthropogenic impact
• selective preservation
• Cosmarium
• Pediastrum

Acknowledgements

We would like to thank Mike Lozon for his excellent drafting work, Volodymyr Sivkov for aiding in the collection of sediment cores, and Cait Garner for help with desmid ecology. We appreciate the comments of Bas van Geel and an anonymous reviewer.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Notes on contributors

Nicholas L. Riddick

Nicholas Riddick is working towards completing his masters of earth science, with Dr Francine McCarthy. He is a graduate of Brock University (geography specialising in geomatics) and Niagara College (environmental technician and geographic information systems) and plans to continue his research in a PhD. Nick is interested in the field of geoarchaeology and is currently working on a thesis which uses microfossils as a proxy of Native American occupation, primarily in Lake Simcoe. He is also working with archaeologists at an excavation in Turkey and plans to use microfossil proxies from this site to better understand the settlement pattern.

Olena Volik

Olena Volik completed her undergraduate and graduate studies in Ukraine in 2003. Her research included the palaeoenvironmental conditions for travertine formation in western Ukraine, caves and other karst features of the Podillya region, Ukraine, and the management of nature reserves. In 2011, Olena joined Dr Francine McCarthy's group at Brock University, Canada, working on non-pollen palynomorphs and thecamoebians as proxies for environmental and anthropogenic changes. Currently, Olena is working on her PhD in geography at University of Waterloo, Canada, with a research focus on temporal and spatial change in carbon accumulation along a salinity gradient within boreal saline wetlands and its implications for constructed wetland management.

Francine M.G. McCarthy

Francine M.G. McCarthy is a professor of earth sciences at Brock University. She has studied a number of microfossil groups, including pollen, dinoflagellate cysts and other non-pollen palynomorphs, planktonic foraminifera and testate amoebae, in marine, lacustrine and wetland sediments of Miocene to Recent age. Most of the work has focused on sea levels, lake levels, or palaeoclimates, but has included providing a palaeoenvironmental context for archaeological sites and evaluating environmental remediation options in the oil sands of Alberta. Current projects include: determining cyst–theca relationships in freshwater dinoflagellates and highlighting the potential of dinocysts as palaeolimnological indicators; studying meromictic lakes to assess the effects of taphonomy on the palynological record; and co-authoring a book with Martin Farley: Microfossils and their practical applications.

Donya C. Danesh

Donya Danesh is working towards completing her PhD in the Department of Biology at Queen's University (ON, Canada) under the supervision of Dr Brian Cumming. Her research focus is on the use of palynological/palaeolimnological techniques to determine long-term climate impacts on freshwater lakes in the boreal forest of northwest Ontario (Canada). She is specifically looking at the relationship between climate, vegetation and fire throughout the Holocene. Moreover, Donya is currently a guest doctoral candidate at the Institute for Biodiversity and Ecosystem Dynamics (IBED) – University of Amsterdam, further developing the use of non-pollen palynomorphs (NPPs) as biological indicator proxies. She is also co-founder/chair of the Water Initiative for the Future (WatIF): Graduate Student Conference, an international conference promoting multidisciplinary collaboration.