Comparing Diatom Paleo-Assemblages to Determine How Different Environments Affect Diversity in the Geological Rock Record

Comparison of Diatom Paleo-Assemblages with Adjacent Limno-Terrestrial Communities on Vega Island, Antarctic Peninsula 

By: Marie Bulínová, Tyler J. Kohler, Jan Kavan, Bart Van de Vijver, Daniel Nývlt, Linda Nedbalová, Silvia H. Coria, Juan M. Lirio, and Kateřina Kopalová

Summarized by: Dani Storms, a senior undergraduate student at the University of South Florida seeking to earn a degree in Geology with a concentration in geophysics. Dani changed her major from English to Geology her sophomore year after taking an Introduction to Earth Science class taught by Dr. Sarah Sheffield. Dr. Sheffield played a significant role in helping her discover a love and curiosity of how geology shapes our everyday lives. She is interested in furthering her education by attending graduate school to earn a master degree in the field of natural hazards, preferably landslides or avalanche control. She hopes to eventually obtain a PhD as well. Dani has loved being in the outdoors her whole life. In her free time, she enjoys hiking, camping, biking, crocheting, and spending quality time with her pup, Zion. 

What data were used? Diatoms are extremely abundant microfossils that are composed of silicon dioxide (SiO2) and can be used to reconstruct different environments through the rock record (i.e., paleoenvironments). This study used statistical analysis to discover if there was a significant difference of diversity within diatom assemblages from core samples derived from two Antarctic lakes, both with varying habitats. Core samples contain sediment that was deposited in paleoenvironments and obtained by drilling into the earth, and these can give insight into how climate has changed over time. The lakes were on Vega Island, in Devil’s Bay (Lake Anónima) and Cape Lamb (Lake Esmeralda). 

The samples were viewed under 100x magnification in order to determine diatom species. The species were then categorized into sub-Antarctic, Maritime Antarctic, and Continental Antarctic groups to determine their biogeographic distribution. Samples were taken from ponds, streams, mosses, and steep habitats to compare the core samples to modern day environments. However, not enough data could be collected from the stream, mosses, and steep habitats for a fair comparison. Therefore, the comparison of habitat differences on different sides of the island was restricted to only ponds.

In order to determine if there was a significant difference between the distributions, the study considered relative abundance, species richness and evenness in relation to diatom counts. Relative abundance refers to the percentage of diatoms found in each sample, richness is the number of species found in the sample, and evenness is comparison of the relative abundance of each individual species. 

Results: Overall, diatom assemblages varied in composition significantly, most likely due to the differences in waterway connections between sites and due to some sites being isolated. Between the categories, Maritime Antarctic Region contained the greatest number of species. Nearly 43% of the taxa found on Vega Island were Antarctic in distribution, while only 6% were found within the Antarctic Continent. Sub-Antarctic accounted for 3% and less than 1% were contributed by the Antarctic Region. The study then addresses the most abundant genus and species within Lake Esmeralda and Lake Anónima. Between the 132 species observed, 100 were found within Lake Esmeralda and only 32 species were found in both lakes. This means that Lake Esmeralda has a greater richness since it contained more species. 

Lake Anónima was found to be most similar to modern environments with the most noteworthy similarities being derived from streams on Devil’s Bay. However, there were several species found only within the core sample containing the paleo-assemblage. The paleo-assemblage consisted primarily of pond and stream species. For Lake Esmeralda, many of the taxa were not found within the modern environment at all. The taxa not found in the modern environment contained both aquatic and terrestrial species of diatoms. 

The authors of the study hypothesize that the connectivity of waterways and habitat type contributed to the difference in diatom assemblage structures. Lake Esmeralda is hydrologically disconnected, meaning it does not have connections to other bodies of water. The disconnect would result in less carbon being funneled into the lake from an outside source, which could alter the chemical makeup of the lake, allowing only certain species to survive as pH levels change. There is also a possibility that Lake Esmeralda might have had a higher preservation rate that allowed for a clearer comparison of paleo-assemblages versus modern, which would explain the higher amounts of diversity between the two. 

However, Lake Anónima is the complete opposite. It is well-connected to a stream that allows for the transportation of diatoms. It also has underground drainage that connects it to other lake-systems and surface streams. The study notes that even if the transport of diatom valves were to not occur, the connection of waterways would result in similar hydrochemistry between the connected water and lakes. This could have led to less viable conditions for preservation that could result in the lower amounts of diversity found. 

Figure 3: These boxplots visualize the comparison of species richness (S), Shannon Diversity (H’), and evenness (J’) between Esmeralda Lake and Anónima Lake core samples. The thicker black lines display median values.

Why is this study important? Looking at the conditions in which modern diatoms are found compared to that of paleo-assemblages can help construct a model, or standard, that can be applied to diatoms in the fossil record. This would allow us to reconstruct paleoclimates and trends associated with how they are affected by waterway connectivity. The differences in diatom-assemblages and species could be used as indicators of climate change. The different modern species that were found to be specific to ponds, mosses, and streams could give insight to what type of habitat fossilized diatoms would have been found in. 

The big picture: As the climate begins to warm and ice melts away, the connection between waterways will more than likely increase, this would cause a more uniform distribution of diatom assemblages and loss of diversity due to the similar chemical compositions of the water. The concept of creating a model that can be applied to the differences of diatom assemblages in modern and paleo-environments could essentially allow for paleoclimate and hydrological connectivity reconstruction. For instance, if we found a more uniform distribution in a general area, we might be able to determine if there was more connectivity between body of waters at some point in the geological record. Another example would be how on average, modern-day dry mosses contain fewer species of diatoms than wet mosses. Knowing this, we could analyze fossilized mosses to give insight into whether they would have been dry or wet. We could also use the findings of this study to determine what bioregion ancient diatoms would have been in based off species type and the diversity of the assemblage. 

Bulínová M, Kohler TJ, Kavan J, Van de Vijver B, Nývlt D, Nedbalová L, Coria SH, Lirio JM, Kopalová K. Comparison of Diatom Paleo-Assemblages with Adjacent Limno-Terrestrial Communities on Vega Island, Antarctic Peninsula. Water. 2020; 12(5):1340.

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