Diatom of the Month October 2018 - Discostella stelligera
Antonia Law makes the Diatom of the Month October 2018 writing about Discostella stelligera and how challenging is to tell apart environmental controls on species' occurrence and abundance in lake sediments
Discostella stelligera (Krammer and Lange-Bertalot 2000; Houk and Klee 2004), formerly Cyclotella stelligera, is a small (< 10µm) planktonic, centric diatom (Figure 1) commonly found in Arctic, Alpine, tropical and temperate lakes. This diatom species, along with other members of the Discostella family is a fascinating, but problematic diatom for palaeolimnologists. Increasing abundances of D. stelligera have been observed with atmospheric temperature increases in northern hemisphere lakes (e.g. in the Northwest Territories, Canada; Smol et al., 2005) due to a longer ice-free period and associated increases in nutrients (Anderson et al., 1996; Ruhland et al., 2003) and changes in mixing depth and light availability (Saros et al., 2015). However, in lakes in south western Greenland, D. stelligera has exhibited fluctuations in relative abundance across time and space over the last century (Perren et al., 2009). Similarly, both increases and decreases in this species’ abundance have been identified during cold and warm periods in the past at temperate and tropical latitudes (Fritz et al., 2018). So, can D. stelligera be used as lake paleo-thermometer indicating global warming and the associated environmental changes (i.e. changes in ice cover, nutrients and thermal stratification)?
Figure 1. a) SEM photograph of Discostella stelligera from a sediment core from Siskiwit Lake (photograph courtesy of Jeffery Stone) and; b) a photograph of D. stelligera taken from lake AT1, SW Greenland (Photograph by the Author).
I first came across D. stelligera whilst analysing diatom slides from sediment cores taken in four lakes in south-western Greenland for my PhD. (Figure 2). The changes in diatom assemblages allowed me to investigate how lake water chemistry (pH) and environment had changed over the last ~10,000 years (the Holocene). I was delighted when I was finally able to identify D. stelligera; then I became intrigued by the contradictions around the environmental preferences of this species and I have been fascinated with it ever since!
|Figure 2. Map of the four lakes studied in south western Greenland (AT1, AT4, SS8, SS1381) with secondary data used by Law et al., (2015). Picture A is lake AT4 and B is lake AT1.|
At lake AT1 (Figure 2), D. stelligera was present in high relative abundances following isolation from the ice-margin up to ~6000 cal. yrs BP (Figure 3), which we interpreted as a response to nutrient rich and alkaline conditions (Law et al., 2015). During the first few millennia following deglaciation, high-latitude lakes are typically rich in nutrients and high in alkalinity, favouring diatoms which thrive under these conditions. Following this early phase of lake development, developing soils and vegetation in the catchment trap nutrients thus, reducing their concentrations in lakes (a phenomenon called oligotrophication) and favoring diatom species that need low nutrients. This suggested that at AT1, oligotrophication began at ~6000 cal. yrs BP and was marked by the near disappearance of D. stelligera (Figure 3).
Figure 3. The diatom assemblages observed at lake AT1. Benthic: Planktonic ratio, DCA Axis 1 scores (indicative of alkalinity) and B carotene (proxy for lake production).
However, at lake AT4, only ~10km away (see Figure 2), the diatoms and proxies indicated that lake alkalinity and nutrients were low during the last ~4000 years; D. stelligera only appeared at ~4000 cal. yrs BP (Figure 4). During this period, benthic diatoms decreased as planktonic diatoms increased thus causing a decline in the benthic: planktonic diatom ratio. This trend suggests that nutrient levels increased (planktonic diatoms require high nutrient concentrations) and light availability in the lake decreased due to turbidity. So, where could nutrients to support planktonic species including D. stelligera come from during this oligotrophic phase and what caused the turbidity? Lake AT4 has a high sided, steep lake catchment. At ~4000 cal. yrs BP a period of cooling (the neoglacial) occurred, which initiated catchment soil deterioration through freeze thaw weathering, resulting in nutrient transfer to the oligotrophic lake that in turn caused an increase in D. stelligera abundance. By contrast, the soils and nutrients were not transported into lake AT1 during the neoglacial due to the low topography of its catchment, leading to a phytobenthos-dominated, clear, oligotrophic lake system (Figure 3).
Figure 4 The diatom assemblages. Benthic: Planktonic ratio, DCA Axis 1 scores (indicative of alkalinity) and B carotene (proxy for lake production) at lake AT4.
Our conclusion is that D. stelligera demonstrated contrasting ecological responses to neoglacial cooling because of differences in lake catchment geomorphology between lakes AT1 and AT4. The data also indicate that D. stelligera responds to nutrient increases and recent studies have suggested that the species responds to the complex interactions between light, mixing depth and nutrients (Saros and Anderson 2015; Malik 2018; Warner et al., 2018). Future research is required to better quantify the relative importance of temperature, nutrients and other factors in determining this species’s abundance and distribution patterns.
*Antonia Law is a Teaching Fellow in Geography at Keele University, UK
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