Diatom of the month - February 2018: Aulacoseira baicalensis
by Sarah Roberts*
Lake Baikal (Siberia, Russia) contains 1/5th of the world’s freshwater and is thus the largest freshwater lake by volume. This ancient lake is over 25 million years old, it harbours many endemic species, and its ecosystem and water quality are increasingly threatened by rising anthropogenic activity. Recent studies have found shoreline pollution at Lake Baikal, with blooms of algae indicative of eutrophic conditions; this littoral water deterioration is a result of nutrient enrichment from outdated sewage treatment plants from shoreline settlements (Timoshkin et al., 2016). However, we do not know the extent of the pollution and whether the pelagic regions are now eutrophic. Sedimentary diatom assemblages at Lake Baikal have been extensively studied in the past, with palaeolimnological studies investigating the impacts of pollution and climate change on diatom community composition (Mackay et al., 1998) and predicting future diatom assemblage change with lake warming (Mackay et al., 2006). As part of my Ph.D. project at the University of Nottingham, I studied diatom assemblages in core tops from sediments collected across Lake Baikal. As some cores were taken at the same sites as previous records (Mackay et al., 1998), we were able to explore recent changes in diatom assemblages over the last c. 20 years. Climate change is impacting Lake Baikal’s freshwater ecosystem, through changes in vertical thermal structure (i.e. mixing layer depth), ice cover dynamics, and through watershed effects. Diatoms dominate this lake’s waters during ice-covered months (November to March). Among other diatom taxa, Aulacoseira baicalensis (K. Meyer) Simonsen is an endemic species that blooms underneath the ice cover in the spring time (Jewson et al., 2009). During the ice-free summer months, water temperatures rise and thus A. baicalensis cells become dormant by producing resting stages at depths of 50 – 150 m, below the mixing layer in the water column (Jewson et al., 2010).
. Microscopic images of Aulacoseria baicalensis (Pictures by the author). Aulacoseria baicalensis cells from long chains and two adajecent frustules are bound together by spines.
In this lake, Aulacoseria baicalensis is a large filamentous planktonic diatom whose size changes with alterations in the mixing depth (Jewson et al., 2010). Shorter valve mantles and thinner walls form in winter, under the ice, as compared to longer valves with thicker walls in the summer months. Thicker cell walls enable this species to survive during the summer, as this diatom cells sink to cooler and deeper waters and help to avoid dissolution during dormancy (Jewson et al., 2010). Thus, when light availability is reduced, as diatom sink within the mixing layer during summer lake stratification, resting cells start forming (Jewson et al., 2010).
Phytoplankton monitoring studies at Lake Baikal have recently found a reduction in the density of this diatom species living under ice (Silow et al., 2016) in the South basin (between 1950 – 2010 AD) during the spring time. This trend may be due to declining ice cover duration and warming waters caused by climate change, which consequent abundance decreases in slow growing endemic taxa, such as A. baicalensis, and increases of faster growing smaller diatoms at Lake Baikal (e.g. Synedra acus and Stephanodiscus meyerii; Mackay et al., 2006).
Figure 2. Fieldwork photo of Lake Baikal in March 2013 with ice thickness of c. 1m over water depths of up to c. 1,200m deep (Picture by the author).
Lake Baikal’s warming is altering its seasonal ice cover dynamics (shortening ice cover periods and reducing ice cover thickness) and its water column thermal structure (deepening of the mixing layer and increasing lake water temperatures). These trends could potentially cause a shift from the current diatom-dominated primary production to one dominated by non-siliceous algae, in particular by picoplankton, all year around (Mackay et al., 2006; Moore et al., 2009), threatening the large endemic diatoms living under the ice. This potential shift towards a picoplankton dominated ecosystem all year around would be as a result of more nutrient rich and warmer lake waters. My Ph.D. findings call for further research on the impact of anthropogenic activities (such as tourism, industry, and mining) and climate change on the world’s largest freshwater lake.
*Ph.D. Graduate Nottingham University
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