Diatom of the Month – July 2018: Ecological uniqueness of diatom communities

by Annika Vilmi*

Humans have altered their surrounding environment to a degree that we are now living in a new geological epoch, the Anthropocene (e.g. Lewis & Maslin 2015). Freshwater ecosystems are among the most severely threatened environments by human activities, such as land use changes, pollution, dam construction, and greenhouse gas emissions causing climate change. Freshwater biodiversity thus suffers from human actions, being a global and urgent problem that needs to be addressed (e.g. Dudgeon et al. 2006).
One of the most common groups of organisms in aquatic systems are diatoms, unicellular algae encased in cell walls made of silica (Fig. 1). Diatoms, like many other algae, photosynthesize: they utilize carbon as energy whilst producing oxygen. These algae also offer resources and habitats to other organisms. For example, they are eaten by invertebrates that are in turn eaten by fish that then enter human diet - we all know people who just love to go out and do some fishing. Consequently, collecting and interpreting data and information on the diversity and ‘health status’ of diatoms is important.

Fig. 1. Freshwater diatoms on a permanent slide. The view has high diatom density and species richness.

Recently we tested a novel approach to study patterns in diatom diversity across streams and lakes (Vilmi et al. 2017) building on the work done by Legendre and De Cáceres (2013) to partition site and species contributions to beta diversity, that is, to detect which localities have exceptional ecological uniqueness. For a site to be “ecologically unique”, it has to have a species composition that in some way differs from (most) other sites. For instance, an ecologically unique site can foster an exceptional combination of species, or perhaps only a few regionally rare species. We tested this approach on diatoms recorded from a number of streams (Fig. 2) and lakes (Fig. 3) across Finland. We were interested in finding out which environmental factors, such as vegetation, bedrock, soil and land use types, were associated with ecological uniqueness calculated based on site-specific diatom communities. Finland is the country of thousands of lakes, but there are also plenty of streams, most of them flowing to the Baltic Sea. Most water bodies in Finland are in good ecological status, but regional differences are present. For instance, agriculture and soil properties can deteriorate water quality of lakes and streams.

Fig. 2. A Finnish pristine stream located in Lapland.

We were partly surprised to find that variation in ecological uniqueness in diatom communities in streams was more related to environmental variables, as compared to spatial factors in lakes. This means that the dispersal ability of diatom species (how well diatoms reach new locations) affects the ecological uniqueness in lake sites.

Fig. 3. A Finnish lake at the Rokua Geopark (UNESCO Global Geopark).

Streams and lakes obviously differ in their connectivity, as lakes tend to be more isolated from one another than streams are. We found that the more isolated a lake site is, the more ecologically unique its species composition is. Therefore, although species richness and rarity should be taken into account in choosing which areas to protect, isolated lakes may be worth conserving for their unique diatom assemblages. We also observed that, in streams, ecologically unique sites had rather low species richness. This means that, if we focus conservation efforts only on species-rich sites, we might overlook sites with exceptional species compositions. As no single index alone is sufficient to guide biodiversity conservation, we recommend a more balanced focus on species diversity (richness and evenness) as well as species combinations (assemblages of diatoms) that may enhance ecosystem functioning.

*State Key Laboratory of Lake Science and Environment, Nanjing Institute of Geography and Limnology, Chinese Academy of Sciences; Finnish Environment Institute

Dudgeon, D., Arthington, A.H., Gessner, M.O., Kawabata, Z., Knowler, D.J., Lévêque, C., Naiman, R.J., Prieur-Richard, A., Soto, D., Stiassny, M.L.J. & Sullivan, C.A. (2006) Freshwater biodiversity: importance, threats, status and conservation challenges. Biological Reviews 81(2): 163–182.
Legendre, P., & De Cáceres, M. (2013) Beta diversity as the variance of community data: Dissimilarity coefficients and partitioning. Ecology Letters 16: 951–963.
Lewis, S.L., & Maslin, M.A. (2015) Defining the Anthropocene. Nature 519: 171–180. doi:10.1038/nature14258. PMID:25762280.
Vilmi, A., Karjalainen, S.M. & Heino, J. (2017) Ecological uniqueness of stream and lake diatom communities shows different macroecological patterns. Diversity and Distributions 23: 1042–1053.