Why diatoms are valuable indicators of water quality

16 December 2010

A small centric diatom

Diatoms provide probably the most versatile and important biomonitoring tool for aquatic environments.  Located at the base of the aquatic foodweb, mostly in epiphytic and/or episammic forms, diatoms are “glued” into their environment and are not nearly as prone to washouts or other changes in hydraulic regimes that directly affect planktonic indicators (vertebrates or fish).  They integrate changes in water chemistry, including pollutants, rapidly and at a fine scale, information that is not provided by aquatic or terrestrial vegetation indicators.  Lastly, their occurrence is not habitat-dependent, as is the case with vertebrates or fish.   They are the easiest indicator to sample, from streams, urban canals or wetlands, being generally equally represented across all surfaces at a single location.

Many countries now use diatoms as a core component of their biomonitoring programmes.  During the 1990s, the value of diatoms as a proxy indicator was integrated into the European Water Framework Directive for the EU and form part of the associated regulatory background as the proxy for phytobenthos.  Diatoms are used in New Zealand as an indicator of Acid Mine Drainage.

Diatoms allow the detection of water quality changes, accompanied by the derivation of ecological inferences, at a scale infinitely more subtle than that possible using conventional water chemistry techniques.  Several South African researchers have identified this versatility.  Many diatom species are sub-cosmpolitan, supporting the global use of generic indices.  Local (endemic) characteristics, such as have been built into the South African Diatom Index (SADI), increase the power of the index for specific local and regional use.

Diatoms are proving, worldwide, to be increasingly important for climate change studies and the derivation of transform functions to inform climate change modeling.  This characteristic is linked to the fact that diatoms are comprised of silica and, like pollen grains, can be paleoecologically-interpreted from sediments, dry river beds and other substrates.  No other biomonitoring indicator provides a fraction of that provided by diatoms for evaluating water stress and climate change, or for benchmarking ecological condition.  Additionally, diatoms reflect teratological and environmental stress changes.

The information that can be drawn from diatoms is supported by decades of information acquired both globally and locally.  South Africa has one of the five largest collections of diatoms and the SADI is the second-largest data-supported diatom index in the world.  This collection provides genuine reference condition information for many South African waterbodies, collected during the 1950s and 1960s.  As such it is the only comprehensive, biological record of conditions in South African waters pre the impact of post-WWII industrial expansion.

Diatoms require a modicum of specialized skills for their identification – a level of skill that is rapidly acquired with appropriate training.  The acquisition of skills is substantially offset by the high level of information that may be derived from diatom samples.  The technique embodies the permanent curation of samples (mounted microscope slides), i.e. it is not a throw-away screening technique such as SASS.

It is this requirement for looking down a microscope that seems to frighten people off. Oddly (because they should know better), it is the opinion of the Water Research Commission that the method needs to be dumbed down for use by non-specialists.  We live in a country where our existence is threatened by limited, degrading water resources, yet the flagship research institution is promoting approaches that are akin to motor mechanics undertaking brain surgery.  It simply does not make sense!  Are we to become a country advised by non-specialists? – I suppose if we can have a Chief of Police who is not a professional policeman, then anything is possible!

South Africa, through the projects funded by the WRC, has available the necessary training skills and tools to support the immediate nationwide adoption of this biomonitoing technique.

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