The relationship between acidity and the species composition of diatoms has been recognised for many decades (e.g. Hustedt 1937-39; Nygaard 1956). They serve as good indicators to changes in environmental parameters such as pH and nutrient availability. They also have short generation times and the community composition reacts relatively quickly to changes in the environmental conditions.

The unit of measurement applied is relative species abundance. Weighted average pH optima and tolerances have been defined for 100s of diatom species by relating species abundances across a number of sites with known contemporary water chemistry (Stevenson et al 1991). From this diatom-pH transfer functions have been developed (Birks et al 1990) which can be used to back-calculate pH from diatom assemblage data.

These can be applied in a number of contexts:

  1. Using epilithic diatoms as a bioindicator of contemporary acidity status of a lake or river (month or two) (Monteith et al. 2000). Epilithic diatoms grow attached to submerged stones making sampling of freshwaters relatively easy. Different sampling methods are applied to standing and running waters.
  2. Using surface sediment diatom assemblages to indicate acidity status of a lake over recent decades (integration of annual to decadal timescales depending on sedimentation rates) to assess recovery from acidification (Allot et al 1997)
  3. Reconstruct past pH over decades to 100s or 1000s of years, using fossil diatom assemblages from a lake sediment core (Flower et al 1997). This approach was used to identify the causes of surface water acidification, in particular to highlight the primary cause of acid deposition (Battarbee 1988).

Previous experience:

Well established and used throughout world since late 1980s.

Suitability to indicate atmospheric concentrations:

Not applicable.

Suitability to indicate atmospheric depositions:

Fossil diatom assemblages from lake sediment cores have been used to identify the causes of surface water acidification, in particular to highlight the primary cause of acid deposition (Battarbee 1988).

Suitability to indicate environmental impacts:

Epilithic diatoms widely used as a bioindicator of acidity status of lakes and rivers (Monteith et al. 2000).

Sensitivity to other factors: 

Changes in light and nutrient availability can influence species assemblages. In rivers, flow is a key factor shaping assemblages.


Epilithic diatoms can indicate a change in pH status over a number of months, while sediment diatom assemblages can show changes over decades and even hundreds or thousands of years.


Epilithic diatoms are best sampled between the months of March and October.

Expertise in field:

No particular expertise required in the field for epilithic sampling. Equipment includes: toothbrush, sample tray, plastic vials, Lugol's iodine and distilled or filtered water.
Specialist expertise is required for lake sediment coring.

Expertise in laboratory:

Expert in diatom taxonomy required to identify diatoms to species level.

Cost (per unit sample):  £10-100

Cost Comment:  Collection of samples: 1 day in field (coring requires at least 2 people)Sample prep: (£10 per sample) + 1 day staff time (up to 50 samples per day) (£100 per day)
Examination by x1000 microscopy of at least 300 diatoms valves from each sample - average 1 day per sample (expert cost: £150-£250 per day)


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