Sampling diatoms from running
waters 1: Introduction
Martyn Kelly and Marian Yallop
Introduction
DARES is a project funded by the Environment Agency and
SNIFFER to develop a diatom-based method for monitoring
ecological status, as required for the WFD.
These presentations are designed to show biologists how to collect
diatom samples during routine GQA surveys. These samples will
then be processed by the DARES team.
Methods are very similar to those described in the TDI manual,
which is itself based on a European standard method. However,
there are a few important differences between the DARES method
and the TDI manual which need to be understood.
This first presentation explains the principles of sample collection
for DARES.
When we sample diatoms for water quality
monitoring we assume that the distribution of
taxa is influenced primarily by water quality
variables …
50
S. pinnata
A. conspicua
% of total
40
30
20
10
0
0.01
0.1
1
10
-1
FRP (mg l )
In this graph, we see Staurosirella (=Fragilaria)
pinnata with a relatively low P optimum and
Achnanthes conspicua with a much higher optimum.
However, as the following presentation will show, other
factors can influence the composition of the diatom
assemblage.
For this reason it is important that we adhere to strict
sampling protocols in order to minimise the impact that
these have.
The DARES protocols are based on
those in the TDI Manual which are
compliant with EN 13946 – the
European Guidance Standard for
diatom sampling.
Diatoms live in a thin, brown ‘biofilm’
which can be found on almost all surfaces
in rivers
This video clip shows a
biofilm that has
developed on a bottle
found submerged in the
River Avon in
Hampshire.
It might not look very exciting, but it is actually a
complex 3-dimensional community of which
diatoms are typically the most abundant group of
photosynthetic organisms exhibiting a range of
ecological strategies.
Plankton “in transit”
Polysaccharide matrix
20 m
Stalked taxa, e.g. Synedra
Motile taxa, e.g. Nitzschia
This biofilm can be found growing on many
surfaces in running waters ….
Rocks plus
filamentous
algae
Manmade
surfaces
rocks
Fine
sediments
Emergent
macrophytes
Submerged
macrophytes
… and contains diatoms with a range of
different growth forms ….
Resistance to scour
Competition for light
Resistance to grazing
Prostrate, e.g. Stalked, e.g.
Arborescent, e.g.
Cocconeis
Achnanthidium Gomphonema
Differences in growth forms can lead to
different assemblages depending upon
the type of substratum.
current
For example, stable rock surfaces favour attached
species such as Achnanthidium and Gomphonema
Whilst less stable surfaces lead to
different assemblages….
Fine sediments favour
motile species such as
Nitzschia ….
… whilst sand grains bear
a distinctive flora of
attached diatoms such as
Staurosira.
Macrophytes and macroalgae also have
distinctive diatom assemblages associated
with them.
Cladophora
filaments, for
example, often
have distinctive
epiphytes such as
Rhoicosphenia
(shown here) and
Cocconeis.
As a result, the diatom assemblage found
in this biofilm varies in both space …
Areas with
relatively low
shear stress
Areas with
low stability
Pioneer species (‘r-strategists’) colonise
bare surfaces and, over time, increase in
quantity. These pioneers are subject to
competition from other species until a
scouring spate removes most of these,
allowing pioneer species to re-establish.
…... and time.
Resource
supply
High
‘r-strategists’
‘c-strategists’
Low
Low
High
Disturbance frequency
This can, in turn, influence the value of
diatom-based water quality indices.
100
TDI
80
60
40
20
0
20
40
60
80
100
Distance from Wearhead (Km)
Data from the River Wear, NE England. Note the scale of
within-site variation in indices. Despite this, longitudinal
trends in the TDI are still apparent.
From Kelly (2002) J. appl. Phycol. 14: 9-18.
From theory to practice
Now that we have
some idea of the
structure and
composition of the
biofilm, we need
to think about how
best to collect
samples for
monitoring
ecological status.
The second part of this presentation will deal with this.
The TDI method: diatoms for
tactical monitoring
• The TDI was developed in response to the
Urban Wastewater Treatment Directive.
• This required an assessment of point sources
of nutrients on a case-by-case basis.
• We asked the question ‘does this sewage
works have a significant effect on this river?’
• Methods needed to be robust but ….
• …. allow flexibility so that biologists can “finetune” sampling to suit local conditions
UK strategy: UWWTD
• The same substrate has to be sampled
at all sites in a river system.
• Cobbles and boulders are the preferred
substrates
• Artificial substrate (polypropylene rope)
is also widely used.
Water Framework Directive
• For this new Directive we need to compare
the biota at a site with that expected in the
absence of significant human impact.
• This means that we need to use substrates
that are ‘typical’ of the river type under
investigation.
• Cobbles and boulders are not characteristic
of all river types, so we need to include other
‘natural’ substrates.
• Assemblages found on an artificial substrata
do not fulfil this criterion.
Sampling protocol
• The sampling strategy outlined in the TDI
manual has had to be modified for DARES in
order to take account of the requirements of
the WFD.
• Cobbles and boulders are still the preferred
substrate
• However, if these are not found, then
emergent macrophytes should be sampled.
• If these are not found, then submerged
macrophytes can be used.
Selection of sampling sites.
• For DARES, samples will be collected from
sites already used for GQA surveys.
• Issues that were considered when selecting
these sites include:
– Location with respect to discharges / confluences
etc.
– Access from road
– Location with respect to other monitoring records
(chemical / macrophyte / invertebrate)
• Glides, runs and riffles are all suitable for
collecting diatom samples but most pools will
be too slow-flowing.
However, once you have arrived at the
site, you need to decide where in the
survey reach to collect your samples.
Aspects to consider include:
• Current speed
• Shade
• Water depth
• Selection of substrate
Factors to consider when selecting
a sample site: 1. current speed
• Sample from main flow of river
– Not sidearms or backwaters
• Avoid areas of very high current velocity
for safety reasons
• Areas of very slow flow (< 0.1 m s-1)
permit accumulation of loosely-attached
species
Factors to consider when selecting
a sample site: 2. depth
• Two rules to follow:
– If you can see the substrate, it is in the euphotic
zone
– If you can wade to it, then the water is not too
deep
• In addition, ensure that surfaces are
permanently submerged
– This will depend, to some extent, on local
knowledge.
– As a ‘rule of thumb’ ensure that the water depth is
at least 10 cm.
Factors to consider when selecting
a sample site: 3. light
• Avoid heavy shade,
unless this is
characteristic of the
system under study
• Try to sample from
sites with similar
light regimes
Selection of substrate
There are three options:
Either cobbles …
... or macrophytes
Emergent
or submerged