Sampling diatoms from running waters: 2. Sampling cobbles

Martyn Kelly and Marian Yallop

© The DARES consortium

Disclaimer

• This presentation does not purport to address all of the health and safety issues associated with sampling diatoms from running waters.

• It is the responsibility of the user to establish appropriate health and safety practices and to ensure compliance with any national regulatory conditions.

Introduction

• The previous presentation introduced the theory and basic principles of diatom sampling in UK rivers.

• In this presentation we will look more closely at how to sample diatoms from cobbles and other hard surfaces.

Photo: Heather Kelly

Selection of cobbles• Cobbles are the preferred substrate because they

combine stability with manoeuvrability.• However, in many lowland rivers, cobbles are

smothered with filamentous green algae (typically Cladophora).

• As we saw in the previous presentation, filamentous green algae carry some distinctive epiphytes which may influence the composition of the sample.

• The easiest approach is to simply avoid cobbles that are smothered with filamentous algae as far as possible….

But …

In enriched lowland streams, this will be difficult. Under such conditions, a stone that lacks filamentous green algal growths can hardly be said to be typical of the reach in question. So for these situations we need to adopt a slightly different approach.

Photo: M

artyn Kelly

Pho

to: H

eath

er K

elly

The stratified sampling approach• The approach adopted in

DARES is to sample cobbles with filamentous algae in proportion to their abundance in the sample reach.

• This involves making an estimate of the percent cover of filamentous green algae and then use the table on the right to decide how many alga-smothered stones to sample.

% coverNumber of

alga-covered

cobbles

< 15 0

15-29 1

30-44 2

45-59 3

60-74 4

75 5

Important!The sample record form has spaces to record the percent cover of filamentous algae and the sampling method.

It is also important to check the identity of the filamentous alga, either in the field or back in the laboratory.

Pho

to: T

im C

olbo

rnP

hoto: Tim

Colborn

Situation 1: cobbles without filamentous algae

• This is the ‘default’ method.• Select five cobbles from the main flow of the

river.• Try to sample different areas of the reach,

bearing in mind the guidance given in the first presentation.

• Remove the stones, taking care to note which is the upper surface, and place in a tray along with a little stream water.

Selection of cobbles

Try to select the stones from different locations within the reach, whilst avoiding the edges and heavily shaded sections. Note, too, how the sampler removes the cobble carefully, so that he can sample its uppermost surface.

If no cobbles are available, use small boulders or large pebbles instead. You may need to sample more than five pebbles.

Cleaning your toothbrush

In order to remove the diatom biofilm, you need to scrub the upper surface of each stone with a toothbrush.

It is not necessary to use a new toothbrush at every site but make sure that you clean the toothbrush before and after sample collection, by rinsing it in stream water. You can also rub the toothbrush on your waders to remove trapped biofilm.

Removing the biofilm

Put a small amount of streamwater (about 50 ml) into your tray and then remove the biofilm from the upper surface of the cobble by scrubbing vigorously with your toothbrush.

Rinse the toothbrush in the stream water regularly. The water in the tray should take on a brown hue as the biofilm is washed into it.

Removing the biofilm (cont.)

Continue this process for all of the cobbles that you removed from the river. The result should be a dark brown suspension in your tray.

If you are sampling pebbles instead of cobbles, you should sample enough to obtain a suspension that looks like this.

Removing the biofilm (cont.)

The final step is to pour the suspension into a well-labelled bottle. Before pouring, swirl the suspension in the tray so that any settled particles are resuspended.

Preservatives can be added now or when you return to the laboratory. The protocol contains full instructions on how to do this.

Situation 2: cobbles plus filamentous algae

• The method for sampling cobbles plus filamentous algae is similar to that for sampling cobbles without algae.

• Filamentous algae are not removed before scrubbing. Note that this is different to the method recommended in the TDI Manual.

• The rationale for this step is outlined in the protocol.

Sampling cobbles with filamentous algae

Scrub the uppermost surface of the stone as before. Don’t worry if you dislodge some filamentous algae in the process.

The filamentous algae often trap sediment particles which will also be removed as you scrub the stone. The result will be a mixture of attached and motile diatoms .

Sampling cobbles with filamentous algae (cont.)

The resulting suspension may contain a few clumps of filamentous algae. These should be removed before pouring into a sample bottle.