Sediment Sampling

Introduction

Sediment samples are generally made by coring or grabbing.  Corers are of several types including hydraulically damped corers (e.g. Craib corer, multi- and mega-corers), box corers and gravity corers.  Hydraulically damped corers should give slow penetration into the sediment with a well preserved surface layer - ideal for quantitative biological work (essential for meiofaunal analysis) and for considering chemical profiles such as redox. Box corers can take samples of precise dimension but usually the sediment surface and its smaller animals are disturbed.  Gravity corers typically homogenise the surface layer and are of little use for this application.

Additionally sediments can be cored by divers. Expert divers can take cores at least as well as hydraulically damped corers.  Hydrualically damped coers are often heavy and difficult to deply beside or underneath fish cages so, provided that the depth allows safe diving, diving may be a good option - see below.

Finally grab samples can be taken and subsampled with short corers.  This is the most common method used in many areas e.g. Scotland, as it is relatively easy and large samples can be collected.   Grab sampling  by van Veen grab is descibed in detail in the following section.

Station selection

Station positions should reflect gradients of impact, with one of the stations in the area of maximum impact. The selection process is aided by modelling (using an appropriate current record) or pre-survey, by means of sediment sampling (e.g. redox, by acoustic methods or by video, etc.) with the objectives of  1) maximising the chance of detecting a gradient and 2) to consider the distance from the farm that farming effects can be detected. A minimum of 5 stations per site were taken (in most cases this included a station at the farm’s edge, and then 25, 50, 75, 100m distance) and in some cases several more. The reference station was selected at a similar depth and sediment type sufficiently distant or upstream not to be influenced by the farm. If this was not possible, an additional station was added to the transect to reveal background conditions by comparing the trend away from the farm, i.e. if stations become more similar with distance. Timing of sampling is important - sampling should be done when the impact is likely to be high.

Replication

The appropriate level of replication can only be assessed by a post hoc analysis of the for example, cumulative species abundance curves and by analyses of the varaibility between samples. The size of the sampler is important. The larger the number of replicates the better.  In general,at each station, 3-5 grab replicates should be taken for macrofauna and sediment chemistry.

Diver sampled cores

Core diameters should be 5cm for sediment chemistry and 10cm for macrofauna. Core depth should be at least 12cm for sediment chemistry and at least 6cm for macrofauna.

1.      The corer should be inserted slowly and smoothly into the sediment in a vertical position.

2.      If necessary, facilitate penetration with the help of a rubber head mallet

3.      The depth of penetration into the sediment must exceed 10 cm[1]

4.      When the desired depth is reached, close the corer’s upper end with a stopper

5.      Remove the corer slowly. Care should be taken to retain the core while removing the corer from the sediment. If needed, remove the sediment around the corer by digging for facilitating core removal.

6.      Close the lower end of the corer with a stopper and place it in a carrying basket

Caution must be exercised to ensure that the cores are not be turned up-side down during transport, as this will mix the enclosed sediments!