Substratum

Hydrological Regime

i. Intertidal sediments

For sediment analysis the size of the sample is not of great importance as long as sufficient material is collected for analysis. A core of approximately 8cm diameter is generally sufficient and at least the top 2 cm of sediment will be representative for analysis of the physico-chemical characteristics. In sediments that are inhomogenous sub-samples may be taken which can be pooled and thoroughly homogenised. Core samples allow an undisturbed sample to be taken and this allows an inspection of the sediment structure. This may be important if for example if the substratum is characterised by a layer of silt overlying coarser sandy material. Information on other features such as the depth of the anoxic layer should also be noted.

ii. Subtidal sandbanks

Subtidal monitoring of the substratum will require the use of large box corers if possible, as they take an undisturbed sample of sediment from which cores can be taken in a similar fashion to that used in intertidal areas. The other possibility is the use of grabs e.g. Day or Van Veen grab (Rees et al, 1990). The grab should take a sample such that disturbance to the sediment structure is minimised and samples which have low quantities of sediment or have lost sediment in transit are rejected. The use of Flow-through corers, e.g. the Craib corer, will prevent surface material being removed by any bow-wave created by the largest sampling devices. A small corer can be used to take the sediment sample and detailed methodologies for sampling subtidal areas are given in Kramer et al (1994) and Holme and McIntyre (1984). Subtidal sandbanks can be heterogeneous and remote operated video (ROV) equipment is useful to visually inspect the nature of the substratum as described above. If sediment samples are collected during biological sampling it is more reliable to take the samples from each biological sample rather than from a separate grab. However, a strict methodology for subsampling the grab must be employed to ensure that the biological sample is uniformly affected.

The use of grabs or corers to sample subtidal areas is somewhat limited and, as mentioned, ROV equipment can greatly enhance the coverage and knowledge of areas where turbidity is not a problem. Other tools include acoustic ground discrimination equipment (Hiscock, 1998b) as described above e.g. RoxAnn^TM or side scan sonar. Ground-truthing should be carried out by grab sampling. This is probably the most effective means at present to gain sufficient coverage of offshore areas and these methods are extremely useful in initial site characterisation of the substratum and consequent monitoring of change.

Particle size analysis can be carried out by several methods. These are dry sieving (Buchanan, 1984), pipette analysis, Coulter counter analysis, and laser granulometry techniques. Laser and pipette/sieve techniques tend to be the most widely used; the former are quick and accurate but costly. In addition, laser techniques cannot however analyse coarser (>2mm) material so are limited to areas of fine sands and silts e.g. intertidal mud and sand flats. Coarser sediments, as often found on more exposed beaches and on offshore sandbanks in areas of higher currents, will require dry sieving which is a time consuming process although it is possible to split samples and use laser techniques on the finer material.

Results of particle size analysis should be shown graphically as percentage per size class and features such as bimodality of sediments noted. Summary statistics e.g. Mean/median grain size, sorting coefficient, skewness and % silt, sand and gravel should also be calculated. Sediment trigons may also be constructed based on the quantity of silt, sand and gravel from which samples can be classified, for example,. sand, muddy sand and muddy gravel. Details of the derivation of these and other parameters are given in Dyer (1979) and Buchanan (1984). These summary statistics provide an easily interpretable analysis of the nature of the substratum although it must be remembered that traditional methods of sedimentary petrography do not take into account the structure of the sediment and in fact tend to destroy aggregated material to allow total dispersion.

Other features which may be examined include porosity which will relate to the water content of the sediment. This is a difficult property to measure with accuracy but is of importance on intertidal mud and sand flats and will influence the abundance of meiofauna. Porosity can be measured gravimetrically (Holme & McIntyre, 1984) and by sonograph interpretation (Taylor et al, 1966). This feature will also be related to the other characteristics of the sediment such as grain size and sorting and due to the difficulties in obtaining an accurate value it is not often measured. Qualitative estimates of the porosity or water content of the intertidal areas may be just as useful.

The organic content of the samples should be analysed. Several methods exist and these are described by Dyer (1979), Holme & McIntyre (1984) and Kramer et al (1994). The most common technique is by loss on ignition (LOI) at 600°C but other techniques e.g. by CHN analyser can be used to determine levels of organic carbon and nitrogen and derive C/N ratios. Chlorophyll levels (as an indicator of phytoplankton biomass) measured by spectrophotometric or fluorimetric techniques and particulate organic phosphorous measured by wet digestion and spectrophotometric analysis can also be measured. The quantity of organic matter is particularly important in determining the food availability for benthic communities and increased organic enrichment can lead to a deterioration in the health of the benthic population (Pearson & Rosenburg, 1978). This parameter is particularly important in intertidal mudflats. In areas of coarser material e.g. subtidal sandbanks, levels of organic material may be of less importance where levels are very low.

Measurements of the redox potential of the sediment can be done using hand held redox probes either directly into the sediment intertidally or into the contents of the grab subtidally if the sediment is relatively undisturbed. Details of determining the redox potential are given by Morris (1983), and Pearson & Stanley (1979). This parameter is useful in identifying the electrochemical regime of the sediment, oxygenation of the sediment and assessing levels of organic enrichment. High levels of microbial activity may be expected in reducing sediments though this should if necessary be determined using more specialised methods e.g. by C14 tracing (Strickland & Parsons, 1972; Dyer, 1979). The redox potential is of more use in intertidal silts than in well oxygenated substrata such as well sorted sands.

e. Trace metal and other persistent contaminants determination

An extensive literature exists on this topic and is considered outside the scope of this review. For example, if it is suspected that the areas to be studied may be subject to influxes of particulate trace metals particularly in finer substratum such as intertidal mudflats. Sediment samples can be totally or partially digested with acid followed by instrumental analysis, Kramer et al (1994) provides detailed methodologies. It should be emphasised that contamination should be kept to a minimum with no metal components used in sampling. If metallic equipment is used e.g. when grab sampling then stainless steel grabs should be used and the sample taken from the centre of the grab away from contact with the grab.

Other contaminants which can be measured from sediments include particulate polycyclic aromatic hydrocarbons (PAHs) measured by fluorescence and UV absorption or gas chromatography, particulate polychlorobiphenyls (PCBs) measured by gas chromatography with mass-spectrometric detection. Precise details of the sampling and analysis are not given in the present report but details are given in Kramer et al (1994) and it is of paramount importance with these techniques that contamination is avoided.

If necessary measurements of current strength/direction, residual flow circulation patterns etc. can be determined with simple drifters or drogues at surface and depth. Eulerian techniques can also be employed using direct reading current meters or recording current meters if long term measurements are necessary. If a SAC may be affected by a potential discharge dye tracer techniques e.g. with Rhodamine B dye may also be utilised to track the direction and extent of the dispersal of the effluent. These techniques utilise fluorimeters e.g. the Chelsea systems Aquapack to measure the concentration of dye. This equipment can be towed either at surface or as a series of transects at various depths depending on the buoyancy of the effluent. A known quantity of the dye should be mixed with saline or fresh water to obtain the correct density and deployed at the depth at which the effluent will be released either directly from a boat or via the discharge installation. Several deployments should be made so that dispersion can be monitored at slack, ebb and flood conditions. Positional data from DGPS can also be linked to output from the fluorimeter and the results logged onto a PC for further processing.

The results of current data profiles and dye tracking may be used with recent computer software to describe circulation patterns, dispersion and to derive peak stress and tidally averaged stress levels on the sea bed. Detailed methodologies for this type of survey are given by Morris (1983) and Dyer (1979). Areas of subtidal sand banks are often subject to considerable stress from tidal currents and this is one of the most important factors controlling the sedimentary and biological environment (Warwick & Davies, 1977) and a detailed knowledge of natural patterns in tidal dynamics is essential in differentiating between natural fluctuations and severe disturbance.

Salinity temperature depth probes can be employed where data are unavailable in order to monitor long term changes in these variables. Turbidity levels should be monitored preferably with transmitometers or nephelometers (McCave, 1979) to assess changes in levels of suspended sediment. The equipment used to for dye tracing mentioned above can often measure a whole suite of parameters simultaneously (for example, temperature, salinity, depth, turbidity, chlorophyll, dissolved oxygen and pH) and can be towed to provide a continuous set of data across an area or deployed throughout the water column at specified sites. Many survey vessels e.g. those employed by the Environment Agency have continuous recording equipment for these parameters. This equipment could be employed whilst carrying out routine monitoring of the subtidal communities as required. Other parameters e.g. dissolved metals, inorganic and organic nutrients etc should be measured when necessary. This will normally involve the use of specialised water samplers which allow water samples to be taken at specified depths without contamination from other parts of the water column. The laboratory procedures required for the analysis of these parameters are complex and details are given in Kramer et al (1994).

References