Substratum

Hydrological Regime

Sediment is often heterogeneous in nature such that a single sample for a physical-chemical characterisation may not provide a reliable description of that area. Multiple sampling and analysis may be required offers better possibilities. Separate samples may be pooled (homogenised) but this will remove indications of structural differences within the site. Samples should be collected concurrently with benthic samples, at the same time and location otherwise interpretation of biological data will be invalid or not possible. Intertidal sand and mud flats will require core sampling to provide sufficient material together with quadrat sampling to indicate surface features such as ripples, casts and heterogeneity. Subtidal mobile sandbanks will require remote sampling via corers or grabs, diver operated corers and underwater photography.

Analysis should be made of the nature of the substratum as this, together with the hydrodynamic regime, are often the most important factor in determining species distribution. Particle Size Analysis (PSA) including % sand, silt and gravel, mean and median grain size, and sorting coefficient will indicate change and other factors such as the organic content and redox potential will indicate sediment health. In some cases where pollution of the SAC from industrial sources is a potential cause of change, levels of contaminants should be monitored. The Appendix discusses these features in detail and the parameters to be analysed are in the table below.

Sedimentary parameters to be analysed

Measurements of the hydrological regime parameters, such as topography and bathymetry, circulation patterns, tidal paths, current speeds, temperature, salinity, and turbidity, may be necessary at some SAC sites if no data area available and if these parameters are expected to change due to anthropogenic activities. In general, available information will be sufficient although this may be either anecdotal from local knowledge, such as fishermen, or from Admiralty Charts. A thorough examination of these latter sources should be made prior to collecting primary hydrographic data at the latter is expensive.

Hydrographic exposure will dictate many of the above conditions and especially the nature of intertidal sand and mudflats and the size and formation of subtidal sandbanks. The exposure index proposed by Thomas (1986), which is derived from wind velocity, direction, duration and the effective fetch, will be of value as:

• it can be calculated from data readily available on charts and in weather records;
• it integrates a range of conditions occurring rather than just the short-term;
• it can indicate different exposure indices for areas within a transect or site if sufficiently detailed topographic data are available;
• it can be used to detect changing responses to exposure resulting from other environmental variables such as tidal range; and
• it can be used to indicate species responses to exposure.

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