Intertidal Sand and Mudflats

Subtidal Mobile Sandbanks

Intertidal areas by definition have low, middle and high tidal areas although in many cases the latter has been constrained by anthropogenic features, e.g. a seawall. The productivity of these areas differ with respect to the tidal elevation and shore slope (Gray, 1981) and most of the infaunal community, in terms of the abundance and biomass, is at the mid-tidal region. Any decrease in tidal height will take the area towards greater current speed near channels whereas an increase in tidal height will increase exposure and thus desiccation of the organisms. Changes in tidal height over the intertidal zone create a less predictable environment where there may be more extreme changes in temperature, salinity, dissolved oxygen and water content than in the sublittoral zone (Hayward, 1994). Such a change in tidal height creates aerial exposure of the sediment which in turn affects desiccation of any organisms and drying of the sediment in warm conditions and salinity stress during rain.

The gradient of a shore reflects the energy conditions - that of mud flats is shallow reflecting the low energy conditions in contrast to more dynamic sandy shores. Steeper shores are associated with larger grains and shallow profiles with fine sediment (Pethick, 1984). Most large shores have a narrow range of grain sizes in the swash zone and they are usually composed of fine to medium sand and although grains of around 200µm are too fine to be easily resuspended, they may be saltated. On a shore with plunging breakers, there is often a concentration of coarser sediment around the plunge point at mean water.

Shore slope has a complex relationship with wave action and the distinction between a beach and a sandflat is subjective and based on a combination of conditions. Although beaches are on the periphery of the biotope complexes covered here, their features are of relevance in understanding sandflat dynamics. High energy wave action and/or fine particles (as with mud flats) result in flatter slopes where the wave energy is more evenly dissipated across the surf and intertidal zone (McLachlan, 1983). Exposed sandy shores can be defined as reflective, intermediate or dissipative based on the sum of their physical parameters.

Figure – Relationship between beach slope and grain size on beaches exposed to low and high energy waves.

Reflective beaches have coarse sand, low wave energy, small tidal ranges, steep slopes, no surf zones and wave energy reflecting out to sea; they tend to be dominated by plunging breakers which dissipate their energy over a short distance (Swart, 1983).

Dissipative beaches develop where there is fine sand, heavy wave action and large tidal ranges (McLachlan, 1990); they have flat slopes and wide surf zones where foaming or spilling breakers gradually dissipate their energy (Swart, 1983).

Intermediate beaches have moderate to heavy wave action with intermediate slopes and surf zones with bars, channels and rip currents (McLachlan, 1990).

These criteria are supported by Pethick (1984) who considers beaches to be energy sinks which must dissipate energy without changing themselves. High energy, steep waves are best buffered by a wide flat beach, whereas low energy waves are countered by a steep shore profile. Steep, coarse beaches can also dissipate the energy of high energy waves through friction and percolation. The aspects are of importance in considering the role of sandflats in coastal defence.

The profile of the shore influences the amount of erosion taking place during storms. Dissipative beaches with flat or even concave profiles are less affected by storms and may even have some consequent accretion. Steeper and possibly convex reflective beaches have more sand cut from the upper profile and this may be either transported off-shore or used to fill the lower profile (Fucella & Dolan, 1996). In addition to on and off-shore movement there is longshore transport which is influenced by wave height, current velocity and grain size (Swart, 1983). The volume of material transported may be considerable daily in storm conditions (Swart, 1983). Grain size characteristics also influence erosion by influencing the mobility of the particles and the sediment cohesiveness.

The depth-related characteristics of sand banks differ depending on whether they are within the infralittoral or circalittoral regions. The former is within the photic zone and thus will sustain attached primary producers whereas the latter will be faunal dominated (Hiscock, 1983). Any increase in either the depth or turbidity of the water will affect the light penetration and thus the primary producers although in the case of the biotope complexes covered here, the only primary producers of concern are the benthic microalgae. The quality of light (as critical depth for primary production) reaching such sandbanks will determine the type of microalgae colonising the sediment but there is little work on these aspects. In shallow or constricted areas the water above the banks may be very turbid (Carter, 1984) thus limiting primary production.

An increase in depth would change characteristics of the sandbank and its interactions with the hydrographic regime. If the depth decreased, the sand bank may become exposed on low spring tides which would decrease survival of subtidal fauna that cannot withstand exposure. The depth of a sand bank is also important for predator populations of birds which are restricted to certain diving depths. For example, the majority of wintering diving birds observed in a survey off the Flamborough coastline (IECS, 1993) were restricted to within 5km of the coastline, in areas of shallow water with a sandy substratum.

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