Hydrophysical regime

Other seasonal influences

The hydrophysical regime is very variable and, while it is possible to consider ‘average’ exposure, it is essential to recognise that for both biotope complexes and particularly subtidal sandbanks, extreme conditions have profound effects (Hiscock, 1983), even when they only persist for a short length of time. Water movement due to wave action is the more erratic because it fluctuates considerably on a seasonal basis. Movements caused by tides and currents varies in regular patterns but it is not only the strength but also the type of movement that affects the distribution of marine organisms. Uni-directional, multi-directional and oscillatory movements each represents a different type of stress or confer particular advantages (Wood, 1987). Storm events will inflict extreme changes in wave action both in terms of strength and direction (Pethick, 1984; Carter, 1984).

Both these intertidal and subtidal biotope complexes will be sensitive to changes in the hydrophysical environment (see Chapter II). For example, periodic increases in wave action can severely alter the appearance of the intertidal region as the top 20cm of sand can be removed by storm events (Dolphin et al, 1995). Such storms are an important mechanism which can re-sort the sediment, leaving coarser particles, and release of sediment-sequestered contaminants (Dolphin et al, 1995).

Increased wave action causes stress to the infauna by disrupting feeding and burrowing activities and reduces species richness, abundance and biomass. The infauna are sensitive to changes in sediment as many are adapted to burrow through certain grades of sediment (Trueman & Ansell, 1969). Coarse material is more difficult to burrow through and species have to be robust in order to survive the stronger currents/wave action in these areas. Changes in sedimentary features may also influence the trophic status of the infauna and the proportions of suspension and deposit feeding animals (Sanders, 1968). The distribution of suspension feeders is greatly affected by sediment instability and they are sensitive to increases in the silt/clay content of the substratum and suspended sediment. In turn, changes to the infauna will affect the predators (Chapter III).

Changes in the hydrophysical regime and thus substratum will change the faunal composition of the biotope complex. Major changes in the former will produce mortality and reduce species richness. Although many species are capable of living in a variety of substrata, the species most affected will be those which are restricted to a particular grade of sediment.

The strength of wave action affects the topography (as flatness/steepness and shore width) of the intertidal area therefore a significant change in wave action will affect the physical and biological integrity of that habitat and the exposure regime. The topography of the shore is equally important as flatter shores dissipate energy and generally have a more stable fauna (McLachlan, 1993). For example, the exposure of the intertidal flats on the Severn estuary to the prevailing gales, together with the large tidal range, caused large areas of surface mud and hence its invertebrates to be removed (Ferns, 1983). Many of the shorebirds moved temporarily to other, more sheltered areas.

Intertidal mudflats are by definition sheltered environments and hence relatively stable in sedimentary terms. Any increase or decrease in grain size, silt content etc. will affect species numbers/richness but these should return to normal levels if the disturbance is temporary. In addition, the sedimentary heterogeneous nature of mudflats reflects the species greater tolerance of different particle sizes. The high bioturbatory potential of mudflat organisms (see Chapter III) will decrease their sensitivity to sediment changes such as smothering by any influx of new sediment.

Intertidal sandflats will be more or less subject to disturbance by hydrodynamic changes depending on their exposure regime. More sheltered sandflats, which may have a large population of tube dwelling polychaetes for instance, will be severely affected by storm events and a large reduction in species richness and abundance may occur. This may lead to the development of a transitional community dominated by opportunist species and more mobile infauna such as Haustoriid amphipods and errant polychaetes (see Chapter III). Recovery of the community will be determined by the degree of change. Longer term changes as an increase in grain-size following intense wave action or a changed wave-field may lead to a more permanent change in the faunal composition of the biotope, with species such as Fabulina fabula, Donax vittatus and Haustoriid amphipods becoming more dominant. Increased deposition of finer material will lead to increased dominance by species preferring finer sediments such as Angulus tenuis. More exposed sandflats have a poorer community with elements more tolerant of increases in wave action so changes will be less severe.

Wave action is also important subtidally in shallower areas as it can disturb the sediment, particularly during storms. Disturbance of this nature may affect shallow and deep sand banks (Perkins, 1974) and may result in large scale sediment transport. In many areas the predominant factor influencing the structure of subtidal sand banks and other shallow subtidal areas are tidal streams. These currents lead to constant change in the size shape and position of sand banks and in some areas e.g. in the Solway Firth where tidal streams are particularly strong, sandbanks may move considerably over one tidal cycle (Perkins, 1974).

Hydrographical changes operate at different spatial scales, for example in the North Sea broad scale communities were influenced by temperature and depth also relating to the presence of different water masses (Glemarec, 1973; Basford et al, 1990). As substratum effects are superimposed over these variables, wide-scale physical effects can lead to community replacement. By definition, subtidal mobile sandbanks are subject to continued reworking of the sediment by wave action and tidal streams and thus are dominated by species capable of tolerating severe changes in the hydrophysical regime.

At more sheltered sandbanks, changes following severe hydrodynamic stress will be greater although continued or regular disturbance may give rise to a transitional community dominated by species such as Chaetozone setosa. As recruitment to subtidal sandbanks is due to chance influx of species from external areas, which will be dictated by the hydrodynamic regime, the post-transitional community may be difficult to determine. In addition, transitional communities by nature are unpredictable. In areas of mixed sediment, the recruitment of species such as Sabellaria spinulosa will stabilise the sediment and allow the influx other species, an thus an increase in diversity, as well as increasing the heterogeneity of the substratum. The timing during the year of the disturbance in relation to the settlement patterns of different species will influence the structure of the community.

Seasonal changes occur in subtidal community structure (e.g. Boesch, 1973) and environments that have characteristic seasonal patterns of species composition are relatively unstable and often ‘physically-controlled’ (Sanders, 1968). In estuarine, intertidal and shallow-subtidal habitats with characteristically large seasonal fluctuations in environmental parameters, changes in the biotope complex are likely. For example, high temperatures and calm conditions may lead to stratification of the water column and to hypoxia in the near bottom water will be exacerbated by high sedimentary organic matter (see below).

The extreme temperatures experienced in the intertidal habitat also influence their populations’ behavioural and reproductive activity and food and oxygen availability (Eltringham, 1971). For example, summer water temperatures may control the number of generations per year of Corophium volutator. Many intertidal species have wide tolerances for temperature and can also alter metabolic activity, or simply burrow deeper in the sediment or move seaward to combat temperature change (Brown, 1983). Severe changes in temperature in intertidal areas will result in a seasonal reduction in benthic species richness and abundance, although the species are well adapted to such changes. Temperature is also an important factor explaining dynamics of microbial activity and microphytobenthic primary production on intertidal mudflats (Blanchard & Guarini, 1996) so microbial activity on intertidal mudflats.

These will not be subjected to such extreme changes in temperature as intertidal areas although fluctuations will occur in stratified waters or on the boundaries of frontal systems. Changes in the salinity or temperature may occur on the sea bed if the stratification of the water column is broken down by storm events or by shifts in the position of front. Despite this, the main source of temperature variation at subtidal sandbanks will be between summer and winter which may be 5° to 10 ° C depending on depth. This variation may be secondary to changes in the sedimentary regime but may have short term but significant effects on diversity (Buchanan & Moore, 1986).

These variations may also affect the succession of macrobenthic species with the occurrence or survival of different groups of species related to periods of mild or cold winter temperatures. For example, Buekema (1992) showed that one third of the macrobenthos of the Wadden Sea is sensitive to winter temperature, with mild winters inducing changes in the structure and functioning of the ecosystem.

Predator use of sedimentary habitats is influenced by seasonal changes. For example, offshore movement of fish populations is caused by avoidance of low temperatures in winter, avoidance of greater turbulence (storm-induced) in autumn and winter, or feeding migration. Juvenile (0-group) plaice recruitment to intertidal and shallow areas is influenced by wind strength (Pihl, 1990; Modin & Pihl, 1996).

Waders generally have little difficulty in meeting energy needs at the end of summer when food is abundant and weather mild. In contrast, in cold periods, waterfowl are close to their energy balance threshold and so are sensitive to additional disturbance. Greater foraging is required through scarcer prey at the same time as energy demand for thermoregulation increases, thus requiring greater food intake (Davidson & Rothwell, 1993) or the use of body reserves. In spring and autumn many waterfowl lay down large stores of fat and protein in preparation for their migration between Arctic breeding grounds and their wintering grounds in Europe and Africa. The direct and indirect effects of disturbance are highest during the autumn moult when energy demands are high because of the growth of new feathers. Waterfowl concentrate on large estuaries during the moulting season and this is considered to be an adaptation to reduce disturbance or predation (Prater, 1981).

As indicated above, intertidal invertebrate prey populations vary with temperature and wading birds differ in their sensitivity to this change. Some birds are less vulnerable to changes in their main prey species, for example, a change in the dietary composition of the black-tailed godwit occurred over the winter, probably in response to changes in the availability of Nereis and small Scrobicularia (Moreira, 1994).

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