236
9 Stahility Properties in the Wadden Sea
At the community level, a severe decrease in the frequency of disturbance
events like ice winters or storms would increase the relative importance of biotic
interactions, because biogenic epipenthic structures (e.g., mussel beds, "tube
lawns" of Lanice conchilega) could exist for longer periods of time and on larger
areas (Nehls & Thiel 1993). This effect is also predicted by the model presented in
Chap. 8. However, without ice winters, the reproduction of mussels might be impaired, which could threaten the existence of their biogenic structures in the longterm. On the other hand, a drastic increase of disturbance frequency would diminish the significance of biotic interactions (Reise 1985). More storms could also
lead to an increased net export of sediments from the Wadden Sea.
9.4.2
Hydrography, Morphology and Sediments
Variables of interest
The most important hydrographical variables of interest are tides and the variation
in levels of low and high tide, current flow and direction and swell. Morphology is
characterized by elevation, i.e. topographic height. The main aspects of morphology in the Wadden Sea are the tidal flats and the tidal channels. Sediments are
ultimately characterized by grain size spectra and by the distribution of these
spectra in space (Chaps. 3.3 and 3.4).
Significance of the ingredients for the Wadden Sea
The tides are the decisive process in the Wadden Sea. They shape the characteristic
morphology comprising the extended tidal flats and the tidal channels. The existence of the large tidal flats is the cardinal property ofthe Wadden Sea.
The tidal flats of the Wadden Sea are a unique habitat. On the one hand, the
regular alternation between low and high tide requires broad physiological and
behavioural flexibility on the part of the organisms living in the tidal t1ats
(Costanza et al. 1993). On the other hand, the low tide increases primary production in the sediment because benthic algae gain more light than in the subtidal.
Moreover, tidal currents and swell allow for an effective exchange of oxygen and
nutrients, as well as organisms, between water column and sediment.
The local hydrographical regime plays, as was also shown by ELA W A T
(Chaps. 5.3 and 8)' a pivotal role for the spatial distribution of macrobenthic species. Settlement, for example, is strongly determined by the local properties of the
tidal currents (Chaps. 5.3 and 5.4). Yet on a regional scale, too, currents are decisive for many species because they enable the transport of larvae over long distances.
In addition to the tidal currents, swell contributes to the kinetic energy of the
water body and thus promotes exchange processes between water column and
sediment. In addition, swell affects sedimentation, and probably also larval settlement and the local availability of food and nutrients. However, there are no detailed empirical records of these effects of swell.
Finally, properties of the sediment are a determining parameter for all organisms living in the sediment. Many benthic species of the Wadden Sea prefer cer-
9 Stahility Properties in the Wadden Sea
At the community level, a severe decrease in the frequency of disturbance
events like ice winters or storms would increase the relative importance of biotic
interactions, because biogenic epipenthic structures (e.g., mussel beds, "tube
lawns" of Lanice conchilega) could exist for longer periods of time and on larger
areas (Nehls & Thiel 1993). This effect is also predicted by the model presented in
Chap. 8. However, without ice winters, the reproduction of mussels might be impaired, which could threaten the existence of their biogenic structures in the longterm. On the other hand, a drastic increase of disturbance frequency would diminish the significance of biotic interactions (Reise 1985). More storms could also
lead to an increased net export of sediments from the Wadden Sea.
9.4.2
Hydrography, Morphology and Sediments
Variables of interest
The most important hydrographical variables of interest are tides and the variation
in levels of low and high tide, current flow and direction and swell. Morphology is
characterized by elevation, i.e. topographic height. The main aspects of morphology in the Wadden Sea are the tidal flats and the tidal channels. Sediments are
ultimately characterized by grain size spectra and by the distribution of these
spectra in space (Chaps. 3.3 and 3.4).
Significance of the ingredients for the Wadden Sea
The tides are the decisive process in the Wadden Sea. They shape the characteristic
morphology comprising the extended tidal flats and the tidal channels. The existence of the large tidal flats is the cardinal property ofthe Wadden Sea.
The tidal flats of the Wadden Sea are a unique habitat. On the one hand, the
regular alternation between low and high tide requires broad physiological and
behavioural flexibility on the part of the organisms living in the tidal t1ats
(Costanza et al. 1993). On the other hand, the low tide increases primary production in the sediment because benthic algae gain more light than in the subtidal.
Moreover, tidal currents and swell allow for an effective exchange of oxygen and
nutrients, as well as organisms, between water column and sediment.
The local hydrographical regime plays, as was also shown by ELA W A T
(Chaps. 5.3 and 8)' a pivotal role for the spatial distribution of macrobenthic species. Settlement, for example, is strongly determined by the local properties of the
tidal currents (Chaps. 5.3 and 5.4). Yet on a regional scale, too, currents are decisive for many species because they enable the transport of larvae over long distances.
In addition to the tidal currents, swell contributes to the kinetic energy of the
water body and thus promotes exchange processes between water column and
sediment. In addition, swell affects sedimentation, and probably also larval settlement and the local availability of food and nutrients. However, there are no detailed empirical records of these effects of swell.
Finally, properties of the sediment are a determining parameter for all organisms living in the sediment. Many benthic species of the Wadden Sea prefer cer-