L)j The Biotic Ingredients of the Wadden Sea
247
more complex and more detailed because we will have to change the level of description from whole functional groups (bacteria, microphytobenthos) to communities and, in particular, populations. On this level of description, the most important variable of interest is the abundance or the density of the species considered.
As a consequence of this new level of description and variables of interest, the
spatial distribution of the organisms must be explicitly discussed in the following.
In soft-bottom communities, distributions are often "patchy", where "patch" is
defined as "an area which, on a specified scale of observation, has some level of
consistency in the density of individuals" (Hall et al. 1994, p. 333; Chap. 8).
Besides single species, there is a long tradition of studying communities in the
Wadden Sea (Reise 1980, 1990a). However, it is often impossible to relate the
classification of communities used in early studies to each other and to the actual
distribution of species (Michaelis & Bbhme 1994). The types of "biofacies" defined by Hertweck (1995; Chap. 3.5) are more suitable because they consider both
the dominant species and the type of sediment in a certain area.
Significance of the ingredient for the Wadden Sea
The extremely high abundance of macrobenthic species (polychaetes, molluscs,
snails, crustacea) is a characteristic property of the Wadden Sea. This high abundance is exploited by fish and birds, as well as by humans (e.g., harvesting mussels). The quantitative aspects of the activity of the macrobenthos are well-known
but still impressive: before their decline in the Lower Saxonian part of the Wadden
Sea (Zens et al. 1997), mussels filtered all the water in their region within just a
few days. The polychaetes Heteromastus filiformis, Arenicola marina, and the
Baltic tellin Macoma balthica move enough sediment within one year to equal a
layer of sediment 35 cm deep (Cadee 1990). The qualitative aspect, i.e. the ecological significance of the macrofauna, is a topic of research in the Wadden Sea.
The questions studied most intensively are the causes and consequences of spatial
and temporal distribution, interactions between species, rates of recovery after
local disturbances, succession, and the concept of community (Reise 1985, Hall et
a1. 1994, Raffaelli & Hawkins 1996).
Stability properties
Constancy and variability - The evaluation of constancy, or the inverse property,
variability, depend upon the spatial and temporal scale considered. At the local
level, e.g., a square metre, the temporal variability of abundance is extremely high.
As the ELA W AT studies show, abundance tluctuates on a scale of just a few days,
as well as within a year and from year to year. At larger spatial scales variability
decreases because small scale heterogeneities average out. However, how much
abundances vary among years 011 the scale of the Spiekeroog backbarrier tidal
basin cannot be assessed because of methodological problems and the short duration of ELA W A T. On a time scale from years to decades, the distributions and
abundances of macrobenthic species are driven by episodic events (storms, ice
winters; Chap. 8) that can cause considerable tluctuations. Besides this, there are
records of historical changes of the distribution patterns and abundances due to
human influence (Reise 1994).
247
more complex and more detailed because we will have to change the level of description from whole functional groups (bacteria, microphytobenthos) to communities and, in particular, populations. On this level of description, the most important variable of interest is the abundance or the density of the species considered.
As a consequence of this new level of description and variables of interest, the
spatial distribution of the organisms must be explicitly discussed in the following.
In soft-bottom communities, distributions are often "patchy", where "patch" is
defined as "an area which, on a specified scale of observation, has some level of
consistency in the density of individuals" (Hall et al. 1994, p. 333; Chap. 8).
Besides single species, there is a long tradition of studying communities in the
Wadden Sea (Reise 1980, 1990a). However, it is often impossible to relate the
classification of communities used in early studies to each other and to the actual
distribution of species (Michaelis & Bbhme 1994). The types of "biofacies" defined by Hertweck (1995; Chap. 3.5) are more suitable because they consider both
the dominant species and the type of sediment in a certain area.
Significance of the ingredient for the Wadden Sea
The extremely high abundance of macrobenthic species (polychaetes, molluscs,
snails, crustacea) is a characteristic property of the Wadden Sea. This high abundance is exploited by fish and birds, as well as by humans (e.g., harvesting mussels). The quantitative aspects of the activity of the macrobenthos are well-known
but still impressive: before their decline in the Lower Saxonian part of the Wadden
Sea (Zens et al. 1997), mussels filtered all the water in their region within just a
few days. The polychaetes Heteromastus filiformis, Arenicola marina, and the
Baltic tellin Macoma balthica move enough sediment within one year to equal a
layer of sediment 35 cm deep (Cadee 1990). The qualitative aspect, i.e. the ecological significance of the macrofauna, is a topic of research in the Wadden Sea.
The questions studied most intensively are the causes and consequences of spatial
and temporal distribution, interactions between species, rates of recovery after
local disturbances, succession, and the concept of community (Reise 1985, Hall et
a1. 1994, Raffaelli & Hawkins 1996).
Stability properties
Constancy and variability - The evaluation of constancy, or the inverse property,
variability, depend upon the spatial and temporal scale considered. At the local
level, e.g., a square metre, the temporal variability of abundance is extremely high.
As the ELA W AT studies show, abundance tluctuates on a scale of just a few days,
as well as within a year and from year to year. At larger spatial scales variability
decreases because small scale heterogeneities average out. However, how much
abundances vary among years 011 the scale of the Spiekeroog backbarrier tidal
basin cannot be assessed because of methodological problems and the short duration of ELA W A T. On a time scale from years to decades, the distributions and
abundances of macrobenthic species are driven by episodic events (storms, ice
winters; Chap. 8) that can cause considerable tluctuations. Besides this, there are
records of historical changes of the distribution patterns and abundances due to
human influence (Reise 1994).