218
8 Grid Based Modelllllg
State variables
Only two species, Mytilus edulis and Lanice conchilega, are considered in
TOPOGRID. The state of a cell is characterized by the abundances of these two
species. Thus, in contrast to the first model, the state variable of a cell is no longer
binary (occupied or not) and both species may be present within one cell. Abundances are specified in relative units which may take values between zero and 255.
For reasons of convenience, Arenicola marina is not considered explicitly; it is
thus assumed that Arenicola does not closely interact with the other two species.
In cells which contain Mytilus, an additional state variable is considered: the
time since the last major spatfall of Mytilus occurred. This variable is recorded
because Mytilus banks at the higher parts of a sandtlat are often dominated by just
one cohort, i.e. they go back to one spatfall (Obert & Michaelis 1991; Ruth 1991).
Accordingly, if the mussels start to grow old, the entire mussel bank will disappear
sooner or later. In the model, the Mytilus population of a cell will die with a probability of pMold (=0.7) if the last spatfall occurred seven or more years ago.
Abiotic factors: topography
The spatial distribution of the macrozoobenthos is mainly affected by larval settlement and disturbance events, which themselves depend on a host of abiotic
factors: the direction and velocity of tidal currents, swell, duration of emergence,
sediment properties, etc. How all these factors affect settlement and sensitivity to
disturbance is not yet known in detail. In particular, little is known about the relative effects of these abiotic factors. For both practical and principal reasons it is
not possible to take into account in a model all factors which are potentially important. But what are the most important factors, i.e. the key factors?
Since there is still no clear, unequivocal answer to this question, we decided to
allow all parameters in the model, which describe settlement, to depend on topographic height (an analogous dependence of local sensitivity to disturbance on
topographic height is - for reasons of simplicity - not considered in the model).
'Topographic height" is thus used as a metaphor for the most important abiotic
factors. In an advanced model version, this metaphor would have to be replaced by
empirical information about real mechanisms and by results of hydrodynamic
models describing the current regime.
Nevertheless, taking into account topographic height is a major achievement
with respect to the "flat" topography of the first model. The assumptions which are
used in the following model rules are mainly based on the conjectures of Hertweck
(1995) that, at least in the backbarrier tidal flat of Spiekeroog, L. conchilega settles
with low densities at intermediate topographic heights, and with high densities in
higher regions. With Mytilus it is assumed that they only become established at the
top parts of the sandflats.
Settlement of Lanice
After ice winters Lanice settles at a certain, "spontaneous" rate Lspontan. Epibenthic substrate for settlement include tubes of adult worms that died during winter,
8 Grid Based Modelllllg
State variables
Only two species, Mytilus edulis and Lanice conchilega, are considered in
TOPOGRID. The state of a cell is characterized by the abundances of these two
species. Thus, in contrast to the first model, the state variable of a cell is no longer
binary (occupied or not) and both species may be present within one cell. Abundances are specified in relative units which may take values between zero and 255.
For reasons of convenience, Arenicola marina is not considered explicitly; it is
thus assumed that Arenicola does not closely interact with the other two species.
In cells which contain Mytilus, an additional state variable is considered: the
time since the last major spatfall of Mytilus occurred. This variable is recorded
because Mytilus banks at the higher parts of a sandtlat are often dominated by just
one cohort, i.e. they go back to one spatfall (Obert & Michaelis 1991; Ruth 1991).
Accordingly, if the mussels start to grow old, the entire mussel bank will disappear
sooner or later. In the model, the Mytilus population of a cell will die with a probability of pMold (=0.7) if the last spatfall occurred seven or more years ago.
Abiotic factors: topography
The spatial distribution of the macrozoobenthos is mainly affected by larval settlement and disturbance events, which themselves depend on a host of abiotic
factors: the direction and velocity of tidal currents, swell, duration of emergence,
sediment properties, etc. How all these factors affect settlement and sensitivity to
disturbance is not yet known in detail. In particular, little is known about the relative effects of these abiotic factors. For both practical and principal reasons it is
not possible to take into account in a model all factors which are potentially important. But what are the most important factors, i.e. the key factors?
Since there is still no clear, unequivocal answer to this question, we decided to
allow all parameters in the model, which describe settlement, to depend on topographic height (an analogous dependence of local sensitivity to disturbance on
topographic height is - for reasons of simplicity - not considered in the model).
'Topographic height" is thus used as a metaphor for the most important abiotic
factors. In an advanced model version, this metaphor would have to be replaced by
empirical information about real mechanisms and by results of hydrodynamic
models describing the current regime.
Nevertheless, taking into account topographic height is a major achievement
with respect to the "flat" topography of the first model. The assumptions which are
used in the following model rules are mainly based on the conjectures of Hertweck
(1995) that, at least in the backbarrier tidal flat of Spiekeroog, L. conchilega settles
with low densities at intermediate topographic heights, and with high densities in
higher regions. With Mytilus it is assumed that they only become established at the
top parts of the sandflats.
Settlement of Lanice
After ice winters Lanice settles at a certain, "spontaneous" rate Lspontan. Epibenthic substrate for settlement include tubes of adult worms that died during winter,