5.3 Settlement and Dispersal of Macrofauna
133
5.3
Settlement, Secondary Dispersal and Turnover Rate of
Benthic Macrofauna
Carmen-Pia Gunther
Abstract: In the three years of investigation, larvae of most benthic macrofauna
species were observed in the plankton, but with variations in the order of magnitude of larval density and in the timing of peak abundance. For each of the investigated species initial settlement occurred regularly at specific main settlement sites.
The abundance of settlers varied in dependence of larval supply, hydrodynamic
conditions and (for Lanice conchilega) availability of hard-substrate. Especially
after the ice winter \995/96 the latter two factors appeared to be of relevance. In
1996 settlement of Heteromastus filiformis took place in wide areas of the
Groninger Plate, thus increasing the area of initial settlement compared to the
preceding years. Recruitment of L. conchilega was limited in 1996 by the low
numbers of larvae available and the lack of suitable substrate for settling.
In the polychaetes under investigation, zonation patterns of adults developed by
initial settlement, while those of adult bivalves developed by postlarval dispersal.
After extreme conditions, such as cold winters with ice formation, secondary dispersal may have some importance in determining zonation patterns of polychaetes
as well.
The turnover rate (i.e. the turnover of benthic organisms on a specific site due to
emigration, resuspension, initial and secondary settlement) was extremely high
even for sedentary polychaetes (e.g. Pygospio elegans). The importance of this
result for recolonization experiments (Chap. 6) in which this species dominated in
all years of investigation, has still to be evaluated in future studies. The in situ
turnover rate of soft sediment macrofauna can also be valuable in estimating the
energy flow through the macrobenthic fauna more precisely.
The order of magnitude of the reproductive output, settlement patterns and settlement behaviour as well as mobility of macrofauna demonstrate that the studied
species react resilient to disturbances in the Wadden Sea by balancing their population level.
5.3.1
Introduction
To persist in a gi ven area, a macrozoobenthic population has to develop mechanisms, which lead either to a high probability of survival for an individual over a
certain period of time, or to a regular replacement of the individuals at this place.
The latter is achieved by colonization processes such as initial settlement (settlement of larvae) or secondary settlement (settlement of postlarvae or adult stages).
Both processes contribute in undisturbed as well as disturbed benthic habitats to
develop and maintain stocks of macrozoobenthic species.
Due to the relatively short study period and the strong interannual variability in
seasonality, no reference dynamics of settlement and secondary dispersal of macrofauna could be determined in the context of ELA W A T. Thus, in the following
chapter the attempt will be made, to identify successive temporal and spatial pat-
133
5.3
Settlement, Secondary Dispersal and Turnover Rate of
Benthic Macrofauna
Carmen-Pia Gunther
Abstract: In the three years of investigation, larvae of most benthic macrofauna
species were observed in the plankton, but with variations in the order of magnitude of larval density and in the timing of peak abundance. For each of the investigated species initial settlement occurred regularly at specific main settlement sites.
The abundance of settlers varied in dependence of larval supply, hydrodynamic
conditions and (for Lanice conchilega) availability of hard-substrate. Especially
after the ice winter \995/96 the latter two factors appeared to be of relevance. In
1996 settlement of Heteromastus filiformis took place in wide areas of the
Groninger Plate, thus increasing the area of initial settlement compared to the
preceding years. Recruitment of L. conchilega was limited in 1996 by the low
numbers of larvae available and the lack of suitable substrate for settling.
In the polychaetes under investigation, zonation patterns of adults developed by
initial settlement, while those of adult bivalves developed by postlarval dispersal.
After extreme conditions, such as cold winters with ice formation, secondary dispersal may have some importance in determining zonation patterns of polychaetes
as well.
The turnover rate (i.e. the turnover of benthic organisms on a specific site due to
emigration, resuspension, initial and secondary settlement) was extremely high
even for sedentary polychaetes (e.g. Pygospio elegans). The importance of this
result for recolonization experiments (Chap. 6) in which this species dominated in
all years of investigation, has still to be evaluated in future studies. The in situ
turnover rate of soft sediment macrofauna can also be valuable in estimating the
energy flow through the macrobenthic fauna more precisely.
The order of magnitude of the reproductive output, settlement patterns and settlement behaviour as well as mobility of macrofauna demonstrate that the studied
species react resilient to disturbances in the Wadden Sea by balancing their population level.
5.3.1
Introduction
To persist in a gi ven area, a macrozoobenthic population has to develop mechanisms, which lead either to a high probability of survival for an individual over a
certain period of time, or to a regular replacement of the individuals at this place.
The latter is achieved by colonization processes such as initial settlement (settlement of larvae) or secondary settlement (settlement of postlarvae or adult stages).
Both processes contribute in undisturbed as well as disturbed benthic habitats to
develop and maintain stocks of macrozoobenthic species.
Due to the relatively short study period and the strong interannual variability in
seasonality, no reference dynamics of settlement and secondary dispersal of macrofauna could be determined in the context of ELA W A T. Thus, in the following
chapter the attempt will be made, to identify successive temporal and spatial pat-