6.5 Discussion
I X9
persal ability (Grant 1981), tolerance towards disturbance induced sediment modifications (Gamenick et al. 1996), season and reproductive mode (Simon & Dauer
1977).
The dependence of macrobenthic recolonization on the season and thus the
availability of larvae and postlarvae became apparent by the spring and autumn
disturbance experiments. Disturbances in spring and summer can be recolonized
quickly due to larvae, the high ambient density of juveniles and the high mobility
of the organisms (Simon & Dauer 1977; Zajac & Whitlatch 1982 a, b; Bonsdorff
& Osterman 1985). Adult macrobenthic organisms can also recolonize areas
through the water column or by sediment transport (Dauer & Simon 1976; Santos
& Simon 1980; Bell & Devlin 1983; Frid 1989). The larger the disturbed site, the
more important is the coloni7.ation by larvae and postlarvae (Probert 1984; Smith
& Brumsickle 1989; Gi.inther 1992; Thrush et al. 1996; Whitlatch et al. 1998). The
experiments on the Griininger Plate were homogeneously colonized in their peripheral and central areas. As the size classes of benthic organisms were not recorded, it was impossible to distinguish between recruitment in those plots and
continuous immigration of postlarval and adult stages. Thus the dependence of
colonization on the mode of dispersal cannot be solved for these experiments.
Several models exist to explain the succession of soft-bottom communities,
taking resource availability, life history strategies, reduced competition and other
species interactions into account (Connell & Slayter 1976; Grassle & Sanders
1973; Thistle 1981; Ambrose 1984; Chesney 1985). Zajac & Whitlatch (1984)
combine these explanations in a hierarchical approach of succession. During the
experiments on the Griininger Plate only seasonally influenced colonization stages
were recorded, but no clear successional stages. The recolonization depended on
the availability of settlers and food resources. In some experiments, abundances of
diatoms and small macrofauna exceeded background values (cf. "peak of opportunists" Pearson & Rosenberg 197R). The plots of the spring and autumn experiments arranged in 1994 had higher abundances of small macrofauna in the following year than the control plots, which could indicate a reduced competition, as
larger macrobenthic species were still missing. Successional stages can be absent,
when the benthic community naturally occurring in an area is characterized by
opportunistic species (Zajac & Whitlatch 1982b; Gamenick et al 1996). All these
findings support the approach by Zajac & Whitlatch (1984) of the combined relevance of environmental factors, life history strategies and biotic interactions for
succession in benthic communities.
6.5.3
Comparison of Recolonization After the Ice Winter and After
Experimental Disturbances
The rapid increase of species numbers after the ice winter compared to the recolonization in the experiments was due to the successful recruitment of many species
in the summer of 1996 and the selective reduction of cold-sensitive species by the
winter (Beukema 1989, Chap. 5.6). Timing of recruitment as well as year class
strength of juvenile macrobenthos (mainly polychaetes and molluscs) was comparable with the recolonization in the spring experiment 1994. Thus, recoloni7.ation
I X9
persal ability (Grant 1981), tolerance towards disturbance induced sediment modifications (Gamenick et al. 1996), season and reproductive mode (Simon & Dauer
1977).
The dependence of macrobenthic recolonization on the season and thus the
availability of larvae and postlarvae became apparent by the spring and autumn
disturbance experiments. Disturbances in spring and summer can be recolonized
quickly due to larvae, the high ambient density of juveniles and the high mobility
of the organisms (Simon & Dauer 1977; Zajac & Whitlatch 1982 a, b; Bonsdorff
& Osterman 1985). Adult macrobenthic organisms can also recolonize areas
through the water column or by sediment transport (Dauer & Simon 1976; Santos
& Simon 1980; Bell & Devlin 1983; Frid 1989). The larger the disturbed site, the
more important is the coloni7.ation by larvae and postlarvae (Probert 1984; Smith
& Brumsickle 1989; Gi.inther 1992; Thrush et al. 1996; Whitlatch et al. 1998). The
experiments on the Griininger Plate were homogeneously colonized in their peripheral and central areas. As the size classes of benthic organisms were not recorded, it was impossible to distinguish between recruitment in those plots and
continuous immigration of postlarval and adult stages. Thus the dependence of
colonization on the mode of dispersal cannot be solved for these experiments.
Several models exist to explain the succession of soft-bottom communities,
taking resource availability, life history strategies, reduced competition and other
species interactions into account (Connell & Slayter 1976; Grassle & Sanders
1973; Thistle 1981; Ambrose 1984; Chesney 1985). Zajac & Whitlatch (1984)
combine these explanations in a hierarchical approach of succession. During the
experiments on the Griininger Plate only seasonally influenced colonization stages
were recorded, but no clear successional stages. The recolonization depended on
the availability of settlers and food resources. In some experiments, abundances of
diatoms and small macrofauna exceeded background values (cf. "peak of opportunists" Pearson & Rosenberg 197R). The plots of the spring and autumn experiments arranged in 1994 had higher abundances of small macrofauna in the following year than the control plots, which could indicate a reduced competition, as
larger macrobenthic species were still missing. Successional stages can be absent,
when the benthic community naturally occurring in an area is characterized by
opportunistic species (Zajac & Whitlatch 1982b; Gamenick et al 1996). All these
findings support the approach by Zajac & Whitlatch (1984) of the combined relevance of environmental factors, life history strategies and biotic interactions for
succession in benthic communities.
6.5.3
Comparison of Recolonization After the Ice Winter and After
Experimental Disturbances
The rapid increase of species numbers after the ice winter compared to the recolonization in the experiments was due to the successful recruitment of many species
in the summer of 1996 and the selective reduction of cold-sensitive species by the
winter (Beukema 1989, Chap. 5.6). Timing of recruitment as well as year class
strength of juvenile macrobenthos (mainly polychaetes and molluscs) was comparable with the recolonization in the spring experiment 1994. Thus, recoloni7.ation