134
5 Spatial and Temporal DistnoutlOn Patterns
terns at species level concerning settlement and secondary dispersal, to relate these
processes to environmental conditions and to estimate the turnover rates of macroZQobenthic organisms due to secondary dispersal.
5.3.2
What Determines Year-Class Strength in Intertidal Soft Bottom
Benthic Macrofauna?
Recruitment of intertidal benthic macrofauna is depending on certain abiotic and
biotic factors which occur every year in a more or less sequential order but with
variable intensity.
The amount of gametes produced each year depends on the age rsp. size structure of the adult population, its abundance and the food availability during gametogenesis. In bivalves, eggs produced by boreal species, such as Macoma halthica,
may be resorbed during mild winters with low food availability, to meet the energy
requirements of the adult. During cold winters, the metabolism of the adults is
assumed to be so much reduced that eggs are not resorbed and a high number of
eggs can be released at the next spawning event (Honkoop & van der Meer 1997).
The authors conclude that rich spatfalls of bivalves after cold winters are the result
of the reduced metabolism of adults in the preceding winter. But this does not
explain, why recruitment success after cold winters varies among bivalve species.
For example, after the ice winter 1995/96 the soft-shell clam Mya arenaria had a
strong recruitment in the Spiekeroog backbarrier tidal flats (Jaklin unpublished
data). In the Sylt-R0m0 area C. edule (Strasser pers. comm.) recruited best of all
infaunal bivalve species. After a preceding ice winter of 1985/86 M. halthica had a
strong recruitment in the tidal flats of the island of Borkum (Gunther 1990). The
latter observations were in agreement with the recruitment pattern observed in
long-term series from Balgzand (Beukema 1979), which showed very good recruitment of this species after cold winters. In the Spiekeroog backbarrier system
the opposite pattern was observed after an ice winter in 1996: only low densities of
M. balthica larvae were recorded in the water column (Chap. 5.1) and subsequently the abundance of initial settlers was low. The reason for this variability in
recruitment success after cold winters is not yet known.
In general, the order of magnitude of bivalve larvae in the water column seemed
to be related to the order of magnitude of initial settlers on the Groninger Plate
(Boysen-Ennen, Jaklin unpublished data). A more precise prediction of the year
class strength from larval densities will not be possible, as initial settlement is
determined by small-scale factors, which vary interannually and depend on the
local conditions (Armonies 1998). For most polychaete species this information is
not available. Results obtained during ELA WAT prove that for the tube-worm
Lanice conchilega no relationship exists between larval density and abundance of
initial settlers (see Sect. 5.3.6).
5 Spatial and Temporal DistnoutlOn Patterns
terns at species level concerning settlement and secondary dispersal, to relate these
processes to environmental conditions and to estimate the turnover rates of macroZQobenthic organisms due to secondary dispersal.
5.3.2
What Determines Year-Class Strength in Intertidal Soft Bottom
Benthic Macrofauna?
Recruitment of intertidal benthic macrofauna is depending on certain abiotic and
biotic factors which occur every year in a more or less sequential order but with
variable intensity.
The amount of gametes produced each year depends on the age rsp. size structure of the adult population, its abundance and the food availability during gametogenesis. In bivalves, eggs produced by boreal species, such as Macoma halthica,
may be resorbed during mild winters with low food availability, to meet the energy
requirements of the adult. During cold winters, the metabolism of the adults is
assumed to be so much reduced that eggs are not resorbed and a high number of
eggs can be released at the next spawning event (Honkoop & van der Meer 1997).
The authors conclude that rich spatfalls of bivalves after cold winters are the result
of the reduced metabolism of adults in the preceding winter. But this does not
explain, why recruitment success after cold winters varies among bivalve species.
For example, after the ice winter 1995/96 the soft-shell clam Mya arenaria had a
strong recruitment in the Spiekeroog backbarrier tidal flats (Jaklin unpublished
data). In the Sylt-R0m0 area C. edule (Strasser pers. comm.) recruited best of all
infaunal bivalve species. After a preceding ice winter of 1985/86 M. halthica had a
strong recruitment in the tidal flats of the island of Borkum (Gunther 1990). The
latter observations were in agreement with the recruitment pattern observed in
long-term series from Balgzand (Beukema 1979), which showed very good recruitment of this species after cold winters. In the Spiekeroog backbarrier system
the opposite pattern was observed after an ice winter in 1996: only low densities of
M. balthica larvae were recorded in the water column (Chap. 5.1) and subsequently the abundance of initial settlers was low. The reason for this variability in
recruitment success after cold winters is not yet known.
In general, the order of magnitude of bivalve larvae in the water column seemed
to be related to the order of magnitude of initial settlers on the Groninger Plate
(Boysen-Ennen, Jaklin unpublished data). A more precise prediction of the year
class strength from larval densities will not be possible, as initial settlement is
determined by small-scale factors, which vary interannually and depend on the
local conditions (Armonies 1998). For most polychaete species this information is
not available. Results obtained during ELA WAT prove that for the tube-worm
Lanice conchilega no relationship exists between larval density and abundance of
initial settlers (see Sect. 5.3.6).