198
7 Effects of the Icc Winter 19<)5/96
the intertidal sediments, later on P. fJouchetii was observed also. This points to a
high sedimentation rate of the bloom of both species.
The total abundance of meiofauna reached maximum values in June with
nematodes being numerically dominant. For nematodes, similar to the macrofauna,
temporal shifts in maximum abundances of some species as well as variations in
dominance structure could be observed in 1996 (compare Chap. 6.4). In spring
species of the nematode family Oncholaimidae, which are potentially scavengers,
dominated. Later in the year, diatom feeding species took over. Total abundances
of nematodes in spring and summer were generally higher compared to preceding
years.
The effects of cold winters on the intertidal and subtidal macrofauna are well
documented (Ziegelmeier 1964; Beukema 1979, 1982, 1985, 1989, 1990; Beukema
& Cadee 1996; Beukema et al. 1978, 1988, 1993; Buhr, 1981; Dorjes 1992 a, b;
Dorjes et al. 1986; Reichert & Dorjes 1980). In both habitats temporary changes in
the macrobenthic association have been observed. Cold sensitive species were
either drastically reduced in abundance or extinguished locally. The following
regeneration varies with the potential of single species to reproduce and disperse
and to the distance of source populations. Thus, depending on the species, regeneration may take up to several years (compare Chaps. 5.3.6 and 6.4).
For example, at the Groninger Plate and Swinnplate the intertidal part of the
L. conchilega population did not recover during the period of field observations.
Measurements of the thorax diameter indicate that some of those specimens present in autumn 1996 may reproduce in the next summer. This implies that the
age/size structure would be faster re-established as the abundance, which is estimated to take 2-4 years (Hertweck 1995; Chap. 5.3.6), provided no further cold
winter occurs.
In summer 1996 mussel beds re-established at the Swinnplate by good recruitment. Similar to L. conchilega patches, the mussel beds consisted only of individuals of the O-group. The development of a mature bed with several age classes takes
several years. A similar regeneration pattern is assumed to take place in C. edule
(Dorjes 1992).
Depending on the late availability of meroplanktonic larvae, settlement and recruitment of nearly all macrobenthic species occurred later in the year when compared to the previous two years of investigation. At the Groninger Plate juveniles
of the soft shell clam, M. arenaria, reached the extreme abundance of up to
200000 ind m 2 • Other species, such as H. filiformis or Ensis americanus recruited
in higher abundances as in 1994 and 1995.
The extraordinarily good recruitment of macrofauna, especially bivalves, after
cold winters was attributed to several non exclusive causes, being probably all
relevant to a different degree:
I. In a cold winter, gametes produced in the preceding summer are not resorbed
for the nutrition of the adults. Thus, the number of eggs spawned is higher and
subsequently the number of recruits is enlarged as well (Honkoop & van der
Meer 1997; compare Chap. 5.1).
2. It is assumed that a reduction of suspension-feeders increases survival of
planktonic larvae, thus positively affecting recruitment (Reichert & Dorjes
19RO). This effect was often postulated, but has so far only been supported by
7 Effects of the Icc Winter 19<)5/96
the intertidal sediments, later on P. fJouchetii was observed also. This points to a
high sedimentation rate of the bloom of both species.
The total abundance of meiofauna reached maximum values in June with
nematodes being numerically dominant. For nematodes, similar to the macrofauna,
temporal shifts in maximum abundances of some species as well as variations in
dominance structure could be observed in 1996 (compare Chap. 6.4). In spring
species of the nematode family Oncholaimidae, which are potentially scavengers,
dominated. Later in the year, diatom feeding species took over. Total abundances
of nematodes in spring and summer were generally higher compared to preceding
years.
The effects of cold winters on the intertidal and subtidal macrofauna are well
documented (Ziegelmeier 1964; Beukema 1979, 1982, 1985, 1989, 1990; Beukema
& Cadee 1996; Beukema et al. 1978, 1988, 1993; Buhr, 1981; Dorjes 1992 a, b;
Dorjes et al. 1986; Reichert & Dorjes 1980). In both habitats temporary changes in
the macrobenthic association have been observed. Cold sensitive species were
either drastically reduced in abundance or extinguished locally. The following
regeneration varies with the potential of single species to reproduce and disperse
and to the distance of source populations. Thus, depending on the species, regeneration may take up to several years (compare Chaps. 5.3.6 and 6.4).
For example, at the Groninger Plate and Swinnplate the intertidal part of the
L. conchilega population did not recover during the period of field observations.
Measurements of the thorax diameter indicate that some of those specimens present in autumn 1996 may reproduce in the next summer. This implies that the
age/size structure would be faster re-established as the abundance, which is estimated to take 2-4 years (Hertweck 1995; Chap. 5.3.6), provided no further cold
winter occurs.
In summer 1996 mussel beds re-established at the Swinnplate by good recruitment. Similar to L. conchilega patches, the mussel beds consisted only of individuals of the O-group. The development of a mature bed with several age classes takes
several years. A similar regeneration pattern is assumed to take place in C. edule
(Dorjes 1992).
Depending on the late availability of meroplanktonic larvae, settlement and recruitment of nearly all macrobenthic species occurred later in the year when compared to the previous two years of investigation. At the Groninger Plate juveniles
of the soft shell clam, M. arenaria, reached the extreme abundance of up to
200000 ind m 2 • Other species, such as H. filiformis or Ensis americanus recruited
in higher abundances as in 1994 and 1995.
The extraordinarily good recruitment of macrofauna, especially bivalves, after
cold winters was attributed to several non exclusive causes, being probably all
relevant to a different degree:
I. In a cold winter, gametes produced in the preceding summer are not resorbed
for the nutrition of the adults. Thus, the number of eggs spawned is higher and
subsequently the number of recruits is enlarged as well (Honkoop & van der
Meer 1997; compare Chap. 5.1).
2. It is assumed that a reduction of suspension-feeders increases survival of
planktonic larvae, thus positively affecting recruitment (Reichert & Dorjes
19RO). This effect was often postulated, but has so far only been supported by